From 45f449ddb2190f5cec1c22f06a535abda2047491 Mon Sep 17 00:00:00 2001
From: mrkkrp <markkarpov@opmbx.org>
Date: Mon, 24 Aug 2015 01:45:12 +0600
Subject: [PATCH] =?UTF-8?q?prefer=20longest=20match=20in=20=E2=80=98mergeE?=
 =?UTF-8?q?rror=E2=80=99,=20fixes=20#23?=
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Since ‘mergeError’ is only used to merge errors from alternative
branches of parsing, longest match should be preferred.
---
 Text/Megaparsec/Error.hs | 9 ++++++---
 tests/Error.hs           | 2 +-
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/Text/Megaparsec/Error.hs b/Text/Megaparsec/Error.hs
index 4484fea..5c0a969 100644
--- a/Text/Megaparsec/Error.hs
+++ b/Text/Megaparsec/Error.hs
@@ -138,14 +138,17 @@ setErrorPos :: SourcePos -> ParseError -> ParseError
 setErrorPos pos (ParseError _ ms) = ParseError pos ms
 
 -- | Merge two error data structures into one joining their collections of
--- messages and preferring shortest match.
+-- messages and preferring longest match. In other words earlier error
+-- message is discarded. This may seem counter-intuitive, but @mergeError@
+-- is only used to merge error messages of alternative branches of parsing
+-- and in this case longest match should be preferred.
 
 mergeError :: ParseError -> ParseError -> ParseError
 mergeError e1@(ParseError pos1 _) e2@(ParseError pos2 ms2) =
   case pos1 `compare` pos2 of
-    LT -> e1
+    LT -> e2
     EQ -> foldr addErrorMessage e1 ms2
-    GT -> e2
+    GT -> e1
 
 -- | @showMessages ms@ transforms list of error messages @ms@ into
 -- their textual representation.
diff --git a/tests/Error.hs b/tests/Error.hs
index ac49b6f..267ace2 100644
--- a/tests/Error.hs
+++ b/tests/Error.hs
@@ -107,7 +107,7 @@ prop_setErrorMessage msg err =
         unique = length (filter (== fromEnum msg) (fromEnum <$> msgs)) == 1
 
 prop_mergeErrorPos :: ParseError -> ParseError -> Bool
-prop_mergeErrorPos e1 e2 = errorPos (mergeError e1 e2) == min pos1 pos2
+prop_mergeErrorPos e1 e2 = errorPos (mergeError e1 e2) == max pos1 pos2
   where pos1 = errorPos e1
         pos2 = errorPos e2