add -fsimpleopt-before-flatten

[ghc-hetmet.git] / compiler / simplCore / SimplEnv.lhs

diff --git a/compiler/simplCore/SimplEnv.lhs b/compiler/simplCore/SimplEnv.lhs
index 9f424cd..d9eea39 100644 (file)
--- a/compiler/simplCore/SimplEnv.lhs
+++ b/compiler/simplCore/SimplEnv.lhs
@@ -46,6 +46,8 @@ import VarEnv
 import VarSet
 import OrdList
 import Id
+import MkCore
+import TysWiredIn
 import qualified CoreSubst
 import qualified Type          ( substTy, substTyVarBndr, substTyVar )
 import Type hiding             ( substTy, substTyVarBndr, substTyVar )
@@ -220,13 +222,38 @@ seIdSubst:
 \begin{code}
 mkSimplEnv :: SimplifierMode -> SimplEnv
 mkSimplEnv mode
-  = SimplEnv { seCC = subsumedCCS,
-              seMode = mode, seInScope = emptyInScopeSet, 
-              seFloats = emptyFloats,
-              seTvSubst = emptyVarEnv, seIdSubst = emptyVarEnv }
+  = SimplEnv { seCC = subsumedCCS
+             , seMode = mode
+             , seInScope = init_in_scope
+             , seFloats = emptyFloats
+             , seTvSubst = emptyVarEnv
+             , seIdSubst = emptyVarEnv }
        -- The top level "enclosing CC" is "SUBSUMED".
 
----------------------
+init_in_scope :: InScopeSet
+init_in_scope = mkInScopeSet (unitVarSet (mkWildValBinder unitTy))
+              -- See Note [WildCard binders]
+\end{code}
+
+Note [WildCard binders]
+~~~~~~~~~~~~~~~~~~~~~~~
+The program to be simplified may have wild binders
+    case e of wild { p -> ... }
+We want to *rename* them away, so that there are no
+occurrences of 'wild-id' (with wildCardKey).  The easy
+way to do that is to start of with a representative
+Id in the in-scope set
+
+There can be be *occurrences* of wild-id.  For example,
+MkCore.mkCoreApp transforms
+   e (a /# b)   -->   case (a /# b) of wild { DEFAULT -> e wild }
+This is ok provided 'wild' isn't free in 'e', and that's the delicate
+thing. Generally, you want to run the simplifier to get rid of the
+wild-ids before doing much else.
+
+It's a very dark corner of GHC.  Maybe it should be cleaned up.
+
+\begin{code}
 getMode :: SimplEnv -> SimplifierMode
 getMode env = seMode env