%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
-\section[Unify]{Unifier}
-
-The unifier is now squarely in the typechecker monad (because of the
-updatable substitution).
+\section{Type subsumption and unification}
\begin{code}
-module TcUnify ( unifyTauTy, unifyTauTyList, unifyTauTyLists,
- unifyFunTy, unifyListTy, unifyTupleTy, unifyUnboxedTupleTy,
- unifyKind, unifyKinds, unifyTypeKind
- ) where
+module TcUnify (
+ -- Full-blown subsumption
+ tcSub, tcGen, subFunTy,
+ checkSigTyVars, sigCtxt, sigPatCtxt,
+
+ -- Various unifications
+ unifyTauTy, unifyTauTyList, unifyTauTyLists,
+ unifyFunTy, unifyListTy, unifyTupleTy,
+ unifyKind, unifyKinds, unifyOpenTypeKind,
+
+ -- Coercions
+ Coercion, ExprCoFn, PatCoFn,
+ (<$>), (<.>), mkCoercion,
+ idCoercion, isIdCoercion
+
+ ) where
#include "HsVersions.h"
--- friends:
-import TcMonad
-import Type ( Type(..), tyVarsOfType, funTyCon,
- mkFunTy, splitFunTy_maybe, splitTyConApp_maybe,
- Kind, boxedTypeKind, typeCon, anyBoxCon, anyBoxKind,
- splitAppTy_maybe,
- tidyOpenType, tidyOpenTypes, tidyTyVar
- )
-import TyCon ( TyCon, isTupleTyCon, isUnboxedTupleTyCon,
- tyConArity )
-import Name ( isSysLocalName )
-import Var ( TyVar, tyVarKind, varName )
-import VarEnv
-import VarSet ( varSetElems )
-import TcType ( TcType, TcTauType, TcTyVar, TcKind,
- newTyVarTy, newOpenTypeKind, newTyVarTy_OpenKind,
- tcGetTyVar, tcPutTyVar, zonkTcType, tcTypeKind
- )
--- others:
-import BasicTypes ( Arity )
-import TysWiredIn ( listTyCon, mkListTy, mkTupleTy, mkUnboxedTupleTy )
-import PprType () -- Instances
-import Util
+
+import HsSyn ( HsExpr(..) )
+import TcHsSyn ( TypecheckedHsExpr, TcPat,
+ mkHsDictApp, mkHsTyApp, mkHsLet )
+import TypeRep ( Type(..), SourceType(..),
+ openKindCon, typeCon )
+
+import TcMonad -- TcType, amongst others
+import TcType ( TcKind, TcType, TcSigmaType, TcPhiType, TcTyVar, TcTauType,
+ TcTyVarSet, TcThetaType,
+ isTauTy, isSigmaTy,
+ tcSplitAppTy_maybe, tcSplitTyConApp_maybe,
+ tcGetTyVar_maybe, tcGetTyVar,
+ mkTyConApp, mkTyVarTys, mkFunTy, tyVarsOfType, mkRhoTy,
+ typeKind, tcSplitFunTy_maybe, mkForAllTys,
+ isHoleTyVar, isSkolemTyVar, isUserTyVar, allDistinctTyVars,
+ tidyOpenType, tidyOpenTypes, tidyOpenTyVar, tidyOpenTyVars,
+ eqKind, openTypeKind, liftedTypeKind, unliftedTypeKind, isTypeKind,
+ hasMoreBoxityInfo, tyVarBindingInfo
+ )
+import qualified Type ( getTyVar_maybe )
+import Inst ( LIE, emptyLIE, plusLIE, mkLIE,
+ newDicts, instToId
+ )
+import TcMType ( getTcTyVar, putTcTyVar, tcInstType,
+ newTyVarTy, newTyVarTys, newBoxityVar, newHoleTyVarTy,
+ zonkTcType, zonkTcTyVars, zonkTcTyVar )
+import TcSimplify ( tcSimplifyCheck )
+import TysWiredIn ( listTyCon, mkListTy, mkTupleTy )
+import TcEnv ( TcTyThing(..), tcExtendGlobalTyVars, tcGetGlobalTyVars, tcLEnvElts )
+import TyCon ( tyConArity, isTupleTyCon, tupleTyConBoxity )
+import PprType ( pprType )
+import CoreFVs ( idFreeTyVars )
+import Id ( mkSysLocal, idType )
+import Var ( Var, varName, tyVarKind )
+import VarSet ( elemVarSet, varSetElems )
+import VarEnv
+import Name ( isSystemName, getSrcLoc )
+import ErrUtils ( Message )
+import BasicTypes ( Boxity, Arity, isBoxed )
+import Util ( isSingleton, equalLength )
+import Maybe ( isNothing )
import Outputable
\end{code}
%************************************************************************
%* *
-\subsection{The Kind variants}
+\subsection{Subsumption}
%* *
%************************************************************************
\begin{code}
-unifyKind :: TcKind -- Expected
- -> TcKind -- Actual
- -> TcM s ()
-unifyKind k1 k2
- = tcAddErrCtxtM (unifyCtxt "kind" k1 k2) $
- uTys k1 k1 k2 k2
+tcSub :: TcSigmaType -- expected_ty; can be a type scheme;
+ -- can be a "hole" type variable
+ -> TcSigmaType -- actual_ty; can be a type scheme
+ -> TcM (ExprCoFn, LIE)
+\end{code}
-unifyKinds :: [TcKind] -> [TcKind] -> TcM s ()
-unifyKinds [] [] = returnTc ()
-unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenTc_`
- unifyKinds ks1 ks2
-unifyKinds _ _ = panic "unifyKinds: length mis-match"
+(tcSub expected_ty actual_ty) checks that
+ 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}
+tcSub expected_ty actual_ty
+ = traceTc (text "tcSub" <+> details) `thenNF_Tc_`
+ tcAddErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
+ (tc_sub expected_ty expected_ty actual_ty actual_ty)
+ where
+ details = vcat [text "Expected:" <+> ppr expected_ty,
+ text "Actual: " <+> ppr actual_ty]
\end{code}
+tc_sub carries the types before and after expanding type synonyms
+
+\begin{code}
+tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
+ -> TcSigmaType -- ..and after
+ -> TcSigmaType -- actual_ty, before
+ -> TcSigmaType -- ..and after
+ -> TcM (ExprCoFn, LIE)
+
+-----------------------------------
+-- Expand synonyms
+tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
+tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
+
+-----------------------------------
+-- "Hole type variable" case
+-- Do this case before unwrapping for-alls in the actual_ty
+
+tc_sub _ (TyVarTy tv) act_sty act_ty
+ | isHoleTyVar tv
+ = -- It's a "hole" type variable
+ getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
+ case maybe_ty of
+
+ Just ty -> -- Already been assigned
+ tc_sub ty ty act_sty act_ty ;
+
+ Nothing -> -- Assign it
+ putTcTyVar tv act_sty `thenNF_Tc_`
+ returnTc (idCoercion, emptyLIE)
+
+
+-----------------------------------
+-- Generalisation case
+-- actual_ty: d:Eq b => b->b
+-- expected_ty: forall a. Ord a => a->a
+-- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
+
+-- It is essential to do this *before* the specialisation case
+-- Example: f :: (Eq a => a->a) -> ...
