X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Fprelude%2FPrelRules.lhs;fp=ghc%2Fcompiler%2Fprelude%2FPrelRules.lhs;h=8243b6e6b1c652fa7f1c238d00948fea29e76275;hb=a4124cf4314794968773cdf8e1ea8cca6e808177;hp=3ab8d6eedca650fc6048e5ea26af04506734f67c;hpb=2cf37ff932f52b68c73b0d6107d872b692a4cbf4;p=ghc-hetmet.git

diff --git a/ghc/compiler/prelude/PrelRules.lhs b/ghc/compiler/prelude/PrelRules.lhs
index 3ab8d6e..8243b6e 100644
--- a/ghc/compiler/prelude/PrelRules.lhs
+++ b/ghc/compiler/prelude/PrelRules.lhs
@@ -20,7 +20,7 @@ module PrelRules ( primOpRules, builtinRules ) where
 #include "HsVersions.h"
 
 import CoreSyn
-import Id		( mkWildId )
+import Id		( mkWildId, isPrimOpId_maybe )
 import Literal		( Literal(..), mkMachInt, mkMachWord
 			, literalType
 			, word2IntLit, int2WordLit
@@ -381,12 +381,15 @@ For dataToTag#, we can reduce if either
 	(b) the argument is a variable whose unfolding is a known constructor
 
 \begin{code}
-dataToTagRule [_, val_arg]
-  = case exprIsConApp_maybe val_arg of
-	Just (dc,_) -> ASSERT( not (isNewTyCon (dataConTyCon dc)) )
-	     	       Just (mkIntVal (toInteger (dataConTag dc - fIRST_TAG)))
+dataToTagRule [ty1, Var tag_to_enum `App` ty2 `App` tag]
+  | Just TagToEnumOp <- isPrimOpId_maybe tag_to_enum
+  , ty1 `coreEqType` ty2
+  = Just tag	-- dataToTag (tagToEnum x)   ==>   x
 
-	other	    -> Nothing
+dataToTagRule [_, val_arg]
+  | Just (dc,_) <- exprIsConApp_maybe val_arg
+  = ASSERT( not (isNewTyCon (dataConTyCon dc)) )
+    Just (mkIntVal (toInteger (dataConTag dc - fIRST_TAG)))
 
 dataToTagRule other = Nothing
 \end{code}