import VarEnv
import VarSet
-import OccName
import Name
import Class
import PrelNames
data TvSubst
= TvSubst InScopeSet -- The in-scope type variables
TvSubstEnv -- The substitution itself
- -- See Note [Apply Once]
+ -- See Note [Apply Once]
+ -- and Note [Extending the TvSubstEnv]
{- ----------------------------------------------------------
- Note [Apply Once]
+Note [Apply Once]
+~~~~~~~~~~~~~~~~~
We use TvSubsts to instantiate things, and we might instantiate
forall a b. ty
\with the types
A TvSubst is not idempotent, but, unlike the non-idempotent substitution
we use during unifications, it must not be repeatedly applied.
+
+Note [Extending the TvSubst]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The following invariant should hold of a TvSubst
+
+ The in-scope set is needed *only* to
+ guide the generation of fresh uniques
+
+ In particular, the *kind* of the type variables in
+ the in-scope set is not relevant
+
+This invariant allows a short-cut when the TvSubstEnv is empty:
+if the TvSubstEnv is empty --- i.e. (isEmptyTvSubt subst) holds ---
+then (substTy subst ty) does nothing.
+
+For example, consider:
+ (/\a. /\b:(a~Int). ...b..) Int
+We substitute Int for 'a'. The Unique of 'b' does not change, but
+nevertheless we add 'b' to the TvSubstEnv, because b's type does change
+
+This invariant has several crucial consequences:
+
+* In substTyVarBndr, we need extend the TvSubstEnv
+ - if the unique has changed
+ - or if the kind has changed
+
+* In substTyVar, we do not need to consult the in-scope set;
+ the TvSubstEnv is enough
+
+* In substTy, substTheta, we can short-circuit when the TvSubstEnv is empty
+
+
-------------------------------------------------------------- -}
emptyTvSubst = TvSubst emptyInScopeSet emptyVarEnv
isEmptyTvSubst :: TvSubst -> Bool
+ -- See Note [Extending the TvSubstEnv]
isEmptyTvSubst (TvSubst _ env) = isEmptyVarEnv env
mkTvSubst :: InScopeSet -> TvSubstEnv -> TvSubst
substTyVar :: TvSubst -> TyVar -> Type
substTyVar subst@(TvSubst in_scope env) tv
= case lookupTyVar subst tv of {
- Nothing -> TyVarTy tv;
+ Nothing -> TyVarTy tv;
Just ty -> ty -- See Note [Apply Once]
}
lookupTyVar :: TvSubst -> TyVar -> Maybe Type
+ -- See Note [Extending the TvSubst]
lookupTyVar (TvSubst in_scope env) tv = lookupVarEnv env tv
substTyVarBndr :: TvSubst -> TyVar -> (TvSubst, TyVar)
substTyVarBndr subst@(TvSubst in_scope env) old_var
= (TvSubst (in_scope `extendInScopeSet` new_var) new_env, new_var)
where
+ is_co_var = isCoVar old_var
new_env | no_change = delVarEnv env old_var
| otherwise = extendVarEnv env old_var (TyVarTy new_var)
no_change = new_var == old_var && not is_co_var
-- no_change means that the new_var is identical in
-- all respects to the old_var (same unique, same kind)
+ -- See Note [Extending the TvSubst]
--
-- In that case we don't need to extend the substitution
-- to map old to new. But instead we must zap any
-- The uniqAway part makes sure the new variable is not already in scope
subst_old_var -- subst_old_var is old_var with the substitution applied to its kind
- -- It's only worth doing the substitution for coercions,
- -- becuase only they can have free type variables
- | is_co_var = setTyVarKind old_var (substTy subst kind)
+ -- It's only worth doing the substitution for coercions,
+ -- becuase only they can have free type variables
+ | is_co_var = setTyVarKind old_var (substTy subst (tyVarKind old_var))
| otherwise = old_var
- kind = tyVarKind old_var
- is_co_var = isCoVar old_var
\end{code}
----------------------------------------------------