\begin{code}
module TcInteract (
solveInteract, AtomicInert,
- InertSet, emptyInert, extendInertSet, extractUnsolved, solveOne,
- listToWorkList
+ InertSet, emptyInert, updInertSet, extractUnsolved, solveOne
) where
#include "HsVersions.h"
+
import BasicTypes
import TcCanonical
import VarSet
import Type
+import TypeRep
import Id
+import VarEnv
import Var
import TcType
import TcRnTypes
import TcErrors
import TcSMonad
-import qualified Bag as Bag
+import 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_eqs :: Bag.Bag CanonicalCt -- Equalities only (CTyEqCan,CFunEqCan)
+ , inert_cts :: Bag.Bag CanonicalCt -- Other constraints
+ , 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_eqs is))
+ , 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_eqs = Bag.emptyBag
+ , 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_eqs = eqs, 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 eqs' = eqs `Bag.snocBag` item
+ fsks' = Map.insertWith (++) tv2 [(tv1, mkCoVarCoercion cv)] fsks
+ -- See Note [InertSet FlattenSkolemEqClass]
+ in IS { inert_eqs = eqs', inert_cts = cts, inert_fsks = fsks' }
+updInertSet (IS { inert_eqs = eqs, inert_cts = cts
+ , inert_fsks = fsks }) item
+ | isEqCCan item
+ = let eqs' = eqs `Bag.snocBag` item
+ in IS { inert_eqs = eqs', inert_cts = cts, inert_fsks = fsks }
+ | otherwise
+ = let cts' = cts `Bag.snocBag` item
+ in IS { inert_eqs = eqs, inert_cts = cts', inert_fsks = fsks }
+
+foldlInertSetM :: (Monad m) => (a -> AtomicInert -> m a) -> a -> InertSet -> m a
+-- Prioritize over the equalities see Note [Prioritizing Equalities]
+foldlInertSetM k z (IS { inert_eqs = eqs, inert_cts = cts })
+ = do { z' <- Bag.foldlBagM k z eqs
+ ; Bag.foldlBagM k z' cts }
+
+extractUnsolved :: InertSet -> (InertSet, CanonicalCts)
+extractUnsolved is@(IS {inert_eqs = eqs, inert_cts = cts})
+ = let is_init = is { inert_eqs = emptyCCan
+ , inert_cts = solved_cts, inert_fsks = Map.empty }
+ is_final = Bag.foldlBag updInertSet is_init solved_eqs -- Add equalities carefully
+ in (is_final, unsolved)
+ where (unsolved_cts, solved_cts) = Bag.partitionBag isWantedCt cts
+ (unsolved_eqs, solved_eqs) = Bag.partitionBag isWantedCt eqs
+ unsolved = unsolved_cts `unionBags` unsolved_eqs
+
+
+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
-
-listToWorkList :: [WorkItem] -> WorkList
-listToWorkList = Bag.listToBag
+-- A mixture of Given, Wanted, and Derived constraints.
+-- We split between equalities and the rest to process equalities first.
+data WorkList = WL { wl_eqs :: CanonicalCts -- Equalities (CTyEqCan, CFunEqCan)
+ , wl_other :: CanonicalCts -- Other
+ }
+type SWorkList = WorkList -- A worklist of solved
unionWorkLists :: WorkList -> WorkList -> WorkList
-unionWorkLists = Bag.unionBags
+unionWorkLists wl1 wl2
+ = WL { wl_eqs = andCCan (wl_eqs wl1) (wl_eqs wl2)
+ , wl_other = andCCan (wl_other wl1) (wl_other wl2) }
foldlWorkListM :: (Monad m) => (a -> WorkItem -> m a) -> a -> WorkList -> m a
-foldlWorkListM = Bag.foldlBagM
+-- This fold prioritizes equalities
+foldlWorkListM f r wl
+ = do { r' <- Bag.foldlBagM f r (wl_eqs wl)
+ ; Bag.foldlBagM f r' (wl_other wl) }
isEmptyWorkList :: WorkList -> Bool
-isEmptyWorkList = Bag.isEmptyBag
+isEmptyWorkList wl = isEmptyCCan (wl_eqs wl) && isEmptyCCan (wl_other wl)
emptyWorkList :: WorkList
-emptyWorkList = Bag.emptyBag
+emptyWorkList = WL { wl_eqs = emptyCCan, wl_other = emptyCCan }
+
+mkEqWorkList :: CanonicalCts -> WorkList
+-- Precondition: *ALL* equalities
+mkEqWorkList eqs = WL { wl_eqs = eqs, wl_other = emptyCCan }
+
+mkNonEqWorkList :: CanonicalCts -> WorkList
