mutable type variables
\begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+-- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
+
module TcMType (
TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,
newFlexiTyVarTys, -- Int -> Kind -> TcM [TcType]
newKindVar, newKindVars,
lookupTcTyVar, LookupTyVarResult(..),
- newMetaTyVar, readMetaTyVar, writeMetaTyVar,
+
+ newMetaTyVar, readMetaTyVar, writeMetaTyVar, isFilledMetaTyVar,
--------------------------------
-- Boxy type variables
--------------------------------
-- Creating new coercion variables
- newCoVars,
+ newCoVars, newMetaCoVar,
--------------------------------
-- Instantiation
tcInstTyVar, tcInstType, tcInstTyVars, tcInstBoxyTyVar,
- tcInstSigTyVars, zonkSigTyVar,
+ tcInstSigTyVars,
tcInstSkolTyVar, tcInstSkolTyVars, tcInstSkolType,
- tcSkolSigType, tcSkolSigTyVars,
+ tcSkolSigType, tcSkolSigTyVars, occurCheckErr,
--------------------------------
-- Checking type validity
- Rank, UserTypeCtxt(..), checkValidType,
+ Rank, UserTypeCtxt(..), checkValidType, checkValidMonoType,
SourceTyCtxt(..), checkValidTheta, checkFreeness,
- checkValidInstHead, checkValidInstance, checkAmbiguity,
- checkInstTermination,
- arityErr,
+ checkValidInstHead, checkValidInstance,
+ checkInstTermination, checkValidTypeInst, checkTyFamFreeness,
+ checkUpdateMeta, updateMeta, checkTauTvUpdate, fillBoxWithTau, unifyKindCtxt,
+ unifyKindMisMatch, validDerivPred, arityErr, notMonoType, notMonoArgs,
--------------------------------
-- Zonking
zonkType, zonkTcPredType,
- zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV,
+ zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkSigTyVar,
zonkQuantifiedTyVar, zonkQuantifiedTyVars,
zonkTcType, zonkTcTypes, zonkTcClassConstraints, zonkTcThetaType,
zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
readKindVar, writeKindVar
-
) where
#include "HsVersions.h"
tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
-> TcType -- Type to instantiate
-> TcM ([TcTyVar], TcThetaType, TcType) -- Result
+ -- (type vars (excl coercion vars), preds (incl equalities), rho)
tcInstType inst_tyvars ty
= case tcSplitForAllTys ty of
([], rho) -> let -- There may be overloading despite no type variables;
%************************************************************************
%* *
+ Updating tau types
+%* *
+%************************************************************************
+
+Can't be in TcUnify, as we also need it in TcTyFuns.
+
+\begin{code}
+type SwapFlag = Bool
+ -- False <=> the two args are (actual, expected) respectively
+ -- True <=> the two args are (expected, actual) respectively
+
+checkUpdateMeta :: SwapFlag
+ -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
+-- Update tv1, which is flexi; occurs check is alrady done
+-- The 'check' version does a kind check too
+-- We do a sub-kind check here: we might unify (a b) with (c d)
+-- where b::*->* and d::*; this should fail
+
+checkUpdateMeta swapped tv1 ref1 ty2
+ = do { checkKinds swapped tv1 ty2
+ ; updateMeta tv1 ref1 ty2 }
+
+updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
+updateMeta tv1 ref1 ty2
+ = ASSERT( isMetaTyVar tv1 )
+ ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
+ do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
+ ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
+ ; writeMutVar ref1 (Indirect ty2)
+ }
+
+----------------
+checkKinds swapped tv1 ty2
+-- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
+-- ty2 has been zonked at this stage, which ensures that
+-- its kind has as much boxity information visible as possible.
+ | tk2 `isSubKind` tk1 = return ()
+
+ | otherwise
+ -- Either the kinds aren't compatible
+ -- (can happen if we unify (a b) with (c d))
+ -- or we are unifying a lifted type variable with an
+ -- unlifted type: e.g. (id 3#) is illegal
+ = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
+ unifyKindMisMatch k1 k2
+ where
+ (k1,k2) | swapped = (tk2,tk1)
+ | otherwise = (tk1,tk2)
+ tk1 = tyVarKind tv1
+ tk2 = typeKind ty2
+
+----------------
+checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
+-- (checkTauTvUpdate tv ty)
+-- We are about to update the TauTv tv with ty.
+-- Check (a) that tv doesn't occur in ty (occurs check)
+-- (b) that ty is a monotype
+-- Furthermore, in the interest of (b), if you find an
+-- empty box (BoxTv that is Flexi), fill it in with a TauTv
+--
+-- We have three possible outcomes:
+-- (1) Return the (non-boxy) type to update the type variable with,
+-- [we know the update is ok!]
+-- (2) return Nothing, or
+-- [we cannot tell whether the update is ok right now]
+-- (3) fails.
+-- [the update is definitely invalid]
+-- We return Nothing in case the tv occurs in ty *under* a type family
+-- application. In this case, we must not update tv (to avoid a cyclic type
+-- term), but we also cannot fail claiming an infinite type. Given
+-- type family F a
+-- type instance F Int = Int
+-- consider
+-- a ~ F a
+-- This is perfectly reasonable, if we later get a ~ Int.
+
+checkTauTvUpdate orig_tv orig_ty
+ = do { result <- go orig_ty
+ ; case result of
+ Right ty -> return $ Just ty
+ Left True -> return $ Nothing
+ Left False -> occurCheckErr (mkTyVarTy orig_tv) orig_ty
+ }
+ where
+ go :: TcType -> TcM (Either Bool TcType)
+ -- go returns
+ -- Right ty if everything is fine
+ -- Left True if orig_tv occurs in orig_ty, but under a type family app
+ -- Left False if orig_tv occurs in orig_ty (with no type family app)
+ -- It fails if it encounters a forall type, except as an argument for a
+ -- closed type synonym that expands to a tau type.