+-- g :: Ord b => b->b
+-- Consider f g !
+
+tc_sub exp_sty expected_ty act_sty actual_ty
+ | isSigmaTy expected_ty
+ = tcGen expected_ty (
+ \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
+ ) `thenTc` \ (gen_fn, co_fn, lie) ->
+ returnTc (gen_fn <.> co_fn, lie)
+
+-----------------------------------
+-- Specialisation case:
+-- actual_ty: forall a. Ord a => a->a
+-- expected_ty: Int -> Int
+-- co_fn e = e Int dOrdInt
+
+tc_sub exp_sty expected_ty act_sty actual_ty
+ | isSigmaTy actual_ty
+ = tcInstType actual_ty `thenNF_Tc` \ (tvs, theta, body_ty) ->
+ newDicts orig theta `thenNF_Tc` \ dicts ->
+ let
+ inst_fn e = mkHsDictApp (mkHsTyApp e (mkTyVarTys tvs))
+ (map instToId dicts)
+ in
+ tc_sub exp_sty expected_ty body_ty body_ty `thenTc` \ (co_fn, lie) ->
+ returnTc (co_fn <.> mkCoercion inst_fn, lie `plusLIE` mkLIE dicts)
+ where
+ orig = Rank2Origin
+
+-----------------------------------
+-- Function case
+
+tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
+ = tcSub_fun exp_arg exp_res act_arg act_res
+
+-----------------------------------
+-- Type variable meets function: imitate
+--
+-- NB 1: we can't just unify the type variable with the type
+-- because the type might not be a tau-type, and we aren't
+-- allowed to instantiate an ordinary type variable with
+-- a sigma-type
+--
+-- NB 2: can we short-cut to an error case?
+-- when the arg/res is not a tau-type?
+-- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
+-- then x = (f,f)
+-- is perfectly fine!
+
+tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ (TyVarTy tv)
+ = getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
+ case maybe_ty of
+ Just ty -> tc_sub exp_sty exp_ty ty ty
+ Nothing -> imitateFun tv exp_sty `thenNF_Tc` \ (act_arg, act_res) ->
+ tcSub_fun exp_arg exp_res act_arg act_res
+
+tc_sub _ (TyVarTy tv) act_sty act_ty@(FunTy act_arg act_res)
+ = getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
+ case maybe_ty of
+ Just ty -> tc_sub ty ty act_sty act_ty
+ Nothing -> imitateFun tv act_sty `thenNF_Tc` \ (exp_arg, exp_res) ->
+ tcSub_fun exp_arg exp_res act_arg act_res
+
+-----------------------------------
+-- Unification case
+-- If none of the above match, we revert to the plain unifier
+tc_sub exp_sty expected_ty act_sty actual_ty
+ = uTys exp_sty expected_ty act_sty actual_ty `thenTc_`
+ returnTc (idCoercion, emptyLIE)
+\end{code}
+
+%************************************************************************
+%* *
+\subsection{Functions}
+%* *
+%************************************************************************
+
+\begin{code}
+tcSub_fun exp_arg exp_res act_arg act_res
+ = tcSub act_arg exp_arg `thenTc` \ (co_fn_arg, lie1) ->
+ tcSub exp_res act_res `thenTc` \ (co_fn_res, lie2) ->
+ tcGetUnique `thenNF_Tc` \ uniq ->
+ let
+ -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
+ -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
+ -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
+ arg_id = mkSysLocal SLIT("sub") uniq exp_arg
+ coercion | isIdCoercion co_fn_arg,
+ isIdCoercion co_fn_res = idCoercion
+ | otherwise = mkCoercion co_fn
+
+ co_fn e = DictLam [arg_id]
+ (co_fn_res <$> (HsApp e (co_fn_arg <$> (HsVar arg_id))))
+ -- Slight hack; using a "DictLam" to get an ordinary simple lambda
+ -- HsVar arg_id :: HsExpr exp_arg
+ -- co_fn_arg $it :: HsExpr act_arg
+ -- HsApp e $it :: HsExpr act_res
+ -- co_fn_res $it :: HsExpr exp_res
+ in
+ returnTc (coercion, lie1 `plusLIE` lie2)
+
+imitateFun :: TcTyVar -> TcType -> NF_TcM (TcType, TcType)
+imitateFun tv ty
+ = ASSERT( not (isHoleTyVar tv) )
+ -- NB: tv is an *ordinary* tyvar and so are the new ones
+
+ -- Check that tv isn't a type-signature type variable
+ -- (This would be found later in checkSigTyVars, but
+ -- we get a better error message if we do it here.)
+ checkTcM (not (isSkolemTyVar tv))
+ (failWithTcM (unifyWithSigErr tv ty)) `thenTc_`
+
+ newTyVarTy openTypeKind `thenNF_Tc` \ arg ->
+ newTyVarTy openTypeKind `thenNF_Tc` \ res ->
+ putTcTyVar tv (mkFunTy arg res) `thenNF_Tc_`
+ returnNF_Tc (arg,res)
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Generalisation}
+%* *
+%************************************************************************
+
+\begin{code}
+tcGen :: TcSigmaType -- expected_ty
+ -> (TcPhiType -> TcM (result, LIE)) -- spec_ty
+ -> TcM (ExprCoFn, result, LIE)
+ -- The expression has type: spec_ty -> expected_ty
+
+tcGen expected_ty thing_inside -- We expect expected_ty to be a forall-type
+ -- If not, the call is a no-op
+ = tcInstType expected_ty `thenNF_Tc` \ (forall_tvs, theta, phi_ty) ->
+
+ -- Type-check the arg and unify with poly type
+ thing_inside phi_ty `thenTc` \ (result, lie) ->
+
+ -- Check that the "forall_tvs" havn't been constrained
+ -- The interesting bit here is that we must include the free variables
+ -- of the expected_ty. Here's an example:
+ -- runST (newVar True)
+ -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
+ -- for (newVar True), with s fresh. Then we unify with the runST's arg type
+ -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
+ -- So now s' isn't unconstrained because it's linked to a.
