- Checking kinds
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-%************************************************************************
-
----------------------------
--- We would like to get a decent error message from
--- (a) Under-applied type constructors
--- f :: (Maybe, Maybe)
--- (b) Over-applied type constructors
--- f :: Int x -> Int x
---
-
-\begin{code}
-checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
--- A fancy wrapper for 'unifyKind', which tries
--- to give decent error messages.
--- (checkExpectedKind ty act_kind exp_kind)
--- checks that the actual kind act_kind is compatible
--- with the expected kind exp_kind
--- The first argument, ty, is used only in the error message generation
-checkExpectedKind ty act_kind exp_kind
- | act_kind `isSubKind` exp_kind -- Short cut for a very common case
- = return ()
- | otherwise = do
- (_errs, mb_r) <- tryTc (unifyKind exp_kind act_kind)
- case mb_r of
- Just _ -> return () -- Unification succeeded
- Nothing -> do
-
- -- So there's definitely an error
- -- Now to find out what sort
- exp_kind <- zonkTcKind exp_kind
- act_kind <- zonkTcKind act_kind
-
- env0 <- tcInitTidyEnv
- let (exp_as, _) = splitKindFunTys exp_kind
- (act_as, _) = splitKindFunTys act_kind
- n_exp_as = length exp_as
- n_act_as = length act_as
-
- (env1, tidy_exp_kind) = tidyKind env0 exp_kind
- (env2, tidy_act_kind) = tidyKind env1 act_kind
-
- err | n_exp_as < n_act_as -- E.g. [Maybe]
- = quotes (ppr ty) <+> ptext (sLit "is not applied to enough type arguments")
-
- -- Now n_exp_as >= n_act_as. In the next two cases,
- -- n_exp_as == 0, and hence so is n_act_as
- | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
- = ptext (sLit "Expecting a lifted type, but") <+> quotes (ppr ty)
- <+> ptext (sLit "is unlifted")
-
- | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
- = ptext (sLit "Expecting an unlifted type, but") <+> quotes (ppr ty)
- <+> ptext (sLit "is lifted")
-
- | otherwise -- E.g. Monad [Int]
- = ptext (sLit "Kind mis-match")
-
- more_info = sep [ ptext (sLit "Expected kind") <+>
- quotes (pprKind tidy_exp_kind) <> comma,
- ptext (sLit "but") <+> quotes (ppr ty) <+>
- ptext (sLit "has kind") <+> quotes (pprKind tidy_act_kind)]
-
- failWithTcM (env2, err $$ more_info)
-\end{code}
-
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