+ go (TyConApp tc tys)
+ | isSynTyCon tc = go_syn tc tys
+ | otherwise = do { tys' <- mapM go tys
+ ; return $ occurs (TyConApp tc) tys' }
+ go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
+ go (PredTy p) = do { p' <- go_pred p
+ ; return $ occurs1 PredTy p' }
+ go (FunTy arg res) = do { arg' <- go arg
+ ; res' <- go res
+ ; return $ occurs2 FunTy arg' res' }
+ go (AppTy fun arg) = do { fun' <- go fun
+ ; arg' <- go arg
+ ; return $ occurs2 mkAppTy fun' arg' }
+ -- NB the mkAppTy; we might have instantiated a
+ -- type variable to a type constructor, so we need
+ -- to pull the TyConApp to the top.
+ go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
+
+ go (TyVarTy tv)
+ | orig_tv == tv = return $ Left False -- (a)
+ | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
+ | otherwise = return $ Right (TyVarTy tv)
+ -- Ordinary (non Tc) tyvars
+ -- occur inside quantified types
+
+ go_pred (ClassP c tys) = do { tys' <- mapM go tys
+ ; return $ occurs (ClassP c) tys' }
+ go_pred (IParam n ty) = do { ty' <- go ty
+ ; return $ occurs1 (IParam n) ty' }
+ go_pred (EqPred t1 t2) = do { t1' <- go t1
+ ; t2' <- go t2
+ ; return $ occurs2 EqPred t1' t2' }
+
+ go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
+ go_tyvar tv (MetaTv box ref)
+ = do { cts <- readMutVar ref
+ ; case cts of
+ Indirect ty -> go ty
+ Flexi -> case box of
+ BoxTv -> do { ty <- fillBoxWithTau tv ref
+ ; return $ Right ty }
+ other -> return $ Right (TyVarTy tv)
+ }
+
+ -- go_syn is called for synonyms only
+ -- See Note [Type synonyms and the occur check]
+ go_syn tc tys
+ | not (isTauTyCon tc)
+ = notMonoType orig_ty -- (b) again
+ | otherwise
+ = do { (msgs, mb_tys') <- tryTc (mapM go tys)
+ ; case mb_tys' of
+
+ -- we had a type error => forall in type parameters
+ Nothing
+ | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
+ -- Synonym families must have monotype args
+ | otherwise -> go (expectJust "checkTauTvUpdate(1)"
+ (tcView (TyConApp tc tys)))
+ -- Try again, expanding the synonym
+
+ -- no type error, but need to test whether occurs check happend
+ Just tys' ->
+ case occurs id tys' of
+ Left _
+ | isOpenTyCon tc -> return $ Left True
+ -- Variable occured under type family application
+ | otherwise -> go (expectJust "checkTauTvUpdate(2)"
+ (tcView (TyConApp tc tys)))
+ -- Try again, expanding the synonym
+ Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
+ -- Retain the synonym (the common case)
+ }
+
+ -- Left results (= occurrence of orig_ty) dominate and
+ -- (Left False) (= fatal occurrence) dominates over (Left True)
+ occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
+ occurs c = either Left (Right . c) . foldr combine (Right [])
+ where
+ combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
+ combine (Right _ ) (Left famInst) = Left famInst
+ combine (Left famInst) (Right _) = Left famInst
+ combine (Right arg) (Right args) = Right (arg:args)
+
+ occurs1 c x = occurs (\[x'] -> c x') [x]
+ occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
+
+fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
+-- (fillBoxWithTau tv ref) fills ref with a freshly allocated
+-- tau-type meta-variable, whose print-name is the same as tv
+-- Choosing the same name is good: when we instantiate a function
+-- we allocate boxy tyvars with the same print-name as the quantified
+-- tyvar; and then we often fill the box with a tau-tyvar, and again
+-- we want to choose the same name.
+fillBoxWithTau tv ref
+ = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
+ ; let tau = mkTyVarTy tv' -- name of the type variable
+ ; writeMutVar ref (Indirect tau)
+ ; return tau }
+\end{code}
+
+Note [Type synonyms and the occur check]
+~~~~~~~~~~~~~~~~~~~~
+Basically we want to update tv1 := ps_ty2
+because ps_ty2 has type-synonym info, which improves later error messages
+
+But consider
+ type A a = ()
+
+ f :: (A a -> a -> ()) -> ()
+ f = \ _ -> ()
+
+ x :: ()
+ x = f (\ x p -> p x)
+
+In the application (p x), we try to match "t" with "A t". If we go
+ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
+an infinite loop later.
+But we should not reject the program, because A t = ().
+Rather, we should bind t to () (= non_var_ty2).
+
+--------------
+
+Error mesages in case of kind mismatch.
+
+\begin{code}
+unifyKindMisMatch ty1 ty2 = do
+ ty1' <- zonkTcKind ty1
+ ty2' <- zonkTcKind ty2
+ let
+ msg = hang (ptext SLIT("Couldn't match kind"))
+ 2 (sep [quotes (ppr ty1'),
+ ptext SLIT("against"),
+ quotes (ppr ty2')])
+ failWithTc msg
+
+unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
+ -- tv1 and ty2 are zonked already
+ = return msg
+ where
+ msg = (env2, ptext SLIT("When matching the kinds of") <+>
+ sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
+
+ (pp_expected, pp_actual) | swapped = (pp2, pp1)
+ | otherwise = (pp1, pp2)
+ (env1, tv1') = tidyOpenTyVar tidy_env tv1
+ (env2, ty2') = tidyOpenType env1 ty2
+ pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
+ pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
+\end{code}
+
+Error message for failure due to an occurs check.