+ -- Conclusion: include the free vars of the expected_ty in the
+ -- list of "free vars" for the signature check.
+
+ tcExtendGlobalTyVars free_tvs $
+ tcAddErrCtxtM (sigCtxt forall_tvs theta phi_ty) $
+
+ newDicts SignatureOrigin theta `thenNF_Tc` \ dicts ->
+ tcSimplifyCheck sig_msg forall_tvs dicts lie `thenTc` \ (free_lie, inst_binds) ->
+ checkSigTyVars forall_tvs free_tvs `thenTc` \ zonked_tvs ->
+
+ let
+ -- This HsLet binds any Insts which came out of the simplification.
+ -- It's a bit out of place here, but using AbsBind involves inventing
+ -- a couple of new names which seems worse.
+ dict_ids = map instToId dicts
+ co_fn e = TyLam zonked_tvs (DictLam dict_ids (mkHsLet inst_binds e))
+ in
+ returnTc (mkCoercion co_fn, result, free_lie)
+ where
+ free_tvs = tyVarsOfType expected_ty
+ sig_msg = ptext SLIT("When generalising the type of an expression")
+\end{code}
+
+
+
+%************************************************************************
+%* *
+\subsection{Coercion functions}
+%* *
+%************************************************************************
+
+\begin{code}
+type Coercion a = Maybe (a -> a)
+ -- Nothing => identity fn
+
+type ExprCoFn = Coercion TypecheckedHsExpr
+type PatCoFn = Coercion TcPat
+
+(<.>) :: Coercion a -> Coercion a -> Coercion a -- Composition
+Nothing <.> Nothing = Nothing
+Nothing <.> Just f = Just f
+Just f <.> Nothing = Just f
+Just f1 <.> Just f2 = Just (f1 . f2)
+
+(<$>) :: Coercion a -> a -> a
+Just f <$> e = f e
+Nothing <$> e = e
+
+mkCoercion :: (a -> a) -> Coercion a
+mkCoercion f = Just f
+
+idCoercion :: Coercion a
+idCoercion = Nothing
+
+isIdCoercion :: Coercion a -> Bool
+isIdCoercion = isNothing
+\end{code}
%************************************************************************
%* *
Unify two @TauType@s. Dead straightforward.
\begin{code}
-unifyTauTy :: TcTauType -> TcTauType -> TcM s ()
+unifyTauTy :: TcTauType -> TcTauType -> TcM ()
unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
- = tcAddErrCtxtM (unifyCtxt "type" ty1 ty2) $
+ = -- The unifier should only ever see tau-types
+ -- (no quantification whatsoever)
+ ASSERT2( isTauTy ty1, ppr ty1 )
+ ASSERT2( isTauTy ty2, ppr ty2 )
+ tcAddErrCtxtM (unifyCtxt "type" ty1 ty2) $
uTys ty1 ty1 ty2 ty2
\end{code}
complain if their lengths differ.
\begin{code}
-unifyTauTyLists :: [TcTauType] -> [TcTauType] -> TcM s ()
+unifyTauTyLists :: [TcTauType] -> [TcTauType] -> TcM ()
unifyTauTyLists [] [] = returnTc ()
unifyTauTyLists (ty1:tys1) (ty2:tys2) = uTys ty1 ty1 ty2 ty2 `thenTc_`
unifyTauTyLists tys1 tys2
lists, when all the elts should be of the same type.
\begin{code}
-unifyTauTyList :: [TcTauType] -> TcM s ()
+unifyTauTyList :: [TcTauType] -> TcM ()
unifyTauTyList [] = returnTc ()
unifyTauTyList [ty] = returnTc ()
unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenTc_`
\begin{code}
uTys :: TcTauType -> TcTauType -- Error reporting ty1 and real ty1
+ -- ty1 is the *expected* type
+
-> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
- -> TcM s ()
+ -- ty2 is the *actual* type
+ -> TcM ()
-- Always expand synonyms (see notes at end)
-uTys ps_ty1 (NoteTy _ ty1) ps_ty2 ty2 = uTys ps_ty1 ty1 ps_ty2 ty2
-uTys ps_ty1 ty1 ps_ty2 (NoteTy _ ty2) = uTys ps_ty1 ty1 ps_ty2 ty2
+ -- (this also throws away FTVs)
+uTys ps_ty1 (NoteTy n1 ty1) ps_ty2 ty2 = uTys ps_ty1 ty1 ps_ty2 ty2
+uTys ps_ty1 ty1 ps_ty2 (NoteTy n2 ty2) = uTys ps_ty1 ty1 ps_ty2 ty2
+
+ -- Ignore usage annotations inside typechecker
+uTys ps_ty1 (UsageTy _ ty1) ps_ty2 ty2 = uTys ps_ty1 ty1 ps_ty2 ty2
+uTys ps_ty1 ty1 ps_ty2 (UsageTy _ ty2) = uTys ps_ty1 ty1 ps_ty2 ty2
-- Variables; go for uVar
uTys ps_ty1 (TyVarTy tyvar1) ps_ty2 ty2 = uVar False tyvar1 ps_ty2 ty2
uTys ps_ty1 ty1 ps_ty2 (TyVarTy tyvar2) = uVar True tyvar2 ps_ty1 ty1
-- "True" means args swapped
+ -- Predicates
+uTys _ (SourceTy (IParam n1 t1)) _ (SourceTy (IParam n2 t2))
+ | n1 == n2 = uTys t1 t1 t2 t2
+uTys _ (SourceTy (ClassP c1 tys1)) _ (SourceTy (ClassP c2 tys2))
+ | c1 == c2 = unifyTauTyLists tys1 tys2
+uTys _ (SourceTy (NType tc1 tys1)) _ (SourceTy (NType tc2 tys2))
+ | tc1 == tc2 = unifyTauTyLists tys1 tys2
+
-- Functions; just check the two parts
uTys _ (FunTy fun1 arg1) _ (FunTy fun2 arg2)
= uTys fun1 fun1 fun2 fun2 `thenTc_` uTys arg1 arg1 arg2 arg2
-- Type constructors must match
uTys ps_ty1 (TyConApp con1 tys1) ps_ty2 (TyConApp con2 tys2)
- = checkTcM (cons_match && length tys1 == length tys2)
- (failWithTcM (unifyMisMatch ps_ty1 ps_ty2)) `thenTc_`
- unifyTauTyLists tys1 tys2
- where
- -- The AnyBox wild card matches anything
- cons_match = con1 == con2
- || con1 == anyBoxCon
- || con2 == anyBoxCon
+ | con1 == con2 && equalLength tys1 tys2
+ = unifyTauTyLists tys1 tys2
+
+ | con1 == openKindCon
+ -- When we are doing kind checking, we might match a kind '?'
