emptySubst, mkEmptySubst, mkSubst, substInScope, isEmptySubst,
extendIdSubst, extendIdSubstList, extendTvSubst, extendTvSubstList,
- extendInScope, extendInScopeIds,
+ extendSubstList, zapSubstEnv,
+ extendInScope, extendInScopeList, extendInScopeIds,
isInScope,
-- Binders
\begin{code}
data Subst
= Subst InScopeSet -- Variables in in scope (both Ids and TyVars)
+ -- *after* applying the substitution
IdSubstEnv -- Substitution for Ids
TvSubstEnv -- Substitution for TyVars
-- - make it empty because all the free vars of the subst are fresh,
-- and hence can't possibly clash.a
--
- -- INVARIANT 2: The substitution is apply-once; see notes with
+ -- INVARIANT 2: The substitution is apply-once; see Note [Apply once] with
-- Types.TvSubstEnv
+ --
+ -- INVARIANT 3: See Note [Extending the Subst]
+
+{-
+Note [Extending the Subst]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+For a core Subst, which binds Ids as well, we make a different choice for Ids
+than we do for TyVars.
+
+For TyVars, see Note [Extending the TvSubst] with Type.TvSubstEnv
+
+For Ids, we have a different invariant
+ The IdSubstEnv is extended *only* when the Unique on an Id changes
+ Otherwise, we just extend the InScopeSet
+
+In consequence:
+
+* In substIdBndr, we extend the IdSubstEnv only when the unique changes
+
+* If the TvSubstEnv and IdSubstEnv are both empty, substExpr does nothing
+ (Note that the above rule for substIdBndr maintains this property. If
+ the incoming envts are both empty, then substituting the type and
+ IdInfo can't change anything.)
+
+* In lookupIdSubst, we *must* look up the Id in the in-scope set, because
+ it may contain non-trivial changes. Example:
+ (/\a. \x:a. ...x...) Int
+ We extend the TvSubstEnv with [a |-> Int]; but x's unique does not change
+ so we only extend the in-scope set. Then we must look up in the in-scope
+ set when we find the occurrence of x.
+
+Why do we make a different choice for the IdSubstEnv than the TvSubstEnv?
+
+* For Ids, we change the IdInfo all the time (e.g. deleting the
+ unfolding), and adding it back later, so using the TyVar convention
+ would entail extending the substitution almost all the time
+
+* The simplifier wants to look up in the in-scope set anyway, in case it
+ can see a better unfolding from an enclosing case expression
+
+* For TyVars, only coercion variables can possibly change, and they are
+ easy to spot
+-}
type IdSubstEnv = IdEnv CoreExpr
substInScope :: Subst -> InScopeSet
substInScope (Subst in_scope _ _) = in_scope
--- zapSubstEnv :: Subst -> Subst
--- zapSubstEnv (Subst in_scope _ _) = Subst in_scope emptyVarEnv emptyVarEnv
+zapSubstEnv :: Subst -> Subst
+zapSubstEnv (Subst in_scope _ _) = Subst in_scope emptyVarEnv emptyVarEnv
-- ToDo: add an ASSERT that fvs(subst-result) is already in the in-scope set
extendIdSubst :: Subst -> Id -> CoreExpr -> Subst
extendTvSubstList :: Subst -> [(TyVar,Type)] -> Subst
extendTvSubstList (Subst in_scope ids tvs) prs = Subst in_scope ids (extendVarEnvList tvs prs)
+extendSubstList :: Subst -> [(Var,CoreArg)] -> Subst
+extendSubstList subst []
+ = subst
+extendSubstList (Subst in_scope ids tvs) ((tv,Type ty):prs)
+ = ASSERT( isTyVar tv ) extendSubstList (Subst in_scope ids (extendVarEnv tvs tv ty)) prs
+extendSubstList (Subst in_scope ids tvs) ((id,expr):prs)
+ = ASSERT( isId id ) extendSubstList (Subst in_scope (extendVarEnv ids id expr) tvs) prs
+
lookupIdSubst :: Subst -> Id -> CoreExpr
lookupIdSubst (Subst in_scope ids tvs) v
| not (isLocalId v) = Var v
- | otherwise = case lookupVarEnv ids v of
- Just e -> e
- Nothing -> Var v
-
-{- We used to have to look up in the in-scope set,
- because GADTs were implicit in the intermediate language
- But with FC, the type of an Id does not change in its scope
- The worst that can happen if we don't look up in the in-scope set
- is that we don't propagate IdInfo as vigorously as we might.
- But that'll happen (when it's useful) in SimplEnv.substId
-
- If you put this back in, you should worry about the
- Just e -> e
- case above too!
-
- case lookupInScope in_scope v of {
- -- Watch out! Must get the Id from the in-scope set,
- -- because its type there may differ
- Just v -> Var v ;
- Nothing -> WARN( True, ptext SLIT("CoreSubst.lookupIdSubst") <+> ppr v )
- Var v
--}
+ | Just e <- lookupVarEnv ids v = e
+ | Just v' <- lookupInScope in_scope v = Var v'
+ -- Vital! See Note [Extending the Subst]
+ | otherwise = WARN( True, ptext SLIT("CoreSubst.lookupIdSubst") <+> ppr v )
+ Var v
lookupTvSubst :: Subst -> TyVar -> Type
lookupTvSubst (Subst _ ids tvs) v = lookupVarEnv tvs v `orElse` Type.mkTyVarTy v
= Subst (in_scope `extendInScopeSet` v)
(ids `delVarEnv` v) (tvs `delVarEnv` v)
+extendInScopeList :: Subst -> [Var] -> Subst
+extendInScopeList (Subst in_scope ids tvs) vs
+ = Subst (in_scope `extendInScopeSetList` vs)
+ (ids `delVarEnvList` vs) (tvs `delVarEnvList` vs)
+
extendInScopeIds :: Subst -> [Id] -> Subst
extendInScopeIds (Subst in_scope ids tvs) vs
= Subst (in_scope `extendInScopeSetList` vs)
new_env | no_change = delVarEnv env old_id
| otherwise = extendVarEnv env old_id (Var new_id)
- no_change = False -- id1 == old_id && isNothing mb_new_info && no_type_change
+ no_change = id1 == old_id
+ -- See Note [Extending the Subst]
+ -- *not* necessary to check mb_new_info and no_type_change
\end{code}
Now a variant that unconditionally allocates a new unique.