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Mesa Language Manual

by James G. Mitchell William Maybury Richard Sweet

Version 5.0 April 1979

CSL-79.3

The Mesa language is one component of a programming system intended for developing and maintaining a wide range of systems and applications programs. Mesa supports the development of systems composed of separate modules with controlled sharing of information among them. The language includes facilities for user-defined data types, strong compile-time checking of both types and interfaces, procedure and coroutine control mechanisms, and control structures for dealing with concurrency and exceptional conditions.

XEROX

PALO ALTO RESEARCH CENTER SYSTEMS DEVELOPMENT DEPARTMENT

3333 Coyote Hill Road / Palo Alto / California 94304


Copyright 1979 by Xerox Corporation


CONTENTS


CHAPTER 1. INTRODUCTION 1.1. Syntax notation

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CHAPTER 2. BASIC DATA TYPES AND EXPRESSIONS

2.1. A slice of Mesa code

2.1.1. Basic lexical structure 2.2. Simple declarations

2.3. The fundamental operations. 4-, and #

2.4. Basic types

2.4.1. The numeric types INTEGER and CARDINAL

2.4.1.1. Numeric literals 2.4.2. Type BOOLEAN

2.4.3. Type CHARACTER

2.4.4. The numeric types LONG INTEGER and LONG CARDINAL 2.4.5. Type REAL

2.4.6. Relations among. basic types 2.5. Expressions

2.5.1. Numeric operators

2.5.1.1. Domains of the numeric operators

2.5.1.2. The operator LONG

2.5.1.3. CHARACTER operators 2.5.2. Relational operators 2.5.3. BOOLEAN operators 2.5.4. Assignment expressions 2.5.5. Operator precedence

2.6. Initializing variables in declarations
2.6.1. Compile-time constants

2.7. More general declarations

CHAPTER 3. COMMON CONSTRUCTED DATA TYPES

3.1. The element types

3.1.1. Enumerated types 3.1.2. Subrange types

3.1.2.1. Subranges of numeric types

3.1.2.2. Range assertions 3.2. Arrays

3.2.1. Declaration of arrays 3.2.2. Array constructors 3.3. Records

3.3.1. Field lists

3.3.2. Declaration of records 3.3.3. Qualified references 3.3.4. Record Constructors 3.3.5. Default field values 3.3.6. Extractors

3.4. Pointers

3.4.1. Constructing pointer types 3.4.2. Pointer operations 3.4.3. Long Pointers

3.4.4. Automatic derefcrencing


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3.5. Type determination

3.5.1. Type conversion

3.5.2. Balancing

3.5.3. Free conformance

3.6. Determination of representation

CHAPTER 4. ORDINARY STATEMENTS

4.1. Assignment statements

4.1.1. Assignment expressions

4.2. IF statements

4.2.1. IF expressions 4.3. SELECT statements

4.3.1. Forms and options for SELECT

4.3.2. The NULL statement

4.3.3. SELECT expressions

4.4. Blocks

4.4.1. GOTO statements 4.4.2. OPEN clauses

4.5. Loop statements 4.5.1. Loop control

4.5.2. GOTOS, LOOPS, EXITS, and loops

CHAPTER 5. PROCEDURES

5.1. Procedure types

5.1.1. Procedure values and compatibility

5.2. Procedure calls

5.2.1. Arguments and parameters

5.2.2. Termination and results

5.3. Procedure bodies

5.3.1. RETURN statements

5.4. A package of procedures
5.4.1. The example

5.4.2. Invoking procedures in other modules

5.5. Nested procedures

5.5.1. Scopes defined by procedures

5.6. Inline procedures

CHAPTER 6. STRINGS, ARRAY DESCRIPTORS, RELATIVE POINTERS, AND VARIANT RECORDS

6.1. Strings

6.1.1. Strine literals and string expressions

6.1.2. Declaring strings

6.1.3. Long strings

6.2. Array descriptors

6.2.1. Array descriptor types 6.2.2. Long descriptors

6.3. Base and relative pointers

6.3.1. Syntax for base and relative pointers

6.3.2. A relative pointer example 6.3.3. Relative pointer types 6.3.4. Relative array descriptors

6.4. Variant records

6.4.1. Declaring variant records 6.4.2. Bound variant types


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6.4.3. Accessing entire variant parts. and ' ariant constructors 6.4.4. Accessing components of variants

CHAPTER 7. MODULES, PROGRAMS, AND CONFIGURATIONS 7.1. Interfaces

7.2. The fundamentals of !lesa modules

7.1.1. Including modules: the DIRECTORY clause

7.2.1.1. Enumerating items from an included module: the USING clause 7.1.1. Accessing items from an included module

7.2.2.1. Qualification

7.2.1.2. OPEN clauses

7.2.3. Scopes for identifiers in a module

7.2.4. Implications of recompiling included modules

7.3. DEFINITIONS modules

7.3.1. Interface variables

7.3.2. Default fields in interfaces

7.3.3. lnline procedures in interfaces

7.3.4. Usage hints for inline procedures in interfaces

7.4. PROGRAM modules: IMPORTS and EXPORTS

7.4.1. IMPORTS, interface types, and interface records

7.4.2. Importing program modules

7.4.3. Exporting interfaces and program modules

7.4.4. IMPORTS in DEFINITIONS modules

7.5. Controlling module interfaces: PUBLIC and PRIVATE

7.5.1. Access attributes in declarations

7.5.1.1. Declared names

7.5.1.2. Names specified in field lists

7.5.1.3. Names for variant parts and for tags in variant records 7.5.2. Access attributes in TYPE definitions

7.5.3. Default global access

7.5.4. Accessing the PRIVATE predefined symbols of other modules 7.6. The Mesa configuration language. an introductory example 7.6.1. Lexicon: a module implementing LexiconDefs

7.6.2. LexiconClient: a client module

7.6.3. Binding, loading, and running a configuration: an overview 7.6.4. A configuration description for running LexiconClient 7.7. C/111esa: syntax and semantics

7.7.1. IMPORTS, EXPORTS, and DIRECTORY in C/Mesa

7.7.2. Explicit naming. IMPORTS, and EXPORTS

7.7.3. Default names for interfaces and instances

7.7.4. Multiple exported interfaces from a single component 7.7.5. Multiple components implementing a single interface 7.7.6. Nested (local) configurations

7.8. Loading modules and configurations: NEW and START

7.8.1. The NEW operation for making copies of modules 7.8.2. How the loader binds interfaces

7.8.3. STARTing, sToPping, and RESTARTing module instances 7.8.4. Loading and starting configurations

CHAPTER 8. SIGNALLING AND SIGNAL DATA TYPES 8.1. Declaring and generating SIGNALS and ERRORS 8.1.1. ERROR in expressions

8.2. Control of generated signals

8.2.1. Preparing to catch signals: catch phrases


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8.2.2. The scope of variables in catch phrases 8.2.3. Catching signals

8.2.4. RETRY and CONTINUE in catch phrases 8.2.5. Resuming from a catch phrase: RESUME 8.3. Signals within signals

CHAPTER 9. PORTS AND CONTROL STRUCTURES

9.1. Syntax and an example of PORTS

9.2. Creating and starting coroutines
9.2.1. The CONNECT statement

9.2.2. Low-level actions for a PORT call 9.2.3. Control faults and linkage faults 9.2.4. Saving arguments during faults

9.3. RESPONDING PORTS

CHAPTER 10. PROCESSES AND CONCURRENCY

10.1. Concurrent execution. FORK and JOIN

10.1.1. A process example

10.1.2. Process language constructs

10.2. Monitors

10.2.1. An overview of monitors 10.2.2. Monitor locks

10.2.3. Declaring monitor modules, ENTRY and INTERNAL procedures 10.2.4. Interfaces to monitors

10.2.5. Interactions of processes and monitors

10.3. Condition variables

10.3.1. Wait, notify, and broadcast

10.3.2. Timeouts

10.4. 1Alore about monitors 10.4.1. The LOCKS clause 10.4.2. Monitored records

10.4.3. Monitors and module instances

10.4.4. Multi-module monitors 10.4.5. Object monitors

10.4.6. Explicit declaration of monitor locks

10.4.7. lnline ENTRY procedures 10.5. Signals

10.5.1. Signals and processes 10.5.2. Signals and monitors 10.6. Initialization

APPENDICES

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Pronouncing Mesa

B.    Programming Conventions

B.1.     Names

B.2.    Layout B.3. Spaces

C. Alto/Mesa Machine Dependencies

C.1.     Numeric limits

C.2.    AltoDefs

C.3.    ASCII character set and ordering of character values

C.4.    Alto/Mesa STRING procedures


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Binder Extensions

Di Code packing 1).1.1. Syntax 1).1.2. Restrictions

112. External links 1/2.1. Syntax 112.2. Restrictions

E.    Mesa Reserved Words F. Collected Grammar

INDEX


Preface


This document describes the Mesa programming language. Its approach is tutorial, and it is intended to be read somewhat as a textbook. It is neither a user's guide nor a reference manual.

Elements of Mesa Style, by James Morris. is a recommended supplement to this manual. The style manual contains several examples of well-constructed Mesa programs, with explanations of their development and commentary on using the language properly. Its purpose is to provide assistance in using the features of Mesa to write programs that work reliably and are easily maintained.

Programmers. should also read the Mesa Users Handbook, which provides an introduction to the use of the Mesa system and a guide to other documentation. The Mesa System Documentation and Mesa Debugger Documentation describe facilities available in the Alto implementation of the Mesa programming system. These include input and output, which are done procedurally and are not built into the language.

Suggestions, corrections and criticisms concerning the style and content of this manual are encouraged and should be sent to your support group.

Acknowledgements

The Mesa language was designed and first implemented by the Computer Science Laboratory of the Xerox Palo Alto Research Center. The principal participants were Butler Lampson, James Mitchell,

Edwin Satterthwaite, Charles Geschke, and Richard Sweet. Subsequent development and

maintenance of the language and compiler have been done by the Systems Development Department of Xerox. In addition to the original participants, John Wick and Richard Johnsson have made major contributions.

The original version of this manual was written by William Maybury and edited by James Mitchell. To reflect changes in the language, many sections have since been added or revised. James Mitchell and Richard Sweet have served as editors, with additional contributions by Edwin Satterthwaite, John Wick and Richard Johnsson. David Redell wrote most of Chapter 10.

Vicki Parish, Gail Pilkington, Janet Farness, and Ode Binkley helped greatly with manuscript preparation and formatting: Gail Pilkington, James Sandman, Barbara Koalkin, and Bruce Malasky assisted in proofreading and indexing.


CHAPTER 1.

INTRODUCTION

This manual concentrates on the Mesa programming language. Mesa is really a programming system of which the language is but one part. Other components of the system are documented separately, as are the details of preparing, compiling, debugging and running Mesa programs.

Each chapter of this manual discusses some aspect of the language, using examples as well as descriptions of semantics and syntax. The chapters emphasize different language features and provide different levels of detail. The complete treatment of some features requires more than one chapter. Generally, earlier chapters introduce topics, and later ones supply additional detail. Titles of chapters, sections and subsections indicate the language issues with which they deal.

In each major section, information is presented at three levels:

(1)     Ordinary usage (motivation, forms and semantics), frequently with examples.

(2)     Syntax equations (when appropriate).

(3) Fine points (if applicable): restrictions, special cases, references to later material, precise semantics, etc.

Level (1) is intended to offer a basic understanding of Mesa. Reading only first level material should be adequate to begin programming in the language. Levels (2) and (3) supply more detail and provide information about the full power of Mesa.

As a rule. these levels of discourse occur separately and in the indicated order. A section with a heading followed by an asterisk (*) deals with specialized material that can be skimmed or skipped entirely on first reading. Occasionally, fine points or syntactic details are presented within first-level material. The reader will be able to distinguish between levels by their appearance. Fine points are written in a small font. like this. Syntax equations and syntactic categories appear in the following font: FontForSyntax.

Any italicized word or phrase is important. If a Mesa technical term is being introduced, it will be in italics: if a term is used before being defined, it will be italicized to warn the reader that it should not be taken lightly and that it has a particular meaning in Mesa. Occurrences of a technical term, once defined, are not distinguished. Lastly, names appearing in programs are italicized in both the program text itself and the explanations of that text.

Programming examples are indented relative to the surrounding text to distinguish them.


1.1. Syntax notation

Mesa's grammar is described by syntax equations written using a variation of Backus-Naur Form (or BNI). For those unfamiliar with BNF, an explanation follows. Reading and understanding that explanation is imperative for full use of this manual; in a first reading. details of the syntax equations can safely be skipped. Those familiar with BNF should scan this section to discover the particular variation being used.

