3 solveInteract, solveInteractGiven, solveInteractWanted,
4 AtomicInert, tyVarsOfInert,
5 InertSet, emptyInert, updInertSet, extractUnsolved, solveOne,
8 #include "HsVersions.h"
22 import Inst( tyVarsOfEvVar )
36 import qualified Data.Map as Map
38 import Control.Monad( when )
40 import FastString ( sLit )
44 Note [InertSet invariants]
45 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
46 An InertSet is a bag of canonical constraints, with the following invariants:
48 1 No two constraints react with each other.
50 A tricky case is when there exists a given (solved) dictionary
51 constraint and a wanted identical constraint in the inert set, but do
52 not react because reaction would create loopy dictionary evidence for
53 the wanted. See note [Recursive dictionaries]
55 2 Given equalities form an idempotent substitution [none of the
56 given LHS's occur in any of the given RHS's or reactant parts]
58 3 Wanted equalities also form an idempotent substitution
60 4 The entire set of equalities is acyclic.
62 5 Wanted dictionaries are inert with the top-level axiom set
64 6 Equalities of the form tv1 ~ tv2 always have a touchable variable
65 on the left (if possible).
67 7 No wanted constraints tv1 ~ tv2 with tv1 touchable. Such constraints
68 will be marked as solved right before being pushed into the inert set.
69 See note [Touchables and givens].
71 8 No Given constraint mentions a touchable unification variable,
74 Note that 6 and 7 are /not/ enforced by canonicalization but rather by
75 insertion in the inert list, ie by TcInteract.
77 During the process of solving, the inert set will contain some
78 previously given constraints, some wanted constraints, and some given
79 constraints which have arisen from solving wanted constraints. For
80 now we do not distinguish between given and solved constraints.
82 Note that we must switch wanted inert items to given when going under an
83 implication constraint (when in top-level inference mode).
87 data CCanMap a = CCanMap { cts_given :: Map.Map a CanonicalCts
88 -- Invariant: all Given
89 , cts_derived :: Map.Map a CanonicalCts
90 -- Invariant: all Derived
91 , cts_wanted :: Map.Map a CanonicalCts }
92 -- Invariant: all Wanted
94 cCanMapToBag :: Ord a => CCanMap a -> CanonicalCts
95 cCanMapToBag cmap = Map.fold unionBags rest_wder (cts_given cmap)
96 where rest_wder = Map.fold unionBags rest_der (cts_wanted cmap)
97 rest_der = Map.fold unionBags emptyCCan (cts_derived cmap)
99 emptyCCanMap :: CCanMap a
100 emptyCCanMap = CCanMap { cts_given = Map.empty
101 , cts_derived = Map.empty, cts_wanted = Map.empty }
103 updCCanMap:: Ord a => (a,CanonicalCt) -> CCanMap a -> CCanMap a
104 updCCanMap (a,ct) cmap
105 = case cc_flavor ct of
107 -> cmap { cts_wanted = Map.insertWith unionBags a this_ct (cts_wanted cmap) }
109 -> cmap { cts_given = Map.insertWith unionBags a this_ct (cts_given cmap) }
111 -> cmap { cts_derived = Map.insertWith unionBags a this_ct (cts_derived cmap) }
112 where this_ct = singleCCan ct
114 getRelevantCts :: Ord a => a -> CCanMap a -> (CanonicalCts, CCanMap a)
115 -- Gets the relevant constraints and returns the rest of the CCanMap
116 getRelevantCts a cmap
117 = let relevant = unionManyBags [ Map.findWithDefault emptyCCan a (cts_wanted cmap)
118 , Map.findWithDefault emptyCCan a (cts_given cmap)
119 , Map.findWithDefault emptyCCan a (cts_derived cmap) ]
120 residual_map = cmap { cts_wanted = Map.delete a (cts_wanted cmap)
121 , cts_given = Map.delete a (cts_given cmap)
122 , cts_derived = Map.delete a (cts_derived cmap) }
123 in (relevant, residual_map)
125 extractUnsolvedCMap :: Ord a => CCanMap a -> (CanonicalCts, CCanMap a)
126 -- Gets the wanted or derived constraints and returns a residual
127 -- CCanMap with only givens.
128 extractUnsolvedCMap cmap =
129 let wntd = Map.fold unionBags emptyCCan (cts_wanted cmap)
130 derd = Map.fold unionBags emptyCCan (cts_derived cmap)
131 in (wntd `unionBags` derd,
132 cmap { cts_wanted = Map.empty, cts_derived = Map.empty })
135 -- See Note [InertSet invariants]
137 = IS { inert_eqs :: CanonicalCts -- Equalities only (CTyEqCan)
138 , inert_dicts :: CCanMap Class -- Dictionaries only
139 , inert_ips :: CCanMap (IPName Name) -- Implicit parameters
140 , inert_frozen :: CanonicalCts
141 , inert_funeqs :: CCanMap TyCon -- Type family equalities only
142 -- This representation allows us to quickly get to the relevant
143 -- inert constraints when interacting a work item with the inert set.
146 tyVarsOfInert :: InertSet -> TcTyVarSet
147 tyVarsOfInert (IS { inert_eqs = eqs
148 , inert_dicts = dictmap
150 , inert_frozen = frozen
151 , inert_funeqs = funeqmap }) = tyVarsOfCanonicals cts
153 cts = eqs `andCCan` frozen `andCCan` cCanMapToBag dictmap
154 `andCCan` cCanMapToBag ipmap `andCCan` cCanMapToBag funeqmap
156 instance Outputable InertSet where
157 ppr is = vcat [ vcat (map ppr (Bag.bagToList $ inert_eqs is))
158 , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_dicts is)))
159 , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_ips is)))
160 , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_funeqs is)))
161 , vcat (map ppr (Bag.bagToList $ inert_frozen is))
164 emptyInert :: InertSet
165 emptyInert = IS { inert_eqs = Bag.emptyBag
166 , inert_frozen = Bag.emptyBag
167 , inert_dicts = emptyCCanMap
168 , inert_ips = emptyCCanMap
169 , inert_funeqs = emptyCCanMap }
171 updInertSet :: InertSet -> AtomicInert -> InertSet
173 | isCTyEqCan item -- Other equality
174 = let eqs' = inert_eqs is `Bag.snocBag` item
175 in is { inert_eqs = eqs' }
176 | Just cls <- isCDictCan_Maybe item -- Dictionary
177 = is { inert_dicts = updCCanMap (cls,item) (inert_dicts is) }
178 | Just x <- isCIPCan_Maybe item -- IP
179 = is { inert_ips = updCCanMap (x,item) (inert_ips is) }
180 | Just tc <- isCFunEqCan_Maybe item -- Function equality
181 = is { inert_funeqs = updCCanMap (tc,item) (inert_funeqs is) }
183 = is { inert_frozen = inert_frozen is `Bag.snocBag` item }
185 extractUnsolved :: InertSet -> (InertSet, CanonicalCts)
186 -- Postcondition: the returned canonical cts are either Derived, or Wanted.
187 extractUnsolved is@(IS {inert_eqs = eqs})
188 = let is_solved = is { inert_eqs = solved_eqs
189 , inert_dicts = solved_dicts
190 , inert_ips = solved_ips
191 , inert_frozen = emptyCCan
192 , inert_funeqs = solved_funeqs }
193 in (is_solved, unsolved)
195 where (unsolved_eqs, solved_eqs) = Bag.partitionBag (not.isGivenCt) eqs
196 (unsolved_ips, solved_ips) = extractUnsolvedCMap (inert_ips is)
197 (unsolved_dicts, solved_dicts) = extractUnsolvedCMap (inert_dicts is)
198 (unsolved_funeqs, solved_funeqs) = extractUnsolvedCMap (inert_funeqs is)
200 unsolved = unsolved_eqs `unionBags` inert_frozen is `unionBags`
201 unsolved_ips `unionBags` unsolved_dicts `unionBags` unsolved_funeqs
204 %*********************************************************************
206 * Main Interaction Solver *
208 **********************************************************************
212 1. Canonicalise (unary)
213 2. Pairwise interaction (binary)
214 * Take one from work list
215 * Try all pair-wise interactions with each constraint in inert
217 As an optimisation, we prioritize the equalities both in the
218 worklist and in the inerts.
