Commit 372dbc4e authored by thomie's avatar thomie
Browse files

Pretty: delete really old changelog

This changelog is very incomplete, and basically useless. I'm removing
it, because it made it harder to compare this copy of `Pretty.hs` with
the copy in `libraries/pretty` (from which a similar changelog was
deleted some time ago).
parent 17d0b84e
{-# LANGUAGE BangPatterns #-}
* *
* John Hughes's and Simon Peyton Jones's Pretty Printer Combinators *
* *
* based on "The Design of a Pretty-printing Library" *
* in Advanced Functional Programming, *
* Johan Jeuring and Erik Meijer (eds), LNCS 925 *
* *
* *
* Heavily modified by Simon Peyton Jones, Dec 96 *
* *
Version 3.0 28 May 1997
* Cured massive performance bug. If you write
foldl <> empty (map ( [1..10000])
you get quadratic behaviour with V2.0. Why? For just the same reason as you get
quadratic behaviour with left-associated (++) chains.
This is really bad news. One thing a pretty-printer abstraction should
certainly guarantee is insensivity to associativity. It matters: suddenly
GHC's compilation times went up by a factor of 100 when I switched to the
new pretty printer.
I fixed it with a bit of a hack (because I wanted to get GHC back on the
road). I added two new constructors to the Doc type, Above and Beside:
<> = Beside
$$ = Above
Then, where I need to get to a "TextBeside" or "NilAbove" form I "force"
the Doc to squeeze out these suspended calls to Beside and Above; but in so
doing I re-associate. It's quite simple, but I'm not satisfied that I've done
the best possible job. I'll send you the code if you are interested.
* Added new exports:
punctuate, hang
int, integer, float, double, rational,
lparen, rparen, lbrack, rbrack, lbrace, rbrace,
* fullRender's type signature has changed. Rather than producing a string it
now takes an extra couple of arguments that tells it how to glue fragments
of output together:
fullRender :: Mode
-> Int -- Line length
-> Float -- Ribbons per line
-> (TextDetails -> a -> a) -- What to do with text
-> a -- What to do at the end
-> Doc
-> a -- Result
The "fragments" are encapsulated in the TextDetails data type:
data TextDetails = Chr Char
| Str String
| PStr FastString
The Chr and Str constructors are obvious enough. The PStr constructor has a packed
string (FastString) inside it. It's generated by using the new "ptext" export.
An advantage of this new setup is that you can get the renderer to do output
directly (by passing in a function of type (TextDetails -> IO () -> IO ()),
rather than producing a string that you then print.
Version 2.0 24 April 1997
* Made empty into a left unit for <> as well as a right unit;
it is also now true that
nest k empty = empty
which wasn't true before.
* Fixed an obscure bug in sep that occasionally gave very weird behaviour
* Added $+$
* Corrected and tidied up the laws and invariants
Relative to John's original paper, there are the following new features:
1. There's an empty document, "empty". It's a left and right unit for
both <> and $$, 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 it 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
{-# LANGUAGE MagicHash #-}
hcat is a list version of <>
hsep is a list version of <+>
vcat is a list version of $$
sep (separate) is either like hsep or like vcat, depending on what fits
cat is behaves like sep, but it uses <> for horizontal conposition
fcat is behaves like fsep, but it uses <> for horizontal conposition
These new ones do the obvious things:
char, semi, comma, colon, space,
parens, brackets, braces,
quotes, quote, doubleQuotes
4. The "above" combinator, $$, 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" $$ nest 5 "there"
lays out as
hi there
rather than
There are two places this is really useful
a) When making labelled blocks, like this:
Left -> code for left
Right -> code for right
LongLongLongLabel ->
code for longlonglonglabel
The block is on the same line as the label if the label is
short, but on the next line otherwise.
b) When laying out lists like this:
[ first
, second
, third
which some people like. But if the list fits on one line
you want [first, second, third]. You can't do this with
John's original combinators, but it's quite easy with the
new $$.
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
{-# LANGUAGE BangPatterns, CPP, MagicHash #-}
-- |
-- Module : Pretty
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file LICENSE)
-- Maintainer : David Terei <>
-- Stability : stable
-- Portability : portable
-- John Hughes's and Simon Peyton Jones's Pretty Printer Combinators
-- Based on /The Design of a Pretty-printing Library/
-- in Advanced Functional Programming,
-- Johan Jeuring and Erik Meijer (eds), LNCS 925
-- <>
module Pretty (
-- * The document type
Doc, TextDetails(..),
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