+
+\begin{code}
+occurCheckErr :: TcType -> TcType -> TcM a
+occurCheckErr ty containingTy
+ = do { env0 <- tcInitTidyEnv
+ ; ty' <- zonkTcType ty
+ ; containingTy' <- zonkTcType containingTy
+ ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
+ (env2, tidy_ty2) = tidyOpenType env1 containingTy'
+ extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
+ ; failWithTcM (env2, hang msg 2 extra) }
+ where
+ msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
+\end{code}
+
+%************************************************************************
+%* *
Kind variables
%* *
%************************************************************************
(mkCoKind ty1 ty2)
| ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
+newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
+newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
+
newKindVar :: TcM TcKind
newKindVar = do { uniq <- newUnique
; ref <- newMutVar Flexi
; return (mkTyVarTy (mkKindVar uniq ref)) }
newKindVars :: Int -> TcM [TcKind]
-newKindVars n = mappM (\ _ -> newKindVar) (nOfThem n ())
+newKindVars n = mapM (\ _ -> newKindVar) (nOfThem n ())
\end{code}
-- Make a new meta tyvar out of thin air
newMetaTyVar box_info kind
= do { uniq <- newUnique
- ; ref <- newMutVar Flexi ;
+ ; ref <- newMutVar Flexi
; let name = mkSysTvName uniq fs
fs = case box_info of
BoxTv -> FSLIT("t")
-- come from an existing TyVar
instMetaTyVar box_info tyvar
= do { uniq <- newUnique
- ; ref <- newMutVar Flexi ;
+ ; ref <- newMutVar Flexi
; let name = setNameUnique (tyVarName tyvar) uniq
kind = tyVarKind tyvar
; return (mkTcTyVar name kind (MetaTv box_info ref)) }
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)
writeMetaTyVar tyvar ty
| not (isMetaTyVar tyvar)
= pprTrace "writeMetaTyVar" (ppr tyvar) $
- returnM ()
+ return ()
| otherwise
= ASSERT( isMetaTyVar tyvar )
- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
+ -- TOM: It should also work for coercions
+ -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
; writeMutVar (metaTvRef tyvar) (Indirect ty) }
where
newFlexiTyVar kind = newMetaTyVar TauTv kind
newFlexiTyVarTy :: Kind -> TcM TcType
-newFlexiTyVarTy kind
- = newFlexiTyVar kind `thenM` \ tc_tyvar ->
- returnM (TyVarTy tc_tyvar)
+newFlexiTyVarTy kind = do
+ tc_tyvar <- newFlexiTyVar kind
+ return (TyVarTy tc_tyvar)
newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
-newFlexiTyVarTys n kind = mappM newFlexiTyVarTy (nOfThem n kind)
+newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
tcInstTyVar :: TyVar -> TcM TcTyVar
-- Instantiate with a META type variable
tcInstTyVars tyvars
= do { tc_tvs <- mapM tcInstTyVar tyvars
; let tys = mkTyVarTys tc_tvs
- ; returnM (tc_tvs, tys, zipTopTvSubst tyvars tys) }
+ ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
-- Since the tyvars are freshly made,
-- they cannot possibly be captured by
-- any existing for-alls. Hence zipTopTvSubst
readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
do { cts <- readMetaTyVar box_tv
; case cts of
- Flexi -> pprPanic "readFilledBox" (ppr box_tv)
+ Flexi -> pprPanic "readFilledBox" (ppr box_tv)
Indirect ty -> return ty }
tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
MetaTv _ ref -> do { meta_details <- readMutVar ref
; case meta_details of
Indirect ty -> return (IndirectTv ty)
- Flexi -> return (DoneTv details) }
+ Flexi -> return (DoneTv details) }
where
details = tcTyVarDetails tyvar
getTcTyVar tyvar
| not (isTcTyVar tyvar)
= pprTrace "getTcTyVar" (ppr tyvar) $
- returnM (Just (mkTyVarTy tyvar))
+ return (Just (mkTyVarTy tyvar))
| otherwise
- = ASSERT2( isTcTyVar tyvar, ppr tyvar )
- readMetaTyVar tyvar `thenM` \ maybe_ty ->
+ = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
+ maybe_ty <- readMetaTyVar tyvar
case maybe_ty of
- Just ty -> short_out ty `thenM` \ ty' ->
- writeMetaTyVar tyvar (Just ty') `thenM_`
- returnM (Just ty')
+ Just ty -> do ty' <- short_out ty
+ writeMetaTyVar tyvar (Just ty')
+ return (Just ty')
- Nothing -> returnM Nothing
+ Nothing -> return Nothing
short_out :: TcType -> TcM TcType
short_out ty@(TyVarTy tyvar)
| not (isTcTyVar tyvar)
- = returnM ty
+ = return ty
- | otherwise
- = readMetaTyVar tyvar `thenM` \ maybe_ty ->
+ | otherwise = do
+ maybe_ty <- readMetaTyVar tyvar
case maybe_ty of
- Just ty' -> short_out ty' `thenM` \ ty' ->
- writeMetaTyVar tyvar (Just ty') `thenM_`
- returnM ty'
+ Just ty' -> do ty' <- short_out ty'
+ writeMetaTyVar tyvar (Just ty')
+ return ty'
- other -> returnM ty
+ other -> return ty
-short_out other_ty = returnM other_ty
+short_out other_ty = return other_ty
-}
\end{code}
\begin{code}
zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
-zonkTcTyVars tyvars = mappM zonkTcTyVar tyvars
+zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
-zonkTcTyVarsAndFV tyvars = mappM zonkTcTyVar tyvars `thenM` \ tys ->
- returnM (tyVarsOfTypes tys)
+zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
zonkTcTyVar :: TcTyVar -> TcM TcType
zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
- zonk_tc_tyvar (\ tv -> returnM (TyVarTy tv)) tyvar
+ zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
\end{code}
----------------- Types
\begin{code}
zonkTcType :: TcType -> TcM TcType
-zonkTcType ty = zonkType (\ tv -> returnM (TyVarTy tv)) ty
+zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
zonkTcTypes :: [TcType] -> TcM [TcType]
-zonkTcTypes tys = mappM zonkTcType tys
+zonkTcTypes tys = mapM zonkTcType tys
-zonkTcClassConstraints cts = mappM zonk cts
- where zonk (clas, tys)
- = zonkTcTypes tys `thenM` \ new_tys ->
- returnM (clas, new_tys)
+zonkTcClassConstraints cts = mapM zonk cts
+ where zonk (clas, tys) = do
+ new_tys <- zonkTcTypes tys
+ return (clas, new_tys)
zonkTcThetaType :: TcThetaType -> TcM TcThetaType
-zonkTcThetaType theta = mappM zonkTcPredType theta
+zonkTcThetaType theta = mapM zonkTcPredType theta
zonkTcPredType :: TcPredType -> TcM TcPredType
-zonkTcPredType (ClassP c ts)
- = zonkTcTypes ts `thenM` \ new_ts ->
- returnM (ClassP c new_ts)
-zonkTcPredType (IParam n t)
- = zonkTcType t `thenM` \ new_t ->
- returnM (IParam n new_t)
-zonkTcPredType (EqPred t1 t2)
- = zonkTcType t1 `thenM` \ new_t1 ->
- zonkTcType t2 `thenM` \ new_t2 ->
- returnM (EqPred new_t1 new_t2)
+zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
+zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
+zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
\end{code}
------------------- These ...ToType, ...ToKind versions
k = tyVarKind tv
default_k = defaultKind k
-zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TyVar]
-zonkQuantifiedTyVars = mappM zonkQuantifiedTyVar
+zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
+zonkQuantifiedTyVars = mapM 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.