220 3. Try to solve spontaneously for equalities involving touchables
221 4. Top-level interaction (binary wrt top-level)
222 Superclass decomposition belongs in (4), see note [Superclasses]
225 type AtomicInert = CanonicalCt -- constraint pulled from InertSet
226 type WorkItem = CanonicalCt -- constraint pulled from WorkList
228 -- A mixture of Given, Wanted, and Derived constraints.
229 -- We split between equalities and the rest to process equalities first.
230 type WorkList = CanonicalCts
232 unionWorkLists :: WorkList -> WorkList -> WorkList
233 unionWorkLists = andCCan
235 isEmptyWorkList :: WorkList -> Bool
236 isEmptyWorkList = isEmptyCCan
238 emptyWorkList :: WorkList
239 emptyWorkList = emptyCCan
241 workListFromCCan :: CanonicalCt -> WorkList
242 workListFromCCan = singleCCan
244 ------------------------
246 = Stop -- Work item is consumed
247 | ContinueWith WorkItem -- Not consumed
249 instance Outputable StopOrContinue where
250 ppr Stop = ptext (sLit "Stop")
251 ppr (ContinueWith w) = ptext (sLit "ContinueWith") <+> ppr w
253 -- Results after interacting a WorkItem as far as possible with an InertSet
255 = SR { sr_inerts :: InertSet
256 -- The new InertSet to use (REPLACES the old InertSet)
257 , sr_new_work :: WorkList
258 -- Any new work items generated (should be ADDED to the old WorkList)
260 -- sr_stop = Just workitem => workitem is *not* in sr_inerts and
261 -- workitem is inert wrt to sr_inerts
262 , sr_stop :: StopOrContinue
265 instance Outputable StageResult where
266 ppr (SR { sr_inerts = inerts, sr_new_work = work, sr_stop = stop })
267 = ptext (sLit "SR") <+>
268 braces (sep [ ptext (sLit "inerts =") <+> ppr inerts <> comma
269 , ptext (sLit "new work =") <+> ppr work <> comma
270 , ptext (sLit "stop =") <+> ppr stop])
272 type SubGoalDepth = Int -- Starts at zero; used to limit infinite
273 -- recursion of sub-goals
274 type SimplifierStage = SubGoalDepth -> WorkItem -> InertSet -> TcS StageResult
276 -- Combine a sequence of simplifier 'stages' to create a pipeline
277 runSolverPipeline :: SubGoalDepth
278 -> [(String, SimplifierStage)]
279 -> InertSet -> WorkItem
280 -> TcS (InertSet, WorkList)
281 -- Precondition: non-empty list of stages
282 runSolverPipeline depth pipeline inerts workItem
283 = do { traceTcS "Start solver pipeline" $
284 vcat [ ptext (sLit "work item =") <+> ppr workItem
285 , ptext (sLit "inerts =") <+> ppr inerts]
287 ; let itr_in = SR { sr_inerts = inerts
288 , sr_new_work = emptyWorkList
289 , sr_stop = ContinueWith workItem }
290 ; itr_out <- run_pipeline pipeline itr_in
292 = case sr_stop itr_out of
293 Stop -> sr_inerts itr_out
294 ContinueWith item -> sr_inerts itr_out `updInertSet` item
295 ; return (new_inert, sr_new_work itr_out) }
297 run_pipeline :: [(String, SimplifierStage)]
298 -> StageResult -> TcS StageResult
299 run_pipeline [] itr = return itr
300 run_pipeline _ itr@(SR { sr_stop = Stop }) = return itr
302 run_pipeline ((name,stage):stages)
303 (SR { sr_new_work = accum_work
305 , sr_stop = ContinueWith work_item })
306 = do { itr <- stage depth work_item inerts
307 ; traceTcS ("Stage result (" ++ name ++ ")") (ppr itr)
308 ; let itr' = itr { sr_new_work = accum_work `unionWorkLists` sr_new_work itr }
309 ; run_pipeline stages itr' }
313 Inert: {c ~ d, F a ~ t, b ~ Int, a ~ ty} (all given)
314 Reagent: a ~ [b] (given)
316 React with (c~d) ==> IR (ContinueWith (a~[b])) True []
317 React with (F a ~ t) ==> IR (ContinueWith (a~[b])) False [F [b] ~ t]
318 React with (b ~ Int) ==> IR (ContinueWith (a~[Int]) True []
321 Inert: {c ~w d, F a ~g t, b ~w Int, a ~w ty}
324 React with (c ~w d) ==> IR (ContinueWith (a~[b])) True []
325 React with (F a ~g t) ==> IR (ContinueWith (a~[b])) True [] (can't rewrite given with wanted!)
329 Inert: {a ~ Int, F Int ~ b} (given)
330 Reagent: F a ~ b (wanted)
332 React with (a ~ Int) ==> IR (ContinueWith (F Int ~ b)) True []
333 React with (F Int ~ b) ==> IR Stop True [] -- after substituting we re-canonicalize and get nothing
336 -- Main interaction solver: we fully solve the worklist 'in one go',
337 -- returning an extended inert set.
339 -- See Note [Touchables and givens].
340 solveInteractGiven :: InertSet -> GivenLoc -> [EvVar] -> TcS InertSet
341 solveInteractGiven inert gloc evs
342 = do { (_, inert_ret) <- solveInteract inert $ listToBag $
347 mk_given ev = mkEvVarX ev flav
349 solveInteractWanted :: InertSet -> [WantedEvVar] -> TcS InertSet
350 solveInteractWanted inert wvs
351 = do { (_,inert_ret) <- solveInteract inert $ listToBag $
352 map wantedToFlavored wvs
355 solveInteract :: InertSet -> Bag FlavoredEvVar -> TcS (Bool, InertSet)
356 -- Post: (True, inert_set) means we managed to discharge all constraints
357 -- without actually doing any interactions!
358 -- (False, inert_set) means some interactions occurred
359 solveInteract inert ws
360 = do { dyn_flags <- getDynFlags
361 ; sctx <- getTcSContext
363 ; traceTcS "solveInteract, before clever canonicalization:" $
364 vcat [ text "ws = " <+> ppr (mapBag (\(EvVarX ev ct)
365 -> (ct,evVarPred ev)) ws)
366 , text "inert = " <+> ppr inert ]
368 ; (flag, inert_ret) <- foldlBagM (tryPreSolveAndInteract sctx dyn_flags) (True,inert) ws
370 ; traceTcS "solveInteract, after clever canonicalization (and interaction):" $
371 vcat [ text "No interaction happened = " <+> ppr flag
372 , text "inert_ret = " <+> ppr inert_ret ]
374 ; return (flag, inert_ret) }
377 tryPreSolveAndInteract :: SimplContext
381 -> TcS (Bool, InertSet)
382 -- Returns: True if it was able to discharge this constraint AND all previous ones
383 tryPreSolveAndInteract sctx dyn_flags (all_previous_discharged, inert)
384 flavev@(EvVarX ev_var fl)
385 = do { let inert_cts = get_inert_cts (evVarPred ev_var)
387 ; this_one_discharged <- dischargeFromCCans inert_cts flavev
389 ; if this_one_discharged
390 then return (all_previous_discharged, inert)
393 { extra_cts <- mkCanonical fl ev_var
394 ; inert_ret <- solveInteractWithDepth (ctxtStkDepth dyn_flags,0,[])
396 ; return (False, inert_ret) } }
399 get_inert_cts (ClassP clas _)
400 | simplEqsOnly sctx = emptyCCan
401 | otherwise = fst (getRelevantCts clas (inert_dicts inert))
402 get_inert_cts (IParam {})
403 = emptyCCan -- We must not do the same thing for IParams, because (contrary
404 -- to dictionaries), work items /must/ override inert items.
405 -- See Note [Overriding implicit parameters] in TcInteract.
