%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
Type subsumption and unification
\begin{code}
module TcUnify (
tcWrapResult, tcSubType, tcGen,
checkConstraints, newImplication,
unifyType, unifyTypeList, unifyTheta, unifyKind, unifyKindEq,
tcInfer,
matchExpectedListTy,
matchExpectedPArrTy,
matchExpectedTyConApp,
matchExpectedAppTy,
matchExpectedFunTys,
matchExpectedFunKind,
wrapFunResCoercion,
mkKindErrorCtxt
) where
#include "HsVersions.h"
import HsSyn
import TypeRep
import TcMType
import TcRnMonad
import TcType
import Type
import TcEvidence
import Name ( isSystemName )
import Inst
import Kind
import TyCon
import TysWiredIn
import Var
import VarSet
import VarEnv
import ErrUtils
import DynFlags
import BasicTypes
import Maybes ( allMaybes )
import Util
import Outputable
import FastString
import Control.Monad
\end{code}
%************************************************************************
%* *
matchExpected functions
%* *
%************************************************************************
Note [Herald for matchExpectedFunTys]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'herald' always looks like:
"The equation(s) for 'f' have"
"The abstraction (\x.e) takes"
"The section (+ x) expects"
"The function 'f' is applied to"
This is used to construct a message of form
The abstraction `\Just 1 -> ...' takes two arguments
but its type `Maybe a -> a' has only one
The equation(s) for `f' have two arguments
but its type `Maybe a -> a' has only one
The section `(f 3)' requires 'f' to take two arguments
but its type `Int -> Int' has only one
The function 'f' is applied to two arguments
but its type `Int -> Int' has only one
Note [matchExpectedFunTys]
~~~~~~~~~~~~~~~~~~~~~~~~~~
matchExpectedFunTys checks that an (Expected rho) has the form
of an n-ary function. It passes the decomposed type to the
thing_inside, and returns a wrapper to coerce between the two types
It's used wherever a language construct must have a functional type,
namely:
A lambda expression
A function definition
An operator section
This is not (currently) where deep skolemisation occurs;
matchExpectedFunTys does not skolmise nested foralls in the
expected type, becuase it expects that to have been done already
\begin{code}
matchExpectedFunTys :: SDoc
-> Arity
-> TcRhoType
-> TcM (TcCoercion, [TcSigmaType], TcRhoType)
matchExpectedFunTys herald arity orig_ty
= go arity orig_ty
where
go n_req ty
| n_req == 0 = return (mkTcReflCo ty, [], ty)
go n_req ty
| Just ty' <- tcView ty = go n_req ty'
go n_req (FunTy arg_ty res_ty)
| not (isPredTy arg_ty)
= do { (co, tys, ty_r) <- go (n_req1) res_ty
; return (mkTcFunCo (mkTcReflCo arg_ty) co, arg_ty:tys, ty_r) }
go n_req ty@(TyVarTy tv)
| ASSERT( isTcTyVar tv) isMetaTyVar tv
= do { cts <- readMetaTyVar tv
; case cts of
Indirect ty' -> go n_req ty'
Flexi -> defer n_req ty }
go n_req ty = defer n_req ty
defer n_req fun_ty
= addErrCtxtM mk_ctxt $
do { arg_tys <- newFlexiTyVarTys n_req openTypeKind
; res_ty <- newFlexiTyVarTy openTypeKind
; co <- unifyType fun_ty (mkFunTys arg_tys res_ty)
; return (co, arg_tys, res_ty) }
mk_ctxt :: TidyEnv -> TcM (TidyEnv, MsgDoc)
mk_ctxt env = do { orig_ty1 <- zonkTcType orig_ty
; let (env', orig_ty2) = tidyOpenType env orig_ty1
(args, _) = tcSplitFunTys orig_ty2
n_actual = length args
; return (env', mk_msg orig_ty2 n_actual) }
mk_msg ty n_args
= herald <+> speakNOf arity (ptext (sLit "argument")) <> comma $$
sep [ptext (sLit "but its type") <+> quotes (pprType ty),
if n_args == 0 then ptext (sLit "has none")
else ptext (sLit "has only") <+> speakN n_args]
\end{code}
\begin{code}
matchExpectedListTy :: TcRhoType -> TcM (TcCoercion, TcRhoType)
matchExpectedListTy exp_ty
= do { (co, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
; return (co, elt_ty) }
matchExpectedPArrTy :: TcRhoType -> TcM (TcCoercion, TcRhoType)
matchExpectedPArrTy exp_ty
= do { (co, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
; return (co, elt_ty) }
matchExpectedTyConApp :: TyCon
-> TcRhoType
-> TcM (TcCoercion,
[TcSigmaType])
matchExpectedTyConApp tc orig_ty
= go orig_ty
where
go ty
| Just ty' <- tcView ty
= go ty'
go ty@(TyConApp tycon args)
| tc == tycon
= return (mkTcReflCo ty, args)
go (TyVarTy tv)
| ASSERT( isTcTyVar tv) isMetaTyVar tv
= do { cts <- readMetaTyVar tv
; case cts of
Indirect ty -> go ty
Flexi -> defer }
go _ = defer
defer = ASSERT2( isLiftedTypeKind res_kind, ppr tc )
do { kappa_tys <- mapM (const newMetaKindVar) kvs
; let arg_kinds' = map (substKiWith kvs kappa_tys) arg_kinds
; tau_tys <- mapM newFlexiTyVarTy arg_kinds'
; co <- unifyType (mkTyConApp tc (kappa_tys ++ tau_tys)) orig_ty
; return (co, kappa_tys ++ tau_tys) }
(kvs, body) = splitForAllTys (tyConKind tc)
(arg_kinds, res_kind) = splitKindFunTys body
matchExpectedAppTy :: TcRhoType
-> TcM (TcCoercion,
(TcSigmaType, TcSigmaType))
matchExpectedAppTy orig_ty
= go orig_ty
where
go ty
| Just ty' <- tcView ty = go ty'
| Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
= return (mkTcReflCo orig_ty, (fun_ty, arg_ty))
go (TyVarTy tv)
| ASSERT( isTcTyVar tv) isMetaTyVar tv
= do { cts <- readMetaTyVar tv
; case cts of
Indirect ty -> go ty
Flexi -> defer }
go _ = defer
defer = do { ty1 <- newFlexiTyVarTy kind1
; ty2 <- newFlexiTyVarTy kind2
; co <- unifyType (mkAppTy ty1 ty2) orig_ty
; return (co, (ty1, ty2)) }
orig_kind = typeKind orig_ty
kind1 = mkArrowKind liftedTypeKind (defaultKind orig_kind)
kind2 = liftedTypeKind
\end{code}
%************************************************************************
%* *
Subsumption checking
%* *
%************************************************************************
All the tcSub calls have the form
tcSub actual_ty expected_ty
which checks
actual_ty <= expected_ty
That is, that a value of type actual_ty is acceptable in
a place expecting a value of type expected_ty.
It returns a coercion function
co_fn :: actual_ty ~ expected_ty
which takes an HsExpr of type actual_ty into one of type
expected_ty.
\begin{code}
tcSubType :: CtOrigin -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
tcSubType origin ctxt ty_actual ty_expected
| isSigmaTy ty_actual
= do { (sk_wrap, inst_wrap)
<- tcGen ctxt ty_expected $ \ _ sk_rho -> do
{ (in_wrap, in_rho) <- deeplyInstantiate origin ty_actual
; cow <- unifyType in_rho sk_rho
; return (coToHsWrapper cow <.> in_wrap) }
; return (sk_wrap <.> inst_wrap) }
| otherwise
= do { cow <- unifyType ty_actual ty_expected
; return (coToHsWrapper cow) }
tcInfer :: (TcType -> TcM a) -> TcM (a, TcType)
tcInfer tc_infer = do { ty <- newFlexiTyVarTy openTypeKind
; res <- tc_infer ty
; return (res, ty) }
tcWrapResult :: HsExpr TcId -> TcRhoType -> TcRhoType -> TcM (HsExpr TcId)
tcWrapResult expr actual_ty res_ty
= do { cow <- unifyType actual_ty res_ty
; return (mkHsWrapCo cow expr) }
wrapFunResCoercion
:: [TcType]
-> HsWrapper
-> TcM HsWrapper
wrapFunResCoercion arg_tys co_fn_res
| isIdHsWrapper co_fn_res
= return idHsWrapper
| null arg_tys
= return co_fn_res
| otherwise
= do { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) }
\end{code}
%************************************************************************
%* *
\subsection{Generalisation}
%* *
%************************************************************************
\begin{code}
tcGen :: UserTypeCtxt -> TcType
-> ([TcTyVar] -> TcRhoType -> TcM result)
-> TcM (HsWrapper, result)
tcGen ctxt expected_ty thing_inside
= do { traceTc "tcGen" empty
; (wrap, tvs', given, rho') <- deeplySkolemise expected_ty
; when debugIsOn $
traceTc "tcGen" $ vcat [
text "expected_ty" <+> ppr expected_ty,
text "inst ty" <+> ppr tvs' <+> ppr rho' ]
; let skol_info = SigSkol ctxt (mkPiTypes given rho')
; (ev_binds, result) <- checkConstraints skol_info tvs' given $
thing_inside tvs' rho'
; return (wrap <.> mkWpLet ev_binds, result) }
checkConstraints :: SkolemInfo
-> [TcTyVar]
-> [EvVar]
-> TcM result
-> TcM (TcEvBinds, result)
checkConstraints skol_info skol_tvs given thing_inside
| null skol_tvs && null given
= do { res <- thing_inside; return (emptyTcEvBinds, res) }
| otherwise
= newImplication skol_info skol_tvs given thing_inside
newImplication :: SkolemInfo -> [TcTyVar]
-> [EvVar] -> TcM result
-> TcM (TcEvBinds, result)
newImplication skol_info skol_tvs given thing_inside
= ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
do { ((result, untch), wanted) <- captureConstraints $
captureUntouchables $
thing_inside
; if isEmptyWC wanted && not (hasEqualities given)
then
return (emptyTcEvBinds, result)
else do
{ ev_binds_var <- newTcEvBinds
; lcl_env <- getLclTypeEnv
; loc <- getCtLoc skol_info
; emitImplication $ Implic { ic_untch = untch
, ic_env = lcl_env
, ic_skols = skol_tvs
, ic_given = given
, ic_wanted = wanted
, ic_insol = insolubleWC wanted
, ic_binds = ev_binds_var
, ic_loc = loc }
; return (TcEvBinds ev_binds_var, result) } }
\end{code}
%************************************************************************
%* *
Boxy unification
%* *
%************************************************************************
The exported functions are all defined as versions of some
non-exported generic functions.
\begin{code}
unifyType :: TcTauType -> TcTauType -> TcM TcCoercion
unifyType ty1 ty2 = uType [] ty1 ty2
unifyPred :: PredType -> PredType -> TcM TcCoercion
unifyPred = unifyType
unifyTheta :: TcThetaType -> TcThetaType -> TcM [TcCoercion]
unifyTheta theta1 theta2
= do { checkTc (equalLength theta1 theta2)
(vcat [ptext (sLit "Contexts differ in length"),
nest 2 $ parens $ ptext (sLit "Use -XRelaxedPolyRec to allow this")])
; zipWithM unifyPred theta1 theta2 }
\end{code}
@unifyTypeList@ takes a single list of @TauType@s and unifies them
all together. It is used, for example, when typechecking explicit
lists, when all the elts should be of the same type.
\begin{code}
unifyTypeList :: [TcTauType] -> TcM ()
unifyTypeList [] = return ()
unifyTypeList [_] = return ()
unifyTypeList (ty1:tys@(ty2:_)) = do { _ <- unifyType ty1 ty2
; unifyTypeList tys }
\end{code}
%************************************************************************
%* *
uType and friends
%* *
%************************************************************************
uType is the heart of the unifier. Each arg occurs twice, because
we want to report errors in terms of synomyms if possible. The first of
the pair is used in error messages only; it is always the same as the
second, except that if the first is a synonym then the second may be a
de-synonym'd version. This way we get better error messages.
\begin{code}
data SwapFlag
= NotSwapped
| IsSwapped
instance Outputable SwapFlag where
ppr IsSwapped = ptext (sLit "Is-swapped")
ppr NotSwapped = ptext (sLit "Not-swapped")
unSwap :: SwapFlag -> (a->a->b) -> a -> a -> b
unSwap NotSwapped f a b = f a b
unSwap IsSwapped f a b = f b a
uType, uType_np, uType_defer
:: [EqOrigin]
-> TcType
-> TcType
-> TcM TcCoercion
uType_defer items ty1 ty2
= ASSERT( not (null items) )
do { eqv <- newEq ty1 ty2
; loc <- getCtLoc (TypeEqOrigin (last items))
; let ctev = Wanted { ctev_wloc = loc, ctev_evar = eqv
, ctev_pred = mkTcEqPred ty1 ty2 }
; emitFlat $ mkNonCanonical ctev
; ifDOptM Opt_D_dump_tc_trace $ do
{ ctxt <- getErrCtxt
; doc <- mkErrInfo emptyTidyEnv ctxt
; traceTc "utype_defer" (vcat [ppr eqv, ppr ty1,
ppr ty2, ppr items, doc])
}
; return (mkTcCoVarCo eqv) }
uType origin ty1 ty2
= uType_np (pushOrigin ty1 ty2 origin) ty1 ty2
uType_np origin orig_ty1 orig_ty2
= do { traceTc "u_tys " $ vcat
[ sep [ ppr orig_ty1, text "~", ppr orig_ty2]
, ppr origin]
; co <- go orig_ty1 orig_ty2
; if isTcReflCo co
then traceTc "u_tys yields no coercion" empty
else traceTc "u_tys yields coercion:" (ppr co)
; return co }
where
go :: TcType -> TcType -> TcM TcCoercion
go (TyVarTy tv1) ty2
= do { lookup_res <- lookupTcTyVar tv1
; case lookup_res of
Filled ty1 -> go ty1 ty2
Unfilled ds1 -> uUnfilledVar origin NotSwapped tv1 ds1 ty2 }
go ty1 (TyVarTy tv2)
= do { lookup_res <- lookupTcTyVar tv2
; case lookup_res of
Filled ty2 -> go ty1 ty2
Unfilled ds2 -> uUnfilledVar origin IsSwapped tv2 ds2 ty1 }
go ty1 ty2
| Just ty1' <- tcView ty1 = go ty1' ty2
| Just ty2' <- tcView ty2 = go ty1 ty2'
go (FunTy fun1 arg1) (FunTy fun2 arg2)
= do { co_l <- uType origin fun1 fun2
; co_r <- uType origin arg1 arg2
; return $ mkTcFunCo co_l co_r }
go ty1@(TyConApp tc1 _) ty2
| isSynFamilyTyCon tc1 = uType_defer origin ty1 ty2
go ty1 ty2@(TyConApp tc2 _)
| isSynFamilyTyCon tc2 = uType_defer origin ty1 ty2
go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
| tc1 == tc2, length tys1 == length tys2
= do { cos <- zipWithM (uType origin) tys1 tys2
; return $ mkTcTyConAppCo tc1 cos }
go (LitTy m) ty@(LitTy n)
| m == n
= return $ mkTcReflCo ty
go (AppTy s1 t1) (AppTy s2 t2)
= go_app s1 t1 s2 t2
go (AppTy s1 t1) (TyConApp tc2 ts2)
| Just (ts2', t2') <- snocView ts2
= ASSERT( isDecomposableTyCon tc2 )
go_app s1 t1 (TyConApp tc2 ts2') t2'
go (TyConApp tc1 ts1) (AppTy s2 t2)
| Just (ts1', t1') <- snocView ts1
= ASSERT( isDecomposableTyCon tc1 )
go_app (TyConApp tc1 ts1') t1' s2 t2
go ty1 ty2
| tcIsForAllTy ty1 || tcIsForAllTy ty2
= unifySigmaTy origin ty1 ty2
go ty1 ty2 = uType_defer origin ty1 ty2
go_app s1 t1 s2 t2
= do { co_s <- uType_np origin s1 s2
; co_t <- uType origin t1 t2
; return $ mkTcAppCo co_s co_t }
unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM TcCoercion
unifySigmaTy origin ty1 ty2
= do { let (tvs1, body1) = tcSplitForAllTys ty1
(tvs2, body2) = tcSplitForAllTys ty2
; defer_or_continue (not (equalLength tvs1 tvs2)) $ do {
(subst1, skol_tvs) <- tcInstSkolTyVars tvs1
; let tys = mkTyVarTys skol_tvs
phi1 = Type.substTy subst1 body1
phi2 = Type.substTy (zipTopTvSubst tvs2 tys) body2
skol_info = UnifyForAllSkol skol_tvs phi1
; (ev_binds, co) <- checkConstraints skol_info skol_tvs [] $
uType origin phi1 phi2
; return (foldr mkTcForAllCo (TcLetCo ev_binds co) skol_tvs) } }
where
defer_or_continue True _ = uType_defer origin ty1 ty2
defer_or_continue False m = m
\end{code}
Note [Care with type applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Note: type applications need a bit of care!