+
%************************************************************************
%* *
= go ty
where
go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
-
- go (TyConApp tc tys) = mappM go tys `thenM` \ tys' ->
- returnM (TyConApp tc tys')
-
- go (PredTy p) = go_pred p `thenM` \ p' ->
- returnM (PredTy p')
-
- go (FunTy arg res) = go arg `thenM` \ arg' ->
- go res `thenM` \ res' ->
- returnM (FunTy arg' res')
-
- go (AppTy fun arg) = go fun `thenM` \ fun' ->
- go arg `thenM` \ arg' ->
- returnM (mkAppTy fun' arg')
+
+ go (TyConApp tc tys) = do tys' <- mapM go tys
+ return (TyConApp tc tys')
+
+ go (PredTy p) = do p' <- go_pred p
+ return (PredTy p')
+
+ go (FunTy arg res) = do arg' <- go arg
+ res' <- go res
+ return (FunTy arg' res')
+
+ go (AppTy fun arg) = do fun' <- go fun
+ arg' <- go arg
+ return (mkAppTy fun' arg')
-- NB the mkAppTy; we might have instantiated a
-- type variable to a type constructor, so we need
-- to pull the TyConApp to the top.
| otherwise = return (TyVarTy tyvar)
-- Ordinary (non Tc) tyvars occur inside quantified types
- go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar )
- go ty `thenM` \ ty' ->
- returnM (ForAllTy tyvar ty')
+ go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
+ ty' <- go ty
+ return (ForAllTy tyvar ty')
- go_pred (ClassP c tys) = mappM go tys `thenM` \ tys' ->
- returnM (ClassP c tys')
- go_pred (IParam n ty) = go ty `thenM` \ ty' ->
- returnM (IParam n ty')
- go_pred (EqPred ty1 ty2) = go ty1 `thenM` \ ty1' ->
- go ty2 `thenM` \ ty2' ->
- returnM (EqPred ty1' ty2')
+ go_pred (ClassP c tys) = do tys' <- mapM go tys
+ return (ClassP c tys')
+ go_pred (IParam n ty) = do ty' <- go ty
+ return (IParam n ty')
+ go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
+ ty2' <- go ty2
+ return (EqPred ty1' ty2')
zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
-> TcTyVar -> TcM TcType
zonk_tc_tyvar unbound_var_fn tyvar
| not (isMetaTyVar tyvar) -- Skolems
- = returnM (TyVarTy tyvar)
+ = return (TyVarTy tyvar)
| otherwise -- Mutables
= do { cts <- readMetaTyVar tyvar
\begin{code}
checkValidType :: UserTypeCtxt -> Type -> TcM ()
-- Checks that the type is valid for the given context
-checkValidType ctxt ty
- = traceTc (text "checkValidType" <+> ppr ty) `thenM_`
- doptM Opt_GlasgowExts `thenM` \ gla_exts ->
+checkValidType ctxt ty = do
+ traceTc (text "checkValidType" <+> ppr ty)
+ unboxed <- doptM Opt_UnboxedTuples
+ rank2 <- doptM Opt_Rank2Types
+ rankn <- doptM Opt_RankNTypes
+ polycomp <- doptM Opt_PolymorphicComponents
let
- rank | gla_exts = Arbitrary
+ rank | rankn = Arbitrary
+ | rank2 = Rank 2
| otherwise
= case ctxt of -- Haskell 98
GenPatCtxt -> Rank 0
TySynCtxt _ -> Rank 0
ExprSigCtxt -> Rank 1
FunSigCtxt _ -> Rank 1
- ConArgCtxt _ -> Rank 1 -- We are given the type of the entire
- -- constructor, hence rank 1
+ ConArgCtxt _ -> if polycomp
+ then Rank 2
+ -- We are given the type of the entire
+ -- constructor, hence rank 1
+ else Rank 1
ForSigCtxt _ -> Rank 1
SpecInstCtxt -> Rank 1
ForSigCtxt _ -> isLiftedTypeKind actual_kind
other -> isSubArgTypeKind actual_kind
- ubx_tup | not gla_exts = UT_NotOk
- | otherwise = case ctxt of
- TySynCtxt _ -> UT_Ok
- ExprSigCtxt -> UT_Ok
- other -> UT_NotOk
- -- Unboxed tuples ok in function results,
- -- but for type synonyms we allow them even at
- -- top level
- in
+ ubx_tup = case ctxt of
+ TySynCtxt _ | unboxed -> UT_Ok
+ ExprSigCtxt | unboxed -> UT_Ok
+ _ -> UT_NotOk
+
-- Check that the thing has kind Type, and is lifted if necessary
- checkTc kind_ok (kindErr actual_kind) `thenM_`
+ checkTc kind_ok (kindErr actual_kind)
-- Check the internal validity of the type itself
- check_poly_type rank ubx_tup ty `thenM_`
+ check_type rank ubx_tup ty
traceTc (text "checkValidType done" <+> ppr ty)
+
+checkValidMonoType :: Type -> TcM ()
+checkValidMonoType ty = check_mono_type ty
\end{code}
decRank Arbitrary = Arbitrary
decRank (Rank n) = Rank (n-1)
+nonZeroRank :: Rank -> Bool
+nonZeroRank (Rank 0) = False
+nonZeroRank _ = True
+
----------------------------------------
data UbxTupFlag = UT_Ok | UT_NotOk
-- The "Ok" version means "ok if -fglasgow-exts is on"
----------------------------------------
-check_poly_type :: Rank -> UbxTupFlag -> Type -> TcM ()
-check_poly_type (Rank 0) ubx_tup ty
- = check_tau_type (Rank 0) ubx_tup ty
-
-check_poly_type rank ubx_tup ty
- | null tvs && null theta
- = check_tau_type rank ubx_tup ty
- | otherwise
- = do { check_valid_theta SigmaCtxt theta
- ; check_poly_type rank ubx_tup tau -- Allow foralls to right of arrow
+check_mono_type :: Type -> TcM () -- No foralls anywhere
+ -- No unlifted types of any kind
+check_mono_type ty
+ = do { check_type (Rank 0) UT_NotOk ty
+ ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
+
+check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
+-- The args say what the *type* context requires, independent
+-- of *flag* settings. You test the flag settings at usage sites.