406 get_inert_cts (EqPred {})
407 = inert_eqs inert `unionBags` cCanMapToBag (inert_funeqs inert)
409 dischargeFromCCans :: CanonicalCts -> FlavoredEvVar -> TcS Bool
410 dischargeFromCCans cans (EvVarX ev fl)
411 = Bag.foldlBagM discharge_ct False cans
412 where discharge_ct :: Bool -> CanonicalCt -> TcS Bool
413 discharge_ct True _ct = return True
414 discharge_ct False ct
415 | evVarPred (cc_id ct) `tcEqPred` evVarPred ev
416 , cc_flavor ct `canSolve` fl
417 = do { when (isWanted fl) $ set_ev_bind ev (cc_id ct)
419 where set_ev_bind x y
420 | EqPred {} <- evVarPred y
421 = setEvBind x (EvCoercion (mkCoVarCoercion y))
422 | otherwise = setEvBind x (EvId y)
423 discharge_ct False _ct = return False
426 Note [Avoiding the superclass explosion]
427 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
428 This note now is not as significant as it used to be because we no
429 longer add the superclasses of Wanted as Derived, except only if they
430 have equality superclasses or superclasses with functional
431 dependencies. The fear was that hundreds of identical wanteds would
432 give rise each to the same superclass or equality Derived's which
433 would lead to a blo-up in the number of interactions.
435 Instead, what we do with tryPreSolveAndCanon, is when we encounter a
436 new constraint, we very quickly see if it can be immediately
437 discharged by a class constraint in our inert set or the previous
438 canonicals. If so, we add nothing to the returned canonical
442 solveOne :: InertSet -> WorkItem -> TcS InertSet
443 solveOne inerts workItem
444 = do { dyn_flags <- getDynFlags
445 ; solveOneWithDepth (ctxtStkDepth dyn_flags,0,[]) inerts workItem
449 solveInteractWithDepth :: (Int, Int, [WorkItem])
450 -> InertSet -> WorkList -> TcS InertSet
451 solveInteractWithDepth ctxt@(max_depth,n,stack) inert ws
456 = solverDepthErrorTcS n stack
459 = do { traceTcS "solveInteractWithDepth" $
460 vcat [ text "Current depth =" <+> ppr n
461 , text "Max depth =" <+> ppr max_depth ]
463 -- Solve equalities first
464 ; let (eqs, non_eqs) = Bag.partitionBag isCTyEqCan ws
465 ; is_from_eqs <- Bag.foldlBagM (solveOneWithDepth ctxt) inert eqs
466 ; Bag.foldlBagM (solveOneWithDepth ctxt) is_from_eqs non_eqs }
469 -- Fully interact the given work item with an inert set, and return a
470 -- new inert set which has assimilated the new information.
471 solveOneWithDepth :: (Int, Int, [WorkItem])
472 -> InertSet -> WorkItem -> TcS InertSet
473 solveOneWithDepth (max_depth, depth, stack) inert work
474 = do { traceFireTcS depth (text "Solving {" <+> ppr work)
475 ; (new_inert, new_work) <- runSolverPipeline depth thePipeline inert work
477 -- Recursively solve the new work generated
478 -- from workItem, with a greater depth
479 ; res_inert <- solveInteractWithDepth (max_depth, depth+1, work:stack)
482 ; traceFireTcS depth (text "Done }" <+> ppr work)
486 thePipeline :: [(String,SimplifierStage)]
487 thePipeline = [ ("interact with inert eqs", interactWithInertEqsStage)
488 , ("interact with inerts", interactWithInertsStage)
489 , ("spontaneous solve", spontaneousSolveStage)
490 , ("top-level reactions", topReactionsStage) ]
493 *********************************************************************************
495 The spontaneous-solve Stage
497 *********************************************************************************
499 Note [Efficient Orientation]
500 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
502 There are two cases where we have to be careful about
503 orienting equalities to get better efficiency.
505 Case 1: In Rewriting Equalities (function rewriteEqLHS)
507 When rewriting two equalities with the same LHS:
510 We have a choice of producing work (xi1 ~ xi2) (up-to the
511 canonicalization invariants) However, to prevent the inert items
512 from getting kicked out of the inerts first, we prefer to
513 canonicalize (xi1 ~ xi2) if (b) comes from the inert set, or (xi2
514 ~ xi1) if (a) comes from the inert set.
516 This choice is implemented using the WhichComesFromInert flag.
518 Case 2: Functional Dependencies
519 Again, we should prefer, if possible, the inert variables on the RHS
521 Case 3: IP improvement work
522 We must always rewrite so that the inert type is on the right.
525 spontaneousSolveStage :: SimplifierStage
526 spontaneousSolveStage depth workItem inerts
527 = do { mSolve <- trySpontaneousSolve workItem
530 SPCantSolve -> -- No spontaneous solution for him, keep going
531 return $ SR { sr_new_work = emptyWorkList
533 , sr_stop = ContinueWith workItem }
536 | not (isGivenCt workItem)
537 -- Original was wanted or derived but we have now made him
538 -- given so we have to interact him with the inerts due to
539 -- its status change. This in turn may produce more work.
540 -- We do this *right now* (rather than just putting workItem'
541 -- back into the work-list) because we've solved
542 -> do { bumpStepCountTcS
543 ; traceFireTcS depth (ptext (sLit "Spontaneous (w/d)") <+> ppr workItem)
544 ; (new_inert, new_work) <- runSolverPipeline depth
545 [ ("recursive interact with inert eqs", interactWithInertEqsStage)
546 , ("recursive interact with inerts", interactWithInertsStage)
548 ; return $ SR { sr_new_work = new_work
549 , sr_inerts = new_inert -- will include workItem'
553 -> -- Original was given; he must then be inert all right, and
554 -- workList' are all givens from flattening
555 do { bumpStepCountTcS
556 ; traceFireTcS depth (ptext (sLit "Spontaneous (g)") <+> ppr workItem)
557 ; return $ SR { sr_new_work = emptyWorkList
558 , sr_inerts = inerts `updInertSet` workItem'
560 SPError -> -- Return with no new work
561 return $ SR { sr_new_work = emptyWorkList
566 data SPSolveResult = SPCantSolve | SPSolved WorkItem | SPError
567 -- SPCantSolve means that we can't do the unification because e.g. the variable is untouchable
568 -- SPSolved workItem' gives us a new *given* to go on
569 -- SPError means that it's completely impossible to solve this equality, eg due to a kind error
572 -- @trySpontaneousSolve wi@ solves equalities where one side is a
573 -- touchable unification variable.
574 -- See Note [Touchables and givens]
575 trySpontaneousSolve :: WorkItem -> TcS SPSolveResult
576 trySpontaneousSolve workItem@(CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi })
579 | Just tv2 <- tcGetTyVar_maybe xi
580 = do { tch1 <- isTouchableMetaTyVar tv1
581 ; tch2 <- isTouchableMetaTyVar tv2
582 ; case (tch1, tch2) of
583 (True, True) -> trySpontaneousEqTwoWay cv gw tv1 tv2
584 (True, False) -> trySpontaneousEqOneWay cv gw tv1 xi
585 (False, True) -> trySpontaneousEqOneWay cv gw tv2 (mkTyVarTy tv1)
586 _ -> return SPCantSolve }
588 = do { tch1 <- isTouchableMetaTyVar tv1
589 ; if tch1 then trySpontaneousEqOneWay cv gw tv1 xi
590 else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:"
592 ; return SPCantSolve }
596 -- trySpontaneousSolve (CFunEqCan ...) = ...
597 -- See Note [No touchables as FunEq RHS] in TcSMonad
598 trySpontaneousSolve _ = return SPCantSolve
601 trySpontaneousEqOneWay :: CoVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
602 -- tv is a MetaTyVar, not untouchable
603 trySpontaneousEqOneWay cv gw tv xi
604 | not (isSigTyVar tv) || isTyVarTy xi
605 = do { let kxi = typeKind xi -- NB: 'xi' is fully rewritten according to the inerts
606 -- so we have its more specific kind in our hands
607 ; if kxi `isSubKind` tyVarKind tv then
608 solveWithIdentity cv gw tv xi
609 else return SPCantSolve
611 else if tyVarKind tv `isSubKind` kxi then
612 return SPCantSolve -- kinds are compatible but we can't solveWithIdentity this way
613 -- This case covers the a_touchable :: * ~ b_untouchable :: ??
