%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 19921998
%
Monadic type operations
This module contains monadic operations over types that contain
mutable type variables
\begin{code}
module TcMType (
TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,
newFlexiTyVar,
newFlexiTyVarTy,
newFlexiTyVarTys,
newKindVar, newKindVars,
lookupTcTyVar, LookupTyVarResult(..),
newMetaTyVar, readMetaTyVar, writeMetaTyVar, isFilledMetaTyVar,
newBoxyTyVar, newBoxyTyVars, newBoxyTyVarTys, readFilledBox,
newCoVars, newMetaCoVar,
tcInstTyVar, tcInstType, tcInstTyVars, tcInstBoxyTyVar,
tcInstSigType,
tcInstSkolTyVars, tcInstSkolType,
tcSkolSigType, tcSkolSigTyVars, occurCheckErr, execTcTyVarBinds,
Rank, UserTypeCtxt(..), checkValidType, checkValidMonoType,
SourceTyCtxt(..), checkValidTheta, checkFreeness,
checkValidInstHead, checkValidInstance,
checkInstTermination, checkValidTypeInst, checkTyFamFreeness, checkKinds,
checkUpdateMeta, updateMeta, checkTauTvUpdate, fillBoxWithTau, unifyKindCtxt,
unifyKindMisMatch, validDerivPred, arityErr, notMonoType, notMonoArgs,
growPredTyVars, growTyVars, growThetaTyVars,
zonkType, zonkTcPredType,
zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkSigTyVar,
zonkQuantifiedTyVar, zonkQuantifiedTyVars,
zonkTcType, zonkTcTypes, zonkTcThetaType,
zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
readKindVar, writeKindVar
) where
#include "HsVersions.h"
import TypeRep
import TcType
import Type
import Coercion
import Class
import TyCon
import Var
import TcRnMonad
import FunDeps
import Name
import VarEnv
import VarSet
import ErrUtils
import DynFlags
import Util
import Bag
import Maybes
import ListSetOps
import UniqSupply
import SrcLoc
import Outputable
import FastString
import Control.Monad
import Data.List ( (\\) )
\end{code}
%************************************************************************
%* *
Instantiation in general
%* *
%************************************************************************
\begin{code}
tcInstType :: ([TyVar] -> TcM [TcTyVar])
-> TcType
-> TcM ([TcTyVar], TcThetaType, TcType)
tcInstType inst_tyvars ty
= case tcSplitForAllTys ty of
([], rho) -> let
(theta, tau) = tcSplitPhiTy rho
in
return ([], theta, tau)
(tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
; return (tyvars', theta, tau) }
\end{code}
%************************************************************************
%* *
Updating tau types
%* *
%************************************************************************
Can't be in TcUnify, as we also need it in TcTyFuns.
\begin{code}
type SwapFlag = Bool
checkUpdateMeta :: SwapFlag
-> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
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 :: Bool -> TyVar -> Type -> TcM ()
checkKinds swapped tv1 ty2
| tk2 `isSubKind` tk1 = return ()
| otherwise
= 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 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 (TyConApp tc tys)
| isSynTyCon tc = go_syn tc tys
| otherwise = do { tys' <- mapM go tys
; return $ occurs (TyConApp tc) tys' }
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' }
go (ForAllTy _ _) = notMonoType orig_ty
go (TyVarTy tv)
| orig_tv == tv = return $ Left False
| isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
| otherwise = return $ Right (TyVarTy tv)
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 }
_ -> return $ Right (TyVarTy tv)
}
go_syn tc tys
| not (isTauTyCon tc)
= notMonoType orig_ty
| otherwise
= do { (_msgs, mb_tys') <- tryTc (mapM go tys)
; case mb_tys' of
Nothing
| isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
| otherwise -> go (expectJust "checkTauTvUpdate(1)"
(tcView (TyConApp tc tys)))
Just tys' ->
case occurs id tys' of
Left _
| isOpenTyCon tc -> return $ Left True
| otherwise -> go (expectJust "checkTauTvUpdate(2)"
(tcView (TyConApp tc tys)))
Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
}
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
= do { tv' <- tcInstTyVar tv
; let tau = mkTyVarTy tv'
; 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 typesynonym 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).
