\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**
+ , inert_cts :: Bag.Bag CanonicalCt -- Other constraints
+ , inert_fds :: FDImprovements -- List of pairwise improvements that have kicked in already
+ -- and reside either in the worklist or in the inerts
+ , inert_fsks :: Map.Map TcTyVar [(TcTyVar,Coercion)] }
+ -- See Note [InertSet FlattenSkolemEqClass]
+
+type FDImprovement = (PredType,PredType)
+type FDImprovements = [(PredType,PredType)]
-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, inert_fds = [] }
+
+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, inert_fds = fdis })
+ 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', inert_fds = fdis }
+updInertSet (IS { inert_eqs = eqs, inert_cts = cts
+ , inert_fsks = fsks, inert_fds = fdis }) item
+ | isTyEqCCan item
+ = let eqs' = eqs `Bag.snocBag` item
+ in IS { inert_eqs = eqs', inert_cts = cts, inert_fsks = fsks, inert_fds = fdis }
+ | otherwise
+ = let cts' = cts `Bag.snocBag` item
+ in IS { inert_eqs = eqs, inert_cts = cts', inert_fsks = fsks, inert_fds = fdis }
+
+updInertSetFDImprs :: InertSet -> Maybe FDImprovement -> InertSet
+updInertSetFDImprs is (Just fdi) = is { inert_fds = fdi : inert_fds is }
+updInertSetFDImprs is Nothing = is
+
+foldISEqCtsM :: Monad m => (a -> AtomicInert -> m a) -> a -> InertSet -> m a
+-- Fold over the equalities of the inerts
+foldISEqCtsM k z IS { inert_eqs = eqs }
+ = Bag.foldlBagM k z eqs
+
+foldISOtherCtsM :: Monad m => (a -> AtomicInert -> m a) -> a -> InertSet -> m a
+-- Fold over other constraints in the inerts
+foldISOtherCtsM k z IS { inert_cts = cts }
+ = Bag.foldlBagM k z cts
+
+extractUnsolved :: InertSet -> (InertSet, CanonicalCts)
+extractUnsolved is@(IS {inert_eqs = eqs, inert_cts = cts, inert_fds = fdis })
+ = let is_init = is { inert_eqs = emptyCCan
+ , inert_cts = solved_cts, inert_fsks = Map.empty, inert_fds = fdis }
+ 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
+
+haveBeenImproved :: FDImprovements -> PredType -> PredType -> Bool
+haveBeenImproved [] _ _ = False
+haveBeenImproved ((pty1,pty2):fdimprs) pty1' pty2'
+ | tcEqPred pty1 pty1' && tcEqPred pty2 pty2'
+ = True
+ | tcEqPred pty1 pty2' && tcEqPred pty2 pty1'
+ = True
+ | otherwise
+ = haveBeenImproved fdimprs pty1' pty2'
+
+getFDImprovements :: InertSet -> FDImprovements
+-- Return a list of the improvements that have kicked in so far
+getFDImprovements = inert_fds
+
{- 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]
2. Pairwise interaction (binary)
* Take one from work list
* Try all pair-wise interactions with each constraint in inert
+
+ As an optimisation, we prioritize the equalities both in the
+ worklist and in the inerts.
+
3. Try to solve spontaneously for equalities involving touchables
4. Top-level interaction (binary wrt top-level)
Superclass decomposition belongs in (4), see note [Superclasses]
\begin{code}
-
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.
+type WorkList = CanonicalCts
+type SWorkList = WorkList -- A worklist of solved
unionWorkLists :: WorkList -> WorkList -> WorkList
-unionWorkLists = Bag.unionBags
-
-foldlWorkListM :: (Monad m) => (a -> WorkItem -> m a) -> a -> WorkList -> m a
-foldlWorkListM = Bag.foldlBagM
+unionWorkLists = andCCan
isEmptyWorkList :: WorkList -> Bool
-isEmptyWorkList = Bag.isEmptyBag
+isEmptyWorkList = isEmptyCCan
emptyWorkList :: WorkList
-emptyWorkList = Bag.emptyBag
+emptyWorkList = emptyCCan
+workListFromCCan :: CanonicalCt -> WorkList
+workListFromCCan = singleCCan
+
+------------------------
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)]
, sr_stop = ContinueWith work_item })
= do { itr <- stage work_item inerts
; traceTcS ("Stage result (" ++ name ++ ")") (ppr itr)
- ; let itr' = itr { sr_new_work = sr_new_work itr
- `unionWorkLists` accum_work }
+ ; let itr' = itr { sr_new_work = accum_work `unionWorkLists` sr_new_work itr }
; run_pipeline stages itr' }
\end{code}
-- 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
+ ; solveInteractWithDepth (ctxtStkDepth dyn_flags,0,[]) inert ws
}
solveOne :: InertSet -> WorkItem -> TcS InertSet
solveOne inerts workItem
| otherwise
= do { traceTcS "solveInteractWithDepth" $
- vcat [ text "Current depth =" <+> ppr n
- , text "Max depth =" <+> ppr max_depth
- ]
- ; foldlWorkListM (solveOneWithDepth ctxt) inert ws }
+ vcat [ text "Current depth =" <+> ppr n
+ , text "Max depth =" <+> ppr max_depth ]
+
+ -- Solve equalities first
+ ; let (eqs, non_eqs) = Bag.partitionBag isTyEqCCan ws
+ ; is_from_eqs <- Bag.foldlBagM (solveOneWithDepth ctxt) inert eqs
+ ; Bag.foldlBagM (solveOneWithDepth ctxt) is_from_eqs non_eqs }
------------------
-- Fully interact the given work item with an inert set, and return a
indent = replicate (2*n) ' '
thePipeline :: [(String,SimplifierStage)]
-thePipeline = [ ("interact with inerts", interactWithInertsStage)
- , ("spontaneous solve", spontaneousSolveStage)
- , ("top-level reactions", topReactionsStage) ]
+thePipeline = [ ("interact with inert eqs", interactWithInertEqsStage)
+ , ("interact with inerts", interactWithInertsStage)
+ , ("spontaneous solve", spontaneousSolveStage)
+ , ("top-level reactions", topReactionsStage) ]
\end{code}
*********************************************************************************
* *
*********************************************************************************
+Note [Efficient Orientation]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are two cases where we have to be careful about
+orienting equalities to get better efficiency.