614 -- which has to be deferred or floated out for someone else to solve
615 -- it in a scope where 'b' is no longer untouchable.
616 else do { addErrorTcS KindError gw (mkTyVarTy tv) xi -- See Note [Kind errors]
620 | otherwise -- Still can't solve, sig tyvar and non-variable rhs
624 trySpontaneousEqTwoWay :: CoVar -> CtFlavor -> TcTyVar -> TcTyVar -> TcS SPSolveResult
625 -- Both tyvars are *touchable* MetaTyvars so there is only a chance for kind error here
626 trySpontaneousEqTwoWay cv gw tv1 tv2
628 , nicer_to_update_tv2 = solveWithIdentity cv gw tv2 (mkTyVarTy tv1)
630 = solveWithIdentity cv gw tv1 (mkTyVarTy tv2)
631 | otherwise -- None is a subkind of the other, but they are both touchable!
633 -- do { addErrorTcS KindError gw (mkTyVarTy tv1) (mkTyVarTy tv2)
634 -- ; return SPError }
638 nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
642 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
643 Consider the wanted problem:
644 alpha ~ (# Int, Int #)
645 where alpha :: ?? and (# Int, Int #) :: (#). We can't spontaneously solve this constraint,
646 but we should rather reject the program that give rise to it. If 'trySpontaneousEqTwoWay'
647 simply returns @CantSolve@ then that wanted constraint is going to propagate all the way and
648 get quantified over in inference mode. That's bad because we do know at this point that the
649 constraint is insoluble. Instead, we call 'recKindErrorTcS' here, which will fail later on.
651 The same applies in canonicalization code in case of kind errors in the givens.
653 However, when we canonicalize givens we only check for compatibility (@compatKind@).
654 If there were a kind error in the givens, this means some form of inconsistency or dead code.
656 You may think that when we spontaneously solve wanteds we may have to look through the
657 bindings to determine the right kind of the RHS type. E.g one may be worried that xi is
658 @alpha@ where alpha :: ? and a previous spontaneous solving has set (alpha := f) with (f :: *).
659 But we orient our constraints so that spontaneously solved ones can rewrite all other constraint
660 so this situation can't happen.
662 Note [Spontaneous solving and kind compatibility]
663 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
664 Note that our canonical constraints insist that *all* equalities (tv ~
665 xi) or (F xis ~ rhs) require the LHS and the RHS to have *compatible*
666 the same kinds. ("compatible" means one is a subKind of the other.)
668 - It can't be *equal* kinds, because
669 b) wanted constraints don't necessarily have identical kinds
671 b) a solved wanted constraint becomes a given
673 - SPJ thinks that *given* constraints (tv ~ tau) always have that
674 tau has a sub-kind of tv; and when solving wanted constraints
675 in trySpontaneousEqTwoWay we re-orient to achieve this.
677 - Note that the kind invariant is maintained by rewriting.
678 Eg wanted1 rewrites wanted2; if both were compatible kinds before,
679 wanted2 will be afterwards. Similarly givens.
682 - Givens from higher-rank, such as:
683 type family T b :: * -> * -> *
684 type instance T Bool = (->)
686 f :: forall a. ((T a ~ (->)) => ...) -> a -> ...
688 Whereas we would be able to apply the type instance, we would not be able to
689 use the given (T Bool ~ (->)) in the body of 'flop'
692 Note [Avoid double unifications]
693 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
694 The spontaneous solver has to return a given which mentions the unified unification
695 variable *on the left* of the equality. Here is what happens if not:
696 Original wanted: (a ~ alpha), (alpha ~ Int)
697 We spontaneously solve the first wanted, without changing the order!
698 given : a ~ alpha [having unified alpha := a]
699 Now the second wanted comes along, but he cannot rewrite the given, so we simply continue.
700 At the end we spontaneously solve that guy, *reunifying* [alpha := Int]
702 We avoid this problem by orienting the resulting given so that the unification
703 variable is on the left. [Note that alternatively we could attempt to
704 enforce this at canonicalization]
706 See also Note [No touchables as FunEq RHS] in TcSMonad; avoiding
707 double unifications is the main reason we disallow touchable
708 unification variables as RHS of type family equations: F xis ~ alpha.
713 solveWithIdentity :: CoVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
714 -- Solve with the identity coercion
715 -- Precondition: kind(xi) is a sub-kind of kind(tv)
716 -- Precondition: CtFlavor is Wanted or Derived
717 -- See [New Wanted Superclass Work] to see why solveWithIdentity
718 -- must work for Derived as well as Wanted
719 -- Returns: workItem where
720 -- workItem = the new Given constraint
721 solveWithIdentity cv wd tv xi
722 = do { traceTcS "Sneaky unification:" $
723 vcat [text "Coercion variable: " <+> ppr wd,
724 text "Coercion: " <+> pprEq (mkTyVarTy tv) xi,
725 text "Left Kind is : " <+> ppr (typeKind (mkTyVarTy tv)),
726 text "Right Kind is : " <+> ppr (typeKind xi)
729 ; setWantedTyBind tv xi
730 ; cv_given <- newGivenCoVar (mkTyVarTy tv) xi xi
732 ; when (isWanted wd) (setWantedCoBind cv xi)
733 -- We don't want to do this for Derived, that's why we use 'when (isWanted wd)'
735 ; return $ SPSolved (CTyEqCan { cc_id = cv_given
736 , cc_flavor = mkGivenFlavor wd UnkSkol
737 , cc_tyvar = tv, cc_rhs = xi }) }
743 *********************************************************************************
745 The interact-with-inert Stage
747 *********************************************************************************
750 -- Interaction result of WorkItem <~> AtomicInert
752 = IR { ir_stop :: StopOrContinue
754 -- => Reagent (work item) consumed.
755 -- ContinueWith new_reagent
756 -- => Reagent transformed but keep gathering interactions.
757 -- The transformed item remains inert with respect
758 -- to any previously encountered inerts.
760 , ir_inert_action :: InertAction
761 -- Whether the inert item should remain in the InertSet.
763 , ir_new_work :: WorkList
764 -- new work items to add to the WorkList
766 , ir_fire :: Maybe String -- Tells whether a rule fired, and if so what
769 -- What to do with the inert reactant.
770 data InertAction = KeepInert | DropInert
772 mkIRContinue :: String -> WorkItem -> InertAction -> WorkList -> TcS InteractResult
773 mkIRContinue rule wi keep newWork
774 = return $ IR { ir_stop = ContinueWith wi, ir_inert_action = keep
775 , ir_new_work = newWork, ir_fire = Just rule }
777 mkIRStop :: String -> WorkList -> TcS InteractResult
778 mkIRStop rule newWork
779 = return $ IR { ir_stop = Stop, ir_inert_action = KeepInert
780 , ir_new_work = newWork, ir_fire = Just rule }
782 noInteraction :: Monad m => WorkItem -> m InteractResult
784 = return $ IR { ir_stop = ContinueWith wi, ir_inert_action = KeepInert
785 , ir_new_work = emptyWorkList, ir_fire = Nothing }
787 data WhichComesFromInert = LeftComesFromInert | RightComesFromInert
788 -- See Note [Efficient Orientation]
791 ---------------------------------------------------
792 -- Interact a single WorkItem with the equalities of an inert set as
793 -- far as possible, i.e. until we get a Stop result from an individual
794 -- reaction (i.e. when the WorkItem is consumed), or until we've
795 -- interact the WorkItem with the entire equalities of the InertSet
797 interactWithInertEqsStage :: SimplifierStage
798 interactWithInertEqsStage depth workItem inert
799 = Bag.foldlBagM (interactNext depth) initITR (inert_eqs inert)
801 initITR = SR { sr_inerts = inert { inert_eqs = emptyCCan }
802 , sr_new_work = emptyWorkList
803 , sr_stop = ContinueWith workItem }
805 ---------------------------------------------------
806 -- Interact a single WorkItem with *non-equality* constraints in the inert set.
807 -- Precondition: equality interactions must have already happened, hence we have
808 -- to pick up some information from the incoming inert, before folding over the
809 -- "Other" constraints it contains!