Execute a bag of type variable bindings.
\begin{code}
execTcTyVarBinds :: TcTyVarBinds -> TcM ()
execTcTyVarBinds = mapM_ execTcTyVarBind . bagToList
where
execTcTyVarBind (TcTyVarBind tv ty)
= do { ASSERTM2( do { details <- readMetaTyVar tv
; return (isFlexi details) }, ppr tv )
; ty' <- if isCoVar tv
then return ty
else do { maybe_ty <- checkTauTvUpdate tv ty
; case maybe_ty of
Nothing -> pprPanic "TcRnMonad.execTcTyBind"
(ppr tv <+> text ":=" <+> ppr ty)
Just ty' -> return ty'
}
; writeMetaTyVar tv ty'
}
\end{code}
Error mesages in case of kind mismatch.
\begin{code}
unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
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 :: Bool -> TyVar -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
unifyKindCtxt swapped tv1 ty2 tidy_env
= 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
%* *
%************************************************************************
\begin{code}
newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
newCoVars spec
= do { us <- newUniqueSupply
; return [ mkCoVar (mkSysTvName uniq (fsLit "co_kv"))
(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 = mapM (\ _ -> newKindVar) (nOfThem n ())
\end{code}
%************************************************************************
%* *
SkolemTvs (immutable)
%* *
%************************************************************************
\begin{code}
mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
| tv <- tyvars ]
tcInstSkolTyVar :: SkolemInfo -> (Name -> SrcSpan) -> TyVar -> TcM TcTyVar
tcInstSkolTyVar info get_loc tyvar
= do { uniq <- newUnique
; let old_name = tyVarName tyvar
kind = tyVarKind tyvar
loc = get_loc old_name
new_name = mkInternalName uniq (nameOccName old_name) loc
; return (mkSkolTyVar new_name kind info) }
tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
tcInstSkolTyVars info tyvars
= do { span <- getSrcSpanM
; mapM (tcInstSkolTyVar info (const span)) tyvars }
tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
tcInstSigType :: Bool -> SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcRhoType)
tcInstSigType use_skols skol_info ty
= tcInstType (mapM inst_tyvar) ty
where
inst_tyvar | use_skols = tcInstSkolTyVar skol_info getSrcSpan
| otherwise = instMetaTyVar (SigTv skol_info)
\end{code}
%************************************************************************
%* *
MetaTvs (meta type variables; mutable)
%* *
%************************************************************************
\begin{code}
newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
newMetaTyVar box_info kind
= do { uniq <- newUnique
; ref <- newMutVar Flexi
; let name = mkSysTvName uniq fs
fs = case box_info of
BoxTv -> fsLit "t"
TauTv -> fsLit "t"
SigTv _ -> fsLit "a"
; return (mkTcTyVar name kind (MetaTv box_info ref)) }
instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
instMetaTyVar box_info tyvar
= do { uniq <- newUnique
; ref <- newMutVar Flexi
; let name = setNameUnique (tyVarName tyvar) uniq
kind = tyVarKind tyvar
; return (mkTcTyVar name kind (MetaTv box_info ref)) }
readMetaTyVar :: TyVar -> TcM MetaDetails
readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
readMutVar (metaTvRef tyvar)
isFilledMetaTyVar :: TyVar -> TcM Bool
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 ()
writeMetaTyVar tyvar ty
| not debugIsOn = writeMutVar (metaTvRef tyvar) (Indirect ty)
writeMetaTyVar tyvar ty
| not (isMetaTyVar tyvar)
= pprTrace "writeMetaTyVar" (ppr tyvar) $
return ()
| otherwise
= ASSERT( isMetaTyVar tyvar )
ASSERT2( isCoVar tyvar || typeKind ty `isSubKind` tyVarKind tyvar,
(ppr tyvar <+> ppr (tyVarKind tyvar))
$$ (ppr ty <+> ppr (typeKind ty)) )
do { if debugIsOn then do { details <- readMetaTyVar tyvar;
; WARN( not (isFlexi details), ppr tyvar )
return () }
else return ()
; traceTc (text "writeMetaTyVar" <+> ppr tyvar <+> text ":=" <+> ppr ty)
; writeMutVar (metaTvRef tyvar) (Indirect ty) }
\end{code}
%************************************************************************
%* *
MetaTvs: TauTvs
%* *
%************************************************************************
\begin{code}
newFlexiTyVar :: Kind -> TcM TcTyVar
newFlexiTyVar kind = newMetaTyVar TauTv kind
newFlexiTyVarTy :: Kind -> TcM TcType
newFlexiTyVarTy kind = do
tc_tyvar <- newFlexiTyVar kind
