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 2d52ee06 Author: firstname.lastname@example.org <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 c9e0e606). 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.
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