811 interactWithInertsStage :: SimplifierStage
812 interactWithInertsStage depth workItem inert
813 = let (relevant, inert_residual) = getISRelevant workItem inert
814 initITR = SR { sr_inerts = inert_residual
815 , sr_new_work = emptyWorkList
816 , sr_stop = ContinueWith workItem }
817 in Bag.foldlBagM (interactNext depth) initITR relevant
819 getISRelevant :: CanonicalCt -> InertSet -> (CanonicalCts, InertSet)
820 getISRelevant (CFrozenErr {}) is = (emptyCCan, is)
821 -- Nothing s relevant; we have alread interacted
822 -- it with the equalities in the inert set
824 getISRelevant (CDictCan { cc_class = cls } ) is
825 = let (relevant, residual_map) = getRelevantCts cls (inert_dicts is)
826 in (relevant, is { inert_dicts = residual_map })
827 getISRelevant (CFunEqCan { cc_fun = tc } ) is
828 = let (relevant, residual_map) = getRelevantCts tc (inert_funeqs is)
829 in (relevant, is { inert_funeqs = residual_map })
830 getISRelevant (CIPCan { cc_ip_nm = nm }) is
831 = let (relevant, residual_map) = getRelevantCts nm (inert_ips is)
832 in (relevant, is { inert_ips = residual_map })
833 -- An equality, finally, may kick everything except equalities out
834 -- because we have already interacted the equalities in interactWithInertEqsStage
835 getISRelevant _eq_ct is -- Equality, everything is relevant for this one
836 -- TODO: if we were caching variables, we'd know that only
837 -- some are relevant. Experiment with this for now.
838 = let cts = cCanMapToBag (inert_ips is) `unionBags`
839 cCanMapToBag (inert_dicts is) `unionBags` cCanMapToBag (inert_funeqs is)
840 in (cts, is { inert_dicts = emptyCCanMap
841 , inert_ips = emptyCCanMap
842 , inert_funeqs = emptyCCanMap })
844 interactNext :: SubGoalDepth -> StageResult -> AtomicInert -> TcS StageResult
845 interactNext depth it inert
846 | ContinueWith work_item <- sr_stop it
847 = do { let inerts = sr_inerts it
849 ; IR { ir_new_work = new_work, ir_inert_action = inert_action
850 , ir_fire = fire_info, ir_stop = stop }
851 <- interactWithInert inert work_item
854 = text rule <+> keep_doc
855 <+> vcat [ ptext (sLit "Inert =") <+> ppr inert
856 , ptext (sLit "Work =") <+> ppr work_item
857 , ppUnless (isEmptyBag new_work) $
858 ptext (sLit "New =") <+> ppr new_work ]
859 keep_doc = case inert_action of
860 KeepInert -> ptext (sLit "[keep]")
861 DropInert -> ptext (sLit "[drop]")
863 Just rule -> do { bumpStepCountTcS
864 ; traceFireTcS depth (mk_msg rule) }
867 -- New inerts depend on whether we KeepInert or not
868 ; let inerts_new = case inert_action of
869 KeepInert -> inerts `updInertSet` inert
872 ; return $ SR { sr_inerts = inerts_new
873 , sr_new_work = sr_new_work it `unionWorkLists` new_work
876 = return $ it { sr_inerts = (sr_inerts it) `updInertSet` inert }
878 -- Do a single interaction of two constraints.
879 interactWithInert :: AtomicInert -> WorkItem -> TcS InteractResult
880 interactWithInert inert workitem
881 = do { ctxt <- getTcSContext
882 ; let is_allowed = allowedInteraction (simplEqsOnly ctxt) inert workitem
885 doInteractWithInert inert workitem
887 noInteraction workitem
890 allowedInteraction :: Bool -> AtomicInert -> WorkItem -> Bool
891 -- Allowed interactions
892 allowedInteraction eqs_only (CDictCan {}) (CDictCan {}) = not eqs_only
893 allowedInteraction eqs_only (CIPCan {}) (CIPCan {}) = not eqs_only
894 allowedInteraction _ _ _ = True
896 --------------------------------------------
897 doInteractWithInert :: CanonicalCt -> CanonicalCt -> TcS InteractResult
898 -- Identical class constraints.
901 (CDictCan { cc_id = d1, cc_flavor = fl1, cc_class = cls1, cc_tyargs = tys1 })
902 workItem@(CDictCan { cc_id = d2, cc_flavor = fl2, cc_class = cls2, cc_tyargs = tys2 })
903 | cls1 == cls2 && (and $ zipWith tcEqType tys1 tys2)
904 = solveOneFromTheOther (d1,fl1) workItem
906 | cls1 == cls2 && (not (isGiven fl1 && isGiven fl2))
907 = -- See Note [When improvement happens]
908 do { let pty1 = ClassP cls1 tys1
909 pty2 = ClassP cls2 tys2
910 inert_pred_loc = (pty1, pprFlavorArising fl1)
911 work_item_pred_loc = (pty2, pprFlavorArising fl2)
912 fd_eqns = improveFromAnother
913 inert_pred_loc -- the template
914 work_item_pred_loc -- the one we aim to rewrite
915 -- See Note [Efficient Orientation]
917 ; m <- rewriteWithFunDeps fd_eqns tys2 fl2
919 Nothing -> noInteraction workItem
920 Just (rewritten_tys2, cos2, fd_work)
922 | tcEqTypes tys1 rewritten_tys2
923 -> -- Solve him on the spot in this case
924 do { let dict_co = mkTyConCoercion (classTyCon cls1) cos2
925 ; when (isWanted fl2) $ setDictBind d2 (EvCast d1 dict_co)
926 ; mkIRStop "Cls/Cls fundep (solved)" fd_work }
929 -> -- We could not quite solve him, but we stil rewrite him
930 -- Example: class C a b c | a -> b
931 -- Given: C Int Bool x, Wanted: C Int beta y
932 -- Then rewrite the wanted to C Int Bool y
933 -- but note that is still not identical to the given
934 -- The important thing is that the rewritten constraint is
935 -- inert wrt the given.
936 -- In fact, it is inert wrt all the previous inerts too, so
937 -- we can keep on going rather than sending it back to the work list
938 do { let dict_co = mkTyConCoercion (classTyCon cls1) cos2
939 ; d2' <- newDictVar cls1 rewritten_tys2
940 ; setDictBind d2 (EvCast d2' dict_co)
941 ; let workItem' = workItem { cc_id = d2', cc_tyargs = rewritten_tys2 }
942 ; mkIRContinue "Cls/Cls fundep (partial)" workItem' KeepInert fd_work }
945 -> ASSERT (isDerived fl2) -- Derived constraints have no evidence,
946 -- so just produce the rewritten constraint
947 let workItem' = workItem { cc_tyargs = rewritten_tys2 }
948 in mkIRContinue "Cls/Cls fundep" workItem' KeepInert fd_work
951 -- Class constraint and given equality: use the equality to rewrite
952 -- the class constraint.
953 doInteractWithInert (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
954 (CDictCan { cc_id = dv, cc_flavor = wfl, cc_class = cl, cc_tyargs = xis })
955 | ifl `canRewrite` wfl
956 , tv `elemVarSet` tyVarsOfTypes xis
957 = do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,wfl,cl,xis)
958 -- Continue with rewritten Dictionary because we can only be in the
959 -- interactWithEqsStage, so the dictionary is inert.
960 ; mkIRContinue "Eq/Cls" rewritten_dict KeepInert emptyWorkList }
962 doInteractWithInert (CDictCan { cc_id = dv, cc_flavor = ifl, cc_class = cl, cc_tyargs = xis })
963 workItem@(CTyEqCan { cc_id = cv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
964 | wfl `canRewrite` ifl
965 , tv `elemVarSet` tyVarsOfTypes xis
966 = do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,ifl,cl,xis)
967 ; mkIRContinue "Cls/Eq" workItem DropInert (workListFromCCan rewritten_dict) }
969 -- Class constraint and given equality: use the equality to rewrite
970 -- the class constraint.
