%
% (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, sigCtxt,
unifyType, unifyTypeList, unifyTheta, unifyKind,
tcInfer,
matchExpectedListTy, matchExpectedPArrTy,
matchExpectedTyConApp, matchExpectedAppTy,
matchExpectedFunTys, matchExpectedFunKind,
wrapFunResCoercion, failWithMisMatch
) where
#include "HsVersions.h"
import HsSyn
import TypeRep
import CoreUtils( mkPiTypes )
import TcErrors ( unifyCtxt )
import TcMType
import TcIface
import TcRnMonad
import TcType
import Type
import Coercion
import Inst
import TyCon
import TysWiredIn
import Var
import VarSet
import VarEnv
import Name
import ErrUtils
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 (Coercion, [TcSigmaType], TcRhoType)
matchExpectedFunTys herald arity orig_ty
= go arity orig_ty
where
go n_req ty
| n_req == 0 = return (mkReflCo 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 { (coi, tys, ty_r) <- go (n_req1) res_ty
; return (mkFunCo (mkReflCo arg_ty) coi, arg_ty:tys, ty_r) }
go _ (TyConApp tc _)
| not (isSynFamilyTyCon tc)
= do { (env,msg) <- mk_ctxt emptyTidyEnv
; failWithTcM (env,msg) }
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 argTypeKind
; res_ty <- newFlexiTyVarTy openTypeKind
; coi <- unifyType fun_ty (mkFunTys arg_tys res_ty)
; return (coi, arg_tys, res_ty) }
mk_ctxt :: TidyEnv -> TcM (TidyEnv, Message)
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 (Coercion, TcRhoType)
matchExpectedListTy exp_ty
= do { (coi, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
; return (coi, elt_ty) }
matchExpectedPArrTy :: TcRhoType -> TcM (Coercion, TcRhoType)
matchExpectedPArrTy exp_ty
= do { (coi, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
; return (coi, elt_ty) }
matchExpectedTyConApp :: TyCon
-> TcRhoType
-> TcM (Coercion,
[TcSigmaType])
matchExpectedTyConApp tc orig_ty
= do { checkWiredInTyCon tc
; go (tyConArity tc) orig_ty [] }
where
go :: Int -> TcRhoType -> [TcSigmaType] -> TcM (Coercion, [TcSigmaType])
go n_req ty tys
| Just ty' <- tcView ty = go n_req ty' tys
go n_req ty@(TyVarTy tv) tys
| ASSERT( isTcTyVar tv) isMetaTyVar tv
= do { cts <- readMetaTyVar tv
; case cts of
Indirect ty -> go n_req ty tys
Flexi -> defer n_req ty tys }
go n_req ty@(TyConApp tycon args) tys
| tc == tycon
= ASSERT( n_req == length args)
return (mkReflCo ty, args ++ tys)
go n_req (AppTy fun arg) tys
| n_req > 0
= do { (coi, args) <- go (n_req 1) fun (arg : tys)
; return (mkAppCo coi (mkReflCo arg), args) }
go n_req ty tys = defer n_req ty tys
defer n_req ty tys
= do { tau_tys <- mapM newFlexiTyVarTy arg_kinds
; coi <- unifyType (mkTyConApp tc tau_tys) ty
; return (coi, tau_tys ++ tys) }
where
(arg_kinds, _) = splitKindFunTysN n_req (tyConKind tc)
matchExpectedAppTy :: TcRhoType
-> TcM (Coercion,
(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 (mkReflCo 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
; coi <- unifyType (mkAppTy ty1 ty2) orig_ty
; return (coi, (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
; coi <- unifyType in_rho sk_rho
; return (coToHsWrapper coi <.> in_wrap) }
; return (sk_wrap <.> inst_wrap) }
| otherwise
= do { coi <- unifyType ty_actual ty_expected
; return (coToHsWrapper coi) }
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 { coi <- unifyType actual_ty res_ty
; return (mkHsWrapCo coi 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 = mkVarSet 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 Coercion
unifyType ty1 ty2 = uType [] ty1 ty2
unifyPred :: PredType -> PredType -> TcM Coercion
unifyPred p1 p2 = uPred [UnifyOrigin (mkPredTy p1) (mkPredTy p2)] p1 p2
unifyTheta :: TcThetaType -> TcThetaType -> TcM [Coercion]
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 Coercion
uType_defer (item : origin) ty1 ty2
= wrapEqCtxt origin $
do { co_var <- newCoVar ty1 ty2
; loc <- getCtLoc (TypeEqOrigin item)
; emitFlat (mkEvVarX co_var loc)
; ctxt <- getErrCtxt
; doc <- mkErrInfo emptyTidyEnv ctxt
; traceTc "utype_defer" (vcat [ppr co_var, ppr ty1, ppr ty2, ppr origin, doc])
; return $ mkCoVarCo co_var }
uType_defer [] _ _
= panic "uType_defer"
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]
; coi <- go orig_ty1 orig_ty2
; if isReflCo coi
then traceTc "u_tys yields no coercion" empty
else traceTc "u_tys yields coercion:" (ppr coi)
; return coi }
where
bale_out :: [EqOrigin] -> TcM a
bale_out origin = failWithMisMatch origin
go :: TcType -> TcType -> TcM Coercion
go (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
go ty1 (TyVarTy tyvar2) = uVar origin IsSwapped tyvar2 ty1
go ty1 ty2
| Just ty1' <- tcView ty1 = go ty1' ty2
| Just ty2' <- tcView ty2 = go ty1 ty2'
go (PredTy p1) (PredTy p2) = uPred origin p1 p2
go (FunTy fun1 arg1) (FunTy fun2 arg2)
= do { coi_l <- uType origin fun1 fun2
; coi_r <- uType origin arg1 arg2
; return $ mkFunCo coi_l coi_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
= do { cois <- uList origin uType tys1 tys2
; return $ mkTyConAppCo tc1 cois }
go (AppTy s1 t1) ty2
| Just (s2,t2) <- tcSplitAppTy_maybe ty2
= do { coi_s <- uType_np origin s1 s2
; coi_t <- uType origin t1 t2
; return $ mkAppCo coi_s coi_t }
go ty1 (AppTy s2 t2)
| Just (s1,t1) <- tcSplitAppTy_maybe ty1
= do { coi_s <- uType_np origin s1 s2
; coi_t <- uType origin t1 t2
; return $ mkAppCo coi_s coi_t }
go ty1 ty2
| tcIsForAllTy ty1 || tcIsForAllTy ty2
= unifySigmaTy origin ty1 ty2
go _ _ = bale_out origin
unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM Coercion
unifySigmaTy origin ty1 ty2
= do { let (tvs1, body1) = tcSplitForAllTys ty1
(tvs2, body2) = tcSplitForAllTys ty2
; unless (equalLength tvs1 tvs2) (failWithMisMatch origin)
; skol_tvs <- tcInstSkolTyVars tvs1
; let tys = mkTyVarTys skol_tvs
in_scope = mkInScopeSet (mkVarSet skol_tvs)
phi1 = Type.substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
phi2 = Type.substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
; ((coi, _untch), lie) <- captureConstraints $
captureUntouchables $
uType origin phi1 phi2
; when (any (`elemVarSet` tyVarsOfWC lie) skol_tvs)
(failWithMisMatch origin)
; emitConstraints lie
; return (foldr mkForAllCo coi skol_tvs) }
uPred :: [EqOrigin] -> PredType -> PredType -> TcM Coercion
uPred origin (IParam n1 t1) (IParam n2 t2)
| n1 == n2
= do { coi <- uType origin t1 t2
; return $ mkPredCo $ IParam n1 coi }
uPred origin (ClassP c1 tys1) (ClassP c2 tys2)
| c1 == c2
= do { cois <- uList origin uType tys1 tys2
; return $ mkPredCo $ ClassP c1 cois }
uPred origin (EqPred ty1a ty1b) (EqPred ty2a ty2b)
= do { coa <- uType origin ty1a ty2a
; cob <- uType origin ty1b ty2b
; return $ mkPredCo $ EqPred coa cob }
uPred origin _ _ = failWithMisMatch origin
uList :: [EqOrigin]
-> ([EqOrigin] -> a -> a -> TcM b)
-> [a] -> [a] -> TcM [b]
uList _ _ [] [] = return []
uList origin unify (ty1:tys1) (ty2:tys2) = do { x <- unify origin ty1 ty2;
; xs <- uList origin unify tys1 tys2
; return (x:xs) }
uList origin _ _ _ = failWithMisMatch origin
\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]
~~~~~~~~~~~~~~~~~~~~~
Considerm 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 [TyCon app]
~~~~~~~~~~~~~~~~
When we find two TyConApps, the argument lists are guaranteed equal
length. Reason: intially the kinds of the two types to be unified is
the same. The only way it can become not the same is when unifying two
AppTys (f1 a1)~(f2 a2). In that case there can't be a TyConApp in
the f1,f2 (because it'd absorb the app). If we unify f1~f2 first,
which we do, that ensures that f1,f2 have the same kind; and that
means a1,a2 have the same kind. And now the argument repeats.
