Tidy up the treatment of dead binders

[ghc-hetmet.git] / compiler / basicTypes / VarEnv.lhs

diff --git a/compiler/basicTypes/VarEnv.lhs b/compiler/basicTypes/VarEnv.lhs
index 4bb00cf..7e28d1a 100644 (file)
--- a/compiler/basicTypes/VarEnv.lhs
+++ b/compiler/basicTypes/VarEnv.lhs
@@ -27,7 +27,6 @@ module VarEnv (
        -- ** Operations on InScopeSets
        emptyInScopeSet, mkInScopeSet, delInScopeSet,
        extendInScopeSet, extendInScopeSetList, extendInScopeSetSet, 
-       modifyInScopeSet,
        getInScopeVars, lookupInScope, elemInScopeSet, uniqAway, 
 
        -- * The RnEnv2 type
@@ -66,7 +65,18 @@ import FastString
 \begin{code}
 -- | A set of variables that are in scope at some point
 data InScopeSet = InScope (VarEnv Var) FastInt
-       -- The Int# is a kind of hash-value used by uniqAway
+       -- The (VarEnv Var) is just a VarSet.  But we write it like
+       -- this to remind ourselves that you can look up a Var in 
+       -- the InScopeSet. Typically the InScopeSet contains the
+       -- canonical version of the variable (e.g. with an informative
+       -- unfolding), so this lookup is useful.
+       --
+       -- INVARIANT: the VarEnv maps (the Unique of) a variable to 
+       --            a variable with the same Uniqua.  (This was not
+       --            the case in the past, when we had a grevious hack
+       --            mapping var1 to var2.     
+       -- 
+       -- The FastInt is a kind of hash-value used by uniqAway
        -- For example, it might be the size of the set
        -- INVARIANT: it's not zero; we use it as a multiplier in uniqAway
 
@@ -94,37 +104,16 @@ extendInScopeSetSet :: InScopeSet -> VarEnv Var -> InScopeSet
 extendInScopeSetSet (InScope in_scope n) vs
    = InScope (in_scope `plusVarEnv` vs) (n +# iUnbox (sizeUFM vs))
 
--- | Replace the first 'Var' with the second in the set of in-scope variables
-modifyInScopeSet :: InScopeSet -> Var -> Var -> InScopeSet
--- Exploit the fact that the in-scope "set" is really a map
---     Make old_v map to new_v
--- QUESTION: shouldn't we add a mapping from new_v to new_v as it is presumably now in scope? - MB 08
-modifyInScopeSet (InScope in_scope n) old_v new_v = InScope (extendVarEnv in_scope old_v new_v) (n +# _ILIT(1))
-
 delInScopeSet :: InScopeSet -> Var -> InScopeSet
 delInScopeSet (InScope in_scope n) v = InScope (in_scope `delVarEnv` v) n
 
 elemInScopeSet :: Var -> InScopeSet -> Bool
 elemInScopeSet v (InScope in_scope _) = v `elemVarEnv` in_scope
 
--- | If the given variable was even added to the 'InScopeSet', or if it was the \"from\" argument
--- of any 'modifyInScopeSet' operation, returns that variable with all appropriate modifications
--- applied to it. Otherwise, return @Nothing@
+-- | Look up a variable the 'InScopeSet'.  This lets you map from 
+-- the variable's identity (unique) to its full value.
 lookupInScope :: InScopeSet -> Var -> Maybe Var
--- It's important to look for a fixed point
--- When we see (case x of y { I# v -> ... })
--- we add  [x -> y] to the in-scope set (Simplify.simplCaseBinder and
--- modifyInScopeSet).
---
--- When we lookup up an occurrence of x, we map to y, but then
--- we want to look up y in case it has acquired more evaluation information by now.
-lookupInScope (InScope in_scope _) v 
-  = go v
-  where
-    go v = case lookupVarEnv in_scope v of
-               Just v' | v == v'   -> Just v'  -- Reached a fixed point
-                       | otherwise -> go v'
-               Nothing             -> Nothing
+lookupInScope (InScope in_scope _) v  = lookupVarEnv in_scope v
 \end{code}
 
 \begin{code}