\begin{code}
module TcSMonad (
CanonicalCts, emptyCCan, andCCan, andCCans,
singleCCan, extendCCans, isEmptyCCan, isCTyEqCan,
isCDictCan_Maybe, isCIPCan_Maybe, isCFunEqCan_Maybe,
isCFrozenErr,
CanonicalCt(..), Xi, tyVarsOfCanonical, tyVarsOfCanonicals, tyVarsOfCDicts,
deCanonicalise, mkFrozenError,
isWanted, isGiven, isDerived,
isGivenCt, isWantedCt, isDerivedCt, pprFlavorArising,
isFlexiTcsTv,
canRewrite, canSolve,
combineCtLoc, mkGivenFlavor, mkWantedFlavor,
getWantedLoc,
TcS, runTcS, failTcS, panicTcS, traceTcS,
traceFireTcS, bumpStepCountTcS,
tryTcS, nestImplicTcS, recoverTcS, wrapErrTcS, wrapWarnTcS,
SimplContext(..), isInteractive, simplEqsOnly, performDefaulting,
newEvVar, newCoVar, newGivenCoVar,
newDerivedId,
newIPVar, newDictVar, newKindConstraint,
setCoBind, setIPBind, setDictBind, setEvBind,
setWantedTyBind,
getInstEnvs, getFamInstEnvs,
getTopEnv, getGblEnv, getTcEvBinds, getUntouchables,
getTcEvBindsBag, getTcSContext, getTcSTyBinds, getTcSTyBindsMap,
newFlattenSkolemTy,
instDFunTypes,
instDFunConstraints,
newFlexiTcSTy, instFlexiTcS,
compatKind,
TcsUntouchables,
isTouchableMetaTyVar,
isTouchableMetaTyVar_InRange,
getDefaultInfo, getDynFlags,
matchClass, matchFam, MatchInstResult (..),
checkWellStagedDFun,
warnTcS,
pprEq
) where
#include "HsVersions.h"
import HscTypes
import BasicTypes
import Inst
import InstEnv
import FamInst
import FamInstEnv
import qualified TcRnMonad as TcM
import qualified TcMType as TcM
import qualified TcEnv as TcM
( checkWellStaged, topIdLvl, tcLookupFamInst, tcGetDefaultTys )
import TcType
import DynFlags
import Coercion
import Class
import TyCon
import TypeRep
import Name
import Var
import VarEnv
import Outputable
import Bag
import MonadUtils
import VarSet
import FastString
import HsBinds
import Id
import TcRnTypes
import Control.Monad
import Data.IORef
\end{code}
%************************************************************************
%* *
%* Canonical constraints *
%* *
%* These are the constraints the low-level simplifier works with *
%* *
%************************************************************************
\begin{code}
type Xi = Type
type CanonicalCts = Bag CanonicalCt
data CanonicalCt
= CDictCan {
cc_id :: EvVar,
cc_flavor :: CtFlavor,
cc_class :: Class,
cc_tyargs :: [Xi]
}
| CIPCan {
cc_id :: EvVar,
cc_flavor :: CtFlavor,
cc_ip_nm :: IPName Name,
cc_ip_ty :: TcTauType
}
| CTyEqCan {
cc_id :: EvVar,
cc_flavor :: CtFlavor,
cc_tyvar :: TcTyVar,
cc_rhs :: Xi
}
| CFunEqCan {
cc_id :: EvVar,
cc_flavor :: CtFlavor,
cc_fun :: TyCon,
cc_tyargs :: [Xi],
cc_rhs :: Xi
}
| CFrozenErr {
cc_id :: EvVar,
cc_flavor :: CtFlavor
}
mkFrozenError :: CtFlavor -> EvVar -> CanonicalCt
mkFrozenError fl ev = CFrozenErr { cc_id = ev, cc_flavor = fl }
compatKind :: Kind -> Kind -> Bool
compatKind k1 k2 = k1 `isSubKind` k2 || k2 `isSubKind` k1
deCanonicalise :: CanonicalCt -> FlavoredEvVar
deCanonicalise ct = mkEvVarX (cc_id ct) (cc_flavor ct)
tyVarsOfCanonical :: CanonicalCt -> TcTyVarSet
tyVarsOfCanonical (CTyEqCan { cc_tyvar = tv, cc_rhs = xi }) = extendVarSet (tyVarsOfType xi) tv
