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This module:

module Q (foo) where
foo :: String -> [String] -> Bool
#ifdef FIRST
foo x _ | x `seq` x == "." = True
#else
foo x _ |         x == "." = True
#endif
foo x xs = x `seq` any (x ==) xs

should be compiled to the same code regardless of whether or not FIRST is defined. However, this is not the case; there is an extra case when FIRST is not defined:

$ ghc --version                                              
The Glorious Glasgow Haskell Compilation System, version 6.8.1
$ ghc -fforce-recomp -ddump-simpl -cpp -O -c Q.hs -DFIRST > 1
$ ghc -fforce-recomp -ddump-simpl -cpp -O -c Q.hs > 2
$ diff -U 1000 1 2
--- 1   2007-11-17 21:02:37.000000000 +0000
+++ 2   2007-11-17 21:02:39.000000000 +0000
@@ -1,33 +1,35 @@
 
 ==================== Tidy Core ====================
 Q.a :: GHC.Base.Char
 [GlobalId]
 [NoCafRefs
  Str: DmdType m]
 Q.a = GHC.Base.C# '.'
 
 Q.lvl :: [GHC.Base.Char]
 [GlobalId]
 [NoCafRefs
  Str: DmdType]
 Q.lvl = GHC.Base.: @ GHC.Base.Char Q.a (GHC.Base.[] @ GHC.Base.Char)
 
 Q.foo :: GHC.Base.String -> [GHC.Base.String] -> GHC.Base.Bool
 [GlobalId]
 [Arity 2
  NoCafRefs
  Str: DmdType SL]
 Q.foo =
-  \ (x_a5D :: GHC.Base.String) (ds_d6y :: [GHC.Base.String]) ->
-    case GHC.Base.eqString x_a5D Q.lvl of wild_Xc {
+  \ (x_a5D :: GHC.Base.String) (ds_d6w :: [GHC.Base.String]) ->
+    case GHC.Base.eqString x_a5D Q.lvl of wild_B1 {
       GHC.Base.False ->
-       GHC.List.any @ GHC.Base.String (GHC.Base.eqString x_a5D) ds_d6y;
+       case x_a5D of tpl_Xd { __DEFAULT ->
+       GHC.List.any @ GHC.Base.String (GHC.Base.eqString tpl_Xd) ds_d6w
+       };
       GHC.Base.True -> GHC.Base.True
     }
 
 
 
 
 ==================== Tidy Core Rules ====================
 
 
$

There is a little discussion in the thread where this was first reported: http://www.haskell.org/pipermail/glasgow-haskell-users/2004-June/006862.html

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown