Add restrictVarEnv :: VarEnv a -> VarSet -> VarEnv a

I needed it, and then didn't need it, so it's not currently
called, but its generally useful kind of thing.

diff --git a/compiler/basicTypes/VarEnv.lhs b/compiler/basicTypes/VarEnv.lhs
index 7e28d1a..2699748 100644 (file)
--- a/compiler/basicTypes/VarEnv.lhs
+++ b/compiler/basicTypes/VarEnv.lhs
@@ -19,7 +19,7 @@ module VarEnv (
        modifyVarEnv, modifyVarEnv_Directly,
        isEmptyVarEnv, foldVarEnv, 
        elemVarEnvByKey, lookupVarEnv_Directly,
-       filterVarEnv_Directly,
+       filterVarEnv_Directly, restrictVarEnv,
 
        -- * The InScopeSet type
        InScopeSet, 
@@ -321,6 +321,7 @@ extendVarEnvList  :: VarEnv a -> [(Var, a)] -> VarEnv a
                  
 lookupVarEnv_Directly :: VarEnv a -> Unique -> Maybe a
 filterVarEnv_Directly :: (Unique -> a -> Bool) -> VarEnv a -> VarEnv a
+restrictVarEnv    :: VarEnv a -> VarSet -> VarEnv a
 delVarEnvList     :: VarEnv a -> [Var] -> VarEnv a
 delVarEnv        :: VarEnv a -> Var -> VarEnv a
 plusVarEnv_C     :: (a -> a -> a) -> VarEnv a -> VarEnv a -> VarEnv a
@@ -361,6 +362,10 @@ foldVarEnv  = foldUFM
 lookupVarEnv_Directly = lookupUFM_Directly
 filterVarEnv_Directly = filterUFM_Directly
 
+restrictVarEnv env vs = filterVarEnv_Directly keep env
+  where
+    keep u _ = u `elemVarSetByKey` vs
+    
 zipVarEnv tyvars tys   = mkVarEnv (zipEqual "zipVarEnv" tyvars tys)
 lookupVarEnv_NF env id = case lookupVarEnv env id of
                          Just xx -> xx