+
+Case 2: In Rewriting Equalities (function rewriteEqLHS)
+
+ When rewriting two equalities with the same LHS:
+ (a) (tv ~ xi1)
+ (b) (tv ~ xi2)
+ We have a choice of producing work (xi1 ~ xi2) (up-to the
+ canonicalization invariants) However, to prevent the inert items
+ from getting kicked out of the inerts first, we prefer to
+ canonicalize (xi1 ~ xi2) if (b) comes from the inert set, or (xi2
+ ~ xi1) if (a) comes from the inert set.
+
+ This choice is implemented using the WhichComesFromInert flag.
+
+Case 2: In Spontaneous Solving
+ Example 2a:
+ Inerts: [w1] : D alpha
+ [w2] : C beta
+ [w3] : F alpha ~ Int
+ [w4] : H beta ~ Int
+ Untouchables = [beta]
+ Then a wanted (beta ~ alpha) comes along.
+ 1) While interacting with the inerts it is going to kick w2,w4
+ out of the inerts
+ 2) Then, it will spontaneoulsy be solved by (alpha := beta)
+ 3) Now (and here is the tricky part), to add him back as
+ solved (alpha ~ beta) is no good because, in the next
+ iteration, it will kick out w1,w3 as well so we will end up
+ with *all* the inert equalities back in the worklist!
+
+ So it is tempting to just add (beta ~ alpha) instead, that is,
+ maintain the original orietnation of the constraint.
+
+ But that does not work very well, because it may cause the
+ "double unification problem" (See Note [Avoid double unifications]).
+ For instance:
+
+ Example 2b:
+ [w1] : fsk1 ~ alpha
+ [w2] : fsk2 ~ alpha
+ ---
+ At the end of the interaction suppose we spontaneously solve alpha := fsk1
+ but keep [Given] fsk1 ~ alpha. Then, the second time around we see the
+ constraint (fsk2 ~ alpha), and we unify *again* alpha := fsk2, which is wrong.
+
+ Our conclusion is that, while in some cases (Example 2a), it makes sense to
+ preserve the original orientation, it is hard to do this in a sound way.
+ So we *don't* do this for now, @solveWithIdentity@ outputs a constraint that
+ is oriented with the unified variable on the left.
+
+Case 3: Functional Dependencies and IP improvement work
+ TODO. Optimisation not yet implemented there.
+
\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
- | isWantedCt workItem
- -- Original was wanted we have now made him given so
- -- we have to ineract him with the inerts again because
- -- of the change in his status. This may produce some work.
- -> do { traceTcS "recursive interact with inerts {" $ vcat
- [ text "work = " <+> ppr workItem'
- , text "inerts = " <+> ppr inerts ]
- ; itr_again <- interactWithInertsStage workItem' inerts
- ; case sr_stop itr_again of
- Stop -> pprPanic "BUG: Impossible to happen" $
- vcat [ text "Original workitem:" <+> ppr workItem
- , text "Spontaneously solved:" <+> ppr workItem'
- , text "Solved was consumed, when reacting with inerts:"
- , nest 2 (ppr inerts) ]
- ContinueWith workItem'' -- Now *this* guy is inert wrt to inerts
- -> do { traceTcS "end recursive interact }" $ ppr workItem''
- ; return $ SR { sr_new_work = sr_new_work itr_again
- , sr_inerts = sr_inerts itr_again
- `extendInertSet` workItem''
- , sr_stop = Stop } }
- }
- | otherwise
- -> return $ SR { sr_new_work = emptyWorkList
- , sr_inerts = inerts `extendInertSet` workItem'
- , sr_stop = Stop } }
+ Just (workItem', workList')
+ | not (isGivenCt workItem)
+ -- Original was wanted or derived but we have now made him
+ -- given so we have to interact him with the inerts due to
+ -- its status change. This in turn may produce more work.