return (TyVarTy tc_tyvar)
newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
tcInstTyVar :: TyVar -> TcM TcTyVar
tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
tcInstTyVars tyvars
= do { tc_tvs <- mapM tcInstTyVar tyvars
; let tys = mkTyVarTys tc_tvs
; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
\end{code}
%************************************************************************
%* *
MetaTvs: SigTvs
%* *
%************************************************************************
\begin{code}
zonkSigTyVar :: TcTyVar -> TcM TcTyVar
zonkSigTyVar sig_tv
| isSkolemTyVar sig_tv
= return sig_tv
| otherwise
= ASSERT( isSigTyVar sig_tv )
do { ty <- zonkTcTyVar sig_tv
; return (tcGetTyVar "zonkSigTyVar" ty) }
\end{code}
%************************************************************************
%* *
MetaTvs: BoxTvs
%* *
%************************************************************************
\begin{code}
newBoxyTyVar :: Kind -> TcM BoxyTyVar
newBoxyTyVar kind = newMetaTyVar BoxTv kind
newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
newBoxyTyVars kinds = mapM newBoxyTyVar kinds
newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
readFilledBox :: BoxyTyVar -> TcM TcType
readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
do { cts <- readMetaTyVar box_tv
; case cts of
Flexi -> pprPanic "readFilledBox" (ppr box_tv)
Indirect ty -> return ty }
tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
\end{code}
%************************************************************************
%* *
\subsection{Putting and getting mutable type variables}
%* *
%************************************************************************
But it's more fun to short out indirections on the way: If this
version returns a TyVar, then that TyVar is unbound. If it returns
any other type, then there might be bound TyVars embedded inside it.
We return Nothing iff the original box was unbound.
\begin{code}
data LookupTyVarResult
= DoneTv TcTyVarDetails
| IndirectTv TcType
lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
lookupTcTyVar tyvar
= ASSERT2( isTcTyVar tyvar, ppr tyvar )
case details of
SkolemTv _ -> return (DoneTv details)
MetaTv _ ref -> do { meta_details <- readMutVar ref
; case meta_details of
Indirect ty -> return (IndirectTv ty)
Flexi -> return (DoneTv details) }
where
details = tcTyVarDetails tyvar
\end{code}
%************************************************************************
%* *
\subsection{Zonking
%* *
%************************************************************************
\begin{code}
zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
zonkTcTyVar :: TcTyVar -> TcM TcType
zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
\end{code}
\begin{code}
zonkTcType :: TcType -> TcM TcType
zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
zonkTcTypes :: [TcType] -> TcM [TcType]
zonkTcTypes tys = mapM zonkTcType tys
zonkTcThetaType :: TcThetaType -> TcM TcThetaType
zonkTcThetaType theta = mapM zonkTcPredType theta
zonkTcPredType :: TcPredType -> TcM TcPredType
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}
are used at the end of type checking
\begin{code}
zonkTopTyVar :: TcTyVar -> TcM TcTyVar
zonkTopTyVar tv
| k `eqKind` default_k = return tv
| otherwise
= do { tv' <- newFlexiTyVar default_k
; writeMetaTyVar tv (mkTyVarTy tv')
; return tv' }
where
k = tyVarKind tv
default_k = defaultKind k
zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
zonkQuantifiedTyVar tv
| ASSERT2( isTcTyVar tv, ppr tv )
isSkolemTyVar tv
= do { kind <- zonkTcType (tyVarKind tv)
; return $ setTyVarKind tv kind
}
| otherwise
= do { details <- readMetaTyVar tv
; let final_kind = defaultKind (tyVarKind tv)
final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
; case details of
Indirect ty -> WARN( True, ppr tv $$ ppr ty )
return ()
Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
; return final_tv }
\end{code}
Note [Silly Type Synonyms]
~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this:
type C u a = u
foo :: (forall a. C u a -> C u a) -> u
foo x = ...
bar :: Num u => u
bar = foo (\t -> t + t)
* From the (\t -> t+t) we get type {Num d} => d -> d
where d is fresh.
* Now unify with type of foo's arg, and we get:
{Num (C d a)} => C d a -> C d a
where a is fresh.