An individual syntax equation defines a portion of the Mesa _grammar. It specifies a rule for forming some class of phrases in the language. A phrase class has a name. e.g., Program. and is defined by one or more syntax equations. Phrase names are always printed in the syntax font when their use is meant to be technically accurate. For example, an OctalDigit, which can be any of 0, 1, 2, . . 7, is defined by the equation:

OctalDigit :: = 011121314151617

Each equation consists of a phrase name on the left, followed by the operator :: (which should be pronounced is defined to be"), in turn followed by a formation rule for that phrase class A formation rule consists of one or more alternatives, separted by the syntactic operator vertical bar, I (which should be pronounced "or"). The ordering of alternatives is not important. In the definition of OctalDigit, "3" is an alternative.

Each alternative is a sequence of symbols, where a symbol is either a phrase name (in the syntax font) or a syntactic literal. In a syntax equation, a literal symbol stands for itself. The reserved words of Mesa, such as BEGIN, appear as literals; they are always written using upper-case characters

in the font shown. The digits 0, 1, 2, etc. and special characters, such as =, + and also are used to form literal symbols. Some composite symbols are formed from more than one special character, e.g., =>. Spaces in syntax equations are used only to separate the items in the rules and have no special significance.

The phrase name empty is often used as one of the alternatives in a formation rule. It means that
the rule permits an ''empty" phrase as one of its alternatives (i.e., an actual phrase is optional; it

may or may not occur in the result of applying the formation rule).

Comments embedded in syntax rules are preceded by a double dash. --, and appear to the right, e.g.,

Digit = OctalDigit 1819 -- a decimal digit is an OctalDigit or

an 8 or a 9

Often, only part of the total definition of a phrase class is given. To indicate that there are other ways of forming phrases of that class, an ellipsis (...) is used as an alternative within the rule. The definition of Statement is distributed throughout much of the manual in this way. When a certain statement form, such as the AssignmentStmt, is being discussed, the following partial rule appears:

Statement = AssignmentStmt I -- this is just an example.

One can read this as, "A Statement is defined to be an AssignmentStmt, among other things."

Within a single alternative, the order of symbols is important. The alternative acts as a "template" for forming an actual phrase; literal names and literal characters are copied, while substitutions are made for the phrase names. Consider the following example:

ReturnStmt = RETURN I RETURN Constructor

("A ReturnStmt is defined to be RETURN or RETURN followed by a Constructor.") The second alternative means that RETURN and some actual phrase defined by Constructor occur in exactly that order.


Syntax equations can indicate recursive substitution: for example:

IdList = identifier I identifier , IdList

In a Mesa program, an identifier is basically a name. This equation defines an IdList to be a list of one or more names, with commas separating them if there is more than a single name in the list.

This result is explained as follows. The formation rule for IdList consists of two alternative rules:

Rule 1: (First alternative) "An IdList is defined to be an identifier". i.e.. any one name can replace an IdList

Rule 2: (Second alternative) "An IdList is defined to be an identifier followed by a comma followed by another IdList". i.e.. name. IdList can replace an IdList.

To derive a single name. use Rule 1 as shown below. (Note: The substitutions are emphasized by writing them in italics.)

IdList ::= name (by Rule 1)
To derive two names separated by a comma:

IdList ::= name, IdList (by Rule 2)

name, name (by Rule 1)

To derive three names separated by commas:


IdList ::= name, IdList (by Rule

name, name, IdList (by Rule

name. name, name (by Rule

To derive n names separated by commas. use Rule 2


2) 2) 1)

n-1 times and then use Rule 1.


The following syntax equation also relies on recursion:

StmtSeries = empty I Statement I Statement ; StmtSeries

The equation is read as, "A StmtSeries is defined to be empty, or a single statement, or a series of statements separated by semicolons: the last statement may be followed by a semicolon."

A trailing semicolon is possible because:

1)      A StmtSeries may take the form specified by the third alternative. "Statement : StmtSeries".

2)      After some number of further substitutions using the third alternative. the recursive reference to StmtSeries may take the "empty" form. i.e.. ".. Statement : empty".

3) empty is replaced by nothing at all, i.e., "... Statement :".

Commas and semicolons are used as major separators for a variety of constructs in Mesa. To distinguish between such constructs, a convention is adopted that the suffix "List" on a phrase name implies a sequence separated by commas, while. "Series" implies a sequence separated by semicolons. This convention is reflected by the phrase names IdList and StmtSeries above.


CHAPTER 2.

BASIC DATA TYPES AND EXPRESSIONS

This chapter presents some of the fundamentals of Mesa. It discusses how to declare, initialize and assign values to variables. It also describes the basic types for numeric, character and Boolean data, as well as the operators used to construct expressions having these types.

The Mesa language is strongly typed. The programmer is given a collection of predefined types and the ability to construct new ones; he is encouraged to choose or invent suitable types for each particular application. Every variable used in a Mesa program must be declared to have one of these types; every constant has a type; and every expression has a type derived from its components and context. All types can be deduced by static analysis of the program, and the language requires that each value be used in a way consistent with its type according to rules specified here and in chapter 3. The type of an object determines its representation and structure as well as the set of applicable operations. In addition, the type system can be used to partition the universe of objects and avoid confusion, even among classes of objects that are represented identically.

2.1. A slice of Mesa code

The example below is an excerpt from a Mesa program. It assigns to gcd the greatest common divisor (GCD) of a pair of integers. in and n (where in, n and gcd are integer variables in the program from which this excerpt was taken; we assume their values need not be preserved). The example uses the Euclidean Algorithm for finding the GCD of two numbers and works as follows:

If both in and n are zero, the GCD is zero (by convention).

Otherwise, repeat the following until ii is zero: find the remainder of dividing in by set in to the value of n: then set n to the remainder. The final value of in is the GCD of the original in and n except that it may be negative; taking its absolute value gives the GCD.

Example. Slice of Mesa Code Using the Euclidean Algorithm

-- Given are integers in and n, which can be altered.

(1)  

IF 111=0 AND 11=0 THEN gcd 4- 0 -- by convention

(2)  

ELSE

(3)  

BEGIN

(4)  

r: INTEGER;

(5)  

UNTIL n= 0

(6)  

DO

(7)  

r in MOD n: r gets remainder of in/

(8)  

in 4- 12: n r -- update variables

(9)  

ENDLOOP ;

(10)  

gcd -- in case one of in or n was negative -- ABS[nij:

(11)  

END;

(12)  


The example contains twelve lines of source code. including comments. The numbers in parentheses at the right side are for reference only and are not part of the source-code. Comments begin with the symbol "--" and terminate at line endings. They may also be completely embedded within lines, in which case they both begin and end with "--''.

Line (2) begins an IF statement that uses the values of in and to select between two alternatives. If
both values are zero, the assignment statement following THEN is executed: it assigns the value 0 to

gcd (the character ".-." is Mesa's assignment operator). If either is nonzero. the assignment is skipped and the compound statement following ELSE (lines (4) through (12) inclusive) is executed. (Distinguishing the two cases is actually unnecessary, but doing so illustrates more features of Mesa.)

The second alternative is a block, a series of declarations followed by a series of statements, all bracketed by "BEGIN" and "END". Line (5) declares a variable r of type INTEGER for use within that block. A semicolon separates the declaration from the statements that follow it.

The iteration in the algorithm is performed by the loop (UNTIL n=0 DO...ENDLOOP). which contains three embedded assignment statements. The loop repeats until n is equal to zero. If it is zero at the outset, the embedded statements are not executed at all. Statements are separated by semicolons. A semicolon at the end of a statement series that is embedded in another statement (such as the series in the loop) is optional; it is permissible to write a semicolon after every statement in the series.

Within the loop, line (8) assigns to r the value of the expression "In MOD a", which gives the remainder of dividing in by n. Line (9) updates In to contain the previous value of n and then updates n for the next iteration, if any. Control transfers from the end of the loop, line (10), back to line (6), where the new value of n is tested. If it is not zero, the loop is repeated; otherwise, execution continues with the first statement following the loop, line (11).

When control reaches the assignment statement in line (11), in either has its original value (if 17 was zero) or contains the value a had just before it became zero. The expression "Aes[n]" has the form used for calling a function and passing it one or more arguments; square brackets enclose the

argument list. Normal parentheses, "(" and '')". are used only for nested expressions, e.g.,
"a*(b+c/(d e)*f)." The assignment places the absolute value of i into gcd; this is the correct

result. At this point, the reader is urged to trace through the example with initial values for m and
n of 15 and 12, respectively; the result should be gcd=3.

2.1.1. Basic lexical structure

The names gcd. in, n and r in the example are called identifiers. The general form of an identifier is given by the following (informal) syntax:

An identifier is a sequence consisting of' any mixture of upper-case letters, lower-case letters or digits, the first of which is a letter. Upper and lower case letters are different and do distinguish identifiers.

The following, valid identifiers are all distinct:

aBc Abc DiskCommandWord display Vector mach 1 x32y40

Identifiers consisting entirely of capital letters are reserved for use by the Mesa language. Some, / such as IF, are punctuation symbols: others name built-in types. such as INTEGER, or functions, such as ABS. All such words that have special meaning and are not to be defined by the programmer are
called reserved words. It is legal for the programmer to use fully capitalized identifiers, but he risks a clash with a reserved word (possibly a new one in some future version of the language). To avoid this. at least one digit or lower case letter should appear in any identifier. Appendix E lists the current set of reserved words.


Mesa uses the blank (or space) character to separate basic lexical units of the language (such as reserved words and identifiers). Blanks are significant separators of lexical units. They may not he embedded in identifiers, composite symbols (such as >= ), or numeric literals (such as 1000). Blanks are meaningful in STRING constants (section 6.1.1). and there is a CHARACTER constant for space (section 2.4.3). As a separator. any sequence of contiguous blanks is equivalent to a single blank. A TAB character also behaves exactly as a blank when used as a separator.

A carriage-return character behaves as a blank for separating lexical units also, but it has one extra function: if the last part of a line is a comment. the carriage return acts as the terminator of that comment. Thus, multiline comments (those containing carriage returns) must begin with "--" on each new line. Line breaks have no significance as statement separators. For example, the single loop statement in the example extends over a number of lines, and a semicolon is used to separate two statements in a series.

Semicolons are used for separating declarations, for separating a series of declarations from following statements, and for separating statements in a series from one another. They cannot be used with abandon, however: care is necessary when writing IF statements (sec. 4.2.1) or SELECT statements (sec. 9.3.1). Multiple statements can be written on a single line, separated by semicolons.

2.2.. Simple declarations

The example (Euclidean Algorithm) contains the following declaration:

r: INTEGER:

This declares r to be a variable of type INTEGER (sec. 2.4.1), one of Mesa's built-in types. More than one variable can be declared at the same time. For instance,

x, y, divisor: INTEGER;

declares identifiers x, y and divisor as variables of type INTEGER. These examples reflect the two primary purposes of every declaration:

to designate one or more identifiers as variables, and to specify their type. -

A declaration always begins with a single identifier or a list of identifiers. Conventionally. "list" is used to denote a single item as well as multiple items separated by commas. An identifier list (IdList) is defined as follows:

IdList = identifier I

identifier, IdList

A declaration begins with an IdList followed by a colon. The colon is followed by a type specification (INTEGER, for instance, is a type specification).

2.3. The fundamental operations: assignment, equality and inequality The example contains the following five assignment statements:

gcd 0

r ni MOD n

m 4- n

n 4- r

gcd ABS[M]


An assignment statement has the following syntax:

AssignmentStmt = LeftSide 4- RightSide I ...

LeftSide = identifier I ... -- plus forms for array indexing. etc.

RightSide = Expression

The RightSide may be any expression (section 2.5) provided that its type conforms to that of the LeftSide. "Conforms" is defined in section 2.4.6 and is discussed further in section 3.5: for now. it can be taken to mean: "is the same as." The LeftSide may be a simple variable or a component of an aggregate variable (such as an element of an array). In any event. a LeftSide denotes a variable, something capable of receiving values. A LeftSide cannot, for example, be a constant, while a

RightSide can.

The assignment operation ( <-.), the equality operation (= ) and the inequality operation (#) are called the fundamental operations. They can be applied to values of most types (including, for instance, entire arrays). The rules governing which pairs of operands may be used in a fundamental operation are detailed in section 3.5.

2.4. Basic types

The types of variables in a Mesa program fall into two broad classifications, built-in types and user-defined types. Chapter 3 describes how a programmer can define new data types using type constructors. This section discusses the basic, built-in types. These include several numeric types

(INTEGER, LONG INTEGER, CARDINAL, LONG CARDINAL and REAL). a type for logical values (BOOLEAN), and a type for individual character values (CHARACTER). The built-in type STRING (for sequences of characters) is described in chapter 6.