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 [Unification and synonyms]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you are tempted to make a short cut on synonyms, as in this
pseudocode...
uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
= if (con1 == con2) then
-- Good news! Same synonym constructors, so we can shortcut
-- by unifying their arguments and ignoring their expansions.
unifyTypepeLists args1 args2
else
-- Never mind. Just expand them and try again
uTys ty1 ty2
then THINK AGAIN. Here is the whole story, as detected and reported
by Chris Okasaki:
Here's a test program that should detect the problem:
type Bogus a = Int
x = (1 :: Bogus Char) :: Bogus Bool
The problem with [the attempted shortcut code] is that
con1 == con2
is not a sufficient condition to be able to use the shortcut!
You also need to know that the type synonym actually USES all
its arguments. For example, consider the following type synonym
which does not use all its arguments.
type Bogus a = Int
If you ever tried unifying, say, (Bogus Char) with )Bogus Bool), the
unifier would blithely try to unify Char with Bool and would fail,
even though the expanded forms (both Int) should match. Similarly,
unifying (Bogus Char) with (Bogus t) would unnecessarily bind t to
Char.
... You could explicitly test for the problem synonyms and mark them
somehow as needing expansion, perhaps also issuing a warning to the
user.
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}
uVar :: [EqOrigin] -> SwapFlag -> TcTyVar -> TcTauType -> TcM Coercion
uVar origin swapped tv1 ty2
= do { traceTc "uVar" (vcat [ ppr origin
, ppr swapped
, ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
, nest 2 (ptext (sLit " ~ "))
, ppr ty2 <+> dcolon <+> ppr (typeKind ty2)])
; details <- lookupTcTyVar tv1
; case details of
Filled ty1 -> unSwap swapped (uType_np origin) ty1 ty2
Unfilled details1 -> uUnfilledVar origin swapped tv1 details1 ty2
}
uUnfilledVar :: [EqOrigin]
-> SwapFlag
-> TcTyVar -> TcTyVarDetails
-> TcTauType
-> TcM Coercion
uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
| tv1 == tv2
= return (mkReflCo (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 Coercion
uUnfilledVars origin swapped tv1 details1 tv2 details2
= case (details1, details2) of
(MetaTv i1 ref1, MetaTv i2 ref2)
| k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1 i1 i2
then updateMeta tv1 ref1 ty2
else updateMeta tv2 ref2 ty1
| k2_sub_k1 -> updateMeta tv1 ref1 ty2
(_, MetaTv _ ref2) | k1_sub_k2 -> updateMeta tv2 ref2 ty1
(MetaTv _ ref1, _) | k2_sub_k1 -> updateMeta tv1 ref1 ty2
(_, _) -> unSwap swapped (uType_defer origin) ty1 ty2
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
k1_sub_k2 = k1 `isSubKind` k2
k2_sub_k1 = k2 `isSubKind` k1
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
; if typeKind ty' `isSubKind` tyVarKind tv then
case ok ty' of
Nothing -> return Nothing
Just ty'' -> return (Just ty'')
else return Nothing }
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 (PredTy sty) | Just sty' <- ok_pred sty = Just (PredTy sty')
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
ok_pred (IParam nm ty) | Just ty' <- ok ty = Just (IParam nm ty')
ok_pred (ClassP cl tys)
| Just tys' <- allMaybes (map ok tys)
= Just (ClassP cl tys')
ok_pred (EqPred ty1 ty2)
| Just ty1' <- ok ty1, Just ty2' <- ok ty2
= Just (EqPred ty1' ty2')
ok_pred _pty = Nothing
\end{code}
Note [Avoid deferring]
~~~~~~~~~~~~~~~~~~~~~~
We try to avoid creating deferred constraints for two reasons.