* Now abstract over the 'a', but float out the Num (C d a) constraint
because it does not 'really' mention a. (see exactTyVarsOfType)
The arg to foo becomes
\/\a -> \t -> t+t
* So we get a dict binding for Num (C d a), which is zonked to give
a = ()
[Note Sept 04: now that we are zonking quantified type variables
on construction, the 'a' will be frozen as a regular tyvar on
quantification, so the floated dict will still have type (C d a).
Which renders this whole note moot; happily!]
* Then the \/\a abstraction has a zonked 'a' in it.
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.
%************************************************************************
%* *
\subsection{Zonking
%* *
%* For internal use only! *
%* *
%************************************************************************
\begin{code}
zonkType :: (TcTyVar -> TcM Type)
-> TcType
-> TcM Type
zonkType unbound_var_fn ty
= go ty
where
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')
go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
| otherwise = liftM TyVarTy $
zonkTyVar unbound_var_fn tyvar
go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
ty' <- go ty
tyvar' <- zonkTyVar unbound_var_fn tyvar
return (ForAllTy tyvar' ty')
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)
-> TcTyVar -> TcM TcType
zonk_tc_tyvar unbound_var_fn tyvar
| not (isMetaTyVar tyvar)
= return (TyVarTy tyvar)
| otherwise
= do { cts <- readMetaTyVar tyvar
; case cts of
Flexi -> unbound_var_fn tyvar
Indirect ty -> zonkType unbound_var_fn ty }
zonkTyVar :: (TcTyVar -> TcM Type)
-> TyVar -> TcM TyVar
zonkTyVar unbound_var_fn tv
| isCoVar tv
= do { kind <- zonkType unbound_var_fn (tyVarKind tv)
; return $ setTyVarKind tv kind
}
| otherwise = return tv
\end{code}
%************************************************************************
%* *
Zonking kinds
%* *
%************************************************************************
\begin{code}
readKindVar :: KindVar -> TcM (MetaDetails)
writeKindVar :: KindVar -> TcKind -> TcM ()
readKindVar kv = readMutVar (kindVarRef kv)
writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
zonkTcKind :: TcKind -> TcM TcKind
zonkTcKind k = zonkTcType k
zonkTcKindToKind :: TcKind -> TcM Kind
zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
\end{code}
%************************************************************************
%* *
\subsection{Checking a user type}
%* *
%************************************************************************
When dealing with a userwritten type, we first translate it from an HsType
to a Type, performing kind checking, and then check various things that should
be true about it. We don't want to perform these checks at the same time
as the initial translation because (a) they are unnecessary for interfacefile
types and (b) when checking a mutually recursive group of type and class decls,
we can't "look" at the tycons/classes yet. Also, the checks are are rather
diverse, and used to really mess up the other code.
One thing we check for is 'rank'.
Rank 0: monotypes (no foralls)
Rank 1: foralls at the front only, Rank 0 inside
Rank 2: foralls at the front, Rank 1 on left of fn arrow,
basic ::= tyvar | T basic ... basic
r2 ::= forall tvs. cxt => r2a
r2a ::= r1 -> r2a | basic
r1 ::= forall tvs. cxt => r0
r0 ::= r0 -> r0 | basic
Another thing is to check that type synonyms are saturated.
This might not necessarily show up in kind checking.