+ -- against a kind '*' or '#'. Notably, CCallable :: ? -> *, and
+ -- (CCallable Int) and (CCallable Int#) are both OK
+ = unifyOpenTypeKind ps_ty2
-- 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
uTys ps_ty1 (AppTy s1 t1) ps_ty2 ty2
- = case splitAppTy_maybe ty2 of
+ = case tcSplitAppTy_maybe ty2 of
Just (s2,t2) -> uTys s1 s1 s2 s2 `thenTc_` uTys t1 t1 t2 t2
- Nothing -> failWithTcM (unifyMisMatch ps_ty1 ps_ty2)
+ Nothing -> unifyMisMatch ps_ty1 ps_ty2
-- Now the same, but the other way round
-- Don't swap the types, because the error messages get worse
uTys ps_ty1 ty1 ps_ty2 (AppTy s2 t2)
- = case splitAppTy_maybe ty1 of
+ = case tcSplitAppTy_maybe ty1 of
Just (s1,t1) -> uTys s1 s1 s2 s2 `thenTc_` uTys t1 t1 t2 t2
- Nothing -> failWithTcM (unifyMisMatch ps_ty1 ps_ty2)
+ Nothing -> unifyMisMatch ps_ty1 ps_ty2
-- Not expecting for-alls in unification
-- ... but the error message from the unifyMisMatch more informative
-- than a panic message!
-- Anything else fails
-uTys ps_ty1 ty1 ps_ty2 ty2 = failWithTcM (unifyMisMatch ps_ty1 ps_ty2)
+uTys ps_ty1 ty1 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
\end{code}
+
Notes on synonyms
~~~~~~~~~~~~~~~~~
If you are tempted to make a short cut on synonyms, as in this
pseudocode...
\begin{verbatim}
-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.
- unifyTauTypeLists args1 args2
- else
- -- Never mind. Just expand them and try again
- uTys ty1 ty2
+-- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
+-- NO = if (con1 == con2) then
+-- NO -- Good news! Same synonym constructors, so we can shortcut
+-- NO -- by unifying their arguments and ignoring their expansions.
+-- NO unifyTauTypeLists args1 args2
+-- NO else
+-- NO -- Never mind. Just expand them and try again
+-- NO uTys ty1 ty2
\end{verbatim}
then THINK AGAIN. Here is the whole story, as detected and reported
-- True => ty is the "expected" thing
-> TcTyVar
-> TcTauType -> TcTauType -- printing and real versions
- -> TcM s ()
+ -> TcM ()
uVar swapped tv1 ps_ty2 ty2
- = tcGetTyVar tv1 `thenNF_Tc` \ maybe_ty1 ->
+ = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenNF_Tc_`
+ getTcTyVar tv1 `thenNF_Tc` \ maybe_ty1 ->
case maybe_ty1 of
Just ty1 | swapped -> uTys ps_ty2 ty2 ty1 ty1 -- Swap back
| otherwise -> uTys ty1 ty1 ps_ty2 ty2 -- Same order
other -> uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
- -- Expand synonyms
-uUnboundVar swapped tv1 maybe_ty1 ps_ty2 (NoteTy _ ty2)
+ -- Expand synonyms; ignore FTVs
+uUnboundVar swapped tv1 maybe_ty1 ps_ty2 (NoteTy n2 ty2)
= uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
-- Distinct type variables
-- ASSERT maybe_ty1 /= Just
| otherwise
- = tcGetTyVar tv2 `thenNF_Tc` \ maybe_ty2 ->
+ = getTcTyVar tv2 `thenNF_Tc` \ maybe_ty2 ->
case maybe_ty2 of
Just ty2' -> uUnboundVar swapped tv1 maybe_ty1 ty2' ty2'
- Nothing -> checkKinds swapped tv1 ty2 `thenTc_`
-
- -- Try to update sys-y type variables in preference to sig-y ones
- -- (the latter respond False to isSysLocalName)
- if isSysLocalName (varName tv2) then
- tcPutTyVar tv2 (TyVarTy tv1) `thenNF_Tc_`
- returnTc ()
- else
- tcPutTyVar tv1 ps_ty2 `thenNF_Tc_`
- returnTc ()
+ Nothing | update_tv2
- -- Second one isn't a type variable
-uUnboundVar swapped tv1 maybe_ty1 ps_ty2 non_var_ty2
- | non_var_ty2 == anyBoxKind
- -- If the
- = returnTc ()
+ -> WARN( not (k1 `hasMoreBoxityInfo` k2), (ppr tv1 <+> ppr k1) $$ (ppr tv2 <+> ppr k2) )
+ putTcTyVar tv2 (TyVarTy tv1) `thenNF_Tc_`
+ returnTc ()
+ | otherwise
- | otherwise
- = checkKinds swapped tv1 non_var_ty2 `thenTc_`
- occur_check non_var_ty2 `thenTc_`
- tcPutTyVar tv1 ps_ty2 `thenNF_Tc_`
- returnTc ()
+ -> WARN( not (k2 `hasMoreBoxityInfo` k1), (ppr tv2 <+> ppr k2) $$ (ppr tv1 <+> ppr k1) )
+ putTcTyVar tv1 ps_ty2 `thenNF_Tc_`
+ returnTc ()
where
- occur_check ty = mapTc occur_check_tv (varSetElems (tyVarsOfType ty)) `thenTc_`
- returnTc ()
+ k1 = tyVarKind tv1
+ k2 = tyVarKind tv2
+ update_tv2 = (k2 `eqKind` openTypeKind) || (not (k1 `eqKind` openTypeKind) && nicer_to_update_tv2)
+ -- Try to get rid of open type variables as soon as poss
- occur_check_tv tv2
- | tv1 == tv2 -- Same tyvar; fail
- = zonkTcType ps_ty2 `thenNF_Tc` \ zonked_ty2 ->
- failWithTcM (unifyOccurCheck tv1 zonked_ty2)
+ nicer_to_update_tv2 = isUserTyVar tv1
+ -- Don't unify a signature type variable if poss
+ || isSystemName (varName tv2)
+ -- Try to update sys-y type variables in preference to sig-y ones
+
+ -- Second one isn't a type variable
+uUnboundVar swapped tv1 maybe_ty1 ps_ty2 non_var_ty2
+ = -- Check that tv1 isn't a type-signature type variable
+ checkTcM (not (isSkolemTyVar tv1))
+ (failWithTcM (unifyWithSigErr tv1 ps_ty2)) `thenTc_`
+
+ -- Check that the kinds match
+ zonkTcType ps_ty2 `thenNF_Tc` \ ps_ty2' ->
+ checkKinds swapped tv1 ps_ty2' `thenTc_`
+
+ -- Occurs check
+ -- Basically we want to update tv1 := ps_ty2
+ -- because ps_ty2 has type-synonym info, which improves later error messages
+ --
+ -- But consider
+ -- type A a = ()
+ --
+ -- f :: (A a -> a -> ()) -> ()
+ -- f = \ _ -> ()
+ --
+ -- x :: ()
+ -- x = f (\ x p -> p x)
+ --
+ -- In the application (p x), we try to match "t" with "A t". If we go
+ -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
+ -- an infinite loop later.