971 doInteractWithInert (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
972 (CIPCan { cc_id = ipid, cc_flavor = wfl, cc_ip_nm = nm, cc_ip_ty = ty })
973 | ifl `canRewrite` wfl
974 , tv `elemVarSet` tyVarsOfType ty
975 = do { rewritten_ip <- rewriteIP (cv,tv,xi) (ipid,wfl,nm,ty)
976 ; mkIRContinue "Eq/IP" rewritten_ip KeepInert emptyWorkList }
978 doInteractWithInert (CIPCan { cc_id = ipid, cc_flavor = ifl, cc_ip_nm = nm, cc_ip_ty = ty })
979 workItem@(CTyEqCan { cc_id = cv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
980 | wfl `canRewrite` ifl
981 , tv `elemVarSet` tyVarsOfType ty
982 = do { rewritten_ip <- rewriteIP (cv,tv,xi) (ipid,ifl,nm,ty)
983 ; mkIRContinue "IP/Eq" workItem DropInert (workListFromCCan rewritten_ip) }
985 -- Two implicit parameter constraints. If the names are the same,
986 -- but their types are not, we generate a wanted type equality
987 -- that equates the type (this is "improvement").
988 -- However, we don't actually need the coercion evidence,
989 -- so we just generate a fresh coercion variable that isn't used anywhere.
990 doInteractWithInert (CIPCan { cc_id = id1, cc_flavor = ifl, cc_ip_nm = nm1, cc_ip_ty = ty1 })
991 workItem@(CIPCan { cc_flavor = wfl, cc_ip_nm = nm2, cc_ip_ty = ty2 })
992 | nm1 == nm2 && isGiven wfl && isGiven ifl
993 = -- See Note [Overriding implicit parameters]
994 -- Dump the inert item, override totally with the new one
995 -- Do not require type equality
996 -- For example, given let ?x::Int = 3 in let ?x::Bool = True in ...
997 -- we must *override* the outer one with the inner one
998 mkIRContinue "IP/IP override" workItem DropInert emptyWorkList
1000 | nm1 == nm2 && ty1 `tcEqType` ty2
1001 = solveOneFromTheOther (id1,ifl) workItem
1004 = -- See Note [When improvement happens]
1005 do { co_var <- newWantedCoVar ty2 ty1 -- See Note [Efficient Orientation]
1006 ; let flav = Wanted (combineCtLoc ifl wfl)
1007 ; cans <- mkCanonical flav co_var
1008 ; mkIRContinue "IP/IP fundep" workItem KeepInert cans }
1010 -- Never rewrite a given with a wanted equality, and a type function
1011 -- equality can never rewrite an equality. We rewrite LHS *and* RHS
1012 -- of function equalities so that our inert set exposes everything that
1013 -- we know about equalities.
1015 -- Inert: equality, work item: function equality
1016 doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi1 })
1017 (CFunEqCan { cc_id = cv2, cc_flavor = wfl, cc_fun = tc
1018 , cc_tyargs = args, cc_rhs = xi2 })
1019 | ifl `canRewrite` wfl
1020 , tv `elemVarSet` tyVarsOfTypes (xi2:args) -- Rewrite RHS as well
1021 = do { rewritten_funeq <- rewriteFunEq (cv1,tv,xi1) (cv2,wfl,tc,args,xi2)
1022 ; mkIRStop "Eq/FunEq" (workListFromCCan rewritten_funeq) }
1023 -- Must Stop here, because we may no longer be inert after the rewritting.
1025 -- Inert: function equality, work item: equality
1026 doInteractWithInert (CFunEqCan {cc_id = cv1, cc_flavor = ifl, cc_fun = tc
1027 , cc_tyargs = args, cc_rhs = xi1 })
1028 workItem@(CTyEqCan { cc_id = cv2, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi2 })
1029 | wfl `canRewrite` ifl
1030 , tv `elemVarSet` tyVarsOfTypes (xi1:args) -- Rewrite RHS as well
1031 = do { rewritten_funeq <- rewriteFunEq (cv2,tv,xi2) (cv1,ifl,tc,args,xi1)
1032 ; mkIRContinue "FunEq/Eq" workItem DropInert (workListFromCCan rewritten_funeq) }
1033 -- One may think that we could (KeepTransformedInert rewritten_funeq)
1034 -- but that is wrong, because it may end up not being inert with respect
1035 -- to future inerts. Example:
1036 -- Original inert = { F xis ~ [a], b ~ Maybe Int }
1037 -- Work item comes along = a ~ [b]
1038 -- If we keep { F xis ~ [b] } in the inert set we will end up with:
1039 -- { F xis ~ [b], b ~ Maybe Int, a ~ [Maybe Int] }
1040 -- At the end, which is *not* inert. So we should unfortunately DropInert here.
1042 doInteractWithInert (CFunEqCan { cc_id = cv1, cc_flavor = fl1, cc_fun = tc1
1043 , cc_tyargs = args1, cc_rhs = xi1 })
1044 workItem@(CFunEqCan { cc_id = cv2, cc_flavor = fl2, cc_fun = tc2
1045 , cc_tyargs = args2, cc_rhs = xi2 })
1046 | fl1 `canSolve` fl2 && lhss_match
1047 = do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
1048 ; mkIRStop "FunEq/FunEq" cans }
1049 | fl2 `canSolve` fl1 && lhss_match
1050 = do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
1051 ; mkIRContinue "FunEq/FunEq" workItem DropInert cans }
1053 lhss_match = tc1 == tc2 && and (zipWith tcEqType args1 args2)
1055 doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
1056 workItem@(CTyEqCan { cc_id = cv2, cc_flavor = fl2, cc_tyvar = tv2, cc_rhs = xi2 })
1057 -- Check for matching LHS
1058 | fl1 `canSolve` fl2 && tv1 == tv2
1059 = do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
1060 ; mkIRStop "Eq/Eq lhs" cans }
1062 | fl2 `canSolve` fl1 && tv1 == tv2
1063 = do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
1064 ; mkIRContinue "Eq/Eq lhs" workItem DropInert cans }
1066 -- Check for rewriting RHS
1067 | fl1 `canRewrite` fl2 && tv1 `elemVarSet` tyVarsOfType xi2
1068 = do { rewritten_eq <- rewriteEqRHS (cv1,tv1,xi1) (cv2,fl2,tv2,xi2)
1069 ; mkIRStop "Eq/Eq rhs" rewritten_eq }
1071 | fl2 `canRewrite` fl1 && tv2 `elemVarSet` tyVarsOfType xi1
1072 = do { rewritten_eq <- rewriteEqRHS (cv2,tv2,xi2) (cv1,fl1,tv1,xi1)
1073 ; mkIRContinue "Eq/Eq rhs" workItem DropInert rewritten_eq }
1075 doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
1076 (CFrozenErr { cc_id = cv2, cc_flavor = fl2 })
1077 | fl1 `canRewrite` fl2 && tv1 `elemVarSet` tyVarsOfEvVar cv2
1078 = do { rewritten_frozen <- rewriteFrozen (cv1, tv1, xi1) (cv2, fl2)
1079 ; mkIRStop "Frozen/Eq" rewritten_frozen }
1081 doInteractWithInert (CFrozenErr { cc_id = cv2, cc_flavor = fl2 })
1082 workItem@(CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
1083 | fl1 `canRewrite` fl2 && tv1 `elemVarSet` tyVarsOfEvVar cv2