type A i = i
data T k = MkT (k Int)
f :: T A
\begin{code}
checkValidType :: UserTypeCtxt -> Type -> TcM ()
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
gen_rank n | rankn = ArbitraryRank
| rank2 = Rank 2
| otherwise = Rank n
rank
= case ctxt of
DefaultDeclCtxt-> MustBeMonoType
ResSigCtxt -> MustBeMonoType
LamPatSigCtxt -> gen_rank 0
BindPatSigCtxt -> gen_rank 0
TySynCtxt _ -> gen_rank 0
GenPatCtxt -> gen_rank 1
ExprSigCtxt -> gen_rank 1
FunSigCtxt _ -> gen_rank 1
ConArgCtxt _ | polycomp -> gen_rank 2
| otherwise -> gen_rank 1
ForSigCtxt _ -> gen_rank 1
SpecInstCtxt -> gen_rank 1
ThBrackCtxt -> gen_rank 1
actual_kind = typeKind ty
kind_ok = case ctxt of
TySynCtxt _ -> True
ThBrackCtxt -> True
ResSigCtxt -> isSubOpenTypeKind actual_kind
ExprSigCtxt -> isSubOpenTypeKind actual_kind
GenPatCtxt -> isLiftedTypeKind actual_kind
ForSigCtxt _ -> isLiftedTypeKind actual_kind
_ -> isSubArgTypeKind actual_kind
ubx_tup = case ctxt of
TySynCtxt _ | unboxed -> UT_Ok
ExprSigCtxt | unboxed -> UT_Ok
ThBrackCtxt | unboxed -> UT_Ok
_ -> UT_NotOk
check_type rank ubx_tup ty
checkTc kind_ok (kindErr actual_kind)
traceTc (text "checkValidType done" <+> ppr ty)
checkValidMonoType :: Type -> TcM ()
checkValidMonoType ty = check_mono_type MustBeMonoType ty
\end{code}
\begin{code}
data Rank = ArbitraryRank
| MustBeMonoType
| TyConArgMonoType
| SynArgMonoType
| Rank Int
decRank :: Rank -> Rank
decRank (Rank 0) = Rank 0
decRank (Rank n) = Rank (n1)
decRank other_rank = other_rank
nonZeroRank :: Rank -> Bool
nonZeroRank ArbitraryRank = True
nonZeroRank (Rank n) = n>0
nonZeroRank _ = False
data UbxTupFlag = UT_Ok | UT_NotOk
check_mono_type :: Rank -> Type -> TcM ()
check_mono_type rank ty
= do { check_type rank UT_NotOk ty
; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
check_type rank ubx_tup ty
| not (null tvs && null theta)
= do { checkTc (nonZeroRank rank) (forAllTyErr rank ty)
; check_valid_theta SigmaCtxt theta
; check_type rank ubx_tup tau
; checkFreeness tvs theta
; checkAmbiguity tvs theta (tyVarsOfType tau) }
where
(tvs, theta, tau) = tcSplitSigmaTy ty
check_type _ _ ty@(PredTy {})
= failWithTc (text "Predicate used as a type:" <+> ppr ty)
check_type _ _ (TyVarTy _) = return ()
check_type rank _ (FunTy arg_ty res_ty)
= do { check_type (decRank rank) UT_NotOk arg_ty
; check_type rank UT_Ok res_ty }
check_type rank _ (AppTy ty1 ty2)
= do { check_arg_type rank ty1
; check_arg_type rank ty2 }
check_type rank ubx_tup ty@(TyConApp tc tys)
| isSynTyCon tc
= do {
checkTc (tyConArity tc <= length tys) arity_msg
; liberal <- doptM Opt_LiberalTypeSynonyms
; if not liberal || isOpenSynTyCon tc then
mapM_ (check_mono_type SynArgMonoType) tys
else
case tcView ty of
Just ty' -> check_type rank ubx_tup ty'
Nothing -> pprPanic "check_tau_type" (ppr ty)
}
| isUnboxedTupleTyCon tc
= 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 ArbitraryRank else TyConArgMonoType
; mapM_ (check_type rank' UT_Ok) tys }
| otherwise
= mapM_ (check_arg_type rank) tys
where
ubx_tup_ok ub_tuples_allowed = case ubx_tup of
UT_Ok -> ub_tuples_allowed
_ -> False
n_args = length tys
tc_arity = tyConArity tc
arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
ubx_tup_msg = ubxArgTyErr ty
check_type _ _ ty = pprPanic "check_type" (ppr ty)
check_arg_type :: Rank -> Type -> TcM ()
check_arg_type rank ty
= do { impred <- doptM Opt_ImpredicativeTypes
; let rank' = case rank of
MustBeMonoType -> MustBeMonoType
_other | impred -> ArbitraryRank
| otherwise -> TyConArgMonoType
; check_type rank' UT_NotOk ty
; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
forAllTyErr :: Rank -> Type -> SDoc