+ -- But we should not reject the program, because A t = ().
+ -- Rather, we should bind t to () (= non_var_ty2).
+ --
+ -- That's why we have this two-state occurs-check
+ if not (tv1 `elemVarSet` tyVarsOfType ps_ty2') then
+ putTcTyVar tv1 ps_ty2' `thenNF_Tc_`
+ returnTc ()
+ else
+ zonkTcType non_var_ty2 `thenNF_Tc` \ non_var_ty2' ->
+ if not (tv1 `elemVarSet` tyVarsOfType non_var_ty2') then
+ -- This branch rarely succeeds, except in strange cases
+ -- like that in the example above
+ putTcTyVar tv1 non_var_ty2' `thenNF_Tc_`
+ returnTc ()
+ else
+ failWithTcM (unifyOccurCheck tv1 ps_ty2')
- | otherwise -- A different tyvar
- = tcGetTyVar tv2 `thenNF_Tc` \ maybe_ty2 ->
- case maybe_ty2 of
- Just ty2' -> occur_check ty2'
- other -> returnTc ()
checkKinds swapped tv1 ty2
- = tcAddErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
+-- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
+-- ty2 has been zonked at this stage
- -- We have to use tcTypeKind not just typeKind to get the
- -- kind of ty2, because there might be mutable kind variables
- -- in the way. For example, suppose that ty2 :: (a b), and
- -- the kind of 'a' is a kind variable 'k' that has (presumably)
- -- been unified with 'k1 -> k2'.
- tcTypeKind ty2 `thenNF_Tc` \ k2 ->
+ | tk1 `eqKind` liftedTypeKind && tk2 `eqKind` unliftedTypeKind
+ -- Check that we don't unify a lifted type variable with an
+ -- unlifted type: e.g. (id 3#) is illegal
+ = tcAddErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
+ unifyMisMatch k1 k2
- if swapped then
- unifyKind k2 (tyVarKind tv1)
- else
- unifyKind (tyVarKind tv1) k2
+ | otherwise
+ = -- Check that we aren't losing boxity info (shouldn't happen)
+ WARN (not (tk2 `hasMoreBoxityInfo` tk1),
+ (ppr tv1 <+> ppr tk1) $$ (ppr ty2 <+> ppr tk2))
+ returnTc ()
+ where
+ (k1,k2) | swapped = (tk2,tk1)
+ | otherwise = (tk1,tk2)
+ tk1 = tyVarKind tv1
+ tk2 = typeKind ty2
\end{code}
+
%************************************************************************
%* *
\subsection[Unify-fun]{@unifyFunTy@}
%* *
%************************************************************************
-@unifyFunTy@ is used to avoid the fruitless creation of type variables.
+@subFunTy@ and @unifyFunTy@ is used to avoid the fruitless
+creation of type variables.
+
+* subFunTy is used when we might be faced with a "hole" type variable,
+ in which case we should create two new holes.