tyVarsOfCanonical (CFunEqCan { cc_tyargs = tys, cc_rhs = xi }) = tyVarsOfTypes (xi:tys)
tyVarsOfCanonical (CDictCan { cc_tyargs = tys }) = tyVarsOfTypes tys
tyVarsOfCanonical (CIPCan { cc_ip_ty = ty }) = tyVarsOfType ty
tyVarsOfCanonical (CFrozenErr { cc_id = ev }) = tyVarsOfEvVar ev
tyVarsOfCDict :: CanonicalCt -> TcTyVarSet
tyVarsOfCDict (CDictCan { cc_tyargs = tys }) = tyVarsOfTypes tys
tyVarsOfCDict _ct = emptyVarSet
tyVarsOfCDicts :: CanonicalCts -> TcTyVarSet
tyVarsOfCDicts = foldrBag (unionVarSet . tyVarsOfCDict) emptyVarSet
tyVarsOfCanonicals :: CanonicalCts -> TcTyVarSet
tyVarsOfCanonicals = foldrBag (unionVarSet . tyVarsOfCanonical) emptyVarSet
instance Outputable CanonicalCt where
ppr (CDictCan d fl cls tys)
= ppr fl <+> ppr d <+> dcolon <+> pprClassPred cls tys
ppr (CIPCan ip fl ip_nm ty)
= ppr fl <+> ppr ip <+> dcolon <+> parens (ppr ip_nm <> dcolon <> ppr ty)
ppr (CTyEqCan co fl tv ty)
= ppr fl <+> ppr co <+> dcolon <+> pprEqPred (mkTyVarTy tv, ty)
ppr (CFunEqCan co fl tc tys ty)
= ppr fl <+> ppr co <+> dcolon <+> pprEqPred (mkTyConApp tc tys, ty)
ppr (CFrozenErr co fl)
= ppr fl <+> pprEvVarWithType co
\end{code}
Note [Canonical implicit parameter constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The type in a canonical implicit parameter constraint doesn't need to
be a xi (type-function-free type) since we can defer the flattening
until checking this type for equality with another type. If we
encounter two IP constraints with the same name, they MUST have the
same type, and at that point we can generate a flattened equality
constraint between the types. (On the other hand, the types in two
class constraints for the same class MAY be equal, so they need to be
flattened in the first place to facilitate comparing them.)
\begin{code}
singleCCan :: CanonicalCt -> CanonicalCts
singleCCan = unitBag
andCCan :: CanonicalCts -> CanonicalCts -> CanonicalCts
andCCan = unionBags
extendCCans :: CanonicalCts -> CanonicalCt -> CanonicalCts
extendCCans = snocBag
andCCans :: [CanonicalCts] -> CanonicalCts
andCCans = unionManyBags
emptyCCan :: CanonicalCts
emptyCCan = emptyBag
isEmptyCCan :: CanonicalCts -> Bool
isEmptyCCan = isEmptyBag
isCTyEqCan :: CanonicalCt -> Bool
isCTyEqCan (CTyEqCan {}) = True
isCTyEqCan (CFunEqCan {}) = False
isCTyEqCan _ = False
isCDictCan_Maybe :: CanonicalCt -> Maybe Class
isCDictCan_Maybe (CDictCan {cc_class = cls }) = Just cls
isCDictCan_Maybe _ = Nothing
isCIPCan_Maybe :: CanonicalCt -> Maybe (IPName Name)
isCIPCan_Maybe (CIPCan {cc_ip_nm = nm }) = Just nm
isCIPCan_Maybe _ = Nothing
isCFunEqCan_Maybe :: CanonicalCt -> Maybe TyCon
isCFunEqCan_Maybe (CFunEqCan { cc_fun = tc }) = Just tc
isCFunEqCan_Maybe _ = Nothing
isCFrozenErr :: CanonicalCt -> Bool
isCFrozenErr (CFrozenErr {}) = True
isCFrozenErr _ = False
\end{code}
%************************************************************************
%* *
CtFlavor
The "flavor" of a canonical constraint
%* *
%************************************************************************
\begin{code}
getWantedLoc :: CanonicalCt -> WantedLoc
getWantedLoc ct