They can match FunTy and TyConApp, so use splitAppTy_maybe
NB: we've already dealt with type variables and Notes,
so if one type is an App the other one jolly well better be too
Note [Unifying AppTy]
~~~~~~~~~~~~~~~~~~~~~
Consider unifying (m Int) ~ (IO Int) where m is a unification variable
that is now bound to (say) (Bool ->). Then we want to report
"Can't unify (Bool -> Int) with (IO Int)
and not
"Can't unify ((->) Bool) with IO"
That is why we use the "_np" variant of uType, which does not alter the error
message.
Note [Mismatched type lists and application decomposition]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we find two TyConApps, you might think that the argument lists
are guaranteed equal length. But they aren't. Consider matching
w (T x) ~ Foo (T x y)
We do match (w ~ Foo) first, but in some circumstances we simply create
a deferred constraint; and then go ahead and match (T x ~ T x y).
This came up in Trac #3950.
So either
(a) either we must check for identical argument kinds
when decomposing applications,
(b) or we must be prepared for ill-kinded unification sub-problems
Currently we adopt (b) since it seems more robust -- no need to maintain
a global invariant.
Note [Expanding synonyms during unification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We expand synonyms during unification, but:
* We expand *after* the variable case so that we tend to unify
variables with un-expanded type synonym. This just makes it
more likely that the inferred types will mention type synonyms
understandable to the user
* We expand *before* the TyConApp case. For example, if we have
type Phantom a = Int
and are unifying
Phantom Int ~ Phantom Char
it is *wrong* to unify Int and Char.
Note [Deferred Unification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
and yet its consistency is undetermined. Previously, there was no way to still
make it consistent. So a mismatch error was issued.
Now these unfications are deferred until constraint simplification, where type
family instances and given equations may (or may not) establish the consistency.
Deferred unifications are of the form
F ... ~ ...
or x ~ ...
where F is a type function and x is a type variable.
E.g.
id :: x ~ y => x -> y
id e = e
involves the unfication x = y. It is deferred until we bring into account the
context x ~ y to establish that it holds.
If available, we defer original types (rather than those where closed type
synonyms have already been expanded via tcCoreView). This is, as usual, to
improve error messages.
%************************************************************************
%* *
uVar and friends
%* *
%************************************************************************
@uVar@ is called when at least one of the types being unified is a
variable. It does {\em not} assume that the variable is a fixed point
of the substitution; rather, notice that @uVar@ (defined below) nips
back into @uTys@ if it turns out that the variable is already bound.
\begin{code}
uUnfilledVar :: [EqOrigin]
-> SwapFlag
-> TcTyVar -> TcTyVarDetails
-> TcTauType
-> TcM TcCoercion
uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
| tv1 == tv2
= return (mkTcReflCo (mkTyVarTy tv1))
| otherwise
= do { lookup2 <- lookupTcTyVar tv2
; case lookup2 of
Filled ty2' -> uUnfilledVar origin swapped tv1 details1 ty2'
Unfilled details2 -> uUnfilledVars origin swapped tv1 details1 tv2 details2
}
uUnfilledVar origin swapped tv1 details1 non_var_ty2
= case details1 of
MetaTv TauTv ref1
-> do { mb_ty2' <- checkTauTvUpdate tv1 non_var_ty2
; case mb_ty2' of
Nothing -> do { traceTc "Occ/kind defer" (ppr tv1); defer }
Just ty2' -> updateMeta tv1 ref1 ty2'
}
_other -> do { traceTc "Skolem defer" (ppr tv1); defer }
where
defer | Just ty2' <- tcView non_var_ty2
= uUnfilledVar origin swapped tv1 details1 ty2'
| otherwise
= unSwap swapped (uType_defer origin) (mkTyVarTy tv1) non_var_ty2
uUnfilledVars :: [EqOrigin]
-> SwapFlag