+ -- We do this *right now* (rather than just putting workItem'
+ -- back into the work-list) because we've solved
+ -> do { (new_inert, new_work) <- runSolverPipeline
+ [ ("recursive interact with inert eqs", interactWithInertEqsStage)
+ , ("recursive interact with inerts", interactWithInertsStage)
+ ] inerts workItem'
+ ; return $ SR { sr_new_work = new_work `unionWorkLists` workList'
+ , sr_inerts = new_inert -- will include workItem'
+ , sr_stop = Stop }
+ }
+ | otherwise
+ -> -- Original was given; he must then be inert all right, and
+ -- workList' are all givens from flattening
+ return $ SR { sr_new_work = workList'
+ , sr_inerts = inerts `updInertSet` 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 (WorkItem, SWorkList))
+trySpontaneousSolve workItem@(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
- else return Nothing }
+ ; if tch1 then trySpontaneousEqOneWay inerts cv gw tv1 xi
+ else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:" (ppr workItem)
+ ; 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 (WorkItem,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
- | otherwise
- = return Nothing
+trySpontaneousEqOneWay inerts cv gw tv xi
+ | not (isSigTyVar tv) || isTyVarTy xi
+ = do { kxi <- zonkTcTypeTcS xi >>= return . typeKind -- Must look through the TcTyBinds
+ -- hence kxi and not typeKind xi
+ -- See Note [Kind Errors]
+ ; if kxi `isSubKind` tyVarKind tv then
+ solveWithIdentity inerts cv gw tv xi
+ else if tyVarKind tv `isSubKind` kxi then
+ return Nothing -- kinds are compatible but we can't solveWithIdentity this way
+ -- This case covers the a_touchable :: * ~ b_untouchable :: ??
+ -- which has to be deferred or floated out for someone else to solve
+ -- it in a scope where 'b' is no longer untouchable.
+ else kindErrorTcS gw (mkTyVarTy tv) xi -- See Note [Kind errors]
+ }
+ | otherwise -- Still can't solve, sig tyvar and non-variable rhs
+ = return Nothing
----------------
-trySpontaneousEqTwoWay :: CoVar -> CtFlavor -> TcTyVar -> TcTyVar
- -> TcS (Maybe SWorkItem)
--- 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 :: InertSet -> CoVar -> CtFlavor -> TcTyVar -> TcTyVar
+ -> TcS (Maybe (WorkItem,SWorkList))
+-- Both tyvars are *touchable* MetaTyvars so there is only a chance for kind error here
+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 -- None is a subkind of the other, but they are both touchable!
+ = kindErrorTcS gw (mkTyVarTy tv1) (mkTyVarTy tv2) -- See Note [Kind errors]
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
\end{code}
-Note [Loopy spontaneous solving]
+Note [Kind errors]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider the wanted problem:
+ alpha ~ (# Int, Int #)
+where alpha :: ?? and (# Int, Int #) :: (#). We can't spontaneously solve this constraint,
+but we should rather reject the program that give rise to it. If 'trySpontaneousEqTwoWay'
+simply returns @Nothing@ then that wanted constraint is going to propagate all the way and
+get quantified over in inference mode. That's bad because we do know at this point that the
+constraint is insoluble. Instead, we call 'kindErrorTcS' here, which immediately fails.
+
+The same applies in canonicalization code in case of kind errors in the givens.
+
+However, when we canonicalize givens we only check for compatibility (@compatKind@).
+If there were a kind error in the givens, this means some form of inconsistency or dead code.
+
+When we spontaneously solve wanteds we may have to look through the bindings, hence we
+call zonkTcTypeTcS above. The reason is that maybe xi is @alpha@ where alpha :: ? and
+a previous spontaneous solving has set (alpha := f) with (f :: *). The reason that xi is
+still alpha and not f is becasue the solved constraint may be oriented as (f ~ alpha) instead
+of (alpha ~ f). Then we should be using @xi@s "real" kind, which is * and not ?, when we try
+to detect whether spontaneous solving is possible.
+
+
+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]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Example 1: [The problem of loopy spontaneous solving]
+----------
Consider the original wanted:
wanted : Maybe (E alpha) ~ alpha
where E is a type family, such that E (T x) = x. After canonicalization,
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:
+
+Example 2: [The need of keeping track of flatten skolem equivalence classes]
+----------
+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!
+
+Example 3: [The need of looking through TyBinds for already spontaneously solved variables]
+----------
+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.
+
+Example 4: [Orientation of (tv ~ xi) equalities]
+----------
+We orient equalities (tv ~ xi) so that flatten skolems appear on the left, if possible. Here
+is an example of why this is needed:
+
+ [Wanted] w1: alpha ~ fsk
+ [Given] g1: F alpha ~ fsk
+ [Given] g2: b ~ fsk
+ Flatten skolem equivalence class = []
+
+Assume that g2 is *not* oriented properly, as shown above. Then we would like to spontaneously
+solve w1 but we can't set alpha := fsk, since fsk hides the type F alpha. However, by using
+the equation g2 it would be possible to solve w1 by setting alpha := b. In other words, it is
+not enough to look at a flatten skolem equivalence class to try to find alternatives to unify
+with. We may have to go to other variables.
+
+By orienting the equalities so that flatten skolems are in the LHS we are eliminating them
+as much as possible from the RHS of other wanted equalities, and hence it suffices to look
+in their flatten skolem equivalence classes.
+
+NB: This situation appears in the IndTypesPerf test case, inside indexed-types/.
+
+Caveat: You may wonder if we should be doing this for unification variables as well.
+However, Note [Efficient Orientation], Case 2, demonstrates that this is not possible
+at least for touchable unification variables which we have to keep oriented with the
+touchable on the LHS to be able to eliminate it. So then, what about untouchables?