forAllTyErr rank ty
= vcat [ hang (ptext (sLit "Illegal polymorphic or qualified type:")) 2 (ppr ty)
, suggestion ]
where
suggestion = case rank of
Rank _ -> ptext (sLit "Perhaps you intended to use -XRankNTypes or -XRank2Types")
TyConArgMonoType -> ptext (sLit "Perhaps you intended to use -XImpredicativeTypes")
SynArgMonoType -> ptext (sLit "Perhaps you intended to use -XLiberalTypeSynonyms")
_ -> empty
unliftedArgErr, ubxArgTyErr :: Type -> SDoc
unliftedArgErr ty = sep [ptext (sLit "Illegal unlifted type:"), ppr ty]
ubxArgTyErr ty = sep [ptext (sLit "Illegal unboxed tuple type as function argument:"), ppr ty]
kindErr :: Kind -> SDoc
kindErr kind = sep [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 forall type, or with a partiallyapplied 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)
%************************************************************************
%* *
\subsection{Checking a theta or source type}
%* *
%************************************************************************
\begin{code}
data SourceTyCtxt
= ClassSCCtxt Name
| SigmaCtxt
| DataTyCtxt Name
| TypeCtxt
| InstThetaCtxt
pprSourceTyCtxt :: SourceTyCtxt -> SDoc
pprSourceTyCtxt (ClassSCCtxt c) = ptext (sLit "the super-classes of class") <+> quotes (ppr c)
pprSourceTyCtxt SigmaCtxt = ptext (sLit "the context of a polymorphic type")
pprSourceTyCtxt (DataTyCtxt tc) = ptext (sLit "the context of the data type declaration for") <+> quotes (ppr tc)
pprSourceTyCtxt InstThetaCtxt = ptext (sLit "the context of an instance declaration")
pprSourceTyCtxt TypeCtxt = ptext (sLit "the context of a type")
\end{code}
\begin{code}
checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
checkValidTheta ctxt theta
= addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
check_valid_theta :: SourceTyCtxt -> [PredType] -> TcM ()
check_valid_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 {
; checkTc (arity == n_tys) arity_err
; mapM_ checkValidMonoType tys
; checkTc (check_class_pred_tys dflags ctxt tys)
(predTyVarErr pred $$ how_to_allow)
}
where
class_name = className cls
arity = classArity cls
n_tys = length tys
arity_err = arityErr "Class" class_name arity n_tys
how_to_allow = parens (ptext (sLit "Use -XFlexibleContexts to permit this"))
check_pred_ty _ (ClassSCCtxt _) (EqPred _ _)
=
failWithTc $
sep [ ptext (sLit "The current implementation of type families does not")
, ptext (sLit "support equality constraints in superclass contexts.")
, ptext (sLit "They are planned for a future release.")
]
check_pred_ty dflags _ pred@(EqPred ty1 ty2)
= do {
; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
; checkValidMonoType ty1
; checkValidMonoType ty2
}
check_pred_ty _ SigmaCtxt (IParam _ ty) = checkValidMonoType ty
check_pred_ty _ _ sty = failWithTc (badPredTyErr sty)
check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
check_class_pred_tys dflags ctxt tys
= case ctxt of
TypeCtxt -> True
InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
_ -> flexible_contexts || all tyvar_head tys
where
flexible_contexts = dopt Opt_FlexibleContexts dflags
undecidable_ok = dopt Opt_UndecidableInstances dflags
tyvar_head :: Type -> Bool
tyvar_head ty
| tcIsTyVarTy ty = True
| otherwise
= case tcSplitAppTy_maybe ty of
Just (ty, _) -> tyvar_head ty
Nothing -> False
\end{code}
Check for ambiguity
~~~~~~~~~~~~~~~~~~~
forall V. P => tau
is ambiguous if P contains generic variables
(i.e. one of the Vs) that are not mentioned in tau
However, we need to take account of functional dependencies
when we speak of 'mentioned in tau'. Example:
class C a b | a -> b where ...
