\begin{code}
module TcInteract (
solveInteract, AtomicInert,
- InertSet, emptyInert, extendInertSet, extractUnsolved, solveOne,
+ InertSet, emptyInert, updInertSet, extractUnsolved, solveOne,
listToWorkList
) where
#include "HsVersions.h"
+
import BasicTypes
import TcCanonical
import VarSet
import Type
+import TypeRep
import Id
import Var
import TcErrors
import TcSMonad
import qualified Bag as Bag
+import qualified Data.Map as Map
+import Maybes
+
import Control.Monad( zipWithM, unless )
import FastString ( sLit )
import DynFlags
\end{code}
-Note [InsertSet invariants]
+Note [InertSet invariants]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
An InertSet is a bag of canonical constraints, with the following invariants:
Note that we must switch wanted inert items to given when going under an
implication constraint (when in top-level inference mode).
+Note [InertSet FlattenSkolemEqClass]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The inert_fsks field of the inert set contains an "inverse map" of all the
+flatten skolem equalities in the inert set. For instance, if inert_cts looks
+like this:
+
+ fsk1 ~ fsk2
+ fsk3 ~ fsk2
+ fsk4 ~ fsk5
+
+Then, the inert_fsks fields holds the following map:
+ fsk2 |-> { fsk1, fsk3 }
+ fsk5 |-> { fsk4 }
+Along with the necessary coercions to convert fsk1 and fsk3 back to fsk2
+and fsk4 back to fsk5. Hence, the invariants of the inert_fsks field are:
+
+ (a) All TcTyVars in the domain and range of inert_fsks are flatten skolems
+ (b) All TcTyVars in the domain of inert_fsk occur naked as rhs in some
+ equalities of inert_cts
+ (c) For every mapping fsk1 |-> { (fsk2,co), ... } it must be:
+ co : fsk2 ~ fsk1
+
+The role of the inert_fsks is to make it easy to maintain the equivalence
+class of each flatten skolem, which is much needed to correctly do spontaneous
+solving. See Note [Loopy Spontaneous Solving]
\begin{code}
-- See Note [InertSet invariants]
+data InertSet
+ = IS { inert_cts :: Bag.Bag CanonicalCt
+ , inert_fsks :: Map.Map TcTyVar [(TcTyVar,Coercion)] }
+ -- See Note [InertSet FlattenSkolemEqClass]
-newtype InertSet = IS (Bag.Bag CanonicalCt)
instance Outputable InertSet where
- ppr (IS cts) = vcat (map ppr (Bag.bagToList cts))
+ ppr is = vcat [ vcat (map ppr (Bag.bagToList $ inert_cts is))
+ , vcat (map (\(v,rest) -> ppr v <+> text "|->" <+> hsep (map (ppr.fst) rest))
+ (Map.toList $ inert_fsks is)
+ )
+ ]
+
+emptyInert :: InertSet
+emptyInert = IS { inert_cts = Bag.emptyBag, inert_fsks = Map.empty }
+
+updInertSet :: InertSet -> AtomicInert -> InertSet
+-- Introduces an element in the inert set for the first time
+updInertSet (IS { inert_cts = cts, inert_fsks = fsks })
+ item@(CTyEqCan { cc_id = cv
+ , cc_tyvar = tv1
+ , cc_rhs = xi })
+ | Just tv2 <- tcGetTyVar_maybe xi,
+ FlatSkol {} <- tcTyVarDetails tv1,
+ FlatSkol {} <- tcTyVarDetails tv2
+ = let cts' = cts `Bag.snocBag` item
+ fsks' = Map.insertWith (++) tv2 [(tv1, mkCoVarCoercion cv)] fsks
+ -- See Note [InertSet FlattenSkolemEqClass]
+ in IS { inert_cts = cts', inert_fsks = fsks' }
+updInertSet (IS { inert_cts = cts
+ , inert_fsks = fsks }) item
+ = let cts' = cts `Bag.snocBag` item
+ in IS { inert_cts = cts', inert_fsks = fsks }
+
+foldlInertSetM :: (Monad m) => (a -> AtomicInert -> m a) -> a -> InertSet -> m a
+foldlInertSetM k z (IS { inert_cts = cts })
+ = Bag.foldlBagM k z cts
+
+extractUnsolved :: InertSet -> (InertSet, CanonicalCts)
+extractUnsolved is@(IS {inert_cts = cts})
+ = (is { inert_cts = cts'}, unsolved)
+ where (unsolved, cts') = Bag.partitionBag isWantedCt cts
+
+
+getFskEqClass :: InertSet -> TcTyVar -> [(TcTyVar,Coercion)]
+-- Precondition: tv is a FlatSkol. See Note [InertSet FlattenSkolemEqClass]
+getFskEqClass (IS { inert_cts = cts, inert_fsks = fsks }) tv
+ = case lkpTyEqCanByLhs of
+ Nothing -> fromMaybe [] (Map.lookup tv fsks)
+ Just ceq ->
+ case tcGetTyVar_maybe (cc_rhs ceq) of
+ Just tv_rhs | FlatSkol {} <- tcTyVarDetails tv_rhs
+ -> let ceq_co = mkSymCoercion $ mkCoVarCoercion (cc_id ceq)
+ mk_co (v,c) = (v, mkTransCoercion c ceq_co)
+ in (tv_rhs, ceq_co): map mk_co (fromMaybe [] $ Map.lookup tv fsks)
+ _ -> []
+ where lkpTyEqCanByLhs = Bag.foldlBag lkp Nothing cts
+ lkp :: Maybe CanonicalCt -> CanonicalCt -> Maybe CanonicalCt
+ lkp Nothing ct@(CTyEqCan {cc_tyvar = tv'}) | tv' == tv = Just ct
+ lkp other _ct = other
+
+
+isWantedCt :: CanonicalCt -> Bool
+isWantedCt ct = isWanted (cc_flavor ct)
{- TODO: Later ...
data Inert = IS { class_inerts :: FiniteMap Class Atomics
Later should we also separate out givens and wanteds?
-}
-emptyInert :: InertSet
-emptyInert = IS Bag.emptyBag
-
-extendInertSet :: InertSet -> AtomicInert -> InertSet
-extendInertSet (IS cts) item = IS (cts `Bag.snocBag` item)
-
-foldlInertSetM :: (Monad m) => (a -> AtomicInert -> m a) -> a -> InertSet -> m a
-foldlInertSetM k z (IS cts) = Bag.foldlBagM k z cts
-
-extractUnsolved :: InertSet -> (InertSet, CanonicalCts)
-extractUnsolved (IS cts)
- = (IS cts', unsolved)
- where (unsolved, cts') = Bag.partitionBag isWantedCt cts
-
-isWantedCt :: CanonicalCt -> Bool
-isWantedCt ct = isWanted (cc_flavor ct)
\end{code}
Note [Touchables and givens]
type AtomicInert = CanonicalCt -- constraint pulled from InertSet
type WorkItem = CanonicalCt -- constraint pulled from WorkList
-type SWorkItem = WorkItem -- a work item we know is solved
type WorkList = CanonicalCts -- A mixture of Given, Wanted, and Solved
+type SWorkList = WorkList -- A worklist of solved
listToWorkList :: [WorkItem] -> WorkList
emptyWorkList :: WorkList
emptyWorkList = Bag.emptyBag
+singletonWorkList :: CanonicalCt -> WorkList
+singletonWorkList ct = singleCCan ct
+
data StopOrContinue
= Stop -- Work item is consumed
| ContinueWith WorkItem -- Not consumed
; let new_inert
= case sr_stop itr_out of
Stop -> sr_inerts itr_out
- ContinueWith item -> sr_inerts itr_out `extendInertSet` item
+ ContinueWith item -> sr_inerts itr_out `updInertSet` item
; return (new_inert, sr_new_work itr_out) }
where
run_pipeline :: [(String, SimplifierStage)]
\begin{code}
spontaneousSolveStage :: SimplifierStage
spontaneousSolveStage workItem inerts
- = do { mSolve <- trySpontaneousSolve workItem
+ = do { mSolve <- trySpontaneousSolve workItem inerts
; case mSolve of
Nothing -> -- no spontaneous solution for him, keep going
return $ SR { sr_new_work = emptyWorkList
, sr_inerts = inerts
, sr_stop = ContinueWith workItem }
- Just workItem' -- He has been solved; workItem' is a Given
+ Just workList' -> -- He has been solved; workList' are all givens
+ return $ SR { sr_new_work = workList'
+ , sr_inerts = inerts
+ , sr_stop = Stop }
+ }
+
+{-- This is all old code, but does not quite work now. The problem is that due to
+ Note [Loopy Spontaneous Solving] we may have unflattened a type, to be able to
+ perform a sneaky unification. This unflattening means that we may have to recanonicalize
+ a given (solved) equality, this is why the result of trySpontaneousSolve is now a list
+ of constraints (instead of an atomic solved constraint). We would have to react all of
+ them once again with the worklist but that is very tiresome. Instead we throw them back
+ in the worklist.
+
| isWantedCt workItem
-- Original was wanted we have now made him given so
-- we have to ineract him with the inerts again because
-> return $ SR { sr_new_work = emptyWorkList
, sr_inerts = inerts `extendInertSet` workItem'
, sr_stop = Stop } }
+--}
-- @trySpontaneousSolve wi@ solves equalities where one side is a
-- touchable unification variable. Returns:
-- * Nothing if we were not able to solve it
-- * Just wi' if we solved it, wi' (now a "given") should be put in the work list.
-- See Note [Touchables and givens]
-trySpontaneousSolve :: WorkItem -> TcS (Maybe SWorkItem)
-trySpontaneousSolve (CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi })
+-- Note, just passing the inerts through for the skolem equivalence classes
+trySpontaneousSolve :: WorkItem -> InertSet -> TcS (Maybe SWorkList)
+trySpontaneousSolve (CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi }) inerts
| Just tv2 <- tcGetTyVar_maybe xi
= do { tch1 <- isTouchableMetaTyVar tv1
; tch2 <- isTouchableMetaTyVar tv2
; case (tch1, tch2) of
- (True, True) -> trySpontaneousEqTwoWay cv gw tv1 tv2
- (True, False) -> trySpontaneousEqOneWay cv gw tv1 xi
+ (True, True) -> trySpontaneousEqTwoWay inerts cv gw tv1 tv2
+ (True, False) -> trySpontaneousEqOneWay inerts cv gw tv1 xi
(False, True) | tyVarKind tv1 `isSubKind` tyVarKind tv2
- -> trySpontaneousEqOneWay cv gw tv2 (mkTyVarTy tv1)
+ -> trySpontaneousEqOneWay inerts cv gw tv2 (mkTyVarTy tv1)
_ -> return Nothing }
| otherwise
= do { tch1 <- isTouchableMetaTyVar tv1
- ; if tch1 then trySpontaneousEqOneWay cv gw tv1 xi
+ ; if tch1 then trySpontaneousEqOneWay inerts cv gw tv1 xi
else return Nothing }
-- No need for
-- trySpontaneousSolve (CFunEqCan ...) = ...
-- See Note [No touchables as FunEq RHS] in TcSMonad
-trySpontaneousSolve _ = return Nothing
+trySpontaneousSolve _ _ = return Nothing
----------------
-trySpontaneousEqOneWay :: CoVar -> CtFlavor -> TcTyVar -> Xi
- -> TcS (Maybe SWorkItem)
+trySpontaneousEqOneWay :: InertSet -> CoVar -> CtFlavor -> TcTyVar -> Xi
+ -> TcS (Maybe SWorkList)
-- tv is a MetaTyVar, not untouchable
-- Precondition: kind(xi) is a sub-kind of kind(tv)
-trySpontaneousEqOneWay cv gw tv xi
+trySpontaneousEqOneWay inerts cv gw tv xi
| not (isSigTyVar tv) || isTyVarTy xi
- = solveWithIdentity cv gw tv xi
+ = solveWithIdentity inerts cv gw tv xi
| otherwise
= return Nothing
----------------
-trySpontaneousEqTwoWay :: CoVar -> CtFlavor -> TcTyVar -> TcTyVar
- -> TcS (Maybe SWorkItem)
+trySpontaneousEqTwoWay :: InertSet -> CoVar -> CtFlavor -> TcTyVar -> TcTyVar
+ -> TcS (Maybe SWorkList)
-- Both tyvars are *touchable* MetaTyvars
-- By the CTyEqCan invariant, k2 `isSubKind` k1
-trySpontaneousEqTwoWay cv gw tv1 tv2
+trySpontaneousEqTwoWay inerts cv gw tv1 tv2
| k1 `eqKind` k2
- , nicer_to_update_tv2 = solveWithIdentity cv gw tv2 (mkTyVarTy tv1)
+ , nicer_to_update_tv2 = solveWithIdentity inerts cv gw tv2 (mkTyVarTy tv1)
| otherwise = ASSERT( k2 `isSubKind` k1 )
- solveWithIdentity cv gw tv1 (mkTyVarTy tv2)
+ solveWithIdentity inerts cv gw tv1 (mkTyVarTy tv2)
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
it and keep it as wanted. In inference mode we'll end up quantifying over
(alpha ~ Maybe (E alpha))
Hence, 'solveWithIdentity' performs a small occurs check before
-actually solving. But this occurs check *must look through* flatten
-skolems.
+actually solving. But this occurs check *must look through* flatten skolems.
+
+However, it may be the case that the flatten skolem in hand is equal to some other
+flatten skolem whith *does not* mention our unification variable. Here's a typical example:
+
+Original wanteds:
+ g: F alpha ~ F beta
+ w: alpha ~ F alpha
+After canonicalization:
+ g: F beta ~ f1
+ g: F alpha ~ f1
+ w: alpha ~ f2
+ g: F alpha ~ f2
+After some reactions:
+ g: f1 ~ f2
+ g: F beta ~ f1
+ w: alpha ~ f2
+ g: F alpha ~ f2
+At this point, we will try to spontaneously solve (alpha ~ f2) which remains as yet unsolved.
+We will look inside f2, which immediately mentions (F alpha), so it's not good to unify! However
+by looking at the equivalence class of the flatten skolems, we can see that it is fine to
+unify (alpha ~ f1) which solves our goals!
+
+A similar problem happens because of other spontaneous solving. Suppose we have the
+following wanteds, arriving in this exact order:
+ (first) w: beta ~ alpha
+ (second) w: alpha ~ fsk
+ (third) g: F beta ~ fsk
+Then, we first spontaneously solve the first constraint, making (beta := alpha), and having
+(beta ~ alpha) as given. *Then* we encounter the second wanted (alpha ~ fsk). "fsk" does not
+obviously mention alpha, so naively we can also spontaneously solve (alpha := fsk). But
+that is wrong since fsk mentions beta, which has already secretly been unified to alpha!
+
+To avoid this problem, the same occurs check must unveil rewritings that can happen because
+of spontaneously having solved other constraints.
+
Note [Avoid double unifications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{code}
----------------
-solveWithIdentity :: CoVar -> CtFlavor -> TcTyVar -> Xi -> TcS (Maybe SWorkItem)
+solveWithIdentity :: InertSet
+ -> CoVar -> CtFlavor -> TcTyVar -> Xi
+ -> TcS (Maybe SWorkList)
-- Solve with the identity coercion
-- Precondition: kind(xi) is a sub-kind of kind(tv)
-- See [New Wanted Superclass Work] to see why we do this for *given* as well
-solveWithIdentity cv gw tv xi
- | tv `elemVarSet` tyVarsOfUnflattenedType xi
- -- Beware of Note [Loopy spontaneous solving]
- -- Can't spontaneously solve loopy equalities
- -- though they are not a type error
- = return Nothing
- | not (isGiven gw) -- Wanted or Derived
- = do { traceTcS "Sneaky unification:" $
- vcat [text "Coercion variable: " <+> ppr gw,
- text "Coercion: " <+> pprEq (mkTyVarTy tv) xi,
- text "Left Kind is : " <+> ppr (typeKind (mkTyVarTy tv)),
- text "Right Kind is : " <+> ppr (typeKind xi)
- ]
- ; setWantedTyBind tv xi -- Set tv := xi
- ; cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi xi
- -- Create new given with identity evidence
-
- ; case gw of
- Wanted {} -> setWantedCoBind cv xi
- Derived {} -> setDerivedCoBind cv xi
- _ -> pprPanic "Can't spontaneously solve *given*" empty
+solveWithIdentity inerts cv gw tv xi
+ | not (isGiven gw)
+ = do { tybnds <- getTcSTyBindsBag
+ ; case occ_check_ok tybnds xi of
+ Nothing -> return Nothing
+ Just (xi_unflat,coi) -- coi : xi_unflat ~ xi
+ -> do { traceTcS "Sneaky unification:" $
+ vcat [text "Coercion variable: " <+> ppr gw,
+ text "Coercion: " <+> pprEq (mkTyVarTy tv) xi,
+ text "Left Kind is : " <+> ppr (typeKind (mkTyVarTy tv)),
+ text "Right Kind is : " <+> ppr (typeKind xi)
+ ]
+ ; setWantedTyBind tv xi_unflat -- Set tv := xi_unflat
+ ; cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat
+ ; let flav = mkGivenFlavor gw UnkSkol
+ ; (cts, co) <- case coi of
+ ACo co -> do { can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi_unflat
+ ; return (can_eqs, co) }
+ IdCo co -> return $
+ (singleCCan (CTyEqCan { cc_id = cv_given
+ , cc_flavor = mkGivenFlavor gw UnkSkol
+ , cc_tyvar = tv, cc_rhs = xi }
+ -- xi, *not* xi_unflat because
+ -- xi_unflat may require flattening!
+ ), co)
+ ; case gw of
+ Wanted {} -> setWantedCoBind cv co
+ Derived {} -> setDerivedCoBind cv co
+ _ -> pprPanic "Can't spontaneously solve *given*" empty
- ; let solved = CTyEqCan { cc_id = cv_given
- , cc_flavor = mkGivenFlavor gw UnkSkol
- , cc_tyvar = tv, cc_rhs = xi }
-- See Note [Avoid double unifications]
- -- The reason that we create a new given variable (cv_given) instead of reusing cv
- -- is because we do not want to end up with coercion unification variables in the givens.
- ; return (Just solved) }
- | otherwise -- Given
+ ; return (Just cts) }
+ }
+ | otherwise
= return Nothing
-tyVarsOfUnflattenedType :: TcType -> TcTyVarSet
--- A version of tyVarsOfType which looks through flatSkols
-tyVarsOfUnflattenedType ty
- = foldVarSet (unionVarSet . do_tv) emptyVarSet (tyVarsOfType ty)
- where
- do_tv :: TyVar -> TcTyVarSet
- do_tv tv = ASSERT( isTcTyVar tv)
- case tcTyVarDetails tv of
- FlatSkol ty -> tyVarsOfUnflattenedType ty
- _ -> unitVarSet tv
+ where occ_check_ok :: Bag.Bag (TcTyVar, TcType) -> TcType -> Maybe (TcType,CoercionI)
+ occ_check_ok ty_binds_bag ty = ok ty
+ where
+ -- @ok ty@
+ -- Traverse @ty@ to make sure that @tv@ does not appear under some flatten skolem.
+ -- If it appears under some flatten skolem look in that flatten skolem equivalence class
+ -- (see Note [InertSet FlattenSkolemEqClass], [Loopy Spontaneous Solving]) to see if you
+ -- can find a different flatten skolem to use, that is, one that does not mention @tv@.
+ --
+ -- Postcondition: Just (ty',coi) <- ok ty
+ -- coi :: ty' ~ ty
+ --
+ -- NB: The returned type may not be flat!
+ --
+ -- NB: There is no need to do the tcView thing here to expand synonyms, because
+ -- expanded synonyms have the same or fewer variables than their expanded definitions,
+ -- but never more.
+ -- See Note [Type synonyms and the occur check] in TcUnify for the handling of type synonyms.
+ ok this_ty@(TyConApp tc tys)
+ | Just tys_cois <- allMaybes (map ok tys)
+ = let (tys',cois') = unzip tys_cois
+ in Just (TyConApp tc tys', mkTyConAppCoI tc cois')
+ | isSynTyCon tc, Just ty_expanded <- tcView this_ty
+ = ok ty_expanded
+ ok (PredTy sty)
+ | Just (sty',coi) <- ok_pred sty
+ = Just (PredTy sty', coi)
+ where ok_pred (ClassP cn tys)
+ | Just tys_cois <- allMaybes $ map ok tys
+ = let (tys', cois') = unzip tys_cois
+ in Just (ClassP cn tys', mkClassPPredCoI cn cois')
+ ok_pred (IParam nm ty)
+ | Just (ty',co') <- ok ty
+ = Just (IParam nm ty', mkIParamPredCoI nm co')
+ ok_pred (EqPred ty1 ty2)
+ | Just (ty1',coi1) <- ok ty1, Just (ty2',coi2) <- ok ty2
+ = Just (EqPred ty1' ty2', mkEqPredCoI coi1 coi2)
+ ok_pred _ = Nothing
+ ok (FunTy arg res)
+ | Just (arg', coiarg) <- ok arg, Just (res', coires) <- ok res
+ = Just (FunTy arg' res', mkFunTyCoI coiarg coires)
+ ok (AppTy fun arg)
+ | Just (fun', coifun) <- ok fun, Just (arg', coiarg) <- ok arg
+ = Just (AppTy fun' arg', mkAppTyCoI coifun coiarg)
+ ok (ForAllTy tv1 ty1)
+ -- WARNING: What if it is a (t1 ~ t2) => t3? It's not handled properly at the moment.
+ | Just (ty1', coi) <- ok ty1
+ = Just (ForAllTy tv1 ty1', mkForAllTyCoI tv1 coi)
+
+ -- Variable cases
+ ok this_ty@(TyVarTy tv')
+ | not $ isTcTyVar tv' = Just (this_ty, IdCo this_ty) -- Bound variable
+ | tv == tv' = Nothing -- Occurs check error
+ -- Flatten skolem
+ ok (TyVarTy fsk) | FlatSkol zty <- tcTyVarDetails fsk
+ = case ok zty of
+ Nothing -> go_down_eq_class $ getFskEqClass inerts fsk
+ Just (zty',ico) -> Just (zty',ico)
+ where go_down_eq_class [] = Nothing
+ go_down_eq_class ((fsk1,co1):rest)
+ = case ok (TyVarTy fsk1) of
+ Nothing -> go_down_eq_class rest
+ Just (ty1,co1i') -> Just (ty1, mkTransCoI co1i' (ACo co1))
+ -- Finally, check if bound already
+ ok this_ty@(TyVarTy tv0)
+ = case Bag.foldlBag find_bind Nothing ty_binds_bag of
+ Nothing -> Just (this_ty, IdCo this_ty)
+ Just ty0 -> ok ty0
+ where find_bind Nothing (tvx,tyx) | tv0 == tvx = Just tyx
+ find_bind m _ = m
+ -- Fall through
+ ok _ty = Nothing
+
\end{code}
, sr_new_work = emptyCCan
, sr_stop = ContinueWith workItem }
+
interactNext :: StageResult -> AtomicInert -> TcS StageResult
interactNext it inert
| ContinueWith workItem <- sr_stop it
= do { ir <- interactWithInert inert workItem
; let inerts = sr_inerts it
; return $ SR { sr_inerts = if ir_inert_action ir == KeepInert
- then inerts `extendInertSet` inert
+ then inerts `updInertSet` inert
else inerts
, sr_new_work = sr_new_work it `unionWorkLists` ir_new_work ir
, sr_stop = ir_stop ir } }
itrAddInert :: AtomicInert -> StageResult -> StageResult
- itrAddInert inert itr = itr { sr_inerts = (sr_inerts itr) `extendInertSet` inert }
+ itrAddInert inert itr = itr { sr_inerts = (sr_inerts itr) `updInertSet` inert }
-- Do a single interaction of two constraints.
interactWithInert :: AtomicInert -> WorkItem -> TcS InteractResult
where
lhss_match = tc1 == tc2 && and (zipWith tcEqType args1 args2)
-doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
+doInteractWithInert
+ inert@(CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
workItem@(CTyEqCan { cc_id = cv2, cc_flavor = fl2, cc_tyvar = tv2, cc_rhs = xi2 })
-- Check for matching LHS
| fl1 `canRewrite` fl2 && tv1 == tv2
= do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
; mkIRStop KeepInert cans }
-{-
- | fl1 `canRewrite` fl2 -- If at all possible, keep the inert,
- , Just tv1_rhs <- tcGetTyVar_maybe xi1 -- special case of inert a~b
- , tv1_rhs == tv2
- = do { cans <- rewriteEqLHS (mkSymCoercion (mkCoVarCoercion cv1), mkTyVarTy tv1)
- (cv2,fl2,xi2)
- ; mkIRStop KeepInert cans }
--}
| fl2 `canRewrite` fl1 && tv1 == tv2
= do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
; mkIRContinue workItem DropInert cans }
= do { rewritten_eq <- rewriteEqRHS (cv2,tv2,xi2) (cv1,fl1,tv1,xi1)
; mkIRContinue workItem DropInert rewritten_eq }
+-- Finally, if workitem is a Flatten Equivalence Class constraint and the
+-- inert is a wanted constraint, even when the workitem cannot rewrite the
+-- inert, drop the inert out because you may have to reconsider solving the
+-- inert *using* the equivalence class you created. See note [Loopy Spontaneous Solving]
+-- and [InertSet FlattenSkolemEqClass]
+
+ | not $ isGiven fl1, -- The inert is wanted or derived
+ isMetaTyVar tv1, -- and has a unification variable lhs
+ FlatSkol {} <- tcTyVarDetails tv2, -- And workitem is a flatten skolem equality
+ Just tv2' <- tcGetTyVar_maybe xi2, FlatSkol {} <- tcTyVarDetails tv2'
+ = mkIRContinue workItem DropInert (singletonWorkList inert)
+
--- Fall-through case for all other cases
+-- Fall-through case for all other situations
doInteractWithInert _ workItem = noInteraction workItem
--------------------------------------------
mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2
; mkCanonical gw cv2' }
--- ->
--- if isWanted gw then
--- do { cv2' <- newWantedCoVar xi1 xi2
--- ; setWantedCoBind cv2 $
--- co1 `mkTransCoercion` mkCoVarCoercion cv2'
--- ; return cv2' }
--- else newGivOrDerCoVar xi1 xi2 $
--- mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2
--- ; mkCanonical gw cv2' }
solveOneFromTheOther :: (EvVar, CtFlavor) -> CanonicalCt -> TcS InteractResult
projects / ghc-hetmet.git / blobdiff