= ASSERT (isWanted (cc_flavor ct))
case cc_flavor ct of
Wanted wl -> wl
_ -> pprPanic "Can't get WantedLoc of non-wanted constraint!" empty
isWantedCt :: CanonicalCt -> Bool
isWantedCt ct = isWanted (cc_flavor ct)
isGivenCt :: CanonicalCt -> Bool
isGivenCt ct = isGiven (cc_flavor ct)
isDerivedCt :: CanonicalCt -> Bool
isDerivedCt ct = isDerived (cc_flavor ct)
canSolve :: CtFlavor -> CtFlavor -> Bool
canSolve (Given {}) _ = True
canSolve (Wanted {}) (Derived {}) = True
canSolve (Wanted {}) (Wanted {}) = True
canSolve (Derived {}) (Derived {}) = True
canSolve _ _ = False
canRewrite :: CtFlavor -> CtFlavor -> Bool
canRewrite = canSolve
combineCtLoc :: CtFlavor -> CtFlavor -> WantedLoc
combineCtLoc (Wanted loc) _ = loc
combineCtLoc _ (Wanted loc) = loc
combineCtLoc (Derived loc ) _ = loc
combineCtLoc _ (Derived loc ) = loc
combineCtLoc _ _ = panic "combineCtLoc: both given"
mkGivenFlavor :: CtFlavor -> SkolemInfo -> CtFlavor
mkGivenFlavor (Wanted loc) sk = Given (setCtLocOrigin loc sk)
mkGivenFlavor (Derived loc) sk = Given (setCtLocOrigin loc sk)
mkGivenFlavor (Given loc) sk = Given (setCtLocOrigin loc sk)
mkWantedFlavor :: CtFlavor -> CtFlavor
mkWantedFlavor (Wanted loc) = Wanted loc
mkWantedFlavor (Derived loc) = Wanted loc
mkWantedFlavor fl@(Given {}) = pprPanic "mkWantedFlavour" (ppr fl)
\end{code}
%************************************************************************
%* *
%* The TcS solver monad *
%* *
%************************************************************************
Note [The TcS monad]
~~~~~~~~~~~~~~~~~~~~
The TcS monad is a weak form of the main Tc monad
All you can do is
* fail
* allocate new variables
* fill in evidence variables
Filling in a dictionary evidence variable means to create a binding
for it, so TcS carries a mutable location where the binding can be
added. This is initialised from the innermost implication constraint.
\begin{code}
data TcSEnv
= TcSEnv {
tcs_ev_binds :: EvBindsVar,
tcs_ty_binds :: IORef (TyVarEnv (TcTyVar, TcType)),
tcs_context :: SimplContext,
tcs_untch :: TcsUntouchables,
tcs_ic_depth :: Int,
tcs_count :: IORef Int
}
type TcsUntouchables = (Untouchables,TcTyVarSet)
\end{code}
\begin{code}
data SimplContext
= SimplInfer
| SimplRuleLhs
| SimplInteractive
| SimplCheck
deriving Eq
instance Outputable SimplContext where
ppr SimplInfer = ptext (sLit "SimplInfer")
ppr SimplRuleLhs = ptext (sLit "SimplRuleLhs")
ppr SimplInteractive = ptext (sLit "SimplInteractive")
ppr SimplCheck = ptext (sLit "SimplCheck")
isInteractive :: SimplContext -> Bool
isInteractive SimplInteractive = True
isInteractive _ = False
simplEqsOnly :: SimplContext -> Bool
simplEqsOnly SimplRuleLhs = True
simplEqsOnly _ = False
performDefaulting :: SimplContext -> Bool
performDefaulting SimplInfer = False
performDefaulting SimplRuleLhs = False
performDefaulting SimplInteractive = True
performDefaulting SimplCheck = True
newtype TcS a = TcS { unTcS :: TcSEnv -> TcM a }
instance Functor TcS where
fmap f m = TcS $ fmap f . unTcS m
instance Monad TcS where
return x = TcS (\_ -> return x)
fail err = TcS (\_ -> fail err)