Then the type
forall x y. (C x y) => x
is not ambiguous because x is mentioned and x determines y
NB; the ambiguity check is only used for *user* types, not for types
coming from inteface files. The latter can legitimately have
ambiguous types. Example
class S a where s :: a -> (Int,Int)
instance S Char where s _ = (1,1)
f:: S a => [a] -> Int -> (Int,Int)
f (_::[a]) x = (a*x,b)
where (a,b) = s (undefined::a)
Here the worker for f gets the type
fw :: forall a. S a => Int -> (# Int, Int #)
If the list of tv_names is empty, we have a monotype, and then we
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
= mapM_ complain (filter is_ambig theta)
where
complain pred = addErrTc (ambigErr pred)
extended_tau_vars = growThetaTyVars theta tau_tyvars
is_ambig pred = isClassPred pred &&
any ambig_var (varSetElems (tyVarsOfPred pred))
ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
not (ct_var `elemVarSet` extended_tau_vars)
ambigErr :: PredType -> SDoc
ambigErr pred
= sep [ptext (sLit "Ambiguous constraint") <+> quotes (pprPred pred),
nest 4 (ptext (sLit "At least one of the forall'd type variables mentioned by the constraint") $$
ptext (sLit "must be reachable from the type after the '=>'"))]
growThetaTyVars :: TcThetaType -> TyVarSet -> TyVarSet
growThetaTyVars theta tvs
| null theta = tvs
| otherwise = fixVarSet mk_next tvs
where
mk_next tvs = foldr growPredTyVars tvs theta
growPredTyVars :: TcPredType -> TyVarSet -> TyVarSet
growPredTyVars (IParam _ ty) tvs = tvs `unionVarSet` tyVarsOfType ty
growPredTyVars pred tvs = growTyVars (tyVarsOfPred pred) tvs
growTyVars :: TyVarSet -> TyVarSet -> TyVarSet
growTyVars new_tvs tvs
| new_tvs `intersectsVarSet` tvs = tvs `unionVarSet` new_tvs
| otherwise = tvs
\end{code}
In addition, GHC insists that at least one type variable
in each constraint is in V. So we disallow a type like
forall a. Eq b => b -> b
even in a scope where b is in scope.
\begin{code}
checkFreeness :: [Var] -> [PredType] -> TcM ()
checkFreeness forall_tyvars 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)))
bound_var ct_var = ct_var `elem` forall_tyvars
complain pred = addErrTc (freeErr pred)
freeErr :: PredType -> SDoc
freeErr pred
= 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}
checkThetaCtxt :: SourceTyCtxt -> ThetaType -> SDoc
checkThetaCtxt ctxt theta
= vcat [ptext (sLit "In the context:") <+> pprTheta theta,
ptext (sLit "While checking") <+> pprSourceTyCtxt ctxt ]
badPredTyErr, eqPredTyErr, predTyVarErr :: PredType -> SDoc
badPredTyErr sty = ptext (sLit "Illegal constraint") <+> pprPred sty
eqPredTyErr sty = ptext (sLit "Illegal equational constraint") <+> pprPred sty
$$
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 :: [[PredType]] -> SDoc
dupPredWarn dups = ptext (sLit "Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
arityErr kind name n m
= hsep [ text kind, quotes (ppr name), ptext (sLit "should have"),
n_arguments <> comma, text "but has been given", int m]
where
n_arguments | n == 0 = ptext (sLit "no arguments")
| n == 1 = ptext (sLit "1 argument")
| True = hsep [int n, ptext (sLit "arguments")]
notMonoType :: TcType -> TcM a
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 :: TcType -> TcM a
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}
%************************************************************************
%* *
\subsection{Checking for a decent instance head type}
%* *
%************************************************************************
@checkValidInstHead@ checks the type {\em and} its syntactic constraints:
it must normally look like: @instance Foo (Tycon a b c ...) ...@
The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
flag is on, or (2)~the instance is imported (they must have been
compiled elsewhere). In these cases, we let them go through anyway.
We can also have instances for functions: @instance Foo (a -> b) ...@.
\begin{code}
checkValidInstHead :: Type -> TcM (Class, [TcType])
checkValidInstHead ty
= case tcSplitPredTy_maybe ty of {
Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
Just pred ->
case getClassPredTys_maybe pred of {
Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
Just (clas,tys) -> do
dflags <- getDOpts
check_inst_head dflags clas tys
return (clas, tys)
}}
check_inst_head :: DynFlags -> Class -> [Type] -> TcM ()
check_inst_head dflags clas tys
= 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_ checkTyFamFreeness tys
; mapM_ checkValidMonoType tys
}
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 :: SDoc -> SDoc -> SDoc
instTypeErr pp_ty msg
= sep [ptext (sLit "Illegal instance declaration for") <+> quotes pp_ty,
nest 4 msg]
\end{code}
%************************************************************************
%* *
\subsection{Checking instance for termination}
%* *
%************************************************************************
\begin{code}
checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
checkValidInstance tyvars theta clas inst_tys
= do { undecidable_ok <- doptM Opt_UndecidableInstances
; checkValidTheta InstThetaCtxt theta
; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
; unless undecidable_ok $
mapM_ addErrTc (checkInstTermination inst_tys theta)
; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
(instTypeErr (pprClassPred clas inst_tys) msg)
}
where
msg = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
undecidableMsg])
\end{code}
Termination test: the socalled "Paterson conditions" (see Section 5 of
"Understanding functionsl dependencies via Constraint Handling Rules,
JFP Jan 2007).
