#1337: Fix wrong indentation from Text.PrettyPrint.HughesPJ fill (fcat and fsep)
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Reporter: thorkilnaur | Owner: thorkilnaur
Type: bug | Status: closed
Priority: normal | Milestone: 6.10.1
Component: libraries/pretty | Version: 6.6.1
Resolution: fixed | Keywords:
Operating System: Unknown/Multiple | Architecture: Unknown/Multiple
Test Case: Pp003 |
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Comment (by Thomas Miedema ):
In [changeset:"bcfae08c0be0fa8604e2025733dfae57e37c2083/ghc"
bcfae08c/ghc]:
{{{
#!CommitTicketReference repository="ghc"
revision="bcfae08c0be0fa8604e2025733dfae57e37c2083"
Pretty: fix potential bad formatting of error message (#10735)
This is a backport of a bug fix by Benedikt Huber for the same problem
in the pretty library (#1337), from commit
8d8866a8379c2fe8108ef034893c59e06d5e752f. The original explanation for
the fix is attached below.
Ticket #1776 originally reported an infinite loop when printing error
message. This promptly got fixed in:
commit 2d52ee06786e5caf0c2d65a4b4bb7c45c6493190
Author: simonpj@microsoft.com <unknown>
Date: Thu Mar 1 11:45:13 2007 +0000
Do not go into an infinite loop when pretty-printer finds a
negative indent (Trac #1176)
SPJ reports in the ticket: "So infinite loop is fixed, but the bad
formatting remains. I've added a test, tcfail177."
tcfail177 however hasn't triggered the formatting problem for years (as
Ian reported in c9e0e6067a47c574d9ff3721afe58e30ca1be3e4).
This patch updates the test to a version that at least still failed with
ghc-7.0 (from #1776#comment:7).
-------------------
From https://mail.haskell.org/pipermail/libraries/2008-June/010013.html,
by Benedikt Huber:
Concerning ticket #1337, we have to change the formal specification of
fill (it doesn't match the implementation):
-- Current Specification:
-- fill [] = empty
-- fill [p] = p
-- fill (p1:p2:ps) = oneLiner p1 <#> nest (length p1)
-- (fill (oneLiner p2 : ps))
-- `union`
-- p1 $$ fill ps
Problem 1: We want to `unnest' the second argument of (p1 $$ fill ps),
but not the first one
In the definition above we have e.g.
> getSecondLayout $
> fillDef False [text "a", text "b", text "a"]
>> text "ab"; nilabove; nest -1; text "a"; empty
>> |ab|
>> |.a|
Problem 2: The overlapping $$ should only be used for those layouts of
p1 which aren't one liners (otherwise violating the invariant "Left
union arg has shorter first line").
I suggest the following specification (i believe it almost matches the
current implementation, modulo [fillNB: fix bug #1337] (see below):
-- Revised Specification:
-- fill g docs = fill' 0 docs
-- gap g = if g then 1 else 0
-- fill' n [] = []
-- fill' n [p] = [p]
-- fill' n (p1:p2:ps) =
-- oneLiner p1 <g> (fill' (n+length p1+gap g) (oneLiner p2 : ps))
-- `union`
-- (p1 $*$ nest (-n) (fill' g ps))
--
-- $*$ is defined for layouts (One-Layout Documents) as
--
-- layout1 $*$ layout2 | isOneLiner layout1 = layout1 $+$ layout2
-- | otherwise = layout1 $$ layout2
I've also implemented the specification in HughesPJQuickCheck.hs,
and checked them against the patched pretty printer.
Concerning Bug #1337:
~~~~~~~~~~~~~~~~~~~~~
If the above formal specification is fine, it is easy to fix:
elide the nests of (oneLiner p2) [see attached patch, record bug
#1337].
> PrettyPrint(0) $ ./Bug1337
> ....ab
> ...c
The (long) explanation follows below.
<snip/>
===========================================================
Explanation of Bug #1337:
Consider
> fcat [ nest 1 $ text "a", nest 2 $ text "b", text "c"]
--> expected: (nest 1; text "a"; text "b"; nest -3; "c")
--> actual : (nest 1; text "a"; text "b"; nest -5; "c")
Reduction:
=== (nest 1; text a) <> (fill (-2) (p2:ps))
==> (nest 2 (text "b") $+$ text "c")
==> (nest 2 (text "b")) `nilabove`
(nest (-3) (text "c"))
==> (nest 1; text a; text b; nest -5 c)
The problem is that if we decide to layout (p1:p2:ps) as
| p1 p2
| ps
(call it layout A), then we want to have
> (p1 <> p2) $+$ ps.
But following law <n6> this means that
> fcat_A [p1:nest k p2:ps]
is equivalent to
> fcat_A [p1,p2,ps]
so the nest of p2 has to be removed.
This is somewhat similar to bug #667, but easier to fix
from a semantic point of view:
p1,p2 and ps are distinct layouts - we only have to preserve the
individual layouts, and no combinations of them.
}}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1337#comment:14
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