m >>= k = TcS (\ebs -> unTcS m ebs >>= \r -> unTcS (k r) ebs)
wrapTcS :: TcM a -> TcS a
wrapTcS = TcS . const
wrapErrTcS :: TcM a -> TcS a
wrapErrTcS = wrapTcS
wrapWarnTcS :: TcM a -> TcS a
wrapWarnTcS = wrapTcS
failTcS, panicTcS :: SDoc -> TcS a
failTcS = wrapTcS . TcM.failWith
panicTcS doc = pprPanic "TcCanonical" doc
traceTcS :: String -> SDoc -> TcS ()
traceTcS herald doc = TcS $ \_env -> TcM.traceTc herald doc
bumpStepCountTcS :: TcS ()
bumpStepCountTcS = TcS $ \env -> do { let ref = tcs_count env
; n <- TcM.readTcRef ref
; TcM.writeTcRef ref (n+1) }
traceFireTcS :: Int -> SDoc -> TcS ()
traceFireTcS depth doc
= TcS $ \env ->
TcM.ifDOptM Opt_D_dump_cs_trace $
do { n <- TcM.readTcRef (tcs_count env)
; let msg = int n
<> text (replicate (tcs_ic_depth env) '>')
<> brackets (int depth) <+> doc
; TcM.dumpTcRn msg }
runTcS :: SimplContext
-> Untouchables
-> TcS a
-> TcM (a, Bag EvBind)
runTcS context untouch tcs
= do { ty_binds_var <- TcM.newTcRef emptyVarEnv
; ev_binds_var@(EvBindsVar evb_ref _) <- TcM.newTcEvBinds
; step_count <- TcM.newTcRef 0
; let env = TcSEnv { tcs_ev_binds = ev_binds_var
, tcs_ty_binds = ty_binds_var
, tcs_context = context
, tcs_untch = (untouch, emptyVarSet)
, tcs_count = step_count
, tcs_ic_depth = 0
}
; res <- unTcS tcs env
; ty_binds <- TcM.readTcRef ty_binds_var
; mapM_ do_unification (varEnvElts ty_binds)
#ifdef DEBUG
; count <- TcM.readTcRef step_count
; TcM.dumpTcRn (ptext (sLit "Constraint solver steps =") <+> int count)
#endif
; ev_binds <- TcM.readTcRef evb_ref
; return (res, evBindMapBinds ev_binds) }
where
do_unification (tv,ty) = TcM.writeMetaTyVar tv ty
nestImplicTcS :: EvBindsVar -> TcsUntouchables -> TcS a -> TcS a
nestImplicTcS ref (inner_range, inner_tcs) (TcS thing_inside)
= TcS $ \ TcSEnv { tcs_ty_binds = ty_binds
, tcs_untch = (_outer_range, outer_tcs)
, tcs_count = count
, tcs_ic_depth = idepth
, tcs_context = ctxt } ->
let
inner_untch = (inner_range, outer_tcs `unionVarSet` inner_tcs)
nest_env = TcSEnv { tcs_ev_binds = ref
, tcs_ty_binds = ty_binds
, tcs_untch = inner_untch
, tcs_count = count
, tcs_ic_depth = idepth+1
, tcs_context = ctxtUnderImplic ctxt }
in
thing_inside nest_env
recoverTcS :: TcS a -> TcS a -> TcS a
recoverTcS (TcS recovery_code) (TcS thing_inside)
= TcS $ \ env ->
TcM.recoverM (recovery_code env) (thing_inside env)
ctxtUnderImplic :: SimplContext -> SimplContext
ctxtUnderImplic SimplRuleLhs = SimplCheck
ctxtUnderImplic ctxt = ctxt
tryTcS :: TcS a -> TcS a
tryTcS tcs
= TcS (\env -> do { ty_binds_var <- TcM.newTcRef emptyVarEnv
; ev_binds_var <- TcM.newTcEvBinds
; let env1 = env { tcs_ev_binds = ev_binds_var
, tcs_ty_binds = ty_binds_var }
; unTcS tcs env1 })
getDynFlags :: TcS DynFlags
getDynFlags = wrapTcS TcM.getDOpts
getTcSContext :: TcS SimplContext
getTcSContext = TcS (return . tcs_context)
getTcEvBinds :: TcS EvBindsVar
getTcEvBinds = TcS (return . tcs_ev_binds)
getUntouchables :: TcS TcsUntouchables
getUntouchables = TcS (return . tcs_untch)
getTcSTyBinds :: TcS (IORef (TyVarEnv (TcTyVar, TcType)))
getTcSTyBinds = TcS (return . tcs_ty_binds)