+
+Example 4a:
+-----------
+ Untouchable = beta, Touchable = alpha
+
+ [Wanted] w1: alpha ~ fsk
+ [Given] g1: F alpha ~ fsk
+ [Given] g2: beta ~ fsk
+ Flatten skolem equivalence class = []
+
+Should we be able to unify alpha := beta to solve the constraint? Arguably yes, but
+that implies that an *untouchable* unification variable (beta) is in the same equivalence
+class as a flatten skolem that mentions @alpha@. I.e. g2 means that:
+ beta ~ F alpha
+But I do not think that there is any way to produce evidence for such a constraint from
+the outside other than beta := F alpha, which violates the OutsideIn-ness.
+
+
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 (WorkItem, 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
+-- Returns: (workItem, workList) where
+-- workItem = the new Given constraint
+-- workList = some additional work that may have been produced as a result of flattening
+-- in case we did some chasing through flatten skolem equivalence classes.
+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
+ ; (ct,cts, co) <- case coi of
+ ACo co -> do { (cc,ccs) <- canEqLeafTyVarLeft flav cv_given tv xi_unflat
+ ; return (cc, ccs, co) }
+ IdCo co -> return $ (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!
+ , emptyWorkList, 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 (ct,cts)
+ }
+
+-- ; let flav = mkGivenFlavor gw UnkSkol
+-- ; (cts, co) <- case coi of
+-- -- TODO: Optimise this, along the way it used to be
+-- ACo co -> do { cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat
+-- ; setWantedTyBind tv xi_unflat
+-- ; can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi_unflat
+-- ; return (can_eqs, co) }
+-- IdCo co -> do { cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi xi
+-- ; setWantedTyBind tv xi
+-- ; can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi
+-- ; return (can_eqs, 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 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}
, ir_new_work :: WorkList
-- new work items to add to the WorkList
+
+ , ir_improvement :: Maybe FDImprovement -- In case improvement kicked in
}
-- What to do with the inert reactant.
deriving Eq
mkIRContinue :: Monad m => WorkItem -> InertAction -> WorkList -> m InteractResult
-mkIRContinue wi keep newWork = return $ IR (ContinueWith wi) keep newWork
+mkIRContinue wi keep newWork = return $ IR (ContinueWith wi) keep newWork Nothing
mkIRStop :: Monad m => InertAction -> WorkList -> m InteractResult
-mkIRStop keep newWork = return $ IR Stop keep newWork
+mkIRStop keep newWork = return $ IR Stop keep newWork Nothing
+
+mkIRStop_RecordImprovement :: Monad m => InertAction -> WorkList -> FDImprovement -> m InteractResult
+mkIRStop_RecordImprovement keep newWork fdimpr = return $ IR Stop keep newWork (Just fdimpr)
+
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
+ -- See Note [Efficient Orientation, Case 2]
---------------------------------------------------
--- Interact a single WorkItem with an InertSet as far as possible, i.e. until we get a Stop
--- result from an individual interaction (i.e. when the WorkItem is consumed), or until we've
--- interacted the WorkItem with the entire InertSet.
---
--- Postcondition: the new InertSet in the resulting StageResult is subset
--- of the input InertSet.
+-- Interact a single WorkItem with the equalities of an inert set as far as possible, i.e. until we
+-- get a Stop result from an individual reaction (i.e. when the WorkItem is consumed), or until we've
+-- interact the WorkItem with the entire equalities of the InertSet
+
+interactWithInertEqsStage :: SimplifierStage
+interactWithInertEqsStage workItem inert
+ = foldISEqCtsM interactNext initITR inert
+ where initITR = SR { sr_inerts = IS { inert_eqs = emptyCCan -- We will fold over the equalities
+ , inert_fsks = Map.empty -- which will generate those two again
+ , inert_cts = inert_cts inert
+ , inert_fds = inert_fds inert
+ }
+ , sr_new_work = emptyWorkList
+ , sr_stop = ContinueWith workItem }
+
+---------------------------------------------------
+-- Interact a single WorkItem with *non-equality* constraints in the inert set.
+-- Precondition: equality interactions must have already happened, hence we have
+-- to pick up some information from the incoming inert, before folding over the
+-- "Other" constraints it contains!
interactWithInertsStage :: SimplifierStage
interactWithInertsStage workItem inert
- = foldlInertSetM interactNext initITR inert
+ = foldISOtherCtsM interactNext initITR inert
where
- initITR = SR { sr_inerts = emptyInert
- , sr_new_work = emptyCCan
+ initITR = SR { -- Pick up: (1) equations, (2) FD improvements, (3) FlatSkol equiv. classes
+ sr_inerts = IS { inert_eqs = inert_eqs inert
+ , inert_cts = emptyCCan
+ , inert_fds = inert_fds inert
+ , inert_fsks = inert_fsks inert }
+ , 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
- else inerts
- , sr_new_work = sr_new_work it `unionWorkLists` ir_new_work ir
- , sr_stop = ir_stop ir } }
- | otherwise = return $ itrAddInert inert it
-
-
- itrAddInert :: AtomicInert -> StageResult -> StageResult
- itrAddInert inert itr = itr { sr_inerts = (sr_inerts itr) `extendInertSet` inert }
+interactNext :: StageResult -> AtomicInert -> TcS StageResult
+interactNext it inert
+ | ContinueWith workItem <- sr_stop it
+ = do { let inerts = sr_inerts it
+ fdimprs_old = getFDImprovements inerts
+
+ ; ir <- interactWithInert fdimprs_old inert workItem
+
+ -- New inerts depend on whether we KeepInert or not and must
+ -- be updated with FD improvement information from the interaction result (ir)
+ ; let inerts_new = updInertSetFDImprs upd_inert (ir_improvement ir)
+ upd_inert = if ir_inert_action ir == KeepInert
+ then inerts `updInertSet` inert else inerts
+
+ ; return $ SR { sr_inerts = inerts_new
+ , sr_new_work = sr_new_work it `unionWorkLists` ir_new_work ir
+ , sr_stop = ir_stop ir } }
+ | otherwise
+ = return $ it { sr_inerts = (sr_inerts it) `updInertSet` inert }
-- Do a single interaction of two constraints.