2.4.1. The numeric types INTEGER and CARDINAL

Mesa provides two standard numeric types, one with values ranging over the signed integers; the other, over the unsigned integers. Neither type completely mirrors the corresponding mathematical abstraction (the integers Z or the natural numbers N, respectively) because a finite representation is used for values of each type. The range of the type INTEGER is (approximately) symmetric about zero, and values of type INTEGER are represented as signed numbers. The range of the type CARDINAL is some finite interval of the natural numbers that includes zero, and values of type CARDINAL are represented as unsigned numbers. "Signed" and "unsigned" are not types; rather, they describe the machine representation of a numeric value.

The programmer must choose an appropriate type for each numeric variable. CARDINALS offer a somewhat greater positive range than INTEGERS, and this is significant in a few applications, e.g., those that manipulate addresses that might be the same size as the word size. More importantly, declaring a variable to have type CARDINAL asserts that its value is always nonnegative; the compiler can use such assertions to perform more checking and to generate better code. Programmers are encouraged to declare as much information about each variable as possible: the ranges of numeric variables can be further constrained by using subrange types (section 3.1.2).

The types INTEGER and CARDINAL are distinct and not interchangable. They are. however, closely related. Mesa allows most combinations of these types to occur within assignments and arithmetic expressions (but not relational expressions). Care is necessary to avoid ambiguity and failures of representation when values with different representations are mixed. This is discussed further in sections 2.4.6 and 2.5.1.1.


2.4.1.1. .Vumeric literals

A !runlet* literal is an instance of the phrase class number, defined as follows:

A number is a sequence of digits. The digits may optionally be followed by the letter B or B. which in turn may optionally be followed by another sequence of digits denoting a scale factor. No spaces arc allowed within numeric literals.

If I) is specified explicitly, or if neither B nor I) appears. the number is treated as decimal. The letter 13 means the number is octal (radix 8). A scale factor indicates the number of zeros to be appended to the first sequence of digits: the scale factor itself is always a decimal number. The literals below all denote the same value:

6400 MOOD 641)2 14400B 144B2

A numeric literal always denotes a nonnegative number (i.e., 5 is considered to be an expression in which the unary negation operator is applied to the literal 5 to produce an INTEGER value). To be valid in a context requiring a CARDINAL, the value of the literal must be a valid CARDINAL number. Similarly, if an INTEGER is required by context, the value must be a valid (positive) INTEGER. (See section 2.4.4 for more details)

2.4.2. Type BOOLEAN

A BOOLEAN value can be either TRUE or FALSE, and these are the only literals of type BOOLEAN: i.e., BooleanLiteral :: = FALSE I TRUE

BOOLEAN expressions are used in conditional statements (following IF) and in certain loop constructs. For instance, the following skeletal form describes the flow of control in Example 1:

IF n?=0 AND n=0 THEN . . . ELSE

. . .

UNTIL 17= 0

DO

. . .

ENDLOOP;

The expression "n=0" is a BOOLEAN expression: its value is TRUE if the value of n is zero and FALSE otherwise. The expression "m=0 AND n=0" is also a BOOLEAN expression: its value is TRUE just if both relations are. The relational and logical operators discussed in sections 2.5.2 and 2.5.3 all yield BOOLEAN values.

Variables of type BOOLEAN can be assigned values and appear as operands (although arithmetic operators) just as any other Mesa variables. For example, the above program could validly be replaced by the following:

mIsZero. nlsZero: BOOLEAN;

mIsZero (m=0): nlsZero n=0: -- compute whether in and n are zero IF ,nlsZero AND nlsZero THEN . .

ELSE

not of outline

UNTIL nIsZero= TRUE DO

. . .

nlsZero n=0;

ENDLOOP;

-- equivalent to just nlsZero by itself

-- recompute whether n is zero just before testing


2.4.3. Tpe CHARACTER

A value of type CHARACTER represents a single character of text. CHARACTER values are ordered I (according to the order specified in appendix C) and can be compared using the normal arithmetic relations. CHARACTER values are distinct from numbers. and they cannot be assigned to variables with numeric types. Limited arithmetic is, however, allowed on characters (section 2.5.1.2).

A characterLiteral is written as an apostrophe C) immediately followed by a single character (which can be a blank, carriage-return, semicolon. apostrophe. or any other character) or as an octal number followed by C. For example:

Text Box: -- mark is set to be a blank. Here a blank is significant end darker is set to be a semicolon
-- an Ascii Carriage Return character:
lowerCaseA 'a: mark 4-

enalarker asciiCR 4- 15C:

2.4.4. The numeric types LONG INTEGER and LONG CARDINAL *

For some applications, the ranges of the numeric types introduced in section 2.4.1 are too limited. Mesa provides both a predefined type LONG INTEGER, with signed representation, and a predefined type LONG CARDINAL, with unsigned representation, for such applications. These types offer greater ranges. but their values occupy more storage and are generally more time-consuming to manipulate than those of the previously introduced numeric types.

In an implementation, values of types INTEGER and CARDINAL are expected to be represented by single machine words, while values of types LONG INTEGER and LONG CARDINAL are expected to occupy two words. For this reason, INTEGER and CARDINAL will be referred to as short numeric types: LONG INTEGER and LONG CARDINAL, as long numeric types. On a machine using two's complement arithmetic and a word length of N bits, the following table indicates the range spanned by each numeric type (".." replaces the mathematician's comma in this interval notation):

INTEGER [ 2N-1

CARDINAL [0 .. 2N)

LONG INTEGER [_2 22N-1)

LONG CARDINAL [0 ..

The actual ranges for these types are given in appendix C. the machine dependencies appendix.

Long numeric constants are denoted by numeric literals defined by the phrase class number (section 2.4.1.1). The allowable type of any decimal or octal literal is determined by its value, as summarized by the following table (using the conventions introduced in the preceding paragraph):

Range

[0 .. 2N-1)

[2N-1 .. 2N)

[2'..2211)

p2N-1 22N)

Allowable Types

INTEGER, CARDINAL, LONG INTEGER, LONG CARDINAL CARDINAL, LONG INTEGER, LONG CARDINAL

LONG INTEGER. LONG CARDINAL

LONG CARDINAL

 

As in the case of short numeric types, the types LONG INTEGER and LONG CARDINAL are distinct but closely related. Mesa allows most combinations of these types and the types INTEGER and CARDINAL to occur within assignments, arithmetic expressions and relational expressions. but care is necessary when this is done (see sections 2.4,6 and 2.5.1.1).


Type REAL (interim) *

The values of Mesa's type REAL are approximations of mathematical real numbers. These approximations arc sometimes called floating-point numbers. For the current version of Mesa. a standard representation for floating-point values has not been chosen. The language nevertheless provides some help with floating-point computation. It allows declaration and assignment of REAL values, and REAL expressions constructed using the standard infix operators are converted to

sequences of procedure applications by the compiler.

A REAL value is assumed to occupy the same amount of storage as a LONG INTEGER (i.e., two words). Beyond this, no assumptions are made about the representation of REALS. There are no literals with type REAL. Users of real arithmetic must provide an appropriate set of procedures for performing the arithmetic and relational operations.

Although Mesa provides no denotations of REAL literals, it does provide automatic conversion from INTEGER, LONG INTEGER, CARDINAL or LONG CARDINAL to REAL (section 2.4.6). Thus numbers (numeric literals) can appear in REAL expressions and provide denotations of certain REAL constants.

2.4.6. Relations among basic types *

If two types are completely interchangable, they are said to be equivalent. A value having a given type is acceptable in any context requiring a value of any other type equivalent to it: there is no operational difference between two equivalent types. None of the basic types discussed in section 2.4 is equivalent to another basic type.

One type is said to conform to another if any value of the first type can be assigned to a variable of the second type. A type trivially conforms to itself or to any type equivalent to itself. In more interesting cases, an automatic application of a conversion function may be required prior to the assignment. Conformance and its implications are discussed further in section 3.5.

There are nontrivial conformance relations involving the types INTEGER, LONG INTEGER, CARDINAL, LONG CARDINAL and REAL. These relations allow certain combinations of the numeric types to be mixed, not only in assignments but also in arithmetic and relational operations (section 2.5). They also permit these types to share denotations of constants (section 2.4.4). The conformance relations can be summarized as follows:

INTEGER and CARDINAL conform to INTEGER.

INTEGER and CARDINAL conform to CARDINAL.

INTEGER, LONG INTEGER, CARDINAL and LONG CARDINAL conform to LONG INTEGER. INTEGER, LONG INTEGER, CARDINAL and LONG CARDINAL conform to LONG CARDINAL. INTEGER, LONG INTEGER, CARDINAL, LONG CARDINAL and REAL conform to REAL.

Pairs of numeric types not on this list do not conform: e.g., it is not possible to assign a LONG

INTEGER to an INTEGER or a REAL to a CARDINAL.

Particular care is required when numeric types with different representations are intermixed. Mathematically, Z D N: however. it is not necessarily true that INTEGER D CARDINAL or that LONG INTEGER D LONG CARDINAL. For instance, with the assumptions above, the intersection of INTEGER and CARDINAL is [0..2/v-1). Within this interval, the signed and unsigned representations agree. and the interpretation of a short numeric value is unambiguous. If a CARDINAL value lies in this range, it can validly be .assigned to an INTEGER variable, and vice-versa: outside this range, the value represented

w

by a given word depends upon whether it is viewed as a CARDINAL or as an INTEGER. Similar


considerations apply to LONG CARDINAL and LONG INTEGER.

Example:

With the assumptions above and A1=16, the unsigned value 177777B and the signed value 1 are encoded by the same bit pattern.

Assignment of an unsigned value to an INTEGER variable. or of a signed value to a CARDINAL variable. implicitly invokes a conversion function, which is just an assertion that the value to be assigned is an element of CARDINAL fl INTEGER. It is the responsibility of the programmer to ensure that the conversion is valid. In many cases this is not too difficult, but programmers are urged to avoid mixing signed and unsigned representations when this is possible. It almost always is.

Mesa does guarantee that LONG T D T for any type T and that LONG INTEGER 3 CARDINAL; thus it is always valid to assign a short numeric value to a LONG INTEGER variable or a short unsigned value to a LONG CARDINAL variable. The properties of conversion to type REAL are not specified by the language.

Some fine points:

A user supplied procedure FLOAT is automatically applied to convert a value from type LONG INTEGER to REAL. Short numeric values are converted first to LONG INTEGER and then to REAL.

Conversion from a short numeric value to a LONG INTEGER (and thus to a REAL) is substantially more efficient when the value has an unsigned representation.

The conversion of a constant to type REAL occurs even time the containing expression is evaluated at run-time.

Neither BOOLEAN nor CHARACTER conforms to any other basic type.

Examples:

INTEGER:

(valid)

n: CARDINAL; LONG INTEGER; x: REAL;

i 4 0;

ii 0:

x F n; x ii;

 

(invalid) i x:

11 4- TRUE;

2.5. Expressions

Expressions are constructs describing rules of computation for evaluating variables and for generating new values by the application of operators. The overall syntactic rule for an expression is given by

Expression = Disjunction AssignmentExpr IfExpr I SelectExpr ...

The Disjunction form includes all the numeric operations, relational operations, and BOOLEAN (logical) operations and is discussed in this section. An AssignmentExpr allows one to write multiple assignments in a single statement and is discussed in section 2.5.4. The IfExpr and SelectExpr forms are discussed in chapter 4.

The basic unit from which expressions are built is called a Primary. This syntactic class includes references to variables, literals, function calls (chapter 5), and any arbitrary expressions embedded in parentheses:


Primary = Variable I Literal I( Expression ) FunctionCall I

Variable = LeftSide

Literal = numbers BooleanLiteral characterLiteral

FunctionCall = BuiltinCall I Call -- defined in chapter 5

Recall that every expression has a well-defined type in Mesa. The general rules for determining the type of an expression from the types of its constituent parts are given in section 3.5. In this section. the types of the basic expression forms (as functions of the types of their operands) will be outlined. For example, the type of a Primary is the type of the Variable or Literal involved or reduces to the type of the Expression within parentheses, or is the type of the value returned by the BuiltinCall (some of which are defined below) or the Call of a user-defined procedure (section 5.1).

A Primary can be of almost any type; this is not true of most of the expression forms built up using Mesa's operators. Some operators are numeric and some are BOOLEAN. The next sections discuss the numeric operations, the relational operations, and the operations applicable only to BOOLEAN values. Considered together, the operators form a single hierarchy with respect to their precedence, which is described with each operator class and summarized in section 2.5.5.

