A pretty printer library in Haskell
Version 3.0, 28 May 1997

I have now bitten the bullet and made an "industrial strength" (I hope) pretty printing library in Haskell, based on John Hughes's paper "The Design of a Pretty-printing Library" (in Advanced Functional Programming, Johan Jeuring and Erik Meijer (eds), LNCS 925).

In this zip file are:

• Pretty.lhs: a version for GHC (0.29 and earlier). The GHC version uses unboxed types to get a little more speed. You need to compile with -glasgow-exts -cpp.

• Pretty.hs: A version for Hugs. This code has been processed by cpp, so it has tons of white space in it. But it works. Look in the GHC verision for nicer layout.

• Main.hs, MainHugs.hs: A test program for GHC and Hugs respectively. Among other things this code shows how you can use the library to print lists like this:

  	[ foo, baz,
  	, this, that,
  	, the other
  	]
  

  which is hard using the original combinators.

Please do try it out, and give me feedback about

• what combinators it lacks
• performance (time and space)
• naming of the combinators

1. There's an empty document, "empty". It's a left and right unit for both <> and $$, and anywhere in the argument list for sep, hcat, hsep, vcat, fcat etc.

   It is Really Useful in practice. (John identified the lack of an empty document as a shortcoming in his paper, but couldn't see how to fit it into the algebra.)

2. There is a paragraph-fill combinator, fsep, that's much like sep, only it keeps fitting things on one line until itc can't fit any more.

3. Some random useful extra combinators are provided.
   • <+> puts its arguments beside each other with a space between them, unless either argument is empty in which case it returns the other

   • hcat is a list version of <>
   • hsep is a list version of <+>
   • vcat is a list version of $$

   • cat is behaves like sep, but it uses <#> for horizontal conposition
   • fcat is behaves like fsep, but it uses <#> for horizontal conposition

   • These new ones do the obvious things: char, semi, comma, colon, space, parens, brackets, braces, quotes, doubleQuotes

4. The "above" combinator, $$, now overlaps its two arguments if the last line of the top argument stops before the first line of the second begins. For example: text "hi" $$ nest 5 "there" lays out as

   			hi   there
   

   rather than

   			hi
   			     there
   

   The combinator $+$ gives the original "never-overlap" behaviour.

5. Several different renderers are provided:
   • a standard one
   • one that uses cut-marks to avoid deeply-nested documents simply piling up in the right-hand margin
   • one that ignores indentation (fewer chars output; good for machines)
   • one that ignores indentation and newlines (ditto, only more so)

6. Numerous implementation tidy-ups, plus (for GHC) the use of unboxed data types to speed up the implementation