-interactWithInert :: AtomicInert -> WorkItem -> TcS InteractResult
-interactWithInert inert workitem
+interactWithInert :: FDImprovements -> AtomicInert -> WorkItem -> TcS InteractResult
+interactWithInert fdimprs inert workitem
= do { ctxt <- getTcSContext
; let is_allowed = allowedInteraction (simplEqsOnly ctxt) inert workitem
inert_ev = cc_id inert
-- We don't have to do this for givens, as we fully know the evidence for them.
; rec_ev_ok <-
case (cc_flavor inert, cc_flavor workitem) of
- (Wanted loc, Derived _) -> isGoodRecEv work_ev (WantedEvVar inert_ev loc)
- (Derived _, Wanted loc) -> isGoodRecEv inert_ev (WantedEvVar work_ev loc)
- _ -> return True
+ (Wanted loc, Derived {}) -> isGoodRecEv work_ev (WantedEvVar inert_ev loc)
+ (Derived {}, Wanted loc) -> isGoodRecEv inert_ev (WantedEvVar work_ev loc)
+ _ -> return True
; if is_allowed && rec_ev_ok then
- doInteractWithInert inert workitem
+ doInteractWithInert fdimprs inert workitem
else
noInteraction workitem
}
allowedInteraction _ _ _ = True
--------------------------------------------
-doInteractWithInert :: CanonicalCt -> CanonicalCt -> TcS InteractResult
+doInteractWithInert :: FDImprovements -> CanonicalCt -> CanonicalCt -> TcS InteractResult
-- Identical class constraints.
-doInteractWithInert
+doInteractWithInert fdimprs
(CDictCan { cc_id = d1, cc_flavor = fl1, cc_class = cls1, cc_tyargs = tys1 })
workItem@(CDictCan { cc_id = d2, cc_flavor = fl2, cc_class = cls2, cc_tyargs = tys2 })
| cls1 == cls2 && (and $ zipWith tcEqType tys1 tys2)
| cls1 == cls2 && (not (isGiven fl1 && isGiven fl2))
= -- See Note [When improvement happens]
- do { let work_item_pred_loc = (ClassP cls2 tys2, ppr d2)
- inert_pred_loc = (ClassP cls1 tys1, ppr d1)
+ do { let pty1 = ClassP cls1 tys1
+ pty2 = ClassP cls2 tys2
+ work_item_pred_loc = (pty2, ppr d2)
+ inert_pred_loc = (pty1, ppr d1)
loc = combineCtLoc fl1 fl2
eqn_pred_locs = improveFromAnother work_item_pred_loc inert_pred_loc
+
; wevvars <- mkWantedFunDepEqns loc eqn_pred_locs
+ ; fd_work <- canWanteds wevvars
-- See Note [Generating extra equalities]
- ; workList <- canWanteds wevvars
- ; mkIRContinue workItem KeepInert workList -- Keep the inert there so we avoid
- -- re-introducing the fundep equalities
+ ; traceTcS "Checking if improvements existed." (ppr fdimprs)
+ ; if isEmptyWorkList fd_work || haveBeenImproved fdimprs pty1 pty2 then
+ -- Must keep going
+ mkIRContinue workItem KeepInert fd_work
+ else do { traceTcS "Recording improvement and throwing item back in worklist." (ppr (pty1,pty2))
+ ; mkIRStop_RecordImprovement KeepInert
+ (fd_work `unionWorkLists` workListFromCCan workItem) (pty1,pty2)
+ }
-- See Note [FunDep Reactions]
}
-- Class constraint and given equality: use the equality to rewrite
-- the class constraint.
-doInteractWithInert (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
+doInteractWithInert _fdimprs
+ (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
(CDictCan { cc_id = dv, cc_flavor = wfl, cc_class = cl, cc_tyargs = xis })
| ifl `canRewrite` wfl
, tv `elemVarSet` tyVarsOfTypes xis
- -- substitute for tv in xis. Note that the resulting class
- -- constraint is still canonical, since substituting xi-types in
- -- xi-types generates xi-types. However, it may no longer be
- -- inert with respect to the inert set items we've already seen.
- -- For example, consider the inert set
- --
- -- D Int (g)
- -- a ~g Int
- --
- -- and the work item D a (w). D a does not interact with D Int.
