pprEq ty1 ty2 = pprPred $ mkEqPred (ty1,ty2)
isTouchableMetaTyVar :: TcTyVar -> TcS Bool
--- is touchable variable!
isTouchableMetaTyVar tv
- | isMetaTyVar tv = do { untch <- getUntouchables
- ; return (inTouchableRange untch tv) }
- | otherwise = return False
+ = case tcTyVarDetails tv of
+ MetaTv TcsTv _ -> return True -- See Note [Touchable meta type variables]
+ MetaTv {} -> do { untch <- getUntouchables
+ ; return (inTouchableRange untch tv) }
+ _ -> return False
+\end{code}
+
+Note [Touchable meta type variables]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Meta type variables allocated *by the constraint solver itself* are always
+touchable. Example:
+ instance C a b => D [a] where...
+if we use this instance declaration we "make up" a fresh meta type
+variable for 'b', which we must later guess. (Perhaps C has a
+functional dependency.) But since we aren't in the constraint *generator*
+we can't allocate a Unique in the touchable range for this implication
+constraint. Instead, we mark it as a "TcsTv", which makes it always-touchable.
+\begin{code}
-- Flatten skolems
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
newFlattenSkolemTy :: TcType -> TcS TcType
newFlattenSkolemTy ty = mkTyVarTy <$> newFlattenSkolemTyVar ty
- where newFlattenSkolemTyVar :: TcType -> TcS TcTyVar
- newFlattenSkolemTyVar ty
- = wrapTcS $ do { uniq <- TcM.newUnique
- ; let name = mkSysTvName uniq (fsLit "f")
- ; return $
- mkTcTyVar name (typeKind ty) (FlatSkol ty)
- }
+
+newFlattenSkolemTyVar :: TcType -> TcS TcTyVar
+newFlattenSkolemTyVar ty
+ = wrapTcS $ do { uniq <- TcM.newUnique
+ ; let name = mkSysTvName uniq (fsLit "f")
+ ; return $ mkTcTyVar name (typeKind ty) (FlatSkol ty) }
-- Instantiations
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
instDFunTypes :: [Either TyVar TcType] -> TcS [TcType]
-instDFunTypes mb_inst_tys =
- let inst_tv :: Either TyVar TcType -> TcS Type
- inst_tv (Left tv) = wrapTcS $ TcM.tcInstTyVar tv >>= return . mkTyVarTy
- inst_tv (Right ty) = return ty
- in mapM inst_tv mb_inst_tys
-
+instDFunTypes mb_inst_tys
+ = mapM inst_tv mb_inst_tys
+ where
+ inst_tv :: Either TyVar TcType -> TcS Type
+ inst_tv (Left tv) = mkTyVarTy <$> newFlexiTcS tv
+ inst_tv (Right ty) = return ty
instDFunConstraints :: TcThetaType -> TcS [EvVar]
instDFunConstraints preds = wrapTcS $ TcM.newWantedEvVars preds
+newFlexiTcS :: TyVar -> TcS TcTyVar
+-- Make a TcsTv meta tyvar; it is always touchable,
+-- but we are supposed to guess its instantiation
+-- See Note [Touchable meta type variables]
+newFlexiTcS tv = wrapTcS $ TcM.instMetaTyVar TcsTv tv
-- Superclasses and recursive dictionaries
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
where
to_work_item :: (Equation, (PredType,SDoc), (PredType,SDoc)) -> TcS [WantedEvVar]
to_work_item ((qtvs, pairs), _, _)
- = do { (_, _, tenv) <- wrapTcS $ TcM.tcInstTyVars (varSetElems qtvs)
- ; mapM (do_one tenv) pairs }
+ = do { let tvs = varSetElems qtvs
+ ; tvs' <- mapM newFlexiTcS tvs
+ ; let subst = zipTopTvSubst tvs (mkTyVarTys tvs')
+ ; mapM (do_one subst) pairs }
- do_one tenv (ty1, ty2) = do { let sty1 = substTy tenv ty1
- sty2 = substTy tenv ty2
+ do_one subst (ty1, ty2) = do { let sty1 = substTy subst ty1
+ sty2 = substTy subst ty2
; ev <- newWantedCoVar sty1 sty2
; return (WantedEvVar ev loc) }