2.5.1. Numeric operators

The operations on numeric values are addition, subtraction, multiplication, division, modulus, and arithmetic negation. The syntax for this group of operations is

Factor = Primary I Primary -- negation

Product = Factor I Product MultiplyingOperator Factor

MultiplyingOperator = *I / I MOD

Sum = Product I Sum AddingOperator Product

AddingOperator = + I

These operators have their usual mathematical meanings. The division operation on integers, /. always truncates toward zero: thus (i/ j)= i/ j= / j. The MOD operator yields the remainder of dividing one number by another (MOD is not applicable to REAL operands). MOD is defined by the relation (i/j)*j+(i MOD j) = i, and the sign of the result of MOD is always the sign of the dividend. (This is the reason that line 11 of Example 1 takes the absolute value of the computed gcd: if m= 12 and n=8 initially, the gcd would be 4 if its absolute value were not taken.)

The built-in function MIN computes the minimum value in a list of expressions; similarly, the MAX function, the maximum value. The built-in function ABS computes the absolute value of its

argument. The syntax for calls on the built-in functions is

BuiltinCall = MIN [ ExpressionList

MAX [ ExpressionList

ABS [Expression ]

. -- other built-in functions later

ExpressionList = Expression I ExpressionList , Expression

For the arithmetic operators and built-in functions. the order in which the operands are evaluated is undefined, but the syntax implies a precedence ordering that controls the association of operators with their operands. In that ordering. unary negation precedes the multiplying operators, which in turn precede the adding operators. Sequences of operators of the same precedence associate from left to right (with the exception of the embedded assignment operator, section 2.5.4). Thus, an


expression such as a+b*c does not specify the order of evaluation of a. b and c but does require that the operations be performed in the following order: negate c: then multiply the result by b; finally, add that result to the value of a.

Examples:

1. j. k: INTEGER; in, n: CARDINAL;

Factors: ri

15

(i+j+ k)

15

ma+, j, k, 15]

Products: /en

i/-15 15

n MOD 8

m/ n*10

k*(i+ 1)/2 moo 3

Sums: 1+1

i+j

i

n n MOD 8 in m/ n*n

-- same as (m/ n)*10 because of left-associativity -- same as a( k)*(i+1))/2) MOD 3

-- same as n(n MOD 8) because of precedence -- same as m MOD n

 

2.5.1.1. Domains of the numeric operators

In principle, each arithmetic operator designates the corresponding mathematical function. Unfortunately, the hardware underlying any implementation of Mesa does not provide this function but only a set of related partial functions. For each operator, the compiler must choose as appropriately as possible from this set. The choice is made by considering the types of the operands.

Example:

With the usual assumptions. 177777B and 1 are represented by the same bit pattern. The value of 177777B > 0 is TRUE, but that of 1 > 0 is FALSE.

Mesa provides the operators +, *, /, MIN. Max and ABS for all the numeric types. The operation moo is defined for all numeric types except REAL; the operation of unary negation. for all but CARDINAL and LONG CARDINAL. For each of these operators. the type of the result is the same as the

type of the operands. In addition, the result of the operation is considered to have signed representation if all the operands have signed representation, and to have unsigned representation if all the operands have unsigned representation. Thus, adding two INTEGER values yields an INTEGER result, and dividing one CARDINAL by another yields a CARDINAL result.

Some fine points:

Division and modulus operations on short numeric values are substantialh more efficient if their operands are unsiened.

Addition. subtraction. and comparison of lone numeric values are fast: multiplication and division are done by software and are relatively slow.

Operations upon REAL values are implemented as calls on user-supplied procedures. These procedures must be assignable to variables declared as follows {chapter 5):

FADD, FSUB. FM UL. FDIV: PROCEDURE [REAL, REAL] RETURNS [REAL]:

FCOMP: PROCEDURE [REAL, REAL] RETURNS [INTEGER]:

-- returns a value that is: 0 if equal. negative if the first is less, positive otherwise

FLOAT: PROCEDURE [LONG INTEGER] RETURNS [REAL]:

All other REAL arithmetic operations are fabricated from these primitives.


Although the mathematical integers (Z) and real numbers are closed under all these operations (except division by zero), the subranges defining the types INTEGER., LONG INTEGER, CARDINAL and LONG CARDINAL generally are not. When the result of an operation falls outside the range of its assumed type, a representational failure called overflow or underflon occurs. In the current version of Mesa, ii is the programmer's responsibility to guard against ovollow and undorflow conditions.

The implications of Mesa's conventions for subtraction are worth emphasizing. If both operands have valid signed representations, the result has a signed representation. If both have only unsigned representations, the result has an unsigned representation and is considered to overflow if the first operand is less than the second.

Example:

INTEGER: m, n: CARDINAL;

I m n: -- should be used only if it is known that m >= n

i IF n7 > n THEN 277 n ELSE (n 1n); -- a safer form (section 3.6)

The arithmetic operations are defined for operands that all have the same type, but it is possible to mix numeric types (and thus representations) within an expression. In this case, operands are convened as necessary to the "smallest" type to which all the operands conform, the operation for that type is applied, and the result also has that type. The rule for expressions involving type REAL is easy to state:

If any operand has type REAL, the REAL operation is used.

The rules governing combination of numeric operands with differing representations involve some additional concepts and are stated in section 3.6. Again, the programmer should try to avoid such combinations when possible. (Recall that literals in INTEGER n CARDINAL have whatever representation is required by context.)

2.5.1.2. The operator LONG

The built-in function LONG converts any value with a short numeric type to a long numeric type. A value with an unsigned representation is converted to LONG CARDINAL; one with a signed

representation, to LONG INTEGER, The syntax is as follows:

BuiltinCall ,.. I LONG [ Expression ]

This operation is necessary when the standard conversion rules do not give the desired result. It can also be used to emphasize the conversion.

Example:

LON G[ni* n] -- "short" multiplication, overflow lost

LONG[nd*LCNG[n] -- "long" multiplication

Some fine points:

Lenethenima a sinw.le-precision expression is substantially more efficient if that expression has an unsigned
representation.

The Mesa implementation provides standard procedures (not part of the languaae) for performing certain multiplication and division operations in which the operands and results do not all have the same length. These procedures provide less expensive equivalents of, e.g LONG[nirLONG[rd.


2.5.13. CHARACTER opera/ors *

Limited CHARACTER arithmetic is possible and is sometimes useful for manipulating the encodings of CHARACTER values. The following arithmetic operations arc defined for operands of type

CHARACTER:

A CHARACTER value plus or minus a short numeric value yields a CHARACTER value. Subtracting two CHARACTER values yields an INTEGER value.

No other arithmetic operations on characters arc allowed. Since the results of character arithmetic depend upon details of the character encoding. such arithmetic should be used with discretion.

Examples:

c: CHARACTER: digit: INTEGER:

digit 4- c -0;

c (- 'A + (c'a)

2.5.2. Relational operators

-- assumes c is lower case

 

The relational operators include = and #, <, <= (less than or equal). >= (greater than or equal), >, and their negatives (e.g., NOT<, <. >=, etc.). These operators always yield BOOLEAN results, depending on the truth or non-truth of the relation expressed. The operators = and # apply to most types: the others, to any ordered type (i.e.. to any type whose values are considered to be ordered). Ordered types include INTEGER, LONG INTEGER, CARDINAL, LONG CARDINAL, REAL, BOOLEAN. CHARACTER (with the ordering given in appendix C). enumerated types (section 3.1), and subranges of ordered types (section 3.1).

The relational operators also include the composite operator IN, which takes a numeric value as its left operand and an interval as its right operand. Its value is TRUE if the left value lies in the interval and FALSE otherwise. The syntax for relational operators is


Text Box: Relation	=
RelationTail
Sum I Sum RelationTail

RelationalOperator Sum

Not RelationalOperator Sum I

IN SubRange

Not IN Su bRange

Text Box: RelationalOperator Not
SubRange SubRangeTC Interval
Text Box:  (1<= =

NOT

SubRangeTC -- explained in chapter 3

Interval ... -- explained in chapter 3

Expression .. Expression ) ( Expression .. Expression ) ( Expression .. Expression ] [ Expression .. Expression ]


The extra syntax for Su bRange and Su bRangeTC is placed here to be consistent with later uses of the class Interval in chapter 3. The syntax for intervals follows mathematical notation: a square bracket indicates the inclusion of the respective end point in the interval, while a parenthesis indicates its exclusion. For example, the following intervals all designate the range from 1 to 5 inclusive:

[I 5] 1-1 .. 6) (-2 .. 6) (-2 .. 5]


In the above examples. 1 is the lower bound of each interval: the upper bound is 5. The hounds of an interval are its end points, reeardless of whether the interval is written as a closed. half-open or open one. The hounds are not required to he constants. An interval with an upper bound less than its lower is said to be empty: no values lie in such an interval. For example, the following are all empty intervals:

2] [-1 .. I) (-2 .. 1) (-2 .. 2]

Examples: Relations:

n = 15

# n

I <= .1

(i < j) = (1 < k) n IN [1 .. 5)

i NOT IN 1 1 .. 5]

-- or m = 11

= with two BOOLEAN operands

>= 1 and n < 5

-- only legal if i is signed (because 1 is)

 

A fine point:

The relational operators, like the arithmetic operators, denote families of hardware operations when they have numeric operands. Anain, there is one operation for each numeric type. If there is a unique "smallest" type to which all the operands conform, they are convened to that type as necessary and then the comparison is performed. There is no unambiguous choice of such a type for numeric operands with different representations:

an attempt to compare two such values is an error. The precise rules appear in section 3.5.

2.5.3. BOOLEAN operators

The operators NOT (logical negation), AND and OR apply only to BOOLEAN values. The syntax is

Negation = Relation I Not Relation

Conjunction = Negation I Conjunction AND Negation

Disjunction = Conjunction I Disjunction OR Conjunction

NOT ?legates the logical value of a BOOLEAN expression. p AND q has the value TRUE if and only if both p and q are TRUE. p OR q is TRUE if at least one of p or q is TRUE.

When evaluating a Boolean expression. evaluation of primaries is guaranteed to take place from left to right. In the operation AND or OR, the second operand is evaluated only if the first operand's value does not determine the value of the expression.

A fine point:

"x AND y" is equivalent to the IfExpr "IF .v THEN v ELSE FALSE": i.e., when x is FALSE. y is not o aluated.

"x OR y" is equivalent to the IfExpr ''IF x THEN TRUE ELSE y'': i.e.. when x is TRUE. y is not evaluated.

It is therefore safe to have expressions of the form "x AND y". where y is defined only when x is TRUE. e.g.. "x#0 AND c/ x > 2". or "p=NIL OR p1=0".

Examples:

Negations: NOT 1=15 -- same as NOT(i=15)

q q must be of type BOOLEAN
(p AND q)

Conjunctions: 1 <= j AND j < k

p AND g

i=5 AND j NOT IN [ 1..1]

Disjunctions: in>n OR in =15

p OR q


2.5.4. Assignment expressions

The assignment operation can be embedded in other expression forms. When it is. the result of the operation has the type of the LeftSide and the value received by the LeftSide in the assignment. The "+-" operator has the lowest precedence of any operator. Its syntax is the same as that of the

AssignmentStmt:

AssignmentExpr = LeftSide (- RightSide

If this form is used to perform multiple assignments. it is important to note that "4-" is right-associative. Thus. the assignment expression a<--b4-b+1 first assigns the value of b+1 to b and then assigns b's new value to a.

Examples: Assignment

Expressions:

m4-15 m4-n*-15

tw-n4-n+1 -- same as m4-(n4-(n+1))

Hj+-(j+1) MOD n)*2 -- all these parentheses are necessary

 

Rules governing assignments of numeric values when the types are not identical are summarized in section 2.4.6.

Fine point:

Because the order of evaluation of the primaries is not defined, expressons such as "(i< j) + (p- kF have unpredictable values and should not be used.

2.5.5. Operator precedence

The following table summarizes the precedences of the unary and binary operators introduced in this section. The order is from highest precedence (tightest binding of operands) to lowest; operators on the same line have the same precedence.

-- unary negation

*, MOD

+, -- subtraction

=, #, <=, >, >=, IN

, NOT

AND

OR

4-

Parentheses can be used to explicitly control the association of operands with operators.

2.6. Initializing variables in declarations

A variable may be given an initial value in a declaration. For example, the Boolean variable delimited could be set initially FALSE by using the declaration:

delimited: BOOLEAN 4- FALSE:


Variables (of the same type) can be initialized collectively:

11. nO: INTEGER 4- 7:

This declares two separate integer variables n and 110 and initializes each to 7.

Any expression that could be used as the Rig htSide of an assignment can be used to initialize a variable:

1: INTEGER 4- ABS[n]; -- this will set i to 7

i.S'quared: INTEGER 4- 14 -- Squared is initialized to 49

j: INTEGER 4- i.S'quared i+1: j is initialized to 49 7+ 1 = 43

All initializations shown so far have taken "assignment" (or "4-") form. There is another form. the "fixed" (or "=") initialization. For example.

octalRadix: INTEGER = 8;

This means that octalRadix is to have a fixed value. It is never valid as the LeftSide of an assignment. We call octalRadix a constant because its value can never change after it is initialized (recall that the number 8 is called a literal). Normally. the term "constant" will include the term "literal"; if the distinction is important, then "literal" will be used.

