Folks, Many of you will know of John Hughes pretty printing library [1]. I recently extended it with two new features: * An "empty document" which is a unit for all the composition operators. In practice this is tremendously useful. * A "paragraph fill" combinator. There are a number of other new features, summarised below. You can find the library on my OGI web page http://www.cse.ogi.edu/~simonpj Please tell me of any bugs you find, or suggestions you have for improving it. Incidentally, I've fixed a rather subtle bug since I first put the library on my Web page, so grab the new copy even if you've come across it already. Simon [1] RJM Hughes "The Design of a Pretty-printing Library", in Advanced Functional Programming, Johan Jeuring and Erik Meijer (eds), LNCS 925 ====================================================================== Relative to John's original paper, there are the following new features: 1. There's an empty document, "empty". It's a left and right unit for both <> and $$, and anywhere in the argument list for sep, hcat, hsep, vcat, fcat etc. It is Really Useful in practice. 2. There is a paragraph-fill combinator, fsep, that's much like sep, only it keeps fitting things on one line until itc can't fit any more. 3. Some random useful extra combinators are provided. <+> puts its arguments beside each other with a space between them, unless either argument is empty in which case it returns the other hcat is a list version of <> hsep is a list version of <+> vcat is a list version of $$ 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 There are two places this is really useful a) When making labelled blocks, like this: Left -> code for left Right -> code for right LongLongLongLabel -> code for longlonglonglabel The block is on the same line as the label if the label is short, but on the next line otherwise. b) When laying out lists like this: [ first , second , third ] which some people like. But if the list fits on one line you want [first, second, third]. You can't do this with John's original combinators, but it's quite easy with the new $$. 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 Use of unboxed data types to speed up the implementation