- -- Next, it does interact with a ~g Int, getting rewritten to D
- -- Int (w). But now we must go back through the rest of the inert
- -- 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) }
+ = if isDerivedSC wfl then
+ mkIRStop KeepInert $ emptyWorkList -- See Note [Adding Derived Superclasses]
+ else do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,wfl,cl,xis)
+ -- Continue with rewritten Dictionary because we can only be in the
+ -- interactWithEqsStage, so the dictionary is inert.
+ ; mkIRContinue rewritten_dict KeepInert emptyWorkList }
-doInteractWithInert (CDictCan { cc_id = dv, cc_flavor = ifl, cc_class = cl, cc_tyargs = xis })
+doInteractWithInert _fdimprs
+ (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) }
+ = if isDerivedSC ifl then
+ mkIRContinue workItem DropInert emptyWorkList -- No need to do any rewriting,
+ -- see Note [Adding Derived Superclasses]
+ else do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,ifl,cl,xis)
+ ; mkIRContinue workItem DropInert (workListFromCCan rewritten_dict) }
-- Class constraint and given equality: use the equality to rewrite
-- the class constraint.
-doInteractWithInert (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
+doInteractWithInert _fdimprs
+ (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
(CIPCan { cc_id = ipid, cc_flavor = wfl, cc_ip_nm = nm, cc_ip_ty = ty })
| ifl `canRewrite` wfl
, tv `elemVarSet` tyVarsOfType ty
= do { rewritten_ip <- rewriteIP (cv,tv,xi) (ipid,wfl,nm,ty)
- ; mkIRStop KeepInert (singleCCan rewritten_ip) }
+ ; mkIRContinue rewritten_ip KeepInert emptyWorkList }
-doInteractWithInert (CIPCan { cc_id = ipid, cc_flavor = ifl, cc_ip_nm = nm, cc_ip_ty = ty })
+doInteractWithInert _fdimprs
+ (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 (workListFromCCan rewritten_ip) }
-- Two implicit parameter constraints. If the names are the same,
-- but their types are not, we generate a wanted type equality
-- that equates the type (this is "improvement").
-- However, we don't actually need the coercion evidence,
-- 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 })
+doInteractWithInert _fdimprs
+ (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 emptyCCan
+ mkIRContinue workItem DropInert emptyWorkList
| nm1 == nm2 && ty1 `tcEqType` ty2
= solveOneFromTheOther (id1,ifl) workItem
= -- 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 cans }
-- Inert: equality, work item: function equality
-- we will ``expose'' x2 and x4 to rewriting.
-- Otherwise, we can try rewriting the type function equality with the equality.
-doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi1 })
+doInteractWithInert _fdimprs
+ (CTyEqCan { cc_id = cv1, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi1 })
(CFunEqCan { cc_id = cv2, cc_flavor = wfl, cc_fun = tc
, cc_tyargs = args, cc_rhs = xi2 })
| 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 (workListFromCCan rewritten_funeq) }
+ -- must Stop here, because we may no longer be inert after the rewritting.
-- Inert: function equality, work item: equality
-
-doInteractWithInert (CFunEqCan {cc_id = cv1, cc_flavor = ifl, cc_fun = tc
+doInteractWithInert _fdimprs
+ (CFunEqCan {cc_id = cv1, cc_flavor = ifl, cc_fun = tc
, cc_tyargs = args, cc_rhs = xi1 })
workItem@(CTyEqCan { cc_id = cv2, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi2 })
| 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 (workListFromCCan rewritten_funeq) }
-doInteractWithInert (CFunEqCan { cc_id = cv1, cc_flavor = fl1, cc_fun = tc1
+doInteractWithInert _fdimprs
+ (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)
+ | fl1 `canSolve` fl2 && lhss_match
+ = do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
; mkIRStop KeepInert cans }
- | fl2 `canRewrite` fl1 && lhss_match
- = do { cans <- rewriteEqLHS (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
+ | fl2 `canSolve` fl1 && lhss_match
+ = do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
; mkIRContinue workItem DropInert 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 _fdimprs
+ 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)
+ | fl1 `canSolve` 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 (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
+ | fl2 `canSolve` fl1 && tv1 == tv2
+ = do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
; mkIRContinue workItem DropInert cans }
-- Check for rewriting RHS
= 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]
--- 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 (workListFromCCan 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 _fdimprs _ workItem = noInteraction workItem
+-------------------------
-- Equational Rewriting
rewriteDict :: (CoVar, TcTyVar, Xi) -> (DictId, CtFlavor, Class, [Xi]) -> TcS CanonicalCt
rewriteDict (cv,tv,xi) (dv,gw,cl,xis)
, cc_rhs = xi2 }) }
-rewriteEqRHS :: (CoVar,TcTyVar,Xi) -> (CoVar,CtFlavor,TcTyVar,Xi) -> TcS CanonicalCts
+rewriteEqRHS :: (CoVar,TcTyVar,Xi) -> (CoVar,CtFlavor,TcTyVar,Xi) -> TcS WorkList
-- Use the first equality to rewrite the second, flavors already checked.