-> TcTyVar -> TcTyVarDetails
-> TcTyVar -> TcTyVarDetails
-> TcM TcCoercion
uUnfilledVars origin swapped tv1 details1 tv2 details2
= do { traceTc "uUnfilledVars" ( text "trying to unify" <+> ppr k1
<+> text "with" <+> ppr k2)
; let ctxt = mkKindErrorCtxt ty1 ty2 k1 k2
; sub_kind <- addErrCtxtM ctxt $ unifyKind k1 k2
; case (sub_kind, details1, details2) of
(LT, _, MetaTv _ ref2) -> updateMeta tv2 ref2 ty1
(GT, MetaTv _ ref1, _) -> updateMeta tv1 ref1 ty2
(EQ, MetaTv i1 ref1, MetaTv i2 ref2)
| nicer_to_update_tv1 i1 i2 -> updateMeta tv1 ref1 ty2
| otherwise -> updateMeta tv2 ref2 ty1
(EQ, MetaTv _ ref1, _) -> updateMeta tv1 ref1 ty2
(EQ, _, MetaTv _ ref2) -> updateMeta tv2 ref2 ty1
(_, _, _) -> unSwap swapped (uType_defer origin) ty1 ty2 }
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
ty1 = mkTyVarTy tv1
ty2 = mkTyVarTy tv2
nicer_to_update_tv1 _ SigTv = True
nicer_to_update_tv1 SigTv _ = False
nicer_to_update_tv1 _ _ = isSystemName (Var.varName tv1)
checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
checkTauTvUpdate tv ty
= do { ty' <- zonkTcType ty
; let k2 = typeKind ty'
; k1 <- zonkTcKind (tyVarKind tv)
; let ctxt = mkKindErrorCtxt (mkTyVarTy tv) ty' k1 k2
; sub_k <- addErrCtxtM ctxt $
unifyKind (tyVarKind tv) (typeKind ty')
; case sub_k of
LT -> return Nothing
_ -> return (ok ty') }
where
ok :: TcType -> Maybe TcType
ok (TyVarTy tv') | not (tv == tv') = Just (TyVarTy tv')
ok this_ty@(TyConApp tc tys)
| not (isSynFamilyTyCon tc), Just tys' <- allMaybes (map ok tys)
= Just (TyConApp tc tys')
| isSynTyCon tc, Just ty_expanded <- tcView this_ty
= ok ty_expanded
ok ty@(LitTy {}) = Just ty
ok (FunTy arg res) | Just arg' <- ok arg, Just res' <- ok res
= Just (FunTy arg' res')
ok (AppTy fun arg) | Just fun' <- ok fun, Just arg' <- ok arg
= Just (AppTy fun' arg')
ok (ForAllTy tv1 ty1) | Just ty1' <- ok ty1 = Just (ForAllTy tv1 ty1')
ok _ty = Nothing
\end{code}
Note [Avoid deferring]
~~~~~~~~~~~~~~~~~~~~~~
We try to avoid creating deferred constraints only for efficiency.
Example (Trac #4917)
a ~ Const a b
where type Const a b = a. We can solve this immediately, even when
'a' is a skolem, just by expanding the synonym.
Note [Type synonyms and the occur check]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Generally speaking we try to update a variable with type synonyms not
expanded, which improves later error messages, unless looking
inside a type synonym may help resolve a spurious occurs check
error. Consider:
type A a = ()
f :: (A a -> a -> ()) -> ()
f = \ _ -> ()
x :: ()
x = f (\ x p -> p x)
We will eventually get a constraint of the form t ~ A t. The ok function above will
properly expand the type (A t) to just (), which is ok to be unified with t. If we had
unified with the original type A t, we would lead the type checker into an infinite loop.
Hence, if the occurs check fails for a type synonym application, then (and *only* then),
the ok function expands the synonym to detect opportunities for occurs check success using
the underlying definition of the type synonym.
The same applies later on in the constraint interaction code; see TcInteract,
function @occ_check_ok@.
Note [Type family sharing]
~~~~~~~~~~~~~~
We must avoid eagerly unifying type variables to types that contain function symbols,
because this may lead to loss of sharing, and in turn, in very poor performance of the
constraint simplifier. Assume that we have a wanted constraint:
{
m1 ~ [F m2],
m2 ~ [F m3],
m3 ~ [F m4],
D m1,
D m2,
D m3
}
where D is some type class. If we eagerly unify m1 := [F m2], m2 := [F m3], m3 := [F m2],
then, after zonking, our constraint simplifier will be faced with the following wanted
constraint:
{
D [F [F [F m4]]],
D [F [F m4]],
D [F m4]
}
which has to be flattened by the constraint solver. However, because the sharing is lost,
an polynomially larger number of flatten skolems will be created and the constraint sets
we are working with will be polynomially larger.