Initial values for fixed initialization can be arbitrary expressions. Paraphrasing the earlier example:

i0: INTEGER = ABS[ octalRadix]: iOSquared: INTEGER = 10* i0: j0: INTEGER = iOSquared i0+1;

The initializing expression can use values that are not known at compile time. In this example, if octalRadix did not have fixed initialization, the values of la iOSquared, and j0 would be computed and assigned at run-time. Variables are initialized in the order of appearance in a declaration, and later declarations can use variables initialized earlier, as shown by the example.

2.6.1. Compile-time constants

Wherever possible, the Mesa compiler evaluates expressions containing only constants. If a variable is initialized using the fixed form and the expression can be evaluated at compile time, then that variable has a known value. Since it can never appear as the LeftSide of an assignment operator, it too becomes a compile-time constant (the variables i0, i0Squared. and JO in the previous section are all compile-time constants).

Example:

beta: INTEGER = 3;

alpha: INTEGER = beta 1:

In this case, alpha is a compile-time constant (with the value 2), since the expression beta-1 involves only compile-time constants. Compile-time constants need not occupy memory at run-time; the compiler can replace references to compile-time constants, such as alpha and beta, by their known values.

Some fine points:

Knowledge of compile-time constant values can also be exploited when analyzing expressions. processing other declarations. or generating object code.

One side effect of this propagation of constants is that the representation of a numeric constant is known at compile-time. For instance. alpha above is declared to be an INTEGER. but because its value is 2. it may also be used as a CARDINAL. However, declaring the type of alpha determines what kind of arithmetic (signed or unsiened) will be used to compute its value. whether at compile-time or run-time (section 2.5.1).


In certain contexts. an expression is required to yield a compile-time constant value. A (sub)expression denotes such a constant if all the operands are compile-time constants and the operation is not one of those listed below (current restrictions):

Conversion of a numeric value to type REAL.

Any arithmetic or relational operation with operands of type LONG INTEGER. LONG CARDINAL or REAL.

Application of an function (chapter 5) other than a built-in function.

The @ operation (section 3.4).

The SELECT operation (section 4.3.31.

2.7. More general declarations

Preceding sections have introduced all the syntactic components of a declaration. The general form is defined as follows:

Declaration :: = IdList : TypeSpecification Initialization

For the moment, TypeSpecification is defined as one of the built-in types: chapter 3 describes other forms of TypeSpecification.

TypeSpecification :: = PredefinedType I .

PredefinedType = INTEGER I CARDINAL I

BOOLEAN I CHARACTER

LONG INTEGER I LONG CARDINAL I REAL I

STRING I -- see chapter 6

WORD I -- see fine point below

UNSPECIFIED -- see fine point below

An Initialization is formally defined as follows:

Initialization :: = empty

<- Expression

= Expression

--other forms are given later

Fine points:

The predefined type WORD is provided to describe values on which bit-by-bit loaical operations are to be performed. Currently. it is a synonym for CARDINAL.

The predefined type UNSPECIFIED is a device for by-passing most type checking. An UNSPECIFIED value is a single machine word. and it matches the type of any object that occupies at most a single machine word. including INTEGER. CARDINAL. CHARACTER. BOOLEAN. UNSPECIFIED, STRING. and any user-defined type (chapter 3) that fits in a single machine word.

For numeric operations. its representation is similarly fluid. If a CARDINAL and an UNSPECIFIED value are the operands of some arithmetic operation. then the UNSPECIFIED value is considered to be unsigned. If an UNSPECIFIED is combined with a signed value. it is treated as if it were signed too. If an UNSPECIFIED is

combined with an UNSPECIFIED. they are both treated as siened.

Less type checking is sacrificed by using LOOPHOLE (section 3.5.1) than by declaring variables with type UNSPECIFIED.


CHAPTER 3.

COMMON CONSTRUCTED DATA TYPES

Mesa encourages the programmer to augment the collection of predefined types by constructing new types. Types can be defined to describe objects that are structured collections of related values (e.g., a vector of Booleans, a table, or a complex number consisting of real and imaginary components). Mesa's type system has other, perhaps less obvious applications. These include expressing some of the programmer's knowledge about a class of variables (e.g., that all take on values restricted to some known interval), separating variables with different semantics into different classes so that they cannot be confused (e.g., to avoid "comparing apples and oranges"), and hiding implementation details of abstractions (e.g., to prevent the user of a table-lookup package from depending upon the internal organization of the table).

Programmer-created types have the same status as Mesa's built-in types. They can be used to declare variables and to construct further new types. In addition, values of most constructed types can be operands of the fundamental operations (4-, =, #).

A new type identifier is declared using the following syntax:

TypeDecla ration :: = idList : TYPE = TypeSpecification

Each identifier in the idList is thereby declared to name the type denoted by the TypeSpecification. If this declaration form is compared to a normal declaration, i.e.,

Declaration = IdList : TypeSpecification Initialization :

it can be seen that "TYPE.' fills the role of a TypeSpecification, and "= TypeSpecification" plays the role of Initialization. In fact, the newly declared identifier has type "TYPE" and a value (which must be constant, hence the "=") that is a TypeSpecification.

Any predefined Mesa type (section 2.7) is a valid TypeSpecification: thus the following are valid type declarations:

SignedNumber: TYPE = INTEGER;
UnsignedNumber: TYPE = CARDINAL;

Truth Value: TYPE = BOOLEAN:
Char: TYPE = CHARACTER;

These type identifiers are now valid type specifications and can be used to declare variables:

j: SignedNumber, n: UnsignedNumber,

b:       Truth Value:

c:       Char;

After this series of declarations, i and j have type SignedNumber, which is equivalent to INTEGER: n has type UnsignedNumber, which is equivalent to CARDINAL; etc. This is a trivial way of defining


new types. A more interesting way uses a type constructor as the TypeSpecification and generates a truly new type. not just an additional name for an existing one. A TypeSpecification can be defined as

TypeSpecification :: = PredefinedType

Typeldentifier TypeConstructor

(TYPE itself is not a TypeSpecification: it can be used only to declare types.)

There is an important point worth emphasizing here. A TypeSpecification that is a PredefinedType or a Typeldentifier denotes an existing type and yields the same type every time it is used. A declaration such as the one of SignedNumber introduces a synonym for the name of an existing type. Synonyms can be more descriptive and thus improve readability, but they do not partition the set of values. The types SignedNumber and INTEGER are fully equivalent, and values with these types can be used interchangably. On the other hand, a TypeConstructor constructs a new type. The rules for equivalence and conformance of constructed types depend upon the forms of their constructors and are discussed as the constructors are introduced. In some cases, each appearance of a constructor generates a unique type, i.e.. writing the same sequence of symbols twice generates two distinct, incompatible types. For this reason, programmers usually should name such a type, using a TypeDeclaration, and thereafter use the type's identifier. Of course, introducing an identifier for a constructed type can make a program easier to read and modify in any case.


Text Box: The predefined types are types in Chapter 10).
Typeldentifier
described in chapter 2 (except for STRING in chapter 6 and process related The simplest form of a Typeldentifier is given by

= identifier I -- which is a declared type

-- other forms given in chapters 6 and 7


The rest of this chapter discusses the attributes and uses of some common constructed types: enumerations, subranges, arrays, records, and pointers. The syntax for TypeConstructor is


TypeConstructor = EnumerationTC

Sub rangeTC A rrayTC

RecordTC PointerTC LongTC

Procedu reTC

A rrayDescriptorTC RelativeTC SignaITC

PortTC

ProcessTC


-- for enumerations -- for subranges

-- for arrays

-- for records

-- for pointers

-- for long pointers, etc

-- see chapter 5 -- see chapter 6 -- see chapter 6 -- see chapter 8 -- see chapter 9 -- see chapter 10


(The suffix "TC" is to be understood as an abbreviation for "TypeConstructor''.)

Enumerations define a set of values by giving a list of identifiers members of an ordered set.

. These identifiers can be viewed as

 

Subranges define types with values drawn from those of a larger, encompassing type but restricted to lie in a specified interval. The subrange takes on the characteristics of the enclosing type: for example. a subrange of INTEGER can be used to declare variables that behave as INTEGERS but are constrained to take values within some interval.


Arrays arc sequences of components that arc homogeneous with respect to type and are accessed by computed indices ("subscripting"). Records are sequences of components that have potentially different types and are accessed using fixed component names ("selection"). Records and arrays are Mesa's aggregate data types.

Pointers arc scalar values used to access data objects indirectly. A pointer value is represented by an address. Pointers can be used to build linked lists, tree structures, etc. Long pointers are pointers capable of spanning a larger address space than ordinary pointers.

Chapter 3 concludes with a discussion of type determination. the process by which Mesa decides whether an expression has an acceptable type for a given operation. This is closely related to questions of the equivalence and conformance of types.

3.1. The element types

This section describes a class of types called element types. Their common properties are the following:

(1)   They are ordered types; values of an element type can be operands of all the relational
operators (section 2.5.2).

(2)   They are scalar types; a value with an element type does not have any visible or directly
accessible internal structure insofar as the language is concerned.

(3)   They can be used to declare subrange types (section 3.1.2).

(4)   They are the only types allowed as index types of arrays (section 3.2). The element types are INTEGER, CARDINAL, CHARACTER. BOOLEAN. the types generated by

EnumerationTC, and the types generated by SubrangeTC. Because of (3) above, this definition is recursive; subranges of subranges are allowed. The definition of the class ElementType is

ElementType = INTEGER I CARDINAL I CHARACTER I BOOLEAN

EnumerationTC Sub rangeTC

A fine point:

Note that LONG INTEGER and LONG CARDINAL. althoueh ordered scalar types, are not element types. It is not possible to declare subranges of these apes or to use lone numeric values as array indices.

3.1.1 Enumerated types

Consider the following declarations and a typical assignment:

channel.S'tate: INTEGER;

disconnected: INTEGER = 0;
busy: INTEGER = 1;

available: INTEGER = 2;

channelS tale f- busy:

Suppose channelState is a variable that is intended to range over a set of three "states" named disconnected, busy, and available, which are represented by values 0, 1, and 2. These values have no real significance; 5, 6, and 7 would serve equally well. Enumerated types are well suited to such an application (where the underlying values are unimportant). The above declarations could be replaced by a single declaration of a variable with an enumerated range:


channelState: {disconnected, busy, available}: channelState - busy:

The effect is the same as before: channelState is a variable with values ranging over the same "states". and similar assignment statements can be used.

The enumeration has some advantages over the original declarations:

It is more convenient: the programmer does not have to provide values for disconnected, busy, and available.

It allows more type checking. In the INTEGER case, one could assign any short numeric value to channelState.

It helps documentation; an enumeration shows all of its possible values.

An enumerated type is constructed by specifying a list of identifiers between braces, "{...}". These identifiers are not variables, but constants of that enumeration called identifier constants. They represent nothing more than their own names.

The type constructor EnumerationTC is defined as follows: EnumerationTC = { IdList }

The IdList supplies all the identifier constants for the enumeration, and duplication of identifiers is illegal. Separately specified enumerations are distinct. Every appearance of an EnumerationTC generates a new type that is not equivalent to, and does not conform to. any other enumeration. Thus the declarations

foreground: {red, orange, yellow, green, blue, violet background: {red, orange, yellow, green, blue, violet

specify two different enumerations. It is illegal to assign background to foreground, despite the fact that the same identifier list appears in each declaration. Occasionally, the inability to declare any further variables with the same type can be used to advantage by the programmer. Otherwise, the best way to avoid such problems is first to declare a type and then to declare variables using the identifier of that type; for example:

Color: TYPE = {red, orange, yellow, green. blue. violet foreground: Color,

background: Color:

This allows the assignment of background to foreground as well as the declaration of further variables with the same type (perhaps initialized differently).

The identifier constants in two different enumerated types have no association whatsoever and do not need to be distinct from one another. To identify unambiguously the enumeration from which a constant is taken, one can, and sometimes must, qualify' the identifier constant by the name of the enumerated type. For example, eiven the additional declaration

Fruit: TYPE = {orange. lemon};

Color[orange] denotes a color and Fruit[orange] denotes a fruit. More generally, the syntax used for this form of qualification is

Primary = I Typeldentifier [ identifier ]

(This adds a new case to the syntactic definition of Primary, which already allows an identifier constant.)