+--
+-- Rank is allowed rank for function args
+-- Rank 0 means no for-alls anywhere
+
+check_type rank ubx_tup ty
+ | not (null tvs && null theta)
+ = do { checkTc (nonZeroRank rank) (forAllTyErr ty)
+ -- Reject e.g. (Maybe (?x::Int => Int)),
+ -- with a decent error message
+ ; check_valid_theta SigmaCtxt theta
+ ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
; checkFreeness tvs theta
; checkAmbiguity tvs theta (tyVarsOfType tau) }
where
(tvs, theta, tau) = tcSplitSigmaTy ty
-----------------------------------------
-check_arg_type :: Type -> TcM ()
--- The sort of type that can instantiate a type variable,
--- or be the argument of a type constructor.
--- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
--- Other unboxed types are very occasionally allowed as type
--- arguments depending on the kind of the type constructor
---
--- For example, we want to reject things like:
---
--- instance Ord a => Ord (forall s. T s a)
--- and
--- g :: T s (forall b.b)
---
--- NB: unboxed tuples can have polymorphic or unboxed args.
--- This happens in the workers for functions returning
--- product types with polymorphic components.
--- But not in user code.
--- Anyway, they are dealt with by a special case in check_tau_type
-
-check_arg_type ty
- = check_poly_type Arbitrary UT_NotOk ty `thenM_`
- checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty)
-
-----------------------------------------
-check_tau_type :: Rank -> UbxTupFlag -> Type -> TcM ()
--- Rank is allowed rank for function args
--- No foralls otherwise
-
-check_tau_type rank ubx_tup ty@(ForAllTy _ _) = failWithTc (forAllTyErr ty)
-check_tau_type rank ubx_tup ty@(FunTy (PredTy _) _) = failWithTc (forAllTyErr ty)
- -- Reject e.g. (Maybe (?x::Int => Int)), with a decent error message
-
-- Naked PredTys don't usually show up, but they can as a result of
-- {-# SPECIALISE instance Ord Char #-}
-- The Right Thing would be to fix the way that SPECIALISE instance pragmas
-- are handled, but the quick thing is just to permit PredTys here.
-check_tau_type rank ubx_tup (PredTy sty) = getDOpts `thenM` \ dflags ->
- check_pred_ty dflags TypeCtxt sty
-
-check_tau_type rank ubx_tup (TyVarTy _) = returnM ()
-check_tau_type rank ubx_tup ty@(FunTy arg_ty res_ty)
- = check_poly_type (decRank rank) UT_NotOk arg_ty `thenM_`
- check_poly_type rank UT_Ok res_ty
-
-check_tau_type rank ubx_tup (AppTy ty1 ty2)
- = check_arg_type ty1 `thenM_` check_arg_type ty2
-
-check_tau_type rank ubx_tup (NoteTy other_note ty)
- = check_tau_type rank ubx_tup ty
-
-check_tau_type rank ubx_tup ty@(TyConApp tc tys)
- | isSynTyCon tc
- = do { -- It's OK to have an *over-applied* type synonym
+check_type rank ubx_tup (PredTy sty)
+ = do { dflags <- getDOpts
+ ; check_pred_ty dflags TypeCtxt sty }
+
+check_type rank ubx_tup (TyVarTy _) = return ()
+check_type rank ubx_tup ty@(FunTy arg_ty res_ty)
+ = do { check_type (decRank rank) UT_NotOk arg_ty
+ ; check_type rank UT_Ok res_ty }
+
+check_type rank ubx_tup (AppTy ty1 ty2)
+ = do { check_arg_type rank ty1
+ ; check_arg_type rank ty2 }
+
+check_type rank ubx_tup (NoteTy other_note ty)
+ = check_type rank ubx_tup ty
+
+check_type rank ubx_tup ty@(TyConApp tc tys)
+ | isSynTyCon tc
+ = do { -- Check that the synonym has enough args
+ -- 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]
-- f :: Foo a b -> ...
- ; case tcView ty of
- Just ty' -> check_tau_type rank ubx_tup ty' -- Check expansion
- Nothing -> unless (isOpenTyCon tc -- No expansion if open
- && tyConArity tc <= length tys) $
- failWithTc arity_msg
-
- ; gla_exts <- doptM Opt_GlasgowExts
- ; if gla_exts && not (isOpenTyCon tc) then
- -- If -fglasgow-exts then don't check the type arguments of
- -- *closed* synonyms.
- -- This allows us to instantiate a synonym defn with a
- -- for-all type, or with a partially-applied type synonym.
- -- e.g. type T a b = a
- -- type S m = m ()
- -- f :: S (T Int)
- -- Here, T is partially applied, so it's illegal in H98.
- -- But if you expand S first, then T we get just
- -- f :: Int
- -- which is fine.
