X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FbasicTypes%2FVarEnv.lhs;h=e7afbeb55a700d97f8a91b7e42aee6ccdaa849d6;hb=3a42f21026077b8b497852ff53288818a28883d2;hp=0b2553a3d4dac90818d05d788c909c74a19f0f9b;hpb=59264221c24a17e7c8ecde3e289882b9620bd5a8;p=ghc-hetmet.git

diff --git a/compiler/basicTypes/VarEnv.lhs b/compiler/basicTypes/VarEnv.lhs
index 0b2553a..e7afbeb 100644
--- a/compiler/basicTypes/VarEnv.lhs
+++ b/compiler/basicTypes/VarEnv.lhs
@@ -28,7 +28,7 @@ module VarEnv (
 	-- RnEnv2 and its operations
 	RnEnv2, mkRnEnv2, rnBndr2, rnBndrs2, rnOccL, rnOccR, inRnEnvL, inRnEnvR,
 		rnBndrL, rnBndrR, nukeRnEnvL, nukeRnEnvR, extendRnInScopeList,
-		rnInScope, lookupRnInScope,
+		rnInScope, rnInScopeSet, lookupRnInScope,
 
 	-- TidyEnvs
 	TidyEnv, emptyTidyEnv
@@ -196,6 +196,9 @@ extendRnInScopeList env vs
 rnInScope :: Var -> RnEnv2 -> Bool
 rnInScope x env = x `elemInScopeSet` in_scope env
 
+rnInScopeSet :: RnEnv2 -> InScopeSet
+rnInScopeSet = in_scope
+
 rnBndrs2 :: RnEnv2 -> [Var] -> [Var] -> RnEnv2
 -- Arg lists must be of equal length
 rnBndrs2 env bsL bsR = foldl2 rnBndr2 env bsL bsR 
@@ -226,20 +229,34 @@ rnBndr2 (RV2 { envL = envL, envR = envR, in_scope = in_scope }) bL bR
 rnBndrL, rnBndrR :: RnEnv2 -> Var -> (RnEnv2, Var)
 -- Used when there's a binder on one side or the other only
 -- Useful when eta-expanding
+-- 
 rnBndrL (RV2 { envL = envL, envR = envR, in_scope = in_scope }) bL
   = (RV2 { envL     = extendVarEnv envL bL new_b
-	 , envR     = envR
+	 , envR     = extendVarEnv envR new_b new_b 	-- Note [rnBndrLR]
 	 , in_scope = extendInScopeSet in_scope new_b }, new_b)
   where
     new_b = uniqAway in_scope bL
 
 rnBndrR (RV2 { envL = envL, envR = envR, in_scope = in_scope }) bR
-  = (RV2 { envL     = envL
+  = (RV2 { envL     = extendVarEnv envL new_b new_b	-- Note [rnBndrLR]
 	 , envR     = extendVarEnv envR bR new_b
 	 , in_scope = extendInScopeSet in_scope new_b }, new_b)
   where
     new_b = uniqAway in_scope bR
 
+-- Note [rnBndrLR] 
+-- ~~~~~~~~~~~~~~~
+-- Notice that in rnBndrL, rnBndrR, we extend envR, envL respectively
+-- with a binding [new_b -> new_b], where new_b is the new binder.
+-- This is important when doing eta expansion; e.g. matching (\x.M) ~ N
+-- In effect we switch to (\x'.M) ~ (\x'.N x'), where x' is new_b
+-- So we must add x' to the env of both L and R.  (x' is fresh, so it
+-- can't capture anything in N.)  
+--
+-- If we don't do this, we can get silly matches like
+--	forall a.  \y.a  ~   v
+-- succeeding with [x -> v y], which is bogus of course 
+
 rnOccL, rnOccR :: RnEnv2 -> Var -> Var
 -- Look up the renaming of an occurrence in the left or right term
 rnOccL (RV2 { envL = env }) v = lookupVarEnv env v `orElse` v