* First, efficiency.
* Second, currently we can only defer some constraints
under a forall. See unifySigmaTy.
So expanding synonyms here is a good thing to do. 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; and we should do so
in case this unification happens inside unifySigmaTy (sigh).
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 Coercion
updateMeta tv1 ref1 ty2
= do { writeMetaTyVarRef tv1 ref1 ty2
; return (mkReflCo 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
wrapEqCtxt :: [EqOrigin] -> TcM a -> TcM a
wrapEqCtxt [] thing_inside = thing_inside
wrapEqCtxt items thing_inside = addErrCtxtM (unifyCtxt (last items)) thing_inside
failWithMisMatch :: [EqOrigin] -> TcM a
failWithMisMatch (item:origin)
= wrapEqCtxt origin $
do { ty_act <- zonkTcType (uo_actual item)
; ty_exp <- zonkTcType (uo_expected item)
; env0 <- tcInitTidyEnv
; let (env1, pp_exp) = tidyOpenType env0 ty_exp
(env2, pp_act) = tidyOpenType env1 ty_act
; failWithTcM (env2, misMatchMsg pp_act pp_exp) }
failWithMisMatch []
= panic "failWithMisMatch"
misMatchMsg :: TcType -> TcType -> SDoc
misMatchMsg ty_act ty_exp
= sep [ ptext (sLit "Couldn't match expected type") <+> quotes (ppr ty_exp)
, nest 12 $ ptext (sLit "with actual type") <+> quotes (ppr ty_act)]
\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.
\begin{code}
matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
matchExpectedFunKind (TyVarTy kvar) = do
maybe_kind <- readKindVar kvar
case maybe_kind of
Indirect fun_kind -> matchExpectedFunKind fun_kind
Flexi ->
do { arg_kind <- newKindVar
; res_kind <- newKindVar
; writeKindVar kvar (mkArrowKind arg_kind res_kind)
; return (Just (arg_kind,res_kind)) }
matchExpectedFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
matchExpectedFunKind _ = return Nothing
unifyKind :: TcKind
-> TcKind
-> TcM ()
unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
| isSubKindCon kc2 kc1 = return ()
unifyKind (FunTy a1 r1) (FunTy a2 r2)
= do { unifyKind a2 a1; unifyKind r1 r2 }
unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
unifyKind k1 k2 = unifyKindMisMatch k1 k2
uKVar :: Bool -> KindVar -> TcKind -> TcM ()
uKVar swapped kv1 k2
= do { mb_k1 <- readKindVar kv1
; case mb_k1 of
Flexi -> uUnboundKVar swapped kv1 k2
Indirect k1 | swapped -> unifyKind k2 k1
| otherwise -> unifyKind k1 k2 }
uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
| kv1 == kv2 = return ()
| otherwise
= do { mb_k2 <- readKindVar kv2
; case mb_k2 of
Indirect k2 -> uUnboundKVar swapped kv1 k2
Flexi -> writeKindVar kv1 k2 }
uUnboundKVar swapped kv1 non_var_k2
= do { k2' <- zonkTcKind non_var_k2
; kindOccurCheck kv1 k2'
; k2'' <- kindSimpleKind swapped k2'
; writeKindVar kv1 k2'' }
kindOccurCheck :: TyVar -> Type -> TcM ()
kindOccurCheck kv1 k2
= checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
where
not_in (TyVarTy kv2) = kv1 /= kv2
not_in (FunTy a2 r2) = not_in a2 && not_in r2
not_in _ = True
kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
kindSimpleKind orig_swapped orig_kind
= go orig_swapped orig_kind
where
go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
; k2' <- go sw k2
; return (mkArrowKind k1' k2') }
go True k
| isOpenTypeKind k = return liftedTypeKind
| isArgTypeKind k = return liftedTypeKind
go _ k
| isLiftedTypeKind k = return liftedTypeKind
| isUnliftedTypeKind k = return unliftedTypeKind
go _ k@(TyVarTy _) = return k
go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
<+> ppr orig_swapped <+> ppr orig_kind)
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
kindOccurCheckErr :: Var -> Type -> SDoc
kindOccurCheckErr tyvar ty
= hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
2 (sep [ppr tyvar, char '=', ppr ty])
\end{code}
%************************************************************************
%* *
\subsection{Checking signature type variables}
%* *
%************************************************************************
@checkSigTyVars@ checks that a set of universally quantified type varaibles
are not mentioned in the environment. In particular:
(a) Not mentioned in the type of a variable in the envt
eg the signature for f in this:
g x = ... where
f :: a->[a]
f y = [x,y]
Here, f is forced to be monorphic by the free occurence of x.
(d) Not (unified with another type variable that is) in scope.
eg f x :: (r->r) = (\y->y) :: forall a. a->r
when checking the expression type signature, we find that
even though there is nothing in scope whose type mentions r,
nevertheless the type signature for the expression isn't right.
Another example is in a class or instance declaration:
class C a where
op :: forall b. a -> b
op x = x
Here, b gets unified with a
Before doing this, the substitution is applied to the signature type variable.
-- \begin{code}
checkSigTyVars :: [TcTyVar] -> TcM ()
checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
-- The extra_tvs can include boxy type variables;
-- e.g. TcMatches.tcCheckExistentialPat
checkSigTyVarsWrt extra_tvs sig_tvs
= do { extra_tvs' <- zonkTcTyVarsAndFV extra_tvs
; check_sig_tyvars extra_tvs' sig_tvs }
check_sig_tyvars
:: TcTyVarSet -- Global type variables. The universally quantified
-- tyvars should not mention any of these
-- Guaranteed already zonked.
-> [TcTyVar] -- Universally-quantified type variables in the signature
-- Guaranteed to be skolems
-> TcM ()
check_sig_tyvars _ []
= return ()
check_sig_tyvars extra_tvs sig_tvs
= ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs )
do { gbl_tvs <- tcGetGlobalTyVars
; traceTc "check_sig_tyvars" $ vcat
[ text "sig_tys" <+> ppr sig_tvs
, text "gbl_tvs" <+> ppr gbl_tvs
, text "extra_tvs" <+> ppr extra_tvs]
; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
; when (any (`elemVarSet` env_tvs) sig_tvs)
(bleatEscapedTvs env_tvs sig_tvs sig_tvs)
}
bleatEscapedTvs :: TcTyVarSet -- The global tvs
-> [TcTyVar] -- The possibly-escaping type variables
-> [TcTyVar] -- The zonked versions thereof
-> TcM ()
-- Complain about escaping type variables
-- We pass a list of type variables, at least one of which
-- escapes. The first list contains the original signature type variable,
-- while the second contains the type variable it is unified to (usually itself)
bleatEscapedTvs globals sig_tvs zonked_tvs
= do { env0 <- tcInitTidyEnv
; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
(env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
where
main_msg = ptext (sLit "Inferred type is less polymorphic than expected")
check (tidy_env, msgs) (sig_tv, zonked_tv)
| not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
| otherwise
= do { lcl_env <- getLclTypeEnv
; (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) lcl_env tidy_env
; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
-----------------------
escape_msg :: Var -> Var -> [SDoc] -> SDoc
escape_msg sig_tv zonked_tv globs
| notNull globs
= vcat [sep [msg, ptext (sLit "is mentioned in the environment:")],
nest 2 (vcat globs)]
| otherwise
= msg <+> ptext (sLit "escapes")
-- Sigh. It's really hard to give a good error message
-- all the time. One bad case is an existential pattern match.
-- We rely on the "When..." context to help.
where
msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
is_bound_to
| sig_tv == zonked_tv = empty
| otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which")
-- \end{code}
These two context are used with checkSigTyVars
\begin{code}
sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
-> TidyEnv -> TcM (TidyEnv, Message)
sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
actual_tau <- zonkTcType sig_tau
let
(env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
(env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
(env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
sub_msg = vcat [ptext (sLit "Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
]
msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),
nest 2 sub_msg]
return (env3, msg)
\end{code}