+
+* unifyFunTy is used when we expect to encounter only "ordinary"
+ type variables, so we should create new ordinary type variables
\begin{code}
-unifyFunTy :: TcType -- Fail if ty isn't a function type
- -> TcM s (TcType, TcType) -- otherwise return arg and result types
+subFunTy :: TcSigmaType -- Fail if ty isn't a function type
+ -> TcM (TcType, TcType) -- otherwise return arg and result types
+subFunTy ty@(TyVarTy tyvar)
+
+ = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
+ case maybe_ty of
+ Just ty -> subFunTy ty
+ Nothing | isHoleTyVar tyvar
+ -> newHoleTyVarTy `thenNF_Tc` \ arg ->
+ newHoleTyVarTy `thenNF_Tc` \ res ->
+ putTcTyVar tyvar (mkFunTy arg res) `thenNF_Tc_`
+ returnTc (arg,res)
+ | otherwise
+ -> unify_fun_ty_help ty
+
+subFunTy ty
+ = case tcSplitFunTy_maybe ty of
+ Just arg_and_res -> returnTc arg_and_res
+ Nothing -> unify_fun_ty_help ty
+
+
+unifyFunTy :: TcPhiType -- Fail if ty isn't a function type
+ -> TcM (TcType, TcType) -- otherwise return arg and result types
unifyFunTy ty@(TyVarTy tyvar)
- = tcGetTyVar tyvar `thenNF_Tc` \ maybe_ty ->
+ = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
case maybe_ty of
Just ty' -> unifyFunTy ty'
- other -> unify_fun_ty_help ty
+ Nothing -> unify_fun_ty_help ty
unifyFunTy ty
- = case splitFunTy_maybe ty of
+ = case tcSplitFunTy_maybe ty of
Just arg_and_res -> returnTc arg_and_res
Nothing -> unify_fun_ty_help ty
unify_fun_ty_help ty -- Special cases failed, so revert to ordinary unification
- = newTyVarTy_OpenKind `thenNF_Tc` \ arg ->
- newTyVarTy_OpenKind `thenNF_Tc` \ res ->
+ = newTyVarTy openTypeKind `thenNF_Tc` \ arg ->
+ newTyVarTy openTypeKind `thenNF_Tc` \ res ->
unifyTauTy ty (mkFunTy arg res) `thenTc_`
returnTc (arg,res)
\end{code}
\begin{code}
unifyListTy :: TcType -- expected list type
- -> TcM s TcType -- list element type
+ -> TcM TcType -- list element type
unifyListTy ty@(TyVarTy tyvar)
- = tcGetTyVar tyvar `thenNF_Tc` \ maybe_ty ->
+ = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
case maybe_ty of
Just ty' -> unifyListTy ty'
- other -> unify_list_ty_help ty
+ other -> unify_list_ty_help ty
unifyListTy ty
- = case splitTyConApp_maybe ty of
+ = case tcSplitTyConApp_maybe ty of
Just (tycon, [arg_ty]) | tycon == listTyCon -> returnTc arg_ty
other -> unify_list_ty_help ty
unify_list_ty_help ty -- Revert to ordinary unification
- = newTyVarTy boxedTypeKind `thenNF_Tc` \ elt_ty ->
+ = newTyVarTy liftedTypeKind `thenNF_Tc` \ elt_ty ->
unifyTauTy ty (mkListTy elt_ty) `thenTc_`
returnTc elt_ty
\end{code}
\begin{code}
-unifyTupleTy :: Arity -> TcType -> TcM s [TcType]
-unifyTupleTy arity ty@(TyVarTy tyvar)
- = tcGetTyVar tyvar `thenNF_Tc` \ maybe_ty ->
+unifyTupleTy :: Boxity -> Arity -> TcType -> TcM [TcType]
+unifyTupleTy boxity arity ty@(TyVarTy tyvar)
+ = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
case maybe_ty of
- Just ty' -> unifyTupleTy arity ty'
- other -> unify_tuple_ty_help arity ty
-
-unifyTupleTy arity ty
- = case splitTyConApp_maybe ty of
- Just (tycon, arg_tys) | isTupleTyCon tycon
- && tyConArity tycon == arity
- -> returnTc arg_tys
- other -> unify_tuple_ty_help arity ty
-
-unify_tuple_ty_help arity ty
- = mapNF_Tc (\ _ -> newTyVarTy boxedTypeKind) [1..arity] `thenNF_Tc` \ arg_tys ->
- unifyTauTy ty (mkTupleTy arity arg_tys) `thenTc_`
+ Just ty' -> unifyTupleTy boxity arity ty'
+ other -> unify_tuple_ty_help boxity arity ty
+
+unifyTupleTy boxity arity ty
+ = case tcSplitTyConApp_maybe ty of
+ Just (tycon, arg_tys)
+ | isTupleTyCon tycon
+ && tyConArity tycon == arity
+ && tupleTyConBoxity tycon == boxity
+ -> returnTc arg_tys
+ other -> unify_tuple_ty_help boxity arity ty
+
+unify_tuple_ty_help boxity arity ty
+ = newTyVarTys arity kind `thenNF_Tc` \ arg_tys ->
+ unifyTauTy ty (mkTupleTy boxity arity arg_tys) `thenTc_`
returnTc arg_tys
+ where
+ kind | isBoxed boxity = liftedTypeKind
+ | otherwise = openTypeKind
\end{code}
+
+%************************************************************************
+%* *
+\subsection{Kind unification}
+%* *
+%************************************************************************
+
\begin{code}
-unifyUnboxedTupleTy :: Arity -> TcType -> TcM s [TcType]
-unifyUnboxedTupleTy arity ty@(TyVarTy tyvar)
- = tcGetTyVar tyvar `thenNF_Tc` \ maybe_ty ->
- case maybe_ty of
- Just ty' -> unifyUnboxedTupleTy arity ty'
- other -> unify_unboxed_tuple_ty_help arity ty
-
-unifyUnboxedTupleTy arity ty
- = case splitTyConApp_maybe ty of
- Just (tycon, arg_tys) | isUnboxedTupleTyCon tycon
- && tyConArity tycon == arity
- -> returnTc arg_tys
- other -> unify_tuple_ty_help arity ty
-
-unify_unboxed_tuple_ty_help arity ty
- = mapNF_Tc (\ _ -> newTyVarTy_OpenKind) [1..arity] `thenNF_Tc` \ arg_tys ->
- unifyTauTy ty (mkUnboxedTupleTy arity arg_tys) `thenTc_`
- returnTc arg_tys
-\end{code}
+unifyKind :: TcKind -- Expected
+ -> TcKind -- Actual
+ -> TcM ()
+unifyKind k1 k2
+ = tcAddErrCtxtM (unifyCtxt "kind" k1 k2) $
+ uTys k1 k1 k2 k2
-Make sure a kind is of the form (Type b) for some boxity b.
+unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
+unifyKinds [] [] = returnTc ()
+unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenTc_`
+ unifyKinds ks1 ks2
+unifyKinds _ _ = panic "unifyKinds: length mis-match"
+\end{code}
\begin{code}
-unifyTypeKind :: TcKind -> TcM s ()
-unifyTypeKind kind@(TyVarTy kv)
- = tcGetTyVar kv `thenNF_Tc` \ maybe_kind ->
- case maybe_kind of
- Just kind' -> unifyTypeKind kind'
- Nothing -> unify_type_kind_help kind
-
-unifyTypeKind kind
- = case splitTyConApp_maybe kind of
- Just (tycon, [_]) | tycon == typeCon -> returnTc ()
- other -> unify_type_kind_help kind
-
-unify_type_kind_help kind
- = newOpenTypeKind `thenNF_Tc` \ expected_kind ->
- unifyKind expected_kind kind
+unifyOpenTypeKind :: TcKind -> TcM ()
+-- Ensures that the argument kind is of the form (Type bx)
+-- for some boxity bx
+
+unifyOpenTypeKind ty@(TyVarTy tyvar)
+ = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
+ case maybe_ty of
+ Just ty' -> unifyOpenTypeKind ty'
+ other -> unify_open_kind_help ty
+
+unifyOpenTypeKind ty
+ | isTypeKind ty = returnTc ()
+ | otherwise = unify_open_kind_help ty
+
+unify_open_kind_help ty -- Revert to ordinary unification
+ = newBoxityVar `thenNF_Tc` \ boxity ->
+ unifyKind ty (mkTyConApp typeCon [boxity])
\end{code}
(env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
- = returnNF_Tc (env2, ptext SLIT("When matching types") <+>
- sep [quotes pp_expected, ptext SLIT("and"), quotes pp_actual])
+ -- tv1 is zonked already
+ = zonkTcType ty2 `thenNF_Tc` \ ty2' ->
+ returnNF_Tc (err ty2')
where
- (pp_expected, pp_actual) | swapped = (pp2, pp1)
- | otherwise = (pp1, pp2)
- (env1, tv1') = tidyTyVar tidy_env tv1
- (env2, ty2') = tidyOpenType env1 ty2
- pp1 = ppr tv1'
- pp2 = ppr ty2'
+ err ty2 = (env2, ptext SLIT("When matching types") <+>
+ sep [quotes pp_expected, ptext SLIT("and"), quotes pp_actual])
+ where
+ (pp_expected, pp_actual) | swapped = (pp2, pp1)
+ | otherwise = (pp1, pp2)
+ (env1, tv1') = tidyOpenTyVar tidy_env tv1
+ (env2, ty2') = tidyOpenType env1 ty2
+ pp1 = ppr tv1'
+ pp2 = ppr ty2'
unifyMisMatch ty1 ty2
- = (env2, hang (ptext SLIT("Couldn't match"))
- 4 (sep [quotes (ppr tidy_ty1), ptext SLIT("against"), quotes (ppr tidy_ty2)]))
+ = zonkTcType ty1 `thenNF_Tc` \ ty1' ->
+ zonkTcType ty2 `thenNF_Tc` \ ty2' ->
+ let
+ (env, [tidy_ty1, tidy_ty2]) = tidyOpenTypes emptyTidyEnv [ty1',ty2']
+ msg = hang (ptext SLIT("Couldn't match"))
+ 4 (sep [quotes (ppr tidy_ty1),
+ ptext SLIT("against"),
+ quotes (ppr tidy_ty2)])
+ in
+ failWithTcM (env, msg)
+
+unifyWithSigErr tyvar ty
+ = (env2, hang (ptext SLIT("Cannot unify the type-signature variable") <+> quotes (ppr tidy_tyvar))
+ 4 (ptext SLIT("with the type") <+> quotes (ppr tidy_ty)))
where
- (env1, tidy_ty1) = tidyOpenType emptyTidyEnv ty1
- (env2, tidy_ty2) = tidyOpenType env1 ty2
+ (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
+ (env2, tidy_ty) = tidyOpenType env1 ty
unifyOccurCheck tyvar ty
= (env2, hang (ptext SLIT("Occurs check: cannot construct the infinite type:"))
4 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
where
- (env1, tidy_tyvar) = tidyTyVar emptyTidyEnv tyvar
- (env2, tidy_ty) = tidyOpenType env1 ty
+ (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
+ (env2, tidy_ty) = tidyOpenType env1 ty
\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Checking signature type variables}
+%* *
+%************************************************************************
+
+@checkSigTyVars@ is used after the type in a type signature has been unified with
+the actual type found. It then checks that the type variables of the type signature
+are
+ (a) Still all type variables
+ eg matching signature [a] against inferred type [(p,q)]
+ [then a will be unified to a non-type variable]
+
+ (b) Still all distinct
+ eg matching signature [(a,b)] against inferred type [(p,p)]
+ [then a and b will be unified together]
+
+ (c) Not mentioned in the environment
+ 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.
+
+We used to have the notion of a "DontBind" type variable, which would
+only be bound to itself or nothing. Then points (a) and (b) were
+self-checking. But it gave rise to bogus consequential error messages.
+For example:
+
+ f = (*) -- Monomorphic
+
+ g :: Num a => a -> a
+ g x = f x x
+
+Here, we get a complaint when checking the type signature for g,
+that g isn't polymorphic enough; but then we get another one when
+dealing with the (Num x) context arising from f's definition;
+we try to unify x with Int (to default it), but find that x has already
+been unified with the DontBind variable "a" from g's signature.
+This is really a problem with side-effecting unification; we'd like to
+undo g's effects when its type signature fails, but unification is done
+by side effect, so we can't (easily).
+
+So we revert to ordinary type variables for signatures, and try to
+give a helpful message in checkSigTyVars.
+
+\begin{code}
+checkSigTyVars :: [TcTyVar] -- Universally-quantified type variables in the signature
+ -> TcTyVarSet -- Tyvars that are free in the type signature
+ -- Not necessarily zonked
+ -- These should *already* be in the free-in-env set,
+ -- and are used here only to improve the error message
+ -> TcM [TcTyVar] -- Zonked signature type variables
+
+checkSigTyVars [] free = returnTc []
+checkSigTyVars sig_tyvars free_tyvars
+ = zonkTcTyVars sig_tyvars `thenNF_Tc` \ sig_tys ->
+ tcGetGlobalTyVars `thenNF_Tc` \ globals ->
+
+ checkTcM (allDistinctTyVars sig_tys globals)
+ (complain sig_tys globals) `thenTc_`
+
+ returnTc (map (tcGetTyVar "checkSigTyVars") sig_tys)
+
+ where
+ complain sig_tys globals
+ = -- "check" checks each sig tyvar in turn
+ foldlNF_Tc check
+ (env2, emptyVarEnv, [])
+ (tidy_tvs `zip` tidy_tys) `thenNF_Tc` \ (env3, _, msgs) ->
+
+ failWithTcM (env3, main_msg $$ vcat msgs)
+ where
+ (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tyvars
+ (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
+
+ main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
+
+ check (tidy_env, acc, msgs) (sig_tyvar,ty)
+ -- sig_tyvar is from the signature;
+ -- ty is what you get if you zonk sig_tyvar and then tidy it
+ --
+ -- acc maps a zonked type variable back to a signature type variable
+ = case tcGetTyVar_maybe ty of {
+ Nothing -> -- Error (a)!
+ returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (quotes (ppr ty)) : msgs) ;
+
+ Just tv ->
+
+ case lookupVarEnv acc tv of {
+ Just sig_tyvar' -> -- Error (b)!
+ returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar thing : msgs)
+ where
+ thing = ptext SLIT("another quantified type variable") <+> quotes (ppr sig_tyvar')
+
+ ; Nothing ->
+
+ if tv `elemVarSet` globals -- Error (c) or (d)! Type variable escapes
+ -- The least comprehensible, so put it last
+ -- Game plan:
+ -- a) get the local TcIds and TyVars from the environment,
+ -- and pass them to find_globals (they might have tv free)
+ -- b) similarly, find any free_tyvars that mention tv
+ then tcGetEnv `thenNF_Tc` \ ve ->
+ find_globals tv tidy_env (tcLEnvElts ve) `thenNF_Tc` \ (tidy_env1, globs) ->
+ find_frees tv tidy_env1 [] (varSetElems free_tyvars) `thenNF_Tc` \ (tidy_env2, frees) ->
+ returnNF_Tc (tidy_env2, acc, escape_msg sig_tyvar tv globs frees : msgs)
+
+ else -- All OK
+ returnNF_Tc (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
+ }}
+
+-----------------------
+-- find_globals looks at the value environment and finds values
+-- whose types mention the offending type variable. It has to be
+-- careful to zonk the Id's type first, so it has to be in the monad.
+-- We must be careful to pass it a zonked type variable, too.
+
+find_globals :: Var
+ -> TidyEnv
+ -> [TcTyThing]
+ -> NF_TcM (TidyEnv, [SDoc])
+
+find_globals tv tidy_env things
+ = go tidy_env [] things
+ where
+ go tidy_env acc [] = returnNF_Tc (tidy_env, acc)
+ go tidy_env acc (thing : things)
+ = find_thing ignore_it tidy_env thing `thenNF_Tc` \ (tidy_env1, maybe_doc) ->
+ case maybe_doc of
+ Just d -> go tidy_env1 (d:acc) things
+ Nothing -> go tidy_env1 acc things
+
+ ignore_it ty = not (tv `elemVarSet` tyVarsOfType ty)
+
+-----------------------
+find_thing ignore_it tidy_env (ATcId id)
+ = zonkTcType (idType id) `thenNF_Tc` \ id_ty ->
+ if ignore_it id_ty then
+ returnNF_Tc (tidy_env, Nothing)
+ else let
+ (tidy_env', tidy_ty) = tidyOpenType tidy_env id_ty
+ msg = sep [ppr id <+> dcolon <+> ppr tidy_ty,
+ nest 2 (parens (ptext SLIT("bound at") <+>
+ ppr (getSrcLoc id)))]
+ in
+ returnNF_Tc (tidy_env', Just msg)
+
+find_thing ignore_it tidy_env (ATyVar tv)
+ = zonkTcTyVar tv `thenNF_Tc` \ tv_ty ->
+ if ignore_it tv_ty then
+ returnNF_Tc (tidy_env, Nothing)
+ else let
+ (tidy_env1, tv1) = tidyOpenTyVar tidy_env tv
+ (tidy_env2, tidy_ty) = tidyOpenType tidy_env1 tv_ty
+ msg = sep [ptext SLIT("Type variable") <+> quotes (ppr tv1) <+> eq_stuff, nest 2 bound_at]
+
+ eq_stuff | Just tv' <- Type.getTyVar_maybe tv_ty, tv == tv' = empty
+ | otherwise = equals <+> ppr tv_ty
+ -- It's ok to use Type.getTyVar_maybe because ty is zonked by now
+
+ bound_at = tyVarBindingInfo tv
+ in
+ returnNF_Tc (tidy_env2, Just msg)
+
+-----------------------
+find_frees tv tidy_env acc []
+ = returnNF_Tc (tidy_env, acc)
+find_frees tv tidy_env acc (ftv:ftvs)
+ = zonkTcTyVar ftv `thenNF_Tc` \ ty ->
+ if tv `elemVarSet` tyVarsOfType ty then
+ let
+ (tidy_env', ftv') = tidyOpenTyVar tidy_env ftv
+ in
+ find_frees tv tidy_env' (ftv':acc) ftvs
+ else
+ find_frees tv tidy_env acc ftvs
+
+
+escape_msg sig_tv tv globs frees
+ = mk_msg sig_tv <+> ptext SLIT("escapes") $$
+ if not (null globs) then
+ vcat [pp_it <+> ptext SLIT("is mentioned in the environment:"),
+ nest 2 (vcat globs)]
+ else if not (null frees) then
+ vcat [ptext SLIT("It is reachable from the type variable(s)") <+> pprQuotedList frees,
+ nest 2 (ptext SLIT("which") <+> is_are <+> ptext SLIT("free in the signature"))
+ ]
+ else
+ empty -- Sigh. It's really hard to give a good error message
+ -- all the time. One bad case is an existential pattern match
+ where
+ is_are | isSingleton frees = ptext SLIT("is")
+ | otherwise = ptext SLIT("are")
+ pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
+ | otherwise = ptext SLIT("It")
+
+ vcat_first :: Int -> [SDoc] -> SDoc
+ vcat_first n [] = empty
+ vcat_first 0 (x:xs) = text "...others omitted..."
+ vcat_first n (x:xs) = x $$ vcat_first (n-1) xs
+
+
+unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> thing
+mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
+\end{code}
+
+These two context are used with checkSigTyVars
+
+\begin{code}
+sigCtxt :: [TcTyVar] -> TcThetaType -> TcTauType
+ -> TidyEnv -> NF_TcM (TidyEnv, Message)
+sigCtxt sig_tyvars sig_theta sig_tau tidy_env
+ = zonkTcType sig_tau `thenNF_Tc` \ actual_tau ->
+ let
+ (env1, tidy_sig_tyvars) = tidyOpenTyVars tidy_env sig_tyvars
+ (env2, tidy_sig_rho) = tidyOpenType env1 (mkRhoTy sig_theta sig_tau)
+ (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
+ msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tyvars tidy_sig_rho),
+ ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
+ ]
+ in
+ returnNF_Tc (env3, msg)
+
+sigPatCtxt bound_tvs bound_ids tidy_env
+ = returnNF_Tc (env1,
+ sep [ptext SLIT("When checking a pattern that binds"),
+ nest 4 (vcat (zipWith ppr_id show_ids tidy_tys))])
+ where
+ show_ids = filter is_interesting bound_ids
+ is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs
+
+ (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
+ ppr_id id ty = ppr id <+> dcolon <+> ppr ty
+ -- Don't zonk the types so we get the separate, un-unified versions
+\end{code}
+
+