1084 = do { rewritten_frozen <- rewriteFrozen (cv1, tv1, xi1) (cv2, fl2)
1085 ; mkIRContinue "Frozen/Eq" workItem DropInert rewritten_frozen }
1087 -- Fall-through case for all other situations
1088 doInteractWithInert _ workItem = noInteraction workItem
1090 -------------------------
1091 -- Equational Rewriting
1092 rewriteDict :: (CoVar, TcTyVar, Xi) -> (DictId, CtFlavor, Class, [Xi]) -> TcS CanonicalCt
1093 rewriteDict (cv,tv,xi) (dv,gw,cl,xis)
1094 = do { let cos = substTysWith [tv] [mkCoVarCoercion cv] xis -- xis[tv] ~ xis[xi]
1095 args = substTysWith [tv] [xi] xis
1097 dict_co = mkTyConCoercion con cos
1098 ; dv' <- newDictVar cl args
1100 Wanted {} -> setDictBind dv (EvCast dv' (mkSymCoercion dict_co))
1101 Given {} -> setDictBind dv' (EvCast dv dict_co)
1102 Derived {} -> return () -- Derived dicts we don't set any evidence
1104 ; return (CDictCan { cc_id = dv'
1107 , cc_tyargs = args }) }
1109 rewriteIP :: (CoVar,TcTyVar,Xi) -> (EvVar,CtFlavor, IPName Name, TcType) -> TcS CanonicalCt
1110 rewriteIP (cv,tv,xi) (ipid,gw,nm,ty)
1111 = do { let ip_co = substTyWith [tv] [mkCoVarCoercion cv] ty -- ty[tv] ~ t[xi]
1112 ty' = substTyWith [tv] [xi] ty
1113 ; ipid' <- newIPVar nm ty'
1115 Wanted {} -> setIPBind ipid (EvCast ipid' (mkSymCoercion ip_co))
1116 Given {} -> setIPBind ipid' (EvCast ipid ip_co)
1117 Derived {} -> return () -- Derived ips: we don't set any evidence
1119 ; return (CIPCan { cc_id = ipid'
1122 , cc_ip_ty = ty' }) }
1124 rewriteFunEq :: (CoVar,TcTyVar,Xi) -> (CoVar,CtFlavor,TyCon, [Xi], Xi) -> TcS CanonicalCt
1125 rewriteFunEq (cv1,tv,xi1) (cv2,gw, tc,args,xi2) -- cv2 :: F args ~ xi2
1126 = do { let arg_cos = substTysWith [tv] [mkCoVarCoercion cv1] args
1127 args' = substTysWith [tv] [xi1] args
1128 fun_co = mkTyConCoercion tc arg_cos -- fun_co :: F args ~ F args'
1130 xi2' = substTyWith [tv] [xi1] xi2
1131 xi2_co = substTyWith [tv] [mkCoVarCoercion cv1] xi2 -- xi2_co :: xi2 ~ xi2'
1132 ; cv2' <- case gw of
1133 Wanted {} -> do { cv2' <- newWantedCoVar (mkTyConApp tc args') xi2'
1134 ; setWantedCoBind cv2 $
1135 fun_co `mkTransCoercion`
1136 mkCoVarCoercion cv2' `mkTransCoercion` mkSymCoercion xi2_co
1138 Given {} -> newGivenCoVar (mkTyConApp tc args') xi2' $
1139 mkSymCoercion fun_co `mkTransCoercion`
1140 mkCoVarCoercion cv2 `mkTransCoercion` xi2_co
1141 Derived {} -> newDerivedId (EqPred (mkTyConApp tc args') xi2')
1143 ; return (CFunEqCan { cc_id = cv2'
1147 , cc_rhs = xi2' }) }
1150 rewriteEqRHS :: (CoVar,TcTyVar,Xi) -> (CoVar,CtFlavor,TcTyVar,Xi) -> TcS WorkList
1151 -- Use the first equality to rewrite the second, flavors already checked.
1152 -- E.g. c1 : tv1 ~ xi1 c2 : tv2 ~ xi2
1153 -- rewrites c2 to give
1154 -- c2' : tv2 ~ xi2[xi1/tv1]
1155 -- We must do an occurs check to sure the new constraint is canonical
1156 -- So we might return an empty bag
1157 rewriteEqRHS (cv1,tv1,xi1) (cv2,gw,tv2,xi2)
1158 | Just tv2' <- tcGetTyVar_maybe xi2'
1159 , tv2 == tv2' -- In this case xi2[xi1/tv1] = tv2, so we have tv2~tv2
1160 = do { when (isWanted gw) (setWantedCoBind cv2 (mkSymCoercion co2'))
1161 ; return emptyCCan }
1166 -> do { cv2' <- newWantedCoVar (mkTyVarTy tv2) xi2'
1167 ; setWantedCoBind cv2 $
1168 mkCoVarCoercion cv2' `mkTransCoercion` mkSymCoercion co2'
1171 -> newGivenCoVar (mkTyVarTy tv2) xi2' $
1172 mkCoVarCoercion cv2 `mkTransCoercion` co2'
1174 -> newDerivedId (EqPred (mkTyVarTy tv2) xi2')
1176 ; canEq gw cv2' (mkTyVarTy tv2) xi2'
1179 xi2' = substTyWith [tv1] [xi1] xi2
1180 co2' = substTyWith [tv1] [mkCoVarCoercion cv1] xi2 -- xi2 ~ xi2[xi1/tv1]
1183 rewriteEqLHS :: WhichComesFromInert -> (Coercion,Xi) -> (CoVar,CtFlavor,Xi) -> TcS WorkList
1184 -- Used to ineract two equalities of the following form:
1185 -- First Equality: co1: (XXX ~ xi1)
1186 -- Second Equality: cv2: (XXX ~ xi2)
1187 -- Where the cv1 `canSolve` cv2 equality
1188 -- We have an option of creating new work (xi1 ~ xi2) OR (xi2 ~ xi1),
1189 -- See Note [Efficient Orientation] for that
1190 rewriteEqLHS which (co1,xi1) (cv2,gw,xi2)
1191 = do { cv2' <- case (isWanted gw, which) of
1192 (True,LeftComesFromInert) ->
1193 do { cv2' <- newWantedCoVar xi2 xi1
1194 ; setWantedCoBind cv2 $
1195 co1 `mkTransCoercion` mkSymCoercion (mkCoVarCoercion cv2')
1197 (True,RightComesFromInert) ->
1198 do { cv2' <- newWantedCoVar xi1 xi2
1199 ; setWantedCoBind cv2 $
1200 co1 `mkTransCoercion` mkCoVarCoercion cv2'
1202 (False,LeftComesFromInert) ->
1204 newGivenCoVar xi2 xi1 $
1205 mkSymCoercion (mkCoVarCoercion cv2) `mkTransCoercion` co1
1206 else newDerivedId (EqPred xi2 xi1)
1207 (False,RightComesFromInert) ->
1209 newGivenCoVar xi1 xi2 $
1210 mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2
1211 else newDerivedId (EqPred xi1 xi2)
1212 ; mkCanonical gw cv2' }
1214 rewriteFrozen :: (CoVar,TcTyVar,Xi) -> (CoVar,CtFlavor) -> TcS WorkList
1215 rewriteFrozen (cv1, tv1, xi1) (cv2, fl2)
1218 Wanted {} -> do { cv2' <- newWantedCoVar ty2a' ty2b'
1219 -- ty2a[xi1/tv1] ~ ty2b[xi1/tv1]
1220 ; setWantedCoBind cv2 $
1221 co2a' `mkTransCoercion`
1222 mkCoVarCoercion cv2' `mkTransCoercion`
1226 Given {} -> newGivenCoVar ty2a' ty2b' $
1227 mkSymCoercion co2a' `mkTransCoercion`
1228 mkCoVarCoercion cv2 `mkTransCoercion`
1231 Derived {} -> newDerivedId (EqPred ty2a' ty2b')
1232 ; return (singleCCan $ CFrozenErr { cc_id = cv2', cc_flavor = fl2 }) }
1234 (ty2a, ty2b) = coVarKind cv2 -- cv2 : ty2a ~ ty2b
1235 ty2a' = substTyWith [tv1] [xi1] ty2a
1236 ty2b' = substTyWith [tv1] [xi1] ty2b
1238 co2a' = substTyWith [tv1] [mkCoVarCoercion cv1] ty2a -- ty2a ~ ty2a[xi1/tv1]
1239 co2b' = substTyWith [tv1] [mkCoVarCoercion cv1] ty2b -- ty2b ~ ty2b[xi1/tv1]
1241 solveOneFromTheOther :: (EvVar, CtFlavor) -> CanonicalCt -> TcS InteractResult
1242 -- First argument inert, second argument work-item. They both represent
1243 -- wanted/given/derived evidence for the *same* predicate so we try here to
1244 -- discharge one directly from the other.