Often qualification is not necessary: for instance, the following is permitted:

hue: Color;

hue orange: -- the type of hue implies Color[orange]

In the following situations, an identifier constant need not be qualified. because the intended

enumerated type is established by the context:

as the RightSide of an assignment

as an initializing Expression

a component in an array or record constructor (sections 3.2.2 and 3.3.4)

--_-_ I as an argument of a procedure (chapter 5)

as an array index (section 3.2)

as the right operand of a Relation, including that part of a Relation used to label an arm in a discrimination (section 4.3)

as the bounds in a Su b rangeTC (section 3.1.2)

The values of an enumeration are ordered. The ordering is given by the order of appearance in the IdList used to construct the enumerated type. The leftmost identifier has the smallest value, and values increase from left to right. The following relations all have the value TRUE:

Color[red] < Color[orange]

Color[red] < violet

hue IN [red .. -- assuming hue = orange

There are two additional built-in functions that are applicable to enumerations:
FIRST [TypeSpecification] yields the smallest value of the specified enumeration; e.g.,

FIRST [Coloij= red. Similarly, LAST [TypeSpecification] produces the greatest value in an

enumeration; e.g., LAST [Color], violet. It is also possible to iterate over all values of an enumeration
(section 4.5).

The predefined type BOOLEAN is really an enumerated type, and its definition is

BOOLEAN: TYPE = {FALSE, TRUE}:

Thus, FALSE<TRUE, FIRST [BOOLEAN] = FALSE, and LAST [BOOLEAN] = TRUE. Note, however, that the BOOLEAN constants TRUE and FALSE may always be used without qualification.

3.1.2. Subrange types

In many cases. the values of a variable are inherently range-limited. For instance, a value for day (of the month) lies in the range [1..31]. In other cases, the range is limited by design. For instance, a value for year might be limited to the range [1900..1999]. Mesa permits the user to declare such variables in the following way:

day: CARDINAL [1 .. 31];

year: CARDINAL [1900 .. 1999];

Since these intervals cover a subrange of CARDINAL, the variables day and year are called subrange variables. It is useful to think of day and year as having type CARDINAL with the additional constraint that values are restricted to the specified intervals.


Subrange types have a number of advantages and uses. Subrange declarations unambiguously document the range of values intended for a variable and thus aid software maintenance. The compiler is able to optimize storage allocation when dealing with range-restricted variables (for example. in arranging the fields of a record. section 3.3) and can take advantage of subrange declarations to generate more efficient object code.

The general form of a Su brangeTC is

Su brangeTC = Typeldentifier Interval I

Interval

The Typeldentifier must evaluate to an ElementType. Thus, one can declare types that are

subranges of INTEGER, CARDINAL. CHARACTER, BOOLEAN, enumerated types, and other subrange types. For example,

SymmetricRange: TYPE = INTEGER [

PositiveInteger TYPE = CARDINAL [1..LAST [INTEGER]];

UpperCaseLetter: TYPE = CHARACTER ['A..'Z];

DegenerciteTjpe: TYPE = BOOLEAN [TRUE..TRUE]:

CoolC olor: TYPE = C oloityellow..LAsT [Color]]: -- excludes red orange, yellow

AthroughM: TYPE = UpperCaseLetter[' A..'M]; subrange of a subrange

The base type for a subrange is that type of which it is a subrange and which is not itself a subrange; e.g., the base type for both UpperCaseLetter and Athroughill is CHARACTER.

The Expressions that define the end points of an interval must have types that conform to the type denoted by the Typeldentifier (or yield short numeric values if the identifier is omitted). Also, for the purpose of defining a subrange type, the end points must be compile-time constants.

A fine point:

It is permissable for the interval defining a subrange type to be empty. It is not legal to use a variable of such a type. but an empty subrange is sometimes useful for specifying the bounds of an array in a record declaration (section 3.2).

A subrange type conforms to its base type, and a base type conforms to any of its subrange types. By extension, any two subrange types with the same base types are mutually conforming (even if they do not overlap in any way). A more revealing point of view is that the value of a subrange variable has the base type as its type. and an assignment of a value to a subrange variable makes an associated assertion that the value is in the appropriate interval. A violation of such an assertion is called a range error. It is the programmer's responsibility to guard against range errors. As implied by this viewpoint, appropriate literals of the base type serve as literals of the subrange type, and any operations defined on the base type automatically extend to the subrange type (but usually without closure).

Examples:

n: CARDINAL [0..10]; in: INTEGER [ 5-5]:

in 0; n <- 0; -- inherited literals

n F n+1; -- not valid if n = 10

17 4" in; -- only valid if m IN [0..5]

The preceding discussion implies that subrange restrictions can be ignored in answering many type-related questions; in this sense. subrange types are "weak." Two subrange types are equivalent if their base types are equivalent and if the corresponding bounds are equal. For these types, equivalence is much stronger than conformance. Equivalence becomes important when subrange types are used in the construction of other types.


FIRST and LAST are applicable to all subrange types and yield the corresponding bound. For example. FIRST [CoolColod= green and LAST [Athroughill]='M. It is also possible to iterate over all values in a subrange (section 4.5).

A fine point:

The operators FIRST and LAST are applicable to all element apes. including INTEGER. CARDINAL and CHARACTER_ as well as LONG INTEGER and LONG CARDINAL. When applied to the numeric apes. they supply information about the range of ialues supported by a particular implementation.

3.1.2.1. Subranges of numeric types *

The description above applies to subranges of both enumerated and numeric types. Numeric subranges introduce one further complication, which is the question of representation. Omission of the initial Typeldentifier in a SubrangeTC is permissable if and only if each bound in the Interval specifies a short numeric value. In that case, INTEGER or CARDINAL is the base type, and the choice depends upon the representations of the bounds.

A numeric subrange type has a signed representation if both bounds are elements of INTEGER and at least one is not an element of INTEGER n CARDINAL. Similarly, it has an unsigned representation if both bounds are elements of CARDINAL and at least one is not an element of INTEGER n CARDINAL. If both bounds are elements of INTEGER n CARDINAL, values of that subrange type are considered to have both representations. Any other combination of bounds is illegal.

Examples:

sl : [ 10..10]; -- signed representation

s2:   [100..33000]; -- unsigned representation (if 33000 > LAST [INTEGER])

s3:   [0..10); -- both representations

With respect to the choice of signed or unsigned versions of arithmetic. and relational operators, a quantity with both representations is treated flexibly. When combined with an unsigned value. it is considered to be unsigned; the unsigned operation and result are chosen. When it is combined with a signed value, the operation and result are signed. The rules governing combinations of values with both representations depend upon the context in which the result is used; the default is to choose signed representation and INTEGER operations. The precise rules are discussed in Section 3.6.

Examples:

i: INTEGER; n: CARDINAL; -- plus the declarations above

(signed)

(unsigned)

sl + 1 sl + s3 s3 i

s2 + 1

s2 + s3

s3 * n

 

A fine point:

The representation assumed for a literal also depends upon context. In fact, any short numeric constant c is treated as if its type were [c..d.


Range assertions *

Assignment to a subrange variable implies an assertion about the range of the expression being assigned. The programmer may make such an assertion explicitly, for any expression, by using a range assertion. If S is an identifier of a subrange type and e is an expression with a type T conforming to 5, the Primary S[e] has the same value as e and is additionally an assertion that e

IN [FIRST rsn LAsT[sn 7]] is TRUE. In addition to user defined types, the basic types INTEGER

and CARDINAL may be used in range assertions.

.4 program that violates one of its range assertions is in error. In addition to providing
documentation and (optional) run-time checking, a subrange assertion affects the attributes attached

to an expression. For example, an assertion of an INTEGER range (or a signed subrange) forces the result to be treated as a value with signed representation. This is useful for controlling the choice of an operation when the intended one cannot correctly be inferred from the operands (section 3.6).

Examples:

1.- INTEGER; n: CARDINAL; S: TYPE = [0..10];

CARDINAL [i] i is asserted to be nonnegative

Sin] -- asserts n IN [0..10]

3.2. Arrays

Arrays are indexable collections of homogeneous components. In other words, the components of a given array all have the same type, and each corresponds to one index value in a range of indices associated with that array. The range of indices (which is actually a type called the index type) and the component type determine the array type. For example:

earningsPerQuarter: ARRAY [1..4]0F INTEGER;

declares a variable with a constructed array type having an index type of [1..4] and a component type of INTEGER. Thus, earningsPerQuarter is an array of four integer elements: earningsPerQuarter[1], earningsPerQuarter[2], , earningsPerQuarter[4]. earningsPerQuarter by itself refers to the entire array variable. (Aggregate variables and components of aggregates are generally called "variables". If a distinction is needed, the term component is used and always means an item contained within an aggregate.)

An index type must be an element type (other than INTEGER or CARDINAL). A one-to-one correspondence exists between the components of an array and the values of the index type. This allows array elements to be accessed via "indexed references". An indexed reference selects and accesses the component corresponding to a particular index value. In its simplest form, it consists of the name of an array followed by a bracketed Expression with a type conforming to the array's index type.

An index type can be specified using a type identifier:

Quarter: TYPE = [1..4];

profit, loss, earnings: ARRAY Quarter OF INTEGER: thisQuarter: Quarter:

earnings [thisQuarter] 4- profit [thisQuarter] loss [thisQuarter];

The arrays profit, loss, and earnings have Quarter as their index types, and thisQuarter is a subrange variable with type Quarter.


Index types may also be enumerations or subrangcs thereof. For example,

CallType: TYPE = {longDistance. tiel.ine. toll, local, inPlant}: nearbyCalls: ARRAY CallType[toll..inPland OF CARDINAL:

nearbtCalls[locall.- nearbyCalls[local+ 1:

Components may be of any desired type. In particular. the component type may itself be an array type. This allows an approximation of multidimensional arrays, which are otherwise absent in Mesa. For example, a two-dimensional data structure can he declared and used as follows:

Matrix3by4; TYPE = ARRAY [1..3] OF ARRAY [1..41 OF INTEGER:

mxy:

nog [3] [4] 4- 0; -- clear last component.

In the assignment statement, tnxy is an expression of array type (with index type [1..3] and component type ARRAY [1..4] OF INTEGER). wxy[3] is an indexed reference to the third component of mxy. This in turn yields an expression of array type (with index type [1..4] and component type INTEGER). Thus, nay [3] [4] is an indexed reference to the fourth component of that subarray. Because of left-associativity, mxy [3] [4] is the same as (mxy [3]) [4].

An array constructor consists of an optional type identifier followed by a list of values (syntactically, Expressions) enclosed in brackets. The list specifies values for components of an array in index order. The declaration below uses an array constructor to initialize an array that can be used as a translation table: i.e., octalChar[n] holds the character denoting octal digit n:

octalChar: ARRAY [0..7] OF CHARACTER = ['0, '1, '2, '3, '4, '5, '6, '7];

Note that the number of values in the list (eight) matches the number of indices in the index type. This is required for array constructors. A special form using the replicator ALL is available for abbreviating array constructors in which all components have the same value. For example, the following two declarations are equivalent:

dashes: ARRAY [0..7] OF CHARACTER 4- -,
dashes: ARRAY [0..7] OF CHARACTER 4- ALL [.-]:

Array variables may also be initialized using other array values. Consider the following example:

freshVector. ARRAY [0..3) OF CARDINAL = ALL [0]: current Vector: ARRAY [0..3) OF CARDINAL 4- fresh Vector,

In this case, current Vector is initialized with fresh Vector's value, i.e., all three of current Vectors elements are initially set to zero.

When the operands of any of the fundamental operations (4-,=.1t) are arrays, the operation is applied on a component-by-component basis. The initialization of current Vector above uses assignment in this way. Similarly, the expression "current Vector = fresh Vector" yields the result TRUE if and only if all three components of each array are equal (as they are in the above example). Because the declaration of fresh Vector uses fixed initialization, assignment either to the entire array or to one of its elements is illegal.


3.2.1. Dechwatiou of arrays

Arrays are declared using the array type constructor. A rrayTC:


Text Box:  A rrayTC PackingOption

IndexType ComponentType


PackingOption ARRAY IndexType OF ComponentType

empty I -- elements word aligned

PACKED -- elements potentially packed within words

ElementType Typeldentifier

TypeSpecification


Two array types are equivalent if both their index types and their component types are equivalent and if they are both packed or both unpacked (see below). An array type conforms to another if the two types are equivalent. Thus it is possible to assign or compare array variables with separately constructed types if those types are structurally identical (see the assignment to currentVector above).

A fine point:

In additon. one array type freely conforms to another if the component type of the first freely conforms to that of the second, the index types are equivalent, and they are both packed or both unpacked (see section 3.5).

Declarations of initialized array variables take the form

IdList : ArrayTC Initialization

The initializing expression must have an array type conforming to the one being declared.
The previous section describes indexed references to array components. A formal definition follows:

IndexedReference Variable I Expression I I

( Expression ) I Expression I

LeftSide IndexedReference

The Variable or the parenthesized Expression must be of some array type, and the bracketed Expression must conform to the index type for that array type. An IndexedReference is itself part of the definition of a Leftside (and therefore of a Variable, section 2.5).