getTcSTyBindsMap :: TcS (TyVarEnv (TcTyVar, TcType))
getTcSTyBindsMap = getTcSTyBinds >>= wrapTcS . (TcM.readTcRef)
getTcEvBindsBag :: TcS EvBindMap
getTcEvBindsBag
= do { EvBindsVar ev_ref _ <- getTcEvBinds
; wrapTcS $ TcM.readTcRef ev_ref }
setCoBind :: CoVar -> Coercion -> TcS ()
setCoBind cv co = setEvBind cv (EvCoercion co)
setWantedTyBind :: TcTyVar -> TcType -> TcS ()
setWantedTyBind tv ty
= do { ref <- getTcSTyBinds
; wrapTcS $
do { ty_binds <- TcM.readTcRef ref
#ifdef DEBUG
; TcM.checkErr (not (tv `elemVarEnv` ty_binds)) $
vcat [ text "TERRIBLE ERROR: double set of meta type variable"
, ppr tv <+> text ":=" <+> ppr ty
, text "Old value =" <+> ppr (lookupVarEnv_NF ty_binds tv)]
#endif
; TcM.writeTcRef ref (extendVarEnv ty_binds tv (tv,ty)) } }
setIPBind :: EvVar -> EvTerm -> TcS ()
setIPBind = setEvBind
setDictBind :: EvVar -> EvTerm -> TcS ()
setDictBind = setEvBind
setEvBind :: EvVar -> EvTerm -> TcS ()
setEvBind ev rhs
= do { tc_evbinds <- getTcEvBinds
; wrapTcS (TcM.addTcEvBind tc_evbinds ev rhs) }
warnTcS :: CtLoc orig -> Bool -> SDoc -> TcS ()
warnTcS loc warn_if doc
| warn_if = wrapTcS $ TcM.setCtLoc loc $ TcM.addWarnTc doc
| otherwise = return ()
getDefaultInfo :: TcS (SimplContext, [Type], (Bool, Bool))
getDefaultInfo
= do { ctxt <- getTcSContext
; (tys, flags) <- wrapTcS (TcM.tcGetDefaultTys (isInteractive ctxt))
; return (ctxt, tys, flags) }
getInstEnvs :: TcS (InstEnv, InstEnv)
getInstEnvs = wrapTcS $ Inst.tcGetInstEnvs
getFamInstEnvs :: TcS (FamInstEnv, FamInstEnv)
getFamInstEnvs = wrapTcS $ FamInst.tcGetFamInstEnvs
getTopEnv :: TcS HscEnv
getTopEnv = wrapTcS $ TcM.getTopEnv
getGblEnv :: TcS TcGblEnv
getGblEnv = wrapTcS $ TcM.getGblEnv
checkWellStagedDFun :: PredType -> DFunId -> WantedLoc -> TcS ()
checkWellStagedDFun pred dfun_id loc
= wrapTcS $ TcM.setCtLoc loc $
do { use_stage <- TcM.getStage
; TcM.checkWellStaged pp_thing bind_lvl (thLevel use_stage) }
where
pp_thing = ptext (sLit "instance for") <+> quotes (ppr pred)
bind_lvl = TcM.topIdLvl dfun_id
pprEq :: TcType -> TcType -> SDoc
pprEq ty1 ty2 = pprPred $ mkEqPred (ty1,ty2)
isTouchableMetaTyVar :: TcTyVar -> TcS Bool
isTouchableMetaTyVar tv
= do { untch <- getUntouchables
; return $ isTouchableMetaTyVar_InRange untch tv }
isTouchableMetaTyVar_InRange :: TcsUntouchables -> TcTyVar -> Bool
isTouchableMetaTyVar_InRange (untch,untch_tcs) tv
= case tcTyVarDetails tv of
MetaTv TcsTv _ -> not (tv `elemVarSet` untch_tcs)
MetaTv {} -> inTouchableRange untch tv
_ -> False
\end{code}
Note [Touchable meta type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Meta type variables allocated *by the constraint solver itself* are always
touchable. Example:
instance C a b => D [a] where...
if we use this instance declaration we "make up" a fresh meta type
variable for 'b', which we must later guess. (Perhaps C has a
functional dependency.) But since we aren't in the constraint *generator*
we can't allocate a Unique in the touchable range for this implication
constraint. Instead, we mark it as a "TcsTv", which makes it always-touchable.