-- E.g. c1 : tv1 ~ xi1 c2 : tv2 ~ xi2
-- rewrites c2 to give
mkCoVarCoercion cv2 `mkTransCoercion` co2'
; xi2'' <- canOccursCheck gw tv2 xi2' -- we know xi2' is *not* tv2
- ; return (singleCCan $ CTyEqCan { cc_id = cv2'
- , cc_flavor = gw
- , cc_tyvar = tv2
- , cc_rhs = xi2'' })
+ ; canEq gw cv2' (mkTyVarTy tv2) 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
--- Used to ineratct two equalities of the following form:
+
+rewriteEqLHS :: WhichComesFromInert -> (Coercion,Xi) -> (CoVar,CtFlavor,Xi) -> TcS WorkList
+-- Used to ineract 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),
+-- See Note [Efficient Orientation] for that
+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
- | 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 `canRewrite` ifl
+ | otherwise -- wfl `canSolve` ifl
= do { unless (isGiven ifl) $ setEvBind iid (EvId wid)
- ; mkIRContinue workItem DropInert emptyCCan }
+ ; mkIRContinue workItem DropInert emptyWorkList }
where
wfl = cc_flavor workItem
doTopReact :: WorkItem -> TcS TopInteractResult
-- The work item does not react with the inert set,
-- so try interaction with top-level instances
+
+-- Given dictionary; just add superclasses
+-- See Note [Given constraint that matches an instance declaration]
+doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = Given loc
+ , cc_class = cls, cc_tyargs = xis })
+ = do { sc_work <- newGivenSCWork dv loc cls xis
+ ; return $ SomeTopInt sc_work (ContinueWith workItem) }
+
+-- Derived dictionary
+-- Do not add any further derived superclasses; their
+-- full transitive closure has already been added.
+-- But do look for functional dependencies
+doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = Derived loc _
+ , cc_class = cls, cc_tyargs = xis })
+ = do { fd_work <- findClassFunDeps dv cls xis loc
+ ; if isEmptyWorkList fd_work then
+ return NoTopInt
+ else return $ SomeTopInt { tir_new_work = fd_work
+ , tir_new_inert = ContinueWith workItem } }
+
doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = Wanted loc
, cc_class = cls, cc_tyargs = xis })
= do { -- See Note [MATCHING-SYNONYMS]
; lkp_inst_res <- matchClassInst cls xis loc
; case lkp_inst_res of
- NoInstance -> do { traceTcS "doTopReact/ no class instance for" (ppr dv)
- ; funDepReact }
+ NoInstance ->
+ do { traceTcS "doTopReact/ no class instance for" (ppr dv)
+ ; fd_work <- findClassFunDeps dv cls xis loc
+ ; if isEmptyWorkList fd_work then
+ do { sc_work <- newDerivedSCWork dv loc cls xis
+ -- See Note [Adding Derived Superclasses]
+ -- 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!
+ ; return $ SomeTopInt
+ { tir_new_work = fd_work `unionWorkLists` sc_work
+ , tir_new_inert = ContinueWith workItem } }
+
+ else -- More fundep work produced, don't do any superclass stuff,
+ -- just thow him back in the worklist, which will prioritize
+ -- the solution of fd equalities
+ return $ SomeTopInt
+ { tir_new_work = fd_work `unionWorkLists`
+ workListFromCCan workItem
+ , tir_new_inert = Stop } }
+
GenInst wtvs ev_term -> -- Solved
-- No need to do fundeps stuff here; the instance
-- matches already so we won't get any more info
-- from functional dependencies
do { traceTcS "doTopReact/ found class instance for" (ppr dv)
; setDictBind dv ev_term
- ; workList <- canWanteds wtvs
+ ; inst_work <- canWanteds wtvs
; if null wtvs
-- 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
+ else do { let solved = makeSolvedByInst workItem
+ ; sc_work <- newDerivedSCWork dv loc cls xis
+ -- See Note [Adding Derived Superclasses]
; return $ SomeTopInt
- { tir_new_work = workList `unionWorkLists` sc_work
+ { tir_new_work = inst_work `unionWorkLists` sc_work
, tir_new_inert = ContinueWith solved } }
- }
- }
- where
- -- Try for a fundep reaction beween the wanted item
- -- and a top-level instance declaration
- funDepReact
- = do { instEnvs <- getInstEnvs
- ; let eqn_pred_locs = improveFromInstEnv (classInstances instEnvs)
- (ClassP cls xis, ppr dv)
- ; 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 }
- -- 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!
-
- -- See Note [FunDep Reactions]
- }
-
--- Otherwise, we have a given or derived
-doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = fl
- , cc_class = cls, cc_tyargs = xis })
- = do { sc_work <- newSCWorkFromFlavored dv fl cls xis
- ; return $ SomeTopInt sc_work (ContinueWith workItem) }
- -- See Note [Given constraint that matches an instance declaration]
+ } }
-- Type functions
doTopReact (CFunEqCan { cc_id = cv, cc_flavor = fl
_ -> newGivOrDerCoVar xi rhs_ty $
mkSymCoercion (mkCoVarCoercion cv) `mkTransCoercion` coe
- ; workList <- mkCanonical fl cv'
- ; return $ SomeTopInt workList Stop }
+ ; can_cts <- mkCanonical fl cv'
+ ; return $ SomeTopInt can_cts Stop }
_
-> panicTcS $ text "TcSMonad.matchFam returned multiple instances!"
