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
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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
