X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2FhsSyn%2FHsUtils.lhs;h=3b9271a633d4996d5fc7a3f54fca43cb38c5aa96;hp=8cef12e97d5e65487e9871ca7fcd795764378ec6;hb=7fc749a43b4b6b85d234fa95d4928648259584f4;hpb=17b297d97d327620ed6bfab942f8992b2446f1bf

diff --git a/compiler/hsSyn/HsUtils.lhs b/compiler/hsSyn/HsUtils.lhs
index 8cef12e..3b9271a 100644
--- a/compiler/hsSyn/HsUtils.lhs
+++ b/compiler/hsSyn/HsUtils.lhs
@@ -13,11 +13,11 @@ which deal with the intantiated versions are located elsewhere:
    Id			typecheck/TcHsSyn	
 
 \begin{code}
-{-# OPTIONS_GHC -w #-}
+{-# OPTIONS -w #-}
 -- The above warning supression flag is a temporary kludge.
 -- While working on this module you are encouraged to remove it and fix
 -- any warnings in the module. See
---     http://hackage.haskell.org/trac/ghc/wiki/WorkingConventions#Warnings
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
 -- for details
 
 module HsUtils where
@@ -413,6 +413,22 @@ collectPatBinders.  In a lazy pattern, for example f ~(C x y) = ...,
 we want to generate bindings for x,y but not for dictionaries bound by
 C.  (The type checker ensures they would not be used.)
 
+Desugaring of arrow case expressions needs these bindings (see DsArrows
+and arrowcase1), but SPJ (Jan 2007) says it's safer for it to use its
+own pat-binder-collector:
+
+Here's the problem.  Consider
+
+data T a where
+   C :: Num a => a -> Int -> T a
+
+f ~(C (n+1) m) = (n,m)
+
+Here, the pattern (C (n+1)) binds a hidden dictionary (d::Num a),
+and *also* uses that dictionary to match the (n+1) pattern.  Yet, the
+variables bound by the lazy pattern are n,m, *not* the dictionary d.
+So in mkSelectorBinds in DsUtils, we want just m,n as the variables bound.
+
 \begin{code}
 collectSigTysFromPats :: [InPat name] -> [LHsType name]
 collectSigTysFromPats pats = foldr collect_lpat [] pats