\begin{code}
newFlattenSkolemTy :: TcType -> TcS TcType
newFlattenSkolemTy ty = mkTyVarTy <$> newFlattenSkolemTyVar ty
newFlattenSkolemTyVar :: TcType -> TcS TcTyVar
newFlattenSkolemTyVar ty
= do { tv <- wrapTcS $ do { uniq <- TcM.newUnique
; let name = TcM.mkTcTyVarName uniq (fsLit "f")
; return $ mkTcTyVar name (typeKind ty) (FlatSkol ty) }
; traceTcS "New Flatten Skolem Born" $
(ppr tv <+> text "[:= " <+> ppr ty <+> text "]")
; return tv }
instDFunTypes :: [Either TyVar TcType] -> TcS [TcType]
instDFunTypes mb_inst_tys
= mapM inst_tv mb_inst_tys
where
inst_tv :: Either TyVar TcType -> TcS Type
inst_tv (Left tv) = mkTyVarTy <$> instFlexiTcS tv
inst_tv (Right ty) = return ty
instDFunConstraints :: TcThetaType -> TcS [EvVar]
instDFunConstraints preds = wrapTcS $ TcM.newWantedEvVars preds
instFlexiTcS :: TyVar -> TcS TcTyVar
instFlexiTcS tv = instFlexiTcSHelper (tyVarName tv) (tyVarKind tv)
newFlexiTcSTy :: Kind -> TcS TcType
newFlexiTcSTy knd
= wrapTcS $
do { uniq <- TcM.newUnique
; ref <- TcM.newMutVar Flexi
; let name = TcM.mkTcTyVarName uniq (fsLit "uf")
; return $ mkTyVarTy (mkTcTyVar name knd (MetaTv TcsTv ref)) }
isFlexiTcsTv :: TyVar -> Bool
isFlexiTcsTv tv
| not (isTcTyVar tv) = False
| MetaTv TcsTv _ <- tcTyVarDetails tv = True
| otherwise = False
newKindConstraint :: TcTyVar -> Kind -> TcS CoVar
newKindConstraint tv knd
= do { tv_k <- instFlexiTcSHelper (tyVarName tv) knd
; let ty_k = mkTyVarTy tv_k
; co_var <- newCoVar (mkTyVarTy tv) ty_k
; return co_var }
instFlexiTcSHelper :: Name -> Kind -> TcS TcTyVar
instFlexiTcSHelper tvname tvkind
= wrapTcS $
do { uniq <- TcM.newUnique
; ref <- TcM.newMutVar Flexi
; let name = setNameUnique tvname uniq
kind = tvkind
; return (mkTcTyVar name kind (MetaTv TcsTv ref)) }
newEvVar :: TcPredType -> TcS EvVar
newEvVar pty = wrapTcS $ TcM.newEvVar pty
newDerivedId :: TcPredType -> TcS EvVar
newDerivedId pty = wrapTcS $ TcM.newEvVar pty
newGivenCoVar :: TcType -> TcType -> Coercion -> TcS EvVar
newGivenCoVar ty1 ty2 co
= do { cv <- newCoVar ty1 ty2
; setEvBind cv (EvCoercion co)
; return cv }
newCoVar :: TcType -> TcType -> TcS EvVar
newCoVar ty1 ty2 = wrapTcS $ TcM.newCoVar ty1 ty2
newIPVar :: IPName Name -> TcType -> TcS EvVar
newIPVar nm ty = wrapTcS $ TcM.newIP nm ty
newDictVar :: Class -> [TcType] -> TcS EvVar
newDictVar cl tys = wrapTcS $ TcM.newDict cl tys
\end{code}
\begin{code}
data MatchInstResult mi
= MatchInstNo
| MatchInstSingle mi
| MatchInstMany
matchClass :: Class -> [Type] -> TcS (MatchInstResult (DFunId, [Either TyVar TcType]))
matchClass clas tys
= do { let pred = mkClassPred clas tys
; instEnvs <- getInstEnvs
; case lookupInstEnv instEnvs clas tys of {
([], unifs)
-> do { traceTcS "matchClass not matching"
(vcat [ text "dict" <+> ppr pred,
text "unifs" <+> ppr unifs ])
; return MatchInstNo
} ;
([(ispec, inst_tys)], [])
-> do { let dfun_id = is_dfun ispec
; traceTcS "matchClass success"
(vcat [text "dict" <+> ppr pred,
text "witness" <+> ppr dfun_id
<+> ppr (idType dfun_id), ppr instEnvs ])
; return $ MatchInstSingle (dfun_id, inst_tys)
} ;
(matches, unifs)
-> do { traceTcS "matchClass multiple matches, deferring choice"
(vcat [text "dict" <+> ppr pred,
text "matches" <+> ppr matches,
text "unifs" <+> ppr unifs])
; return MatchInstMany
}
}
}
matchFam :: TyCon
-> [Type]
-> TcS (MatchInstResult (TyCon, [Type]))
matchFam tycon args
= do { mb <- wrapTcS $ TcM.tcLookupFamInst tycon args
; case mb of
Nothing -> return MatchInstNo
Just res -> return $ MatchInstSingle res
}
\end{code}