Instead, if we defer the unifications m1 := [F m2], etc. we will only be generating three
flatten skolems, which is the maximum possible sharing arising from the original constraint.
\begin{code}
data LookupTyVarResult
= Unfilled TcTyVarDetails
| Filled TcType
lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
lookupTcTyVar tyvar
| MetaTv _ ref <- details
= do { meta_details <- readMutVar ref
; case meta_details of
Indirect ty -> return (Filled ty)
Flexi -> do { is_untch <- isUntouchable tyvar
; let
ret_details | is_untch = vanillaSkolemTv
| otherwise = details
; return (Unfilled ret_details) } }
| otherwise
= return (Unfilled details)
where
details = ASSERT2( isTcTyVar tyvar, ppr tyvar )
tcTyVarDetails tyvar
updateMeta :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM TcCoercion
updateMeta tv1 ref1 ty2
= do { writeMetaTyVarRef tv1 ref1 ty2
; return (mkTcReflCo ty2) }
\end{code}
Note [Unifying untouchables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We treat an untouchable type variable as if it was a skolem. That
ensures it won't unify with anything. It's a slight had, because
we return a made-up TcTyVarDetails, but I think it works smoothly.
%************************************************************************
%* *
Errors and contexts
%* *
%************************************************************************
\begin{code}
pushOrigin :: TcType -> TcType -> [EqOrigin] -> [EqOrigin]
pushOrigin ty_act ty_exp origin
= UnifyOrigin { uo_actual = ty_act, uo_expected = ty_exp } : origin
\end{code}
-----------------------------------------
UNUSED FOR NOW
-----------------------------------------
----------------
----------------
-- If an error happens we try to figure out whether the function
-- function has been given too many or too few arguments, and say so.
addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a
addSubCtxt orig actual_res_ty expected_res_ty thing_inside
= addErrCtxtM mk_err thing_inside
where
mk_err tidy_env
= do { exp_ty' <- zonkTcType expected_res_ty
; act_ty' <- zonkTcType actual_res_ty
; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
(env2, act_ty'') = tidyOpenType env1 act_ty'
(exp_args, _) = tcSplitFunTys exp_ty''
(act_args, _) = tcSplitFunTys act_ty''
len_act_args = length act_args
len_exp_args = length exp_args
message = case orig of
OccurrenceOf fun
| len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
| len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
_ -> mkExpectedActualMsg act_ty'' exp_ty''
; return (env2, message) }
%************************************************************************
%* *
Kind unification
%* *
%************************************************************************
Unifying kinds is much, much simpler than unifying types.
One small wrinkle is that as far as the user is concerned, types of kind
Constraint should only be allowed to occur where we expect *exactly* that kind.
We SHOULD NOT allow a type of kind fact to appear in a position expecting
one of argTypeKind or openTypeKind.
The situation is different in the core of the compiler, where we are perfectly
happy to have types of kind Constraint on either end of an arrow.
\begin{code}
matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
matchExpectedFunKind (FunTy arg_kind res_kind)
= return (Just (arg_kind,res_kind))
matchExpectedFunKind (TyVarTy kvar)
| isTcTyVar kvar, isMetaTyVar kvar
= do { maybe_kind <- readMetaTyVar kvar
; case maybe_kind of
Indirect fun_kind -> matchExpectedFunKind fun_kind
Flexi ->