We check that each assertion in the context satisfies:
(1) no variable has more occurrences in the assertion than in the head, and
(2) the assertion has fewer constructors and variables (taken together
and counting repetitions) than the head.
This is only needed with -fglasgow-exts, as Haskell 98 restrictions
(which have already been checked) guarantee termination.
The underlying idea is that
for any ground substitution, each assertion in the
context has fewer type constructors than the head.
\begin{code}
checkInstTermination :: [TcType] -> ThetaType -> [Message]
checkInstTermination tys theta
= mapCatMaybes check theta
where
fvs = fvTypes tys
size = sizeTypes tys
check pred
| not (null (fvPred pred \\ fvs))
= Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
| sizePred pred >= size
= Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
| otherwise
= Nothing
predUndecErr :: PredType -> SDoc -> SDoc
predUndecErr pred msg = sep [msg,
nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
nomoreMsg, smallerMsg, undecidableMsg :: SDoc
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 XUndecidableInstances 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 typevariable constraints.
Here is another example:
data Fix f = In (f (Fix f)) deriving( Eq )
Here, if we are prepared to allow XUndecidableInstances 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 _ tys) = hasNoDups fvs && sizeTypes tys == length fvs
where fvs = fvTypes tys
validDerivPred _ = False
\end{code}
%************************************************************************
%* *
Checking type instance wellformedness and termination
%* *
%************************************************************************
\begin{code}
checkValidTypeInst :: [Type] -> Type -> TcM ()
checkValidTypeInst typats rhs
= do {
; mapM_ checkTyFamFreeness typats
; checkValidMonoType rhs
; undecidable_ok <- doptM Opt_UndecidableInstances
; unless undecidable_ok $
mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
}
checkFamInst :: [Type]
-> [(TyCon, [Type])]
-> [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
checkTyFamFreeness :: Type -> TcM ()
checkTyFamFreeness ty
= checkTc (isTyFamFree ty) $
tyFamInstIllegalErr ty
isTyFamFree :: Type -> Bool
isTyFamFree = null . tyFamInsts
tyFamInstIllegalErr :: Type -> SDoc
tyFamInstIllegalErr ty
= hang (ptext (sLit "Illegal type synonym family application in instance") <>
colon) 4 $
ppr ty
famInstUndecErr :: Type -> SDoc -> SDoc
famInstUndecErr ty msg
= sep [msg,
nest 2 (ptext (sLit "in the type family application:") <+>
pprType ty)]
nestedMsg, nomoreVarMsg, smallerAppMsg :: SDoc
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}
fvType :: Type -> [TyVar]
fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
fvType (TyVarTy tv) = [tv]
fvType (TyConApp _ tys) = fvTypes tys
fvType (PredTy pred) = fvPred pred
fvType (FunTy arg res) = fvType arg ++ fvType res
fvType (AppTy fun arg) = fvType fun ++ fvType arg
fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
fvTypes :: [Type] -> [TyVar]
fvTypes tys = concat (map fvType tys)
fvPred :: PredType -> [TyVar]
fvPred (ClassP _ tys') = fvTypes tys'
fvPred (IParam _ ty) = fvType ty
fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
sizeType :: Type -> Int
sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
sizeType (TyVarTy _) = 1
sizeType (TyConApp _ tys) = sizeTypes tys + 1
sizeType (PredTy pred) = sizePred pred
sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
sizeType (AppTy fun arg) = sizeType fun + sizeType arg
sizeType (ForAllTy _ ty) = sizeType ty
sizeTypes :: [Type] -> Int
sizeTypes xs = sum (map sizeType xs)
sizePred :: PredType -> Int
sizePred (ClassP _ tys') = sizeTypes tys'
sizePred (IParam _ ty) = sizeType ty
sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2 1
\end{code}