- returnM ()
- else
- -- For H98, do check the type args
- mappM_ check_arg_type tys
- }
+ checkTc (tyConArity tc <= length tys) arity_msg
+
+ -- See Note [Liberal type synonyms]
+ ; liberal <- doptM Opt_LiberalTypeSynonyms
+ ; if not liberal || isOpenSynTyCon tc then
+ -- For H98 and synonym families, do check the type args
+ mapM_ check_mono_type tys
+
+ else -- In the liberal case (only for closed syns), expand then check
+ case tcView ty of
+ Just ty' -> check_type rank ubx_tup ty'
+ Nothing -> pprPanic "check_tau_type" (ppr ty)
+ }
| isUnboxedTupleTyCon tc
- = doptM Opt_GlasgowExts `thenM` \ gla_exts ->
- checkTc (ubx_tup_ok gla_exts) ubx_tup_msg `thenM_`
- mappM_ (check_tau_type (Rank 0) UT_Ok) tys
- -- Args are allowed to be unlifted, or
+ = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
+ ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
+
+ ; impred <- doptM Opt_ImpredicativeTypes
+ ; let rank' = if impred then rank else Rank 0
+ -- c.f. check_arg_type
+ -- However, args are allowed to be unlifted, or
-- more unboxed tuples, so can't use check_arg_ty
+ ; mapM_ (check_type rank' UT_Ok) tys }
| otherwise
- = mappM_ check_arg_type tys
+ = mapM_ (check_arg_type rank) tys
where
- ubx_tup_ok gla_exts = case ubx_tup of { UT_Ok -> gla_exts; other -> False }
+ ubx_tup_ok ub_tuples_allowed = case ubx_tup of { UT_Ok -> ub_tuples_allowed; other -> False }
n_args = length tys
tc_arity = tyConArity tc
ubx_tup_msg = ubxArgTyErr ty
----------------------------------------
+check_arg_type :: Rank -> Type -> TcM ()
+-- The sort of type that can instantiate a type variable,
+-- or be the argument of a type constructor.
+-- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
+-- Other unboxed types are very occasionally allowed as type
+-- arguments depending on the kind of the type constructor
+--
+-- For example, we want to reject things like:
+--
+-- instance Ord a => Ord (forall s. T s a)
+-- and
+-- g :: T s (forall b.b)
+--
+-- NB: unboxed tuples can have polymorphic or unboxed args.
+-- This happens in the workers for functions returning
+-- product types with polymorphic components.
+-- But not in user code.
+-- Anyway, they are dealt with by a special case in check_tau_type
+
+check_arg_type rank ty
+ = do { impred <- doptM Opt_ImpredicativeTypes
+ ; let rank' = if impred then rank else Rank 0 -- Monotype unless impredicative
+ ; check_type rank' UT_NotOk ty
+ ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
+
+----------------------------------------
forAllTyErr ty = ptext SLIT("Illegal polymorphic or qualified type:") <+> ppr ty
-unliftedArgErr ty = ptext SLIT("Illegal unlifted type argument:") <+> ppr ty
+unliftedArgErr ty = ptext SLIT("Illegal unlifted type:") <+> ppr ty
ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as function argument:") <+> ppr ty
kindErr kind = ptext SLIT("Expecting an ordinary type, but found a type of kind") <+> ppr kind
\end{code}
+Note [Liberal type synonyms]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
+doing validity checking. This allows us to instantiate a synonym defn
+with a for-all type, or with a partially-applied type synonym.
+ e.g. type T a b = a
+ type S m = m ()
+ f :: S (T Int)
+Here, T is partially applied, so it's illegal in H98. But if you
+expand S first, then T we get just
+ f :: Int
+which is fine.
+
+IMPORTANT: suppose T is a type synonym. Then we must do validity
+checking on an appliation (T ty1 ty2)
+
+ *either* before expansion (i.e. check ty1, ty2)
+ *or* after expansion (i.e. expand T ty1 ty2, and then check)
+ BUT NOT BOTH
+
+If we do both, we get exponential behaviour!!
+
+ data TIACons1 i r c = c i ::: r c
+ type TIACons2 t x = TIACons1 t (TIACons1 t x)
+ type TIACons3 t x = TIACons2 t (TIACons1 t x)
+ type TIACons4 t x = TIACons2 t (TIACons2 t x)
+ type TIACons7 t x = TIACons4 t (TIACons3 t x)
%************************************************************************
-------------------------
check_valid_theta ctxt []
- = returnM ()
-check_valid_theta ctxt theta
- = getDOpts `thenM` \ dflags ->
- warnTc (notNull dups) (dupPredWarn dups) `thenM_`
- mappM_ (check_pred_ty dflags ctxt) theta
+ = return ()
+check_valid_theta ctxt theta = do
+ dflags <- getDOpts
+ warnTc (notNull dups) (dupPredWarn dups)
+ mapM_ (check_pred_ty dflags ctxt) theta
where
(_,dups) = removeDups tcCmpPred theta
-------------------------
+check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
check_pred_ty dflags ctxt pred@(ClassP cls tys)
= do { -- Class predicates are valid in all contexts
; checkTc (arity == n_tys) arity_err
-- Check the form of the argument types
- ; mappM_ check_arg_type tys
+ ; mapM_ check_mono_type tys
; checkTc (check_class_pred_tys dflags ctxt tys)
(predTyVarErr pred $$ how_to_allow)
}
arity = classArity cls
n_tys = length tys
arity_err = arityErr "Class" class_name arity n_tys
- how_to_allow = parens (ptext SLIT("Use -fglasgow-exts to permit this"))
+ how_to_allow = parens (ptext SLIT("Use -XFlexibleContexts to permit this"))
check_pred_ty dflags ctxt pred@(EqPred ty1 ty2)
= do { -- Equational constraints are valid in all contexts if type
; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
-- Check the form of the argument types
- ; check_eq_arg_type ty1
- ; check_eq_arg_type ty2
+ ; check_mono_type ty1
+ ; check_mono_type ty2
}
- where
- check_eq_arg_type = check_poly_type (Rank 0) UT_NotOk
-check_pred_ty dflags SigmaCtxt (IParam _ ty) = check_arg_type ty
+check_pred_ty dflags SigmaCtxt (IParam _ ty) = check_mono_type ty
-- Implicit parameters only allowed in type
-- signatures; not in instance decls, superclasses etc
-- The reason for not allowing implicit params in instances is a bit
check_pred_ty dflags ctxt sty = failWithTc (badPredTyErr sty)
-------------------------
+check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
check_class_pred_tys dflags ctxt tys
= case ctxt of
TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
- InstThetaCtxt -> gla_exts || undecidable_ok || all tcIsTyVarTy tys
+ InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
-- Further checks on head and theta in
-- checkInstTermination
- other -> gla_exts || all tyvar_head tys
+ other -> flexible_contexts || all tyvar_head tys
where
- gla_exts = dopt Opt_GlasgowExts dflags
- undecidable_ok = dopt Opt_AllowUndecidableInstances dflags
+ flexible_contexts = dopt Opt_FlexibleContexts dflags
+ undecidable_ok = dopt Opt_UndecidableInstances dflags
-------------------------
tyvar_head ty -- Haskell 98 allows predicates of form
don't need to check for ambiguity either, because the test can't fail
(see is_ambig).