do { arg_kind <- newMetaKindVar
; res_kind <- newMetaKindVar
; writeMetaTyVar kvar (mkArrowKind arg_kind res_kind)
; return (Just (arg_kind,res_kind)) } }
matchExpectedFunKind _ = return Nothing
unifyKind :: TcKind
-> TcKind
-> TcM Ordering
unifyKind (TyVarTy kv1) k2 = uKVar False unifyKind EQ kv1 k2
unifyKind k1 (TyVarTy kv2) = uKVar True unifyKind EQ kv2 k1
unifyKind k1 k2
| Just k1' <- tcView k1 = unifyKind k1' k2
| Just k2' <- tcView k2 = unifyKind k1 k2'
unifyKind k1@(TyConApp kc1 []) k2@(TyConApp kc2 [])
| kc1 == kc2 = return EQ
| kc1 `tcIsSubKindCon` kc2 = return LT
| kc2 `tcIsSubKindCon` kc1 = return GT
| otherwise = unifyKindMisMatch k1 k2
unifyKind k1 k2 = do { unifyKindEq k1 k2; return EQ }
uKVar :: Bool -> (TcKind -> TcKind -> TcM a) -> a
-> MetaKindVar -> TcKind -> TcM a
uKVar isFlipped unify_kind eq_res kv1 k2
| isTcTyVar kv1, isMetaTyVar kv1
= do { mb_k1 <- readMetaTyVar kv1
; case mb_k1 of
Flexi -> do { uUnboundKVar kv1 k2; return eq_res }
Indirect k1 -> if isFlipped then unify_kind k2 k1
else unify_kind k1 k2 }
| TyVarTy kv2 <- k2, kv1 == kv2
= return eq_res
| TyVarTy kv2 <- k2, isTcTyVar kv2, isMetaTyVar kv2
= uKVar (not isFlipped) unify_kind eq_res kv2 (TyVarTy kv1)
| otherwise = if isFlipped
then unifyKindMisMatch k2 (TyVarTy kv1)
else unifyKindMisMatch (TyVarTy kv1) k2
unifyKindEq :: TcKind -> TcKind -> TcM ()
unifyKindEq (TyVarTy kv1) k2 = uKVar False unifyKindEq () kv1 k2
unifyKindEq k1 (TyVarTy kv2) = uKVar True unifyKindEq () kv2 k1
unifyKindEq (FunTy a1 r1) (FunTy a2 r2)
= do { unifyKindEq a1 a2; unifyKindEq r1 r2 }
unifyKindEq (TyConApp kc1 k1s) (TyConApp kc2 k2s)
| kc1 == kc2
= ASSERT (length k1s == length k2s)
zipWithM_ unifyKindEq k1s k2s
unifyKindEq k1 k2 = unifyKindMisMatch k1 k2
uUnboundKVar :: MetaKindVar -> TcKind -> TcM ()
uUnboundKVar kv1 k2@(TyVarTy kv2)
| kv1 == kv2 = return ()
| isTcTyVar kv2, isMetaTyVar kv2
= do { mb_k2 <- readMetaTyVar kv2
; case mb_k2 of
Indirect k2 -> uUnboundKVar kv1 k2
Flexi -> writeMetaTyVar kv1 k2 }
| otherwise = writeMetaTyVar kv1 k2
uUnboundKVar kv1 non_var_k2
= do { k2' <- zonkTcKind non_var_k2
; let k2'' = defaultKind k2'
; kindUnifCheck kv1 k2''
; writeMetaTyVar kv1 k2'' }
kindUnifCheck :: TyVar -> Type -> TcM ()
kindUnifCheck kv1 k2
| elemVarSet kv1 (tyVarsOfType k2)
= failWithTc (kindOccurCheckErr kv1 k2)
| isSigTyVar kv1
= failWithTc (kindSigVarErr kv1 k2)
| otherwise
= return ()
mkKindErrorCtxt :: Type -> Type -> Kind -> Kind -> TidyEnv -> TcM (TidyEnv, SDoc)
mkKindErrorCtxt ty1 ty2 k1 k2 env0
= let (env1, ty1') = tidyOpenType env0 ty1
(env2, ty2') = tidyOpenType env1 ty2
(env3, k1' ) = tidyOpenKind env2 k1
(env4, k2' ) = tidyOpenKind env3 k2
in do ty1 <- zonkTcType ty1'
ty2 <- zonkTcType ty2'
k1 <- zonkTcKind k1'
k2 <- zonkTcKind k2'
return (env4,
vcat [ ptext (sLit "Kind incompatibility when matching types:")
, nest 2 (vcat [ ppr ty1 <+> dcolon <+> ppr k1
, ppr ty2 <+> dcolon <+> ppr k2 ]) ])
unifyKindMisMatch :: TcKind -> TcKind -> TcM a
unifyKindMisMatch ki1 ki2 = do
ki1' <- zonkTcKind ki1
ki2' <- zonkTcKind ki2
let msg = hang (ptext (sLit "Couldn't match kind"))
2 (sep [quotes (ppr ki1'),
ptext (sLit "against"),
quotes (ppr ki2')])
failWithTc msg
kindOccurCheckErr :: Var -> Type -> SDoc
kindOccurCheckErr tyvar ty
= hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
2 (sep [ppr tyvar, char '=', ppr ty])
kindSigVarErr :: Var -> Type -> SDoc
kindSigVarErr tv ty
= hang (ptext (sLit "Cannot unify the kind variable") <+> quotes (ppr tv))
2 (ptext (sLit "with the kind") <+> quotes (ppr ty))
\end{code}