+-- Precondition: *NO* equalities
+mkNonEqWorkList cts = WL { wl_eqs = emptyCCan, wl_other = cts }
+
+workListFromCCans :: CanonicalCts -> WorkList
+-- Generic, no precondition
+workListFromCCans cts = WL eqs others
+ where (eqs, others) = Bag.partitionBag isEqCCan cts
+
+-- Convenience
+singleEqWL :: CanonicalCt -> WorkList
+singleNonEqWL :: CanonicalCt -> WorkList
+singleEqWL = mkEqWorkList . singleCCan
+singleNonEqWL = mkNonEqWorkList . singleCCan
+
data StopOrContinue
= Stop -- Work item is consumed
, ptext (sLit "new work =") <+> ppr work <> comma
, ptext (sLit "stop =") <+> ppr stop])
+instance Outputable WorkList where
+ ppr (WL eqcts othercts) = vcat [ppr eqcts, ppr othercts]
+
type SimplifierStage = WorkItem -> InertSet -> TcS StageResult
-- Combine a sequence of simplifier 'stages' to create a pipeline
; 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)]
-- returning an extended inert set.
--
-- See Note [Touchables and givens].
-solveInteract :: InertSet -> WorkList -> TcS InertSet
+solveInteract :: InertSet -> CanonicalCts -> TcS InertSet
solveInteract inert ws
= do { dyn_flags <- getDynFlags
- ; solveInteractWithDepth (ctxtStkDepth dyn_flags,0,[]) inert ws
+ ; let worklist = workListFromCCans ws
+ ; solveInteractWithDepth (ctxtStkDepth dyn_flags,0,[]) inert worklist
}
solveOne :: InertSet -> WorkItem -> TcS InertSet
solveOne inerts workItem
\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
+ 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 })
+-- NB: 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
+ | isGiven gw
+ = return Nothing
| 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
- (False, True) | tyVarKind tv1 `isSubKind` tyVarKind tv2
- -> trySpontaneousEqOneWay cv gw tv2 (mkTyVarTy tv1)
+ (True, True) -> trySpontaneousEqTwoWay inerts cv gw tv1 tv2
+ (True, False) -> trySpontaneousEqOneWay inerts cv gw tv1 xi
+ (False, True) -> 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
- | not (isSigTyVar tv) || isTyVarTy xi
- = solveWithIdentity cv gw tv xi
+trySpontaneousEqOneWay inerts cv gw tv xi
+ | not (isSigTyVar tv) || isTyVarTy xi,
+ typeKind xi `isSubKind` tyVarKind tv
+ = 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
- | k1 `eqKind` k2
- , nicer_to_update_tv2 = solveWithIdentity cv gw tv2 (mkTyVarTy tv1)
- | otherwise = ASSERT( k2 `isSubKind` k1 )
- solveWithIdentity cv gw tv1 (mkTyVarTy tv2)
+trySpontaneousEqTwoWay inerts cv gw tv1 tv2
+ | k1 `isSubKind` k2
+ , nicer_to_update_tv2 = solveWithIdentity inerts cv gw tv2 (mkTyVarTy tv1)
+ | k2 `isSubKind` k1
+ = solveWithIdentity inerts cv gw tv1 (mkTyVarTy tv2)
+ | otherwise = return Nothing
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
\end{code}
+
+Note [Spontaneous solving and kind compatibility]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Note that our canonical constraints insist that only *given* equalities (tv ~ xi)
+or (F xis ~ rhs) require the LHS and the RHS to have exactly the same kinds.
+
+ - We have to require this because:
+ Given equalities can be freely used to rewrite inside
+ other types or constraints.
+ - We do not have to do the same for wanteds because:
+ First, wanted equations (tv ~ xi) where tv is a touchable unification variable
+ may have kinds that do not agree (the kind of xi must be a sub kind of the kind of tv).
+ Second, any potential kind mismatch will result in the constraint not being soluble,
+ which will be reported anyway. This is the reason that @trySpontaneousOneWay@ and
+ @trySpontaneousTwoWay@ will perform a kind compatibility check, and only then will
+ they proceed to @solveWithIdentity@.
+
+Caveat:
+ - Givens from higher-rank, such as:
+ type family T b :: * -> * -> *
+ type instance T Bool = (->)
+
+ f :: forall a. ((T a ~ (->)) => ...) -> a -> ...
+ flop = f (...) True
+ Whereas we would be able to apply the type instance, we would not be able to
+ use the given (T Bool ~ (->)) in the body of 'flop'
+
Note [Loopy spontaneous solving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the original wanted:
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]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
variable *on the left* of the equality. Here is what happens if not:
Original wanted: (a ~ alpha), (alpha ~ Int)
We spontaneously solve the first wanted, without changing the order!
- given : a ~ alpha [having unifice alpha := a]
+ given : a ~ alpha [having unified alpha := a]
Now the second wanted comes along, but he cannot rewrite the given, so we simply continue.
At the end we spontaneously solve that guy, *reunifying* [alpha := Int]
-We avoid this problem by orienting the given so that the unification variable is on the left.
-[Note that alternatively we could attempt to enforce this at canonicalization]
+We avoid this problem by orienting the given so that the unification
+variable is on the left. [Note that alternatively we could attempt to
+enforce this at canonicalization]
-Avoiding double unifications is yet another reason to disallow touchable unification variables
-as RHS of type family equations: F xis ~ alpha. Consider having already spontaneously solved
-a wanted (alpha ~ [b]) by setting alpha := [b]. So the inert set looks like:
- given : alpha ~ [b]
-And now a new wanted (F tau ~ alpha) comes along. Since it does not react with anything
-we will be left with a constraint (F tau ~ alpha) that must cause a unification of
-(alpha := F tau) at some point (either in spontaneous solving, or at the end). But alpha
-is *already* unified so we must not do anything to it. By disallowing naked touchables in
-the RHS of constraints (in favor of introduced flatten skolems) we do not have to worry at
-all about unifying or spontaneously solving (F xis ~ alpha) by unification.
+See also Note [No touchables as FunEq RHS] in TcSMonad; avoiding
+double unifications is the main reason we disallow touchable
+unification variables as RHS of type family equations: F xis ~ alpha.
\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
-
- ; 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 Nothing
-
-tyVarsOfUnflattenedType :: TcType -> TcTyVarSet
--- A version of tyVarsOfType which looks through flatSkols
-tyVarsOfUnflattenedType ty
- = foldVarSet (unionVarSet . do_tv) emptyVarSet (tyVarsOfType ty)
+-- Precondition: CtFlavor is Wanted or Derived
+-- See [New Wanted Superclass Work] to see why solveWithIdentity
+-- must work for Derived as well as Wanted
+solveWithIdentity inerts cv gw tv xi
+ = do { tybnds <- getTcSTyBindsMap
+ ; case occurCheck tybnds inerts tv xi of
+ Nothing -> return Nothing
+ Just (xi_unflat,coi) -> solve_with xi_unflat coi }
where
- do_tv :: TyVar -> TcTyVarSet
- do_tv tv = ASSERT( isTcTyVar tv)
- case tcTyVarDetails tv of
- FlatSkol ty -> tyVarsOfUnflattenedType ty
- _ -> unitVarSet tv
+ solve_with 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
+ -- See Note [Avoid double unifications]
+ ; return $ Just (mkEqWorkList cts) }
+
+occurCheck :: VarEnv (TcTyVar, TcType) -> InertSet
+ -> TcTyVar -> TcType -> Maybe (TcType,CoercionI)
+-- 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) = occurCheck binds inerts tv ty
+-- coi :: ty' ~ ty
+-- NB: The returned type ty' may not be flat!
+
+occurCheck ty_binds inerts the_tv the_ty
+ = ok emptyVarSet the_ty
+ where
+ -- If (fsk `elem` bad) then tv occurs in any rendering
+ -- of the type under the expansion of fsk
+ ok bad this_ty@(TyConApp tc tys)
+ | Just tys_cois <- allMaybes (map (ok bad) tys)
+ , (tys',cois') <- unzip tys_cois
+ = Just (TyConApp tc tys', mkTyConAppCoI tc cois')
+ | isSynTyCon tc, Just ty_expanded <- tcView this_ty
+ = ok bad ty_expanded -- See Note [Type synonyms and the occur check] in TcUnify
+ ok bad (PredTy sty)
+ | Just (sty',coi) <- ok_pred bad sty
+ = Just (PredTy sty', coi)
+ ok bad (FunTy arg res)
+ | Just (arg', coiarg) <- ok bad arg, Just (res', coires) <- ok bad res
+ = Just (FunTy arg' res', mkFunTyCoI coiarg coires)
+ ok bad (AppTy fun arg)
+ | Just (fun', coifun) <- ok bad fun, Just (arg', coiarg) <- ok bad arg
+ = Just (AppTy fun' arg', mkAppTyCoI coifun coiarg)
+ ok bad (ForAllTy tv1 ty1)
+ -- WARNING: What if it is a (t1 ~ t2) => t3? It's not handled properly at the moment.
+ | Just (ty1', coi) <- ok bad ty1
+ = Just (ForAllTy tv1 ty1', mkForAllTyCoI tv1 coi)
+
+ -- Variable cases
+ ok bad this_ty@(TyVarTy tv)
+ | tv == the_tv = Nothing -- Occurs check error
+ | not (isTcTyVar tv) = Just (this_ty, IdCo this_ty) -- Bound var
+ | FlatSkol zty <- tcTyVarDetails tv = ok_fsk bad tv zty
+ | Just (_,ty) <- lookupVarEnv ty_binds tv = ok bad ty
+ | otherwise = Just (this_ty, IdCo this_ty)
+
+ -- Check if there exists a ty bind already, as a result of sneaky unification.
+ -- Fall through
+ ok _bad _ty = Nothing
+
+ -----------
+ ok_pred bad (ClassP cn tys)
+ | Just tys_cois <- allMaybes $ map (ok bad) tys
+ = let (tys', cois') = unzip tys_cois
+ in Just (ClassP cn tys', mkClassPPredCoI cn cois')
+ ok_pred bad (IParam nm ty)
+ | Just (ty',co') <- ok bad ty
+ = Just (IParam nm ty', mkIParamPredCoI nm co')
+ ok_pred bad (EqPred ty1 ty2)
+ | Just (ty1',coi1) <- ok bad ty1, Just (ty2',coi2) <- ok bad ty2
+ = Just (EqPred ty1' ty2', mkEqPredCoI coi1 coi2)
+ ok_pred _ _ = Nothing
+
+ -----------
+ ok_fsk bad fsk zty
+ | fsk `elemVarSet` bad
+ -- We are already trying to find a rendering of fsk,
+ -- and to do that it seems we need a rendering, so fail
+ = Nothing
+ | otherwise
+ = firstJusts (ok new_bad zty : map (go_under_fsk new_bad) fsk_equivs)
+ where
+ fsk_equivs = getFskEqClass inerts fsk
+ new_bad = bad `extendVarSetList` (fsk : map fst fsk_equivs)
+
+ -----------
+ go_under_fsk bad_tvs (fsk,co)
+ | FlatSkol zty <- tcTyVarDetails fsk
+ = case ok bad_tvs zty of
+ Nothing -> Nothing
+ Just (ty,coi') -> Just (ty, mkTransCoI coi' (ACo co))
+ | otherwise = pprPanic "go_down_equiv" (ppr fsk)
\end{code}
mkIRStop keep newWork = return $ IR Stop keep newWork
dischargeWorkItem :: Monad m => m InteractResult
-dischargeWorkItem = mkIRStop KeepInert emptyCCan
+dischargeWorkItem = mkIRStop KeepInert emptyWorkList
noInteraction :: Monad m => WorkItem -> m InteractResult
-noInteraction workItem = mkIRContinue workItem KeepInert emptyCCan
+noInteraction workItem = mkIRContinue workItem KeepInert emptyWorkList
+data WhichComesFromInert = LeftComesFromInert | RightComesFromInert
---------------------------------------------------
-- Interact a single WorkItem with an InertSet as far as possible, i.e. until we get a Stop
= foldlInertSetM interactNext initITR inert
where
initITR = SR { sr_inerts = emptyInert
- , sr_new_work = emptyCCan
+ , sr_new_work = emptyWorkList
, 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
inert_pred_loc = (ClassP cls1 tys1, ppr d1)
loc = combineCtLoc fl1 fl2
eqn_pred_locs = improveFromAnother work_item_pred_loc inert_pred_loc
+
; wevvars <- mkWantedFunDepEqns loc eqn_pred_locs
+ ; fd_cts <- canWanteds wevvars
+ ; let fd_work = mkEqWorkList fd_cts
-- See Note [Generating extra equalities]
- ; workList <- canWanteds wevvars
- ; mkIRContinue workItem KeepInert workList -- Keep the inert there so we avoid
- -- re-introducing the fundep equalities
+ ; if isEmptyCCan fd_cts || not (isWanted fl2) then -- || or impr. had previously kicked in
+ -- No improvement kicked in, keep going
+ mkIRContinue workItem KeepInert fd_work
+ else -- Improvement kicked in, throw him back into the worklist so that he
+ -- gets rewritten. The reason is that we do not want to let him fall off
+ -- at the end and then add its potential un-improved superclasses. This
+ -- optimisation crucially relies on prioritizing the equalities in the
+ -- worklist.
+
+ -- The termination of this relies on wanteds being able to rewrite wanteds.
+ -- Since the class may be at the bottom of an equality worklist, which may
+ -- consist of insoluble wanteds, if these wanteds *never* become solved or given
+ -- (because they mention untouchables), the workitem will *never* be rewritten
+ -- so next time we meet him we will be once again producing FunDep equalities
+ -- for ever and ever!
+ mkIRStop KeepInert $ fd_work `unionWorkLists` singleNonEqWL workItem
+
-- See Note [FunDep Reactions]
}
-- set again, to find that it can now be discharged by the given D
-- Int instance.
= do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,wfl,cl,xis)
- ; mkIRStop KeepInert (singleCCan rewritten_dict) }
+ ; mkIRStop KeepInert $ singleNonEqWL rewritten_dict }
doInteractWithInert (CDictCan { cc_id = dv, cc_flavor = ifl, cc_class = cl, cc_tyargs = xis })
workItem@(CTyEqCan { cc_id = cv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
| wfl `canRewrite` ifl
, tv `elemVarSet` tyVarsOfTypes xis
= do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,ifl,cl,xis)
- ; mkIRContinue workItem DropInert (singleCCan rewritten_dict) }
+ ; mkIRContinue workItem DropInert (singleNonEqWL rewritten_dict) }
-- Class constraint and given equality: use the equality to rewrite
-- the class constraint.
| ifl `canRewrite` wfl
, tv `elemVarSet` tyVarsOfType ty
= do { rewritten_ip <- rewriteIP (cv,tv,xi) (ipid,wfl,nm,ty)
- ; mkIRStop KeepInert (singleCCan rewritten_ip) }
+ ; mkIRStop KeepInert (singleNonEqWL rewritten_ip) }
doInteractWithInert (CIPCan { cc_id = ipid, cc_flavor = ifl, cc_ip_nm = nm, cc_ip_ty = ty })
workItem@(CTyEqCan { cc_id = cv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
| wfl `canRewrite` ifl
, tv `elemVarSet` tyVarsOfType ty
= do { rewritten_ip <- rewriteIP (cv,tv,xi) (ipid,ifl,nm,ty)
- ; mkIRContinue workItem DropInert (singleCCan rewritten_ip) }
+ ; mkIRContinue workItem DropInert (singleNonEqWL rewritten_ip) }
-- Two implicit parameter constraints. If the names are the same,
-- but their types are not, we generate a wanted type equality
-- so we just generate a fresh coercion variable that isn't used anywhere.
doInteractWithInert (CIPCan { cc_id = id1, cc_flavor = ifl, cc_ip_nm = nm1, cc_ip_ty = ty1 })
workItem@(CIPCan { cc_flavor = wfl, cc_ip_nm = nm2, cc_ip_ty = ty2 })
+ | nm1 == nm2 && isGiven wfl && isGiven ifl
+ = -- See Note [Overriding implicit parameters]
+ -- Dump the inert item, override totally with the new one
+ -- Do not require type equality
+ mkIRContinue workItem DropInert emptyWorkList
+
| nm1 == nm2 && ty1 `tcEqType` ty2
= solveOneFromTheOther (id1,ifl) workItem
- | nm1 == nm2 && (not (isGiven ifl && isGiven wfl))
+ | nm1 == nm2
= -- See Note [When improvement happens]
do { co_var <- newWantedCoVar ty1 ty2
; let flav = Wanted (combineCtLoc ifl wfl)
- ; mkCanonical flav co_var >>= mkIRContinue workItem KeepInert }
+ ; cans <- mkCanonical flav co_var
+ ; mkIRContinue workItem KeepInert (mkEqWorkList cans) }
-- Inert: equality, work item: function equality
| ifl `canRewrite` wfl
, tv `elemVarSet` tyVarsOfTypes args
= do { rewritten_funeq <- rewriteFunEq (cv1,tv,xi1) (cv2,wfl,tc,args,xi2)
- ; mkIRStop KeepInert (singleCCan rewritten_funeq) }
+ ; mkIRStop KeepInert (singleEqWL rewritten_funeq) }
-- Inert: function equality, work item: equality
| wfl `canRewrite` ifl
, tv `elemVarSet` tyVarsOfTypes args
= do { rewritten_funeq <- rewriteFunEq (cv2,tv,xi2) (cv1,ifl,tc,args,xi1)
- ; mkIRContinue workItem DropInert (singleCCan rewritten_funeq) }
+ ; mkIRContinue workItem DropInert (singleEqWL rewritten_funeq) }
doInteractWithInert (CFunEqCan { cc_id = cv1, cc_flavor = fl1, cc_fun = tc1
, cc_tyargs = args1, cc_rhs = xi1 })
workItem@(CFunEqCan { cc_id = cv2, cc_flavor = fl2, cc_fun = tc2
, cc_tyargs = args2, cc_rhs = xi2 })
- | fl1 `canRewrite` fl2 && lhss_match
- = do { cans <- rewriteEqLHS (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
- ; mkIRStop KeepInert cans }
- | fl2 `canRewrite` fl1 && lhss_match
- = do { cans <- rewriteEqLHS (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
- ; mkIRContinue workItem DropInert cans }
+ | fl1 `canSolve` fl2 && lhss_match
+ = do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
+ ; mkIRStop KeepInert (mkEqWorkList cans) }
+ | fl2 `canSolve` fl1 && lhss_match
+ = do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
+ ; mkIRContinue workItem DropInert (mkEqWorkList cans) }
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 (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 (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
- ; mkIRContinue workItem DropInert cans }
+ | fl1 `canSolve` fl2 && tv1 == tv2
+ = do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
+ ; mkIRStop KeepInert (mkEqWorkList cans) }
+
+ | fl2 `canSolve` fl1 && tv1 == tv2
+ = do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
+ ; mkIRContinue workItem DropInert (mkEqWorkList cans) }
-- Check for rewriting RHS
| fl1 `canRewrite` fl2 && tv1 `elemVarSet` tyVarsOfType xi2
= do { rewritten_eq <- rewriteEqRHS (cv1,tv1,xi1) (cv2,fl2,tv2,xi2)
- ; mkIRStop KeepInert rewritten_eq }
+ ; mkIRStop KeepInert (mkEqWorkList rewritten_eq) }
| fl2 `canRewrite` fl1 && tv2 `elemVarSet` tyVarsOfType xi1
= do { rewritten_eq <- rewriteEqRHS (cv2,tv2,xi2) (cv1,fl1,tv1,xi1)
- ; mkIRContinue workItem DropInert rewritten_eq }
+ ; mkIRContinue workItem DropInert (mkEqWorkList 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]
--- Fall-through case for all other cases
-doInteractWithInert _ workItem = noInteraction workItem
+ | 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 (singleEqWL inert)
---------------------------------------------
-combineCtLoc :: CtFlavor -> CtFlavor -> WantedLoc
--- Precondition: At least one of them should be wanted
-combineCtLoc (Wanted loc) _ = loc
-combineCtLoc _ (Wanted loc) = loc
-combineCtLoc _ _ = panic "Expected one of wanted constraints (BUG)"
+-- Fall-through case for all other situations
+doInteractWithInert _ workItem = noInteraction workItem
+-------------------------
-- Equational Rewriting
rewriteDict :: (CoVar, TcTyVar, Xi) -> (DictId, CtFlavor, Class, [Xi]) -> TcS CanonicalCt
rewriteDict (cv,tv,xi) (dv,gw,cl,xis)
; return (singleCCan $ CTyEqCan { cc_id = cv2'
, cc_flavor = gw
, cc_tyvar = tv2
- , cc_rhs = xi2'' })
+ , cc_rhs = xi2'' })
}
where
xi2' = substTyWith [tv1] [xi1] xi2
co2' = substTyWith [tv1] [mkCoVarCoercion cv1] xi2 -- xi2 ~ xi2[xi1/tv1]
-rewriteEqLHS :: (Coercion,Xi) -> (CoVar,CtFlavor,Xi) -> TcS CanonicalCts
+
+rewriteEqLHS :: WhichComesFromInert -> (Coercion,Xi) -> (CoVar,CtFlavor,Xi) -> TcS CanonicalCts
-- Used to ineratct two equalities of the following form:
-- First Equality: co1: (XXX ~ xi1)
-- Second Equality: cv2: (XXX ~ xi2)
--- Where the cv1 `canRewrite` cv2 equality
-rewriteEqLHS (co1,xi1) (cv2,gw,xi2)
- = do { 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' }
-
-
+-- Where the cv1 `canSolve` cv2 equality
+-- We have an option of creating new work (xi1 ~ xi2) OR (xi2 ~ xi1). This
+-- depends on whether the left or the right equality comes from the inert set.
+-- We must:
+-- prefer to create (xi2 ~ xi1) if the first comes from the inert
+-- prefer to create (xi1 ~ xi2) if the second comes from the inert
+rewriteEqLHS which (co1,xi1) (cv2,gw,xi2)
+ = do { cv2' <- case (isWanted gw, which) of
+ (True,LeftComesFromInert) ->
+ do { cv2' <- newWantedCoVar xi2 xi1
+ ; setWantedCoBind cv2 $
+ co1 `mkTransCoercion` mkSymCoercion (mkCoVarCoercion cv2')
+ ; return cv2' }
+ (True,RightComesFromInert) ->
+ do { cv2' <- newWantedCoVar xi1 xi2
+ ; setWantedCoBind cv2 $
+ co1 `mkTransCoercion` mkCoVarCoercion cv2'
+ ; return cv2' }
+ (False,LeftComesFromInert) ->
+ newGivOrDerCoVar xi2 xi1 $
+ mkSymCoercion (mkCoVarCoercion cv2) `mkTransCoercion` co1
+ (False,RightComesFromInert) ->
+ newGivOrDerCoVar xi1 xi2 $
+ mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2
+ ; mkCanonical gw cv2'
+ }
+
solveOneFromTheOther :: (EvVar, CtFlavor) -> CanonicalCt -> TcS InteractResult
-- First argument inert, second argument workitem. They both represent
-- wanted/given/derived evidence for the *same* predicate so we try here to
| isDerived ifl && isDerived wfl
= noInteraction workItem
- | wfl `canRewrite` ifl
- = do { unless (isGiven ifl) $ setEvBind iid (EvId wid)
- -- Overwrite the binding, if one exists
- -- (For Givens, they are lambda-bound so nothing to overwrite,
- -- but we still drop the overridden one and replace it in
- -- the inert set with the new one
- ; mkIRContinue workItem DropInert emptyCCan }
- -- The order is important here: must do (wfl `canRewrite` ifl) first
- -- so that we override the inert item with an inner given if possible.
- -- See Note [Overriding implicit parameters]
-
- | otherwise -- ifl `canRewrite` wfl
+ | ifl `canSolve` wfl
= do { unless (isGiven wfl) $ setEvBind wid (EvId iid)
+ -- Overwrite the binding, if one exists
+ -- For Givens, which are lambda-bound, nothing to overwrite,
; dischargeWorkItem }
+
+ | otherwise -- wfl `canSolve` ifl
+ = do { unless (isGiven ifl) $ setEvBind iid (EvId wid)
+ ; mkIRContinue workItem DropInert emptyWorkList }
+
where
wfl = cc_flavor workItem
wid = cc_id workItem
-- Solved in one step and no new wanted work produced.
-- i.e we directly matched a top-level instance
-- No point in caching this in 'inert', nor in adding superclasses
- then return $ SomeTopInt { tir_new_work = emptyCCan
+ then return $ SomeTopInt { tir_new_work = emptyWorkList
, tir_new_inert = Stop }
-- Solved and new wanted work produced, you may cache the
-- (tentatively solved) dictionary as Derived and its superclasses
else do { let solved = makeSolved workItem
; sc_work <- newSCWorkFromFlavored dv (Derived loc) cls xis
+ ; let inst_work = workListFromCCans workList
; return $ SomeTopInt
- { tir_new_work = workList `unionWorkLists` sc_work
+ { tir_new_work = inst_work `unionWorkLists` sc_work
, tir_new_inert = ContinueWith solved } }
}
}
; wevvars <- mkWantedFunDepEqns loc eqn_pred_locs
-- NB: fundeps generate some wanted equalities, but
-- we don't use their evidence for anything
- ; fd_work <- canWanteds wevvars
- ; sc_work <- newSCWorkFromFlavored dv (Derived loc) cls xis
- ; return $ SomeTopInt { tir_new_work = fd_work `unionWorkLists` sc_work
- , tir_new_inert = ContinueWith workItem }
+ ; fd_cts <- canWanteds wevvars
+ ; let fd_work = mkEqWorkList fd_cts
+
+ ; if isEmptyCCan fd_cts then
+ do { sc_work <- newSCWorkFromFlavored dv (Derived loc) cls xis
+ ; return $ SomeTopInt { tir_new_work = fd_work `unionWorkLists` sc_work
+ , tir_new_inert = ContinueWith workItem }
+ }
+ else -- More fundep work produced, don't do any superlcass stuff, just
+ -- thow him back in the worklist prioritizing the solution of fd equalities
+ return $
+ SomeTopInt { tir_new_work = fd_work `unionWorkLists` singleNonEqWL workItem
+ , tir_new_inert = Stop }
+
-- NB: workItem is inert, but it isn't solved
-- keep it as inert, although it's not solved because we
-- have now reacted all its top-level fundep-induced equalities!
_ -> newGivOrDerCoVar xi rhs_ty $
mkSymCoercion (mkCoVarCoercion cv) `mkTransCoercion` coe
- ; workList <- mkCanonical fl cv'
+ ; can_cts <- mkCanonical fl cv'
+ ; let workList = mkEqWorkList can_cts
; return $ SomeTopInt workList Stop }
_
-> panicTcS $ text "TcSMonad.matchFam returned multiple instances!"
d2 := d1 |> D Int cv
But in general it's a bit painful to figure out the necessary coercion,
-so we just take the first approach.
+so we just take the first approach. Here is a better example. Consider:
+ class C a b c | a -> b
+And:
+ [Given] d1 : C T Int Char
+ [Wanted] d2 : C T beta Int
+In this case, it's *not even possible* to solve the wanted immediately.
+So we should simply output the functional dependency and add this guy
+[but NOT its superclasses] back in the worklist. Even worse:
+ [Given] d1 : C T Int beta
+ [Wanted] d2: C T beta Int
+Then it is solvable, but its very hard to detect this on the spot.
It's exactly the same with implicit parameters, except that the
"aggressive" approach would be much easier to implement.
Now suppose that we have:
given: C a b
wanted: C a beta
- By interacting the given we will get that (F a ~ b) which is not
+ By interacting the given we will get given (F a ~ b) which is not
enough by itself to make us discharge (C a beta). However, we
- may create a new given equality from the super-class that we promise
- to solve: (F a ~ beta). Now we may interact this with the rest of
- constraint to finally get:
- given : beta ~ b
-
+ may create a new derived equality from the super-class of the
+ wanted constraint (C a beta), namely derived (F a ~ beta).
+ Now we may interact this with given (F a ~ b) to get:
+ derived : beta ~ b
But 'beta' is a touchable unification variable, and hence OK to
- unify it with 'b', replacing the given evidence with the identity.
+ unify it with 'b', replacing the derived evidence with the identity.
- This requires trySpontaneousSolve to solve given equalities that
- have a touchable in their RHS, *in addition* to solving wanted
- equalities.
+ This requires trySpontaneousSolve to solve *derived*
+ equalities that have a touchable in their RHS, *in addition*
+ to solving wanted equalities.
Here is another example where this is useful.
-- Add *all* its superclasses (equalities or not) as new given work
-- See Note [New Wanted Superclass Work]
; sc_vars <- zipWithM inst_one sc_theta1 [0..]
- ; mkCanonicals flavor sc_vars }
+ ; can_cts <- mkCanonicals flavor sc_vars
+ ; return $ workListFromCCans can_cts
+ }
where
inst_one pred n = newGivOrDerEvVar pred (EvSuperClass ev n)
}
}
\end{code}
-
-
projects / ghc-hetmet.git / blobdiff