Some fine points:

Unless an array is packed, each component is "aligned". i.e.. begins on a word boundary. Currently, a byte is the smallest unit into which the elements are packed. Thus a packed array of CHARACTER wastes no space. but a packed array of BOOLEAN has considerable overhead.

Since packed array elements are not necessarily word aligned. one cannot use the C operator (section 3.4) to generate the address of an clement.

The length of an array is the number of its elements. For variables with an array type. the length is fixed and known at compile-time. (Dynamic arrays are possible in Mesa through the use of array descriptors, discussed in section 6.2.1.)

The IndexType of an array may legally be an empty interval. In this case, no storm is allocated for the array. This is useful when the array appears as the last component of a MACHINE DEPENDENT RECORD (section 3.3) and the user will be obtaining storage for each record plus some number of array elements from a free storage manager. Note that [0..0) is not equivalent to [1..1). since the intervals specify different initial indices for the array.

Three function-like operators are relevant to arrays (and more relevant to array descriptors): LENGTH. BASE, and DESCRIPTOR. These are discussed in section 6.2. but a brief summary is provided below. For this summary . mg denotes an expression with some array type.

LENGTH [arg] -- yields the number of array elements.

BASE [arg] yields a pointer value for locating the first array elemenL

DESCRIPTOR [arg] yields argis array descriptor value (consisting of base and length).


Text Box: 3.2.2. Array constructors
In the preceding examples, array for arrays may be used in any
Text Box: Primary Constructor OptionalTypeld ComponentListText Box:  constructors are used only for initialization. Actually, constructors RightSide context. An array constructor is defined as follows:

Constructor I ...

OptionalTypeld [ ComponentList ] I ALL [ Component ] Typeldentifier I empty

PositionalComponentList

Text Box: PositionalComponentList
Component
-- other forms for record constructors

= Component

PositionalComponentList , Component

= empty I -- elided component
Expression I

NULL


The empty components in a constructor are said to be elided, and NULL components are said to be voided. The values of both elided and voided components are undefined. In the first form of array constructor, the number of Expressions plus elided or voided components must match the length implied by the array type. The type of each Expression must conform to the array's component type. The expressions (and elided or voided components) are taken in order to form a sequence that is the constructed array value.

Consider the following example:

Triple: TYPE = ARRAY [1..3] OF CARDINAL;

triplet: Triple 4- Triple [11, 12, 13];

The declaration assigns 11 to triplet [1], 12 to triplet [2] and 13 to triplet [3].

When the array type is implied by context, the Typeldentifier may be omitted (see the discussion of record constructors, section 3.3.4). Thus the declaration above could be written as

triplet: Triple 4- [11, 12, 13]:

Taken out of context, the constructor [11, 12. 13] is ambiguous: it could be assigned to any array of three numeric elements: for example:

trio: ARRAY {Patt}, Laverne. Maxine} OF LONG INTEGER +- [11, 12, 13];

The second form of constructor, using ALL, is only valid when the array type is implied by context. The type of the Expression must conform to the array's component type. The value of the constructor is an array in which the specified value is replicated a number of times equal to the length of the array. The expression is evaluated just once. In the case of an array of arrays, the structure must be mirrored by nesting in the constructor, as shown by the following example:

allOnes: Alatrix3by4 4- ALL [ALL [1]]:

Some fine points:

The value of an elided or voided component of an array constructor is not defined, but it will have some value.

In particular. if the statement

triplet +- [1. , 3]:

is executed after the previous assignment to triplet. the value of triplet [2] is undefined.

Any array constructor in which all components are compile-time constants is a compile-time constant_ Also, selection from an array that is a compile-time constant using a constant index yields a compile-time constant.


3.3. Records

A record is an aggregate that allows a group of related data items of different types to be packaged together. In the definition of a record type, the type of each individual component must be supplied. as in the following example:

MilitaryTime: TYPE = RECORD [hrs: [0..24). [0..60)1:
oldTime. newTime:

HereIlditwyTime is a newly defined type, and oldTime and neitime are record variables of that type. IlilitaryTime is a two-component record type, where the first record component is named hrs and the second rains. Every AlilitaryTime record contains both components, but different record objects have their own values for these components.

A constructor of a record type contains a field list after the word RECORD. Each element in the list specifies one (or more) components of the record. For Militaryrime, the field list is [hrs: [0..24), mins: [0..604 The component names, hrs and rains, are called field names. They are used to refer to components in any IllilitarTime record. For instance, the first component of oldTime may be selected using the qualified reference, "oldTime.hrs".

One can construct an entire record value using a record constructor. For instance, the constructors below yield MilitafryTime values with hrs components that have the value 13 and mins components that have the value of the expression "y+1":

AfilitaryTime[13, y+ 1] AlilitagTime[hrs: 13, mins: y+ 1]

The second constructor is an example of a keyword constructor, since it specifies the name of the component (e.g., as "hrs:") with which a value is to be associated.

A default value can be specified for any field in the definition of a record type. The default is used in constructing records of that type when no value is specified in the constructor. Defaults are useful for suppressing detail and ensuring initialization of fields. In the following example, the two constructors have the same value:

Datum: TYPE = RECORD

value: INTEGER, nReads: CARDINAL nWrites: CARDINAL 1:

Datum [x]

Datum [value: x, nReads: 0, nWrites: 1]

The basic operations on (non-variant) record values include the fundamental operations (=, #, 4-), as well as qualification and extraction for accessing the record's components.

3.3.1. Field lists

There are two kinds of field lists, depending on whether the fields are "named" or "unnamed". (Field lists used to construct multi-component record types are almost always named).

Syntax equations:

FieldList = [ UnnamedFieldList [ NamedFieldList

UnnamedFieldList = TypeSpecification

TypeSpecification UnnamedFieldList


Text Box: = IdList FieldDescription DefaultOption
NamedFieldList IdList : FieldDescription DefaultOption
= TypeSpecification
= empty I 4- DefaultSpecification -- section 3.3.5
NamedFieldList

FieldDescription DefaultOption

Examples:

[i: INTEGER, b: BOOLEAN, c: CHARACTER] [INTEGER. BOOLEAN, CHARACTER]

[17: CHARACTER, f2. f3: INTEGER]

[fl: CHARACTER, f2: INTEGER. fl: INTEGER]

a named field list

a similar, but unnamed field list components listed and declared together equivalent to the above


Note that if one field is named, all must be named. Also, field names must be unique within a given field list. (The same identifiers may be used as field names in other field lists, however, or as names of declared variables.)

Field descriptions in a named field list contain a type specification, indicating the type of the field. Any type may be specified, including an array type or (some other) record type.

Some fine points:

A field's type specification must not imply an infinite nesting of records. For instance, the following is illegal:

A:     TYPE = RECORD [b: B]:

B:     TYPE = RECORD [a: A]:

Field lists occur in constructors of types other than records, such as PROCEDUREs (chapter 5). SIGNALs (chapter 8). and in variant record specifications (chapter 6).

Unnamed field lists are normally used when component names would be ignored if they were present. This is common for functions that return single-component results. "Unnamed field lists are sometimes used in specifying the input parameters for procedure variables that are to be set to one of several actual procedures. (However, an unnamed field list does not allow Calls using such a procedure variable to refer to the parameters by name.)

3.3.2. Declaration of records

The type constructor RecordTC is defined as follows:

RecordTC = RECORD FieldList

-- plus variant records (chapter 6)

where FieldList is defined in the previous section. Separately declared record types are unique, even if they look the same. Every appearance of a record constructor creates a new type that is not

equivalent to, and does not conform to, any other record type. In the example:

ReCT-Jpei: TYPE RECORD [a,b: INTEGER];

reel: RecTypel;

RecType2: TYPE RECORD [a,b: INTEGER]:

rec2:     RecType2;

rec3:        RECORD [a,b: INTEGER];

rec4: RECORD [a,b: INTEGER];

the record variables reel, rec2, rec3, and rec4 all have different, non-conforming types. None of these can be assigned to any of the others (despite the similarity of their components). It is, of course, legal to assign to a component any value with a conforming type. For example:

recl.a rec2.b rec3.a 5:

rec4.a recl.a; rec4.b recl.b;


Any single-component record type conforms to the type of its single component, but not vice versa. The automatic conversion in this case requires no computation.

Example:

Bundle: TYPE = RECORD [value: INTEGER]:

recEar: Bundle;

iniVar: INTEGER;

intVar 4- recVar; -- means intVar 4-- recVar.value

intVar recVar+1; -- operand conversion

recVar F Bundle[intVat]; -- a constructor

recVar.value intVar,

This conversion simplifies dealing with functions that return single-component records (chapter 5). It also provides a way of partitioning a set of variables that can be checked by the type system. In the example above, a direct assignment of intVar to recVar is invalid. Furthermore, no other single-component record type, such as

Prime: TYPE = RECORD [value: INTEGER];

can be confused with Bundle; assignment of a Bundle value to a Prime, or a Prime to a Bundle, is illegal. Either a Bundle or a Prime can, however, appear as a numeric operand. Defining Bundle and Prime as synonyms for INTEGER would not provide this additional checking.

Because of the uniqueness of constructed record types, record variables are typically declared in two steps: first the record type, then the record variables. The general form is:

identifier : TYPE = RecordTC ; -- define record type.

IdList : identifier Initialization ; -- same identifier as just defined

Record variables can also be declared directly:

IdList : RecordTC Initialization ;

This form is not very useful because the (unnamable) record type is not available for purposes such as declaring other records of the same type or writing constructors.

The Initialization shown in these general forms applies to the entire record variable, not to individual components. Any Initialization must have the proper record type. Initialization of

record variables is shown in the next example.

noon: AfilitatyTime = [hrs:12, mins:0];

midnight: AfilitatyTime = [hrs:0, mins:0];

time: AlilitaryTime 4- midnight; -- start time at midnight.

Some fine points:

The Mesa compiler packs record components into machine words The components may be arranged in an order that differs from the left-to-right order of the fields in the type constructor. All records of the same type have the same component arrangement.

Norman.. the user is unconcerned with the actual arrangement of record components. When component

arrangement is important, the user may specify "MACHINE DEPENDENT" records. An example is:

InterruptWord: TYPE = MACHINE DEPENDENT RECORD

device: DeviceNumber,

channel: [0..7].

stopCode: {finishedOk. errorStop. powerOffl, command: ChannelCommand


In this case. the user takes full responsibility for component arrangement. Components are positioned exactly as green. from left to right in machine words. In general. "fill" components are needed to ensure that no field crosses a word boundary (unless it starts on one). Components (such as ChannelCommand) may themselves be aggregates occupying more than one word

It is also the user's responsibility to "fill out the record to a full word if the record crosses a word boundar3. (Interrupt Word might be correct for a 16-bit machine. but not for a machine having a larger word length).

Except in MACHINE DEPENDENT records. components are packed for storaee efficiency. Some fields ma} be aligned (to the beginning of a word boundary) and some may not. Components occupying a full word or more are always aligned: arrays. INTEGERs and pointers. for example. Subrecords may or may not be aliened.

depending on their size. Packed arrays are always aliened. even if there would have been space in the

preceding word for a b3te-sized element.

The function-like operator SIZE is often used to find the number of machine words occupied by a record of some type. The general form is: SIZE [TypeSpecification]. The result is a CARDINAL value. the number of words required by an object with the type specified by the argument. SIZE may be used to find the number of words required for any type of object.

3.3.3. Qualified references

Qualification is used to refer unambiguously to a named component of some record. The general form (which extends the definition of a LeftSide) is

QualifiedReference = Variable . identifier I

( Expression ). identifier

LeftSide = I QualifiedReference

The field name is said co be ''qualified by" the record value (the Variable or Expression) to the left of the dot. The operator associates from left-to-right in the case of multiple qualification. For example:

Latitude: TYPE = RECORD [degs: [0..360), mins, secs: [0..60)];

Longitude: TYPE = RECORD [degs: I 90..90], mins, secs: [O..60)]:

Position: TYPE = RECORD [latitude: Latitude, longitude: Longitude]:
somePosition: Position;

Some of the possible qualified references to components of somePosition are listed below:

Qualified Reference Refers To

somePosition.latitude 1st sub-record

somePosition.longitude 2nd sub-record

sornePosition.latitude.degs 1st component of 1st sub-record

somePosition.longitude.secs 3rd component of 2nd sub-record

The association order for qualification means that names must occur in the proper sequence: e.g.. somePositiolunins.longitude is incorrect. Also, a qualified reference must be complete, i.e., names may not be skipped (as in somePosition.secs, which would be ambiguous in any event).