+
\begin{code}
checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
checkAmbiguity forall_tyvars theta tau_tyvars
- = mappM_ complain (filter is_ambig theta)
+ = mapM_ complain (filter is_ambig theta)
where
complain pred = addErrTc (ambigErr pred)
extended_tau_vars = grow theta tau_tyvars
- -- Only a *class* predicate can give rise to ambiguity
- -- An *implicit parameter* cannot. For example:
- -- foo :: (?x :: [a]) => Int
- -- foo = length ?x
- -- is fine. The call site will suppply a particular 'x'
+ -- See Note [Implicit parameters and ambiguity] in TcSimplify
is_ambig pred = isClassPred pred &&
any ambig_var (varSetElems (tyVarsOfPred pred))
\begin{code}
checkFreeness forall_tyvars theta
- = do { gla_exts <- doptM Opt_GlasgowExts
- ; if gla_exts then return () -- New! Oct06
- else mappM_ complain (filter is_free theta) }
+ = do { flexible_contexts <- doptM Opt_FlexibleContexts
+ ; unless flexible_contexts $ mapM_ complain (filter is_free theta) }
where
is_free pred = not (isIPPred pred)
&& not (any bound_var (varSetElems (tyVarsOfPred pred)))
complain pred = addErrTc (freeErr pred)
freeErr pred
- = sep [ptext SLIT("All of the type variables in the constraint") <+> quotes (pprPred pred) <+>
- ptext SLIT("are already in scope"),
- nest 4 (ptext SLIT("(at least one must be universally quantified here)"))
- ]
+ = sep [ ptext SLIT("All of the type variables in the constraint") <+>
+ quotes (pprPred pred)
+ , ptext SLIT("are already in scope") <+>
+ ptext SLIT("(at least one must be universally quantified here)")
+ , nest 4 $
+ ptext SLIT("(Use -XFlexibleContexts to lift this restriction)")
+ ]
\end{code}
\begin{code}
badPredTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
eqPredTyErr sty = ptext SLIT("Illegal equational constraint") <+> pprPred sty
$$
- parens (ptext SLIT("Use -ftype-families to permit this"))
+ parens (ptext SLIT("Use -XTypeFamilies to permit this"))
predTyVarErr pred = sep [ptext SLIT("Non type-variable argument"),
nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
dupPredWarn dups = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
n_arguments | n == 0 = ptext SLIT("no arguments")
| n == 1 = ptext SLIT("1 argument")
| True = hsep [int n, ptext SLIT("arguments")]
+
+-----------------
+notMonoType ty
+ = do { ty' <- zonkTcType ty
+ ; env0 <- tcInitTidyEnv
+ ; let (env1, tidy_ty) = tidyOpenType env0 ty'
+ msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
+ ; failWithTcM (env1, msg) }
+
+notMonoArgs ty
+ = do { ty' <- zonkTcType ty
+ ; env0 <- tcInitTidyEnv
+ ; let (env1, tidy_ty) = tidyOpenType env0 ty'
+ msg = ptext SLIT("Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
+ ; failWithTcM (env1, msg) }
\end{code}
case getClassPredTys_maybe pred of {
Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
- Just (clas,tys) ->
+ Just (clas,tys) -> do
- getDOpts `thenM` \ dflags ->
- mappM_ check_arg_type tys `thenM_`
- check_inst_head dflags clas tys `thenM_`
- returnM (clas, tys)
+ dflags <- getDOpts
+ mapM_ check_mono_type tys
+ check_inst_head dflags clas tys
+ return (clas, tys)
}}
check_inst_head dflags clas tys
-- If GlasgowExts then check at least one isn't a type variable
- | dopt Opt_GlasgowExts dflags
- = mapM_ check_one tys
-
- -- WITH HASKELL 98, MUST HAVE C (T a b c)
- | otherwise
- = checkTc (isSingleton tys && tcValidInstHeadTy first_ty)
- (instTypeErr (pprClassPred clas tys) head_shape_msg)
-
- where
- (first_ty : _) = tys
-
- head_shape_msg = parens (text "The instance type must be of form (T a1 ... an)" $$
- text "where T is not a synonym, and a1 ... an are distinct type *variables*")
-
+ = do checkTc (dopt Opt_TypeSynonymInstances dflags ||
+ all tcInstHeadTyNotSynonym tys)
+ (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
+ checkTc (dopt Opt_FlexibleInstances dflags ||
+ all tcInstHeadTyAppAllTyVars tys)
+ (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
+ checkTc (dopt Opt_MultiParamTypeClasses dflags ||
+ isSingleton tys)
+ (instTypeErr (pprClassPred clas tys) head_one_type_msg)
+ mapM_ check_mono_type tys
-- For now, I only allow tau-types (not polytypes) in
-- the head of an instance decl.
-- E.g. instance C (forall a. a->a) is rejected
-- One could imagine generalising that, but I'm not sure
-- what all the consequences might be
- check_one ty = do { check_tau_type (Rank 0) UT_NotOk ty
- ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
+
+ where
+ head_type_synonym_msg = parens (
+ text "All instance types must be of the form (T t1 ... tn)" $$
+ text "where T is not a synonym." $$
+ text "Use -XTypeSynonymInstances if you want to disable this.")
+
+ head_type_args_tyvars_msg = parens (vcat [
+ text "All instance types must be of the form (T a1 ... an)",
+ text "where a1 ... an are type *variables*,",
+ text "and each type variable appears at most once in the instance head.",
+ text "Use -XFlexibleInstances if you want to disable this."])
+
+ head_one_type_msg = parens (
+ text "Only one type can be given in an instance head." $$
+ text "Use -XMultiParamTypeClasses if you want to allow more.")
instTypeErr pp_ty msg
= sep [ptext SLIT("Illegal instance declaration for") <+> quotes pp_ty,
\begin{code}
checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
checkValidInstance tyvars theta clas inst_tys
- = do { gla_exts <- doptM Opt_GlasgowExts
- ; undecidable_ok <- doptM Opt_AllowUndecidableInstances
+ = do { undecidable_ok <- doptM Opt_UndecidableInstances
; checkValidTheta InstThetaCtxt theta
; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
-- Check that instance inference will terminate (if we care)
- -- For Haskell 98, checkValidTheta has already done that
- ; when (gla_exts && not undecidable_ok) $
+ -- For Haskell 98 this will already have been done by checkValidTheta,
+ -- but as we may be using other extensions we need to check.