}
-- Any other work item does not react with any top-level equations
doTopReact _workItem = return NoTopInt
+
+----------------------
+findClassFunDeps :: EvVar -> Class -> [Xi] -> WantedLoc -> TcS WorkList
+-- Look for a fundep reaction beween the wanted item
+-- and a top-level instance declaration
+findClassFunDeps dv cls xis loc
+ = do { instEnvs <- getInstEnvs
+ ; let eqn_pred_locs = improveFromInstEnv (classInstances instEnvs)
+ (ClassP cls xis, ppr dv)
+ ; wevvars <- mkWantedFunDepEqns loc eqn_pred_locs
+ -- NB: fundeps generate some wanted equalities, but
+ -- we don't use their evidence for anything
+ ; canWanteds wevvars }
\end{code}
+Note [Adding Derived Superclasses]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Generally speaking, we want to be able to add derived superclasses of
+unsolved wanteds, and wanteds that have been partially being solved
+via an instance. This is important to be able to simplify the inferred
+constraints more (and to allow for recursive dictionaries, less
+importantly). Example:
+
+Inferred wanted constraint is (Eq a, Ord a), but we'd only like to
+quantify over Ord a, hence we would like to be able to add the
+superclass of Ord a as Derived and use it to solve the wanted Eq a.
+
+Hence we will add Derived superclasses in the following two cases:
+ (1) When we meet an unsolved wanted in top-level reactions
+ (2) When we partially solve a wanted in top-level reactions using an instance decl.
+
+At that point, we have two options:
+ (1) Add transitively add *ALL* of the superclasses of the Derived
+ (2) Add only the immediate ones, but whenever we meet a Derived in
+ the future, add its own superclasses as Derived.
+
+Option (2) is terrible, because deriveds may be rewritten or kicked
+out of the inert set, which will result in slightly rewritten
+superclasses being reintroduced in the worklist and the inert set. Eg:
+
+ class C a => B a
+ instance Foo a => B [a]
+
+Original constraints:
+[Wanted] d : B [a]
+[Given] co : a ~ Int
+
+We apply the instance to the wanted and put it and its superclasses as
+as Deriveds in the inerts:
+
+[Derived] d : B [a]
+[Derived] (sel d) : C [a]
+
+The work is now:
+[Given] co : a ~ Int
+[Wanted] d' : Foo a
+
+Now, suppose that we interact the Derived with the Given equality, and
+kick him out of the inert, the next time around a superclass C [Int]
+will be produced -- but we already *have* C [a] in the inerts which
+will anyway get rewritten to C [Int].
+
+So we choose (1), and *never* introduce any more superclass work from
+Deriveds. This enables yet another optimisation: If we ever meet an
+equality that can rewrite a Derived, if that Derived is a superclass
+derived (like C [a] above), i.e. not a partially solved one (like B
+[a]) above, we may simply completely *discard* that Derived. The
+reason is because somewhere in the inert lies the original wanted, or
+partially solved constraint that gave rise to that superclass, and
+that constraint *will* be kicked out, and *will* result in the
+rewritten superclass to be added in the inerts later on, anyway.
+
+
+
Note [FunDep and implicit parameter reactions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Currently, our story of interacting two dictionaries (or a dictionary
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.
that participate in recursive dictionary bindings.
\begin{code}
-newSCWorkFromFlavored :: EvVar -> CtFlavor -> Class -> [Xi]
- -> TcS WorkList
-newSCWorkFromFlavored ev flavor cls xis
- | Given loc <- flavor -- The NoScSkol says "don't add superclasses"
- , NoScSkol <- ctLocOrigin loc -- Very important!
+
+newGivenSCWork :: EvVar -> GivenLoc -> Class -> [Xi] -> TcS WorkList
+newGivenSCWork ev loc cls xis
+ | NoScSkol <- ctLocOrigin loc -- Very important!
= return emptyWorkList
-
| otherwise
+ = newImmSCWorkFromFlavored ev (Given loc) cls xis >>= return
+
+newDerivedSCWork :: EvVar -> WantedLoc -> Class -> [Xi] -> TcS WorkList
+newDerivedSCWork ev loc cls xis
+ = do { ims <- newImmSCWorkFromFlavored ev flavor cls xis
+ ; rec_sc_work ims }
+ where
+ rec_sc_work :: CanonicalCts -> TcS CanonicalCts
+ rec_sc_work cts
+ = do { bg <- mapBagM (\c -> do { ims <- imm_sc_work c
+ ; recs_ims <- rec_sc_work ims
+ ; return $ consBag c recs_ims }) cts
+ ; return $ concatBag bg }
+ imm_sc_work (CDictCan { cc_id = dv, cc_flavor = fl, cc_class = cls, cc_tyargs = xis })
+ = newImmSCWorkFromFlavored dv fl cls xis
+ imm_sc_work _ct = return emptyCCan
+
+ flavor = Derived loc DerSC
+
+newImmSCWorkFromFlavored :: EvVar -> CtFlavor -> Class -> [Xi] -> TcS WorkList
+-- Returns immediate superclasses
+newImmSCWorkFromFlavored ev flavor cls xis
= do { let (tyvars, sc_theta, _, _) = classBigSig cls
sc_theta1 = substTheta (zipTopTvSubst tyvars xis) sc_theta
- -- 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 }
+ ; mkCanonicals flavor sc_vars }
where
inst_one pred n = newGivOrDerEvVar pred (EvSuperClass ev n)
+
data LookupInstResult
= NoInstance
| GenInst [WantedEvVar] EvTerm
}
}
\end{code}
-
-
projects / ghc-hetmet.git / blobdiff