Qualified references and indexed references have the same precedence (the highest possible) and may be intermixed. For example:

recordOfArrays: RECORD [a.b: ARRAY [0..100) OF CARDINAL];

arravOfRecords: ARRAY [1..5] OF RECORD CARDINAL];

array0fRecords[5].i3 record0fArrays.a[0]; -- ("last" gets "first")

A fine point:

Qualification briefly opens up a given "name scope". For instance. in the record qualification, qualified name. x. must name a field of rec and selects that field_ Scope is treated more fully in

rec.x. the chapter 7.


3.3.4. Record coirstnictors

A record constructor assembles a record value from a set of component values. In the following example. a constructor is used as a RightSide of an assignment.

Month:Vow: TYPE = {Jan, Feb. Mar, Apr. .11 ay. Jun..lul. Aug, .Sep. Oct. Nov. Dec}: Date: TYPE = RECORD

day: [1 .. 31].

month: MonthName, year: [1900 .. 2000)

birth Day: Date:

dd: [1..31]: mm: MonthName: Sy: [1900..2000); now: [1900..2000) 1976: birthDay r Date[25, Apr, now 33];

This constructor yields a record value with type Date. The record assigned to birthDay contains the following component values:

Component Value

day 25

month Apr

yearnow-33 (which is 1943)

A Constructor is a Primary and may not be used as the LeftSide of an assignment.

Record constructors are of two kinds: keyword constructors and positional constructors. Within both kinds, the component value for a particular field may either be supplied or be omitted. If it is omitted, the value of the field is determined by the DefaultOption appearing in the declaration of

the field (section 3.3.5).

Text Box: Syntax equations:
Primary Constructor OptionalTypeld CornponentList
Text Box:  ... I Constructor

OptionalTypeld [ ConnponentList Typeldentifier I empty

KeywordComponentList I

PositionalComponentList

KeywordComponentList = KeywordComponent

KeywordComponentList KeywordComponent

PositionalComponentList = Component I

PositionalComponentList , Component

KeywordComponent = identifier : Component

Component empty I -- elided component

Expression I -- explicit component

NULL -- voided component

The initial Typeldentifier, if present, must name the type of the record being constructed.

In keyword constructors, the correspondence between constructor components and record components is strictly "by name". Keyword names may not be repeated in a constructor, but the order is irrelevant. For example. the following keyword constructors are equivalent:

Date[day: 25, month: Apr, year now 33] Date[month: Apr, day: 25, year: now 33]


All of these keyword constructors specify values for all the components. In the following example. the first keyword constructor elides the month component (the place for the component value is specified, but no value is given): the second voids the 117011111 component (by specifying NULL instead of a value):

Text Box: -- month is elided
-- month is voided
Date[da: 25. month:, rear: now 33]

25, month: NULL. }ear: now 331

The distinction between an elided and a voided a field arises in the treatment of defaults (section 3.3.5). Since the declaration of Date specifies no default value fir month, both of these examples construct records with a second component having an undefined value.

In a positional constructor, the correspondence between constructor components and record components is strictly "by position". The first constructor component corresponds to the first record component, the second value to the second component. etc. Positional constructors may be used for both records and arrays (section 3.2.2). It does not matter whether or not fields are named in the

definition of the record type. The following three constructors are equivalent:

Date[day: 25, month: rear: now 33] -- value of month is undefined

Date[25 now-33] -- value of 2nd component is undefined (elided)

Date[25, NULL. now 33] -- value of 2nd component is undefined (voided)

Positional constructors may elide or void components as shown above. but components not supplied at the end of the list must be elided by supplying a sufficient number of trailing commas.

Keyword and positional 1101111i011.5 may not be mixed in a single constructor. The order of evaluation of components is not specified for either kind of constructor.

The initial Typeldentifier in a constructor may be omitted when the constructor is used as: the RightSide of an assignment (unless the LeftSide is an extractor, section 3.3.5) an expression in an Initialization

a component of an enclosing record or array constructor

an areument of a procedure

the right operand of a Relation.

In other cases, an initial Typeldentifier must appear. It is never incorrect to supply the identifier, and sometimes doing so improves readability.

A fine point:

Any record constructor in which all components are compile-time constants is a compile-time constant. Also, a field selected from a record that is a compile-time constant is itself a compile-time constant

3.3.5. Default field values

The definition of a record type may specify a default value for each field. These default specifications are optional; if present, they are used in constructing records of that type when no values for the corresponding fields are specified in constructors. An elided field, as discussed in the preceding section, supplies no value. Specifying some default treatment of a field also allows complete omission of that field in a constructor. In a keyword constructor, a field is omitted by omitting the keyword entirely: in a positional constructor. trailing fields (only) can be omitted by omitting the final commas. The positional constructor "[ ]" is considered to omit, not elide, its first component (if any).


 

In the following example, all constructors have Interval: TYPE = RECORD

range: INTEGER.

origin: INTEGER *- 0.

direction: {up. down} r up

Interval[range: 10. origin: 0. direction: up] Interval[range: 10. origin:. direction: ] Interval[range: 10]

nterval[10]

the same nalue.

all fields specified

origin. direction elided

origin. direction omitted

origin. direction omitted (positional form)

 

Text Box:  Text Box: JIText Box: 0 ttf,,t The syntax for specifying defaults in a NamedFieldList follows: DefaultOption = empty I DefaultSpecification

DefaultSpecification = empty I

Expression I

NULL

Expression I NULL

Note: In the final line, the vertical bar denotes itself and is embedded within an alternative.

Only named fields may have default values. In a DefaultSpecification, the Expression must

have a type that conforms to the type of the corresponding field.

Suppose that R is a record type with a field v of type T. The above syntax allows five forms for the DefaultOption in the declaration of v. No matter which form is used, a constructor of an R may explicitly specify a value for the field v. The various options control whether the existence of the field must be made evident in the constructor, whether an explicit value must be supplied and. if not_ what action is taken. The options are interpreted as follows:

(1)      v: T

In a constructor, the value of v can be left undefined, but that must be indicated explicitly, by eliding or voiding the field. This Wile also applies to an unnamed field.

(2)      v: T

Every constructor must supply an explicit value (not NULL) for v.

(3)      v: T t e

If a constructor elides or omits v. the value of the expression e is used: voiding the field is not permitted.

(4)            T <- NULL

As in (1) above. except that the constructor may omit v entirely. If the field is omitted, elided or voided, its value is undefined.

(5) v: T F e I NULL

As in (4) above, except that a constructor may explicitly void v. If the field is omitted or elided. the value of e is used: if it is voided. its value is undefined.

If the first or second form is used, the field cannot be omitted from a constructor: these forms are useful when such omission is likely to indicate a programming error. Omission is permitted by the other forms, which differ in the default action for an omitted or elided field. These forms are appropriate when a field has some common and meaningful default value (the third and fifth cases) or. alternatively, is relevant only in unusual circumstances (the fourth case). The last three forms are particularly suitable for extending the definition of a record type: constructors in existing programs need not be modified.

I-a,:3-


Chapter 3: Common Constructed Data Types

Fine points:

The second form of field declaration guarantees that the field r has a well-defined value in am record created 1)i a constructor (but not otherwise).

if ihe Expression form of a default specification is used. that expression is evaluated at the time of construction but in the context of the declaration of the record. i.e.. the expression is treated as a parameterless procedure invoked e\ aluation of the constructor (see chapter 5).

The default value of a field cannot be specified in terms of other fields in the same record. Default values for fields of record types defined in DEFINITIONS modules (section 7.3.2) must be compile-time constants.

Examples:

R: TYPE = RECORD

I: CARDINAL,

v2: CARDINAL -,

v3:  CARDINAL 4- 3,

v4: CARDINAL 4- NULL, CARDINAL 4- 5 I NULL

-- the following, are valid R[v1: 1. v2: 2]

R[v1: . v2: 2, v5:]

R[vl: 1, v2: 2, v5: NULL]

-- the following are not valid R[]

R[v1: 1, v2: NULL, v3: NULL]

-- means R[v/: 1, v2: 2, v3: 3, thi: NULL. v.5: 5] -- means R[v , v2: 2, v3: 3, v4: NULL, v5: 5]

-- means R[v1: 1, v2: 2, v3: 3, v4: NULL, v5: NULL]

-- neither vl nor v2 may be omitted -- neither v2 nor v3 may be voided

 

3.3.6. Extractors

Extractors are used to "explode" record objects and assign their components to individual variables in a single statement. For example, the extractor below assigns the components of birthDay (defined

in the previous section) to the variables dd, mm, and yy, in that order:

[dd, mm, yy] birthDay:

This has the same effect as the following three separate assignments, except that birthDay is evaluated only once:

dd birthDay.day: mm birthDayanotah; yy birthDay.year

An extractor resembles a constructor in form. but there are some important differences:

An extractor may only be used as a LeftSide, never as an Expression.
The "components" of an extractor specify LeftSides, not Expressions. Extractors always begin with a left bracket, never with a Typeldentifier.

The type of the record value assigned to an extractor must be known to the compiler. This means that the following (rather useless) statement is invalid because the constructor's type cannot be determined:

[dd, mm, yy] [25, Apr, 1943]: -- invalid

The statement should specify the type of the constructed value: [dd, mm, YY] 4- Date[25, Apr, 194.3]: -- valid


Extractors, like constructors. may use keywords. This allows an extractor to he written without regard to the record's component order. For instance, the following statements are equivalent to the

first one in this section:

[day: dd, month: mm. year: yv] birthDay: [month: mm, day: dd. year: n'] birthDay:

Extractors may elide or omit any item. in vnhich case the corresponding record component is not assigned. The extractors shown below arc equivalent:

[day: dd. month: , year: jy] F birthDay: [day: dd, year: yy] 4- birthDay;

[dd. yy] 4- birthDay:

-- month elided

-- month omitted

-- 2nd component elided

 

A positional extractor may omit trailing components without supplying trailing, commas. The year component of birthDay is omitted below.

[dd, mm] birth Day:

An extraction operation (unlike an ordinary assignment) yields no value. This means that an extractor may not be embedded within an expression. For example, the first statement following is illegal: the second is a valid alternative:

r [x. y, 4 44. s; -- invalid

[x, y, 4 r s: -- valid

Syntax equations:

AssignmentStmt = I Extractor 4- RightSide

Extractor = KeywordExtractList I

[ PositionalExtractList

KeywordExtractList = KeywordExtract

KeywordExtract, KeywordExtractList

KeywordExtract - identifier : Extractltem

PositionalExtractList = ExtractItem I

Extractitem PositionalExtractList

Extractitem empty I -- component is ignored

LeftSide -- component is assigned to LeftSide

The identifiers in a KeywordExtractList must be field names for the record type. Note that an extraction list can be empty, in which case the effect is to discard a record value. Extractors cannot be nested.

3.4. Pointers

Pointers allow efficient indirect access to objects. A pointer may refer to only one specific type of item. For instance, the following pointer provides access only to objects of type INTEGER:

intPtr: POINTER TO INTEGER:

Another pointer might be specified to point only to BOOLEAN objects:

boolPtr: POINTER TO BOOLEAN:

These are different types of pointers since they have different reference types, INTEGER and BOOLEAN. Furthermore, since INTEGER and BOOLEAN are incompatible types. these pointer types are also incompatible; i.e., assignment of boolPtr to intPtr, or vice versa, is disallowed.


A pointer value-is represented by the address of some data object. This object is called the pointer's referent. The postfix operator T may be applied to a pointer value of any type to yield that value's referent. The process of "following" a pointer to its referent is called dereferencing.

A dereferenced pointer designates a variable. When the pointer is declared as above, the variable can be used as a LeftSide or as a Primary. Thus iniPtrr and boolPtrt are variables of type INTEGER and BOOLEAN respecti% ely. 'the statement

60011'1,1 (inthrt = 0):

is executed by following iniPir to obtain a INTEGER value, testing that value, and assigning the result to the BOOLEAN variable referenced by boolPir.

Sometimes a pointer is created simply to identify an object or to allow indirect access to a value that is not to be modified. Mesa provides readonly pointers for such applications. A value with a readonly pointer type cannot be used to update its referent. For example, the declaration

ROiniPtr: POINTER TO READONLY INTEGER:

declares a readonly pointer. ROintPirt is a Primary with type INTEGER but not a valid LeftSide.

Any type specification is permitted as the reference type of a pointer type. The pointers declared below reference a named record type.

Person: TYPE = RECORD

age: [0..200],

sex: {male, female},

party: {Democratic, Republican}
1:

candidate!, candidate2: Person:

winner, loser: POINTER TO Person;

Pointers to record objects may be used to qualify field names. if record candidate] is the referent of winner, then qualifications such as

wirnter:age winner. sex winnerparty

select the corresponding components of candidate'. However, if candidate2 were the referent, these same qualifications would select components of candidate2. When applied to a pointer, the operation of selection implies dereferencing. For example, winner.age specifies dereferencing winner to obtain a record variable of type Person and then performing normal field selection on that record. Thus winner.age is an abbreviation of winnert.age.