+ ; unless undecidable_ok $
mapM_ addErrTc (checkInstTermination inst_tys theta)
-- The Coverage Condition
nomoreMsg = ptext SLIT("Variable occurs more often in a constraint than in the instance head")
smallerMsg = ptext SLIT("Constraint is no smaller than the instance head")
undecidableMsg = ptext SLIT("Use -fallow-undecidable-instances to permit this")
+\end{code}
+
+
+%************************************************************************
+%* *
+ Checking the context of a derived instance declaration
+%* *
+%************************************************************************
+
+Note [Exotic derived instance contexts]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In a 'derived' instance declaration, we *infer* the context. It's a
+bit unclear what rules we should apply for this; the Haskell report is
+silent. Obviously, constraints like (Eq a) are fine, but what about
+ data T f a = MkT (f a) deriving( Eq )
+where we'd get an Eq (f a) constraint. That's probably fine too.
+
+One could go further: consider
+ data T a b c = MkT (Foo a b c) deriving( Eq )
+ instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
+
+Notice that this instance (just) satisfies the Paterson termination
+conditions. Then we *could* derive an instance decl like this:
+
+ instance (C Int a, Eq b, Eq c) => Eq (T a b c)
+
+even though there is no instance for (C Int a), because there just
+*might* be an instance for, say, (C Int Bool) at a site where we
+need the equality instance for T's.
+
+However, this seems pretty exotic, and it's quite tricky to allow
+this, and yet give sensible error messages in the (much more common)
+case where we really want that instance decl for C.
+
+So for now we simply require that the derived instance context
+should have only type-variable constraints.
+
+Here is another example:
+ data Fix f = In (f (Fix f)) deriving( Eq )
+Here, if we are prepared to allow -fallow-undecidable-instances we
+could derive the instance
+ instance Eq (f (Fix f)) => Eq (Fix f)
+but this is so delicate that I don't think it should happen inside
+'deriving'. If you want this, write it yourself!
+
+NB: if you want to lift this condition, make sure you still meet the
+termination conditions! If not, the deriving mechanism generates
+larger and larger constraints. Example:
+ data Succ a = S a
+ data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
+
+Note the lack of a Show instance for Succ. First we'll generate
+ instance (Show (Succ a), Show a) => Show (Seq a)
+and then
+ instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
+and so on. Instead we want to complain of no instance for (Show (Succ a)).
+
+The bottom line
+~~~~~~~~~~~~~~~
+Allow constraints which consist only of type variables, with no repeats.
+
+\begin{code}
+validDerivPred :: PredType -> Bool
+validDerivPred (ClassP cls tys) = hasNoDups fvs && sizeTypes tys == length fvs
+ where fvs = fvTypes tys
+validDerivPred otehr = False
+\end{code}
+
+%************************************************************************
+%* *
+ Checking type instance well-formedness and termination
+%* *
+%************************************************************************
+\begin{code}
+-- Check that a "type instance" is well-formed (which includes decidability
+-- unless -fallow-undecidable-instances is given).
+--
+checkValidTypeInst :: [Type] -> Type -> TcM ()
+checkValidTypeInst typats rhs
+ = do { -- left-hand side contains no type family applications
+ -- (vanilla synonyms are fine, though)
+ ; mapM_ checkTyFamFreeness typats
+
+ -- the right-hand side is a tau type
+ ; checkTc (isTauTy rhs) $
+ polyTyErr rhs
+
+ -- we have a decidable instance unless otherwise permitted
+ ; undecidable_ok <- doptM Opt_UndecidableInstances
+ ; unless undecidable_ok $
+ mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
+ }
+
+-- Make sure that each type family instance is
+-- (1) strictly smaller than the lhs,
+-- (2) mentions no type variable more often than the lhs, and
+-- (3) does not contain any further type family instances.
+--
+checkFamInst :: [Type] -- lhs
+ -> [(TyCon, [Type])] -- type family instances
+ -> [Message]
+checkFamInst lhsTys famInsts
+ = mapCatMaybes check famInsts
+ where
+ size = sizeTypes lhsTys
+ fvs = fvTypes lhsTys
+ check (tc, tys)
+ | not (all isTyFamFree tys)
+ = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
+ | not (null (fvTypes tys \\ fvs))
+ = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
+ | size <= sizeTypes tys
+ = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
+ | otherwise
+ = Nothing
+ where
+ famInst = TyConApp tc tys
+
+-- Ensure that no type family instances occur in a type.
+--
+checkTyFamFreeness :: Type -> TcM ()
+checkTyFamFreeness ty
+ = checkTc (isTyFamFree ty) $
+ tyFamInstInIndexErr ty
+
+-- Check that a type does not contain any type family applications.
+--
+isTyFamFree :: Type -> Bool
+isTyFamFree = null . tyFamInsts
+
+-- Error messages
+
+tyFamInstInIndexErr ty
+ = hang (ptext SLIT("Illegal type family application in type instance") <>
+ colon) 4 $
+ ppr ty
+
+polyTyErr ty
+ = hang (ptext SLIT("Illegal polymorphic type in type instance") <> colon) 4 $
+ ppr ty
+
+famInstUndecErr ty msg
+ = sep [msg,
+ nest 2 (ptext SLIT("in the type family application:") <+>
+ pprType ty)]
+
+nestedMsg = ptext SLIT("Nested type family application")
+nomoreVarMsg = ptext SLIT("Variable occurs more often than in instance head")
+smallerAppMsg = ptext SLIT("Application is no smaller than the instance head")
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Auxiliary functions}
+%* *
+%************************************************************************
+
+\begin{code}
-- Free variables of a type, retaining repetitions, and expanding synonyms
fvType :: Type -> [TyVar]
fvType ty | Just exp_ty <- tcView ty = fvType exp_ty