newFlexiTyVarTys, -- Int -> Kind -> TcM [TcType]
newKindVar, newKindVars,
lookupTcTyVar, LookupTyVarResult(..),
- newMetaTyVar, readMetaTyVar, writeMetaTyVar,
+
+ newMetaTyVar, readMetaTyVar, writeMetaTyVar, isFilledMetaTyVar,
--------------------------------
-- Boxy type variables
--------------------------------
-- Instantiation
tcInstTyVar, tcInstType, tcInstTyVars, tcInstBoxyTyVar,
- tcInstSigTyVars, zonkSigTyVar,
+ tcInstSigTyVars,
tcInstSkolTyVar, tcInstSkolTyVars, tcInstSkolType,
tcSkolSigType, tcSkolSigTyVars, occurCheckErr,
--------------------------------
-- Zonking
zonkType, zonkTcPredType,
- zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV,
+ zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkSigTyVar,
zonkQuantifiedTyVar, zonkQuantifiedTyVars,
zonkTcType, zonkTcTypes, zonkTcClassConstraints, zonkTcThetaType,
zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
readMutVar (metaTvRef tyvar)
+isFilledMetaTyVar :: TyVar -> TcM Bool
+-- True of a filled-in (Indirect) meta type variable
+isFilledMetaTyVar tv
+ | not (isTcTyVar tv) = return False
+ | MetaTv _ ref <- tcTyVarDetails tv
+ = do { details <- readMutVar ref
+ ; return (isIndirect details) }
+ | otherwise = return False
+
writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
#ifndef DEBUG
writeMetaTyVar tyvar ty = writeMutVar (metaTvRef tyvar) (Indirect ty)
k = tyVarKind tv
default_k = defaultKind k
-zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TyVar]
+zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
zonkQuantifiedTyVars = mappM zonkQuantifiedTyVar
-zonkQuantifiedTyVar :: TcTyVar -> TcM TyVar
+zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
-- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
--
-- The quantified type variables often include meta type variables
-- we want to freeze them into ordinary type variables, and
-- default their kind (e.g. from OpenTypeKind to TypeKind)
-- -- see notes with Kind.defaultKind
--- The meta tyvar is updated to point to the new regular TyVar. Now any
+-- The meta tyvar is updated to point to the new skolem TyVar. Now any
-- bound occurences of the original type variable will get zonked to
-- the immutable version.
--
| otherwise -- It's a meta-type-variable
= do { details <- readMetaTyVar tv
- -- Create the new, frozen, regular type variable
+ -- Create the new, frozen, skolem type variable
+ -- We zonk to a skolem, not to a regular TcVar
+ -- See Note [Zonking to Skolem]
; let final_kind = defaultKind (tyVarKind tv)
- final_tv = mkTyVar (tyVarName tv) final_kind
+ final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
-- Bind the meta tyvar to the new tyvar
; case details of
; return final_tv }
\end{code}
-[Silly Type Synonyms]
-
+Note [Silly Type Synonyms]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this:
type C u a = u -- Note 'a' unused
All very silly. I think its harmless to ignore the problem. We'll end up with
a /\a in the final result but all the occurrences of a will be zonked to ()
+Note [Zonking to Skolem]
+~~~~~~~~~~~~~~~~~~~~~~~~
+We used to zonk quantified type variables to regular TyVars. However, this
+leads to problems. Consider this program from the regression test suite:
+
+ eval :: Int -> String -> String -> String
+ eval 0 root actual = evalRHS 0 root actual
+
+ evalRHS :: Int -> a
+ evalRHS 0 root actual = eval 0 root actual
+
+It leads to the deferral of an equality
+
+ (String -> String -> String) ~ a
+
+which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
+In the meantime `a' is zonked and quantified to form `evalRHS's signature.
+This has the *side effect* of also zonking the `a' in the deferred equality
+(which at this point is being handed around wrapped in an implication
+constraint).
+
+Finally, the equality (with the zonked `a') will be handed back to the
+simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
+If we zonk `a' with a regular type variable, we will have this regular type
+variable now floating around in the simplifier, which in many places assumes to
+only see proper TcTyVars.
+
+We can avoid this problem by zonking with a skolem. The skolem is rigid
+(which we requirefor a quantified variable), but is still a TcTyVar that the
+simplifier knows how to deal with.
+
%************************************************************************
%* *
check_tau_type rank ubx_tup ty@(TyConApp tc tys)
| isSynTyCon tc
= do { -- Check that the synonym has enough args
- -- This applies eqaually to open and closed synonyms
+ -- This applies equally to open and closed synonyms
-- It's OK to have an *over-applied* type synonym
-- data Tree a b = ...
-- type Foo a = Tree [a]
-- See Note [Liberal type synonyms]
; liberal <- doptM Opt_LiberalTypeSynonyms
- ; if not liberal then
- -- For H98, do check the type args
+ ; if not liberal || isOpenSynTyCon tc then
+ -- For H98 and synonym families, do check the type args
mappM_ check_arg_type tys
- else -- In the liberal case, expand then check
+ else -- In the liberal case (only for closed syns), expand then check
case tcView ty of
Just ty' -> check_tau_type rank ubx_tup ty'
- Nothing -> pprPanic "check_tau_type" (ppr ty)
+ Nothing -> pprPanic "check_tau_type" (ppr ty)
}
| isUnboxedTupleTyCon tc