1246 -- Precondition: value evidence only (implicit parameters, classes)
1248 solveOneFromTheOther (iid,ifl) workItem
1250 = mkIRStop "Solved (derived)" emptyWorkList
1252 | ifl `canSolve` wfl
1253 = do { when (isWanted wfl) $ setEvBind wid (EvId iid)
1254 -- Overwrite the binding, if one exists
1255 -- For Givens, which are lambda-bound, nothing to overwrite,
1256 ; mkIRStop "Solved" emptyWorkList }
1258 | wfl `canSolve` ifl
1259 = do { when (isWanted ifl) $ setEvBind iid (EvId wid)
1260 ; mkIRContinue "Solved inert" workItem DropInert emptyWorkList }
1262 | otherwise -- The inert item is Derived, we can just throw it away,
1263 = mkIRContinue "Discard derived inert" workItem DropInert emptyWorkList
1266 wfl = cc_flavor workItem
1267 wid = cc_id workItem
1270 Note [Superclasses and recursive dictionaries]
1271 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1272 Overlaps with Note [SUPERCLASS-LOOP 1]
1273 Note [SUPERCLASS-LOOP 2]
1274 Note [Recursive instances and superclases]
1275 ToDo: check overlap and delete redundant stuff
1277 Right before adding a given into the inert set, we must
1278 produce some more work, that will bring the superclasses
1279 of the given into scope. The superclass constraints go into
1282 When we simplify a wanted constraint, if we first see a matching
1283 instance, we may produce new wanted work. To (1) avoid doing this work
1284 twice in the future and (2) to handle recursive dictionaries we may ``cache''
1285 this item as given into our inert set WITHOUT adding its superclass constraints,
1286 otherwise we'd be in danger of creating a loop [In fact this was the exact reason
1287 for doing the isGoodRecEv check in an older version of the type checker].
1289 But now we have added partially solved constraints to the worklist which may
1290 interact with other wanteds. Consider the example:
1294 class Eq b => Foo a b --- 0-th selector
1295 instance Eq a => Foo [a] a --- fooDFun
1297 and wanted (Foo [t] t). We are first going to see that the instance matches
1298 and create an inert set that includes the solved (Foo [t] t) but not its superclasses:
1299 d1 :_g Foo [t] t d1 := EvDFunApp fooDFun d3
1300 Our work list is going to contain a new *wanted* goal
1303 Ok, so how do we get recursive dictionaries, at all:
1307 data D r = ZeroD | SuccD (r (D r));
1309 instance (Eq (r (D r))) => Eq (D r) where
1310 ZeroD == ZeroD = True
1311 (SuccD a) == (SuccD b) = a == b
1314 equalDC :: D [] -> D [] -> Bool;
1317 We need to prove (Eq (D [])). Here's how we go:
1321 by instance decl, holds if
1325 *BUT* we have an inert set which gives us (no superclasses):
1327 By the instance declaration of Eq we can show the 'd2' goal if
1329 where d2 = dfEqList d3
1331 Now, however this wanted can interact with our inert d1 to set:
1333 and solve the goal. Why was this interaction OK? Because, if we chase the
1334 evidence of d1 ~~> dfEqD d2 ~~-> dfEqList d3, so by setting d3 := d1 we
1336 d3 := dfEqD2 (dfEqList d3)
1337 which is FINE because the use of d3 is protected by the instance function
1340 So, our strategy is to try to put solved wanted dictionaries into the
1341 inert set along with their superclasses (when this is meaningful,
1342 i.e. when new wanted goals are generated) but solve a wanted dictionary
1343 from a given only in the case where the evidence variable of the
1344 wanted is mentioned in the evidence of the given (recursively through
1345 the evidence binds) in a protected way: more instance function applications
1346 than superclass selectors.
1348 Here are some more examples from GHC's previous type checker
1352 This code arises in the context of "Scrap Your Boilerplate with Class"
1356 instance Sat (ctx Char) => Data ctx Char -- dfunData1
1357 instance (Sat (ctx [a]), Data ctx a) => Data ctx [a] -- dfunData2
1359 class Data Maybe a => Foo a
1361 instance Foo t => Sat (Maybe t) -- dfunSat
1363 instance Data Maybe a => Foo a -- dfunFoo1
1364 instance Foo a => Foo [a] -- dfunFoo2
1365 instance Foo [Char] -- dfunFoo3
1367 Consider generating the superclasses of the instance declaration
1368 instance Foo a => Foo [a]
1370 So our problem is this
1372 d1 :_w Data Maybe [t]
1374 We may add the given in the inert set, along with its superclasses
1375 [assuming we don't fail because there is a matching instance, see
1376 tryTopReact, given case ]
1380 d01 :_g Data Maybe t -- d2 := EvDictSuperClass d0 0
1381 d1 :_w Data Maybe [t]
1382 Then d2 can readily enter the inert, and we also do solving of the wanted
1385 d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
1387 d2 :_w Sat (Maybe [t])
1389 d01 :_g Data Maybe t
1390 Now, we may simplify d2 more:
1393 d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
1394 d1 :_g Data Maybe [t]
1395 d2 :_g Sat (Maybe [t]) d2 := dfunSat d4
1399 d01 :_g Data Maybe t
1401 Now, we can just solve d3.
1404 d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
1405 d2 :_g Sat (Maybe [t]) d2 := dfunSat d4
1408 d01 :_g Data Maybe t
1409 And now we can simplify d4 again, but since it has superclasses we *add* them to the worklist:
1412 d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
1413 d2 :_g Sat (Maybe [t]) d2 := dfunSat d4
1414 d4 :_g Foo [t] d4 := dfunFoo2 d5
1417 d6 :_g Data Maybe [t] d6 := EvDictSuperClass d4 0
1418 d01 :_g Data Maybe t
1419 Now, d5 can be solved! (and its superclass enter scope)
1422 d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
1423 d2 :_g Sat (Maybe [t]) d2 := dfunSat d4
1424 d4 :_g Foo [t] d4 := dfunFoo2 d5
1425 d5 :_g Foo t d5 := dfunFoo1 d7
1428 d6 :_g Data Maybe [t]
1429 d8 :_g Data Maybe t d8 := EvDictSuperClass d5 0
1430 d01 :_g Data Maybe t
1433 [1] Suppose we pick d8 and we react him with d01. Which of the two givens should
1434 we keep? Well, we *MUST NOT* drop d01 because d8 contains recursive evidence
1435 that must not be used (look at case interactInert where both inert and workitem
1436 are givens). So we have several options:
1437 - Drop the workitem always (this will drop d8)
1438 This feels very unsafe -- what if the work item was the "good" one
1439 that should be used later to solve another wanted?
1440 - Don't drop anyone: the inert set may contain multiple givens!
1441 [This is currently implemented]
1443 The "don't drop anyone" seems the most safe thing to do, so now we come to problem 2:
1444 [2] We have added both d6 and d01 in the inert set, and we are interacting our wanted
1445 d7. Now the [isRecDictEv] function in the ineration solver
1446 [case inert-given workitem-wanted] will prevent us from interacting d7 := d8
1447 precisely because chasing the evidence of d8 leads us to an unguarded use of d7.
1449 So, no interaction happens there. Then we meet d01 and there is no recursion
1450 problem there [isRectDictEv] gives us the OK to interact and we do solve d7 := d01!
1452 Note [SUPERCLASS-LOOP 1]
1453 ~~~~~~~~~~~~~~~~~~~~~~~~
1454 We have to be very, very careful when generating superclasses, lest we
1455 accidentally build a loop. Here's an example:
1459 class S a => C a where { opc :: a -> a }
1460 class S b => D b where { opd :: b -> b }
1462 instance C Int where
1465 instance D Int where
1468 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1469 Simplifying, we may well get:
1470 $dfCInt = :C ds1 (opd dd)
1473 Notice that we spot that we can extract ds1 from dd.
1475 Alas! Alack! We can do the same for (instance D Int):
1477 $dfDInt = :D ds2 (opc dc)
1481 And now we've defined the superclass in terms of itself.
1482 Two more nasty cases are in
1487 - Satisfy the superclass context *all by itself*
1488 (tcSimplifySuperClasses)
1489 - And do so completely; i.e. no left-over constraints
1490 to mix with the constraints arising from method declarations
1493 Note [SUPERCLASS-LOOP 2]
1494 ~~~~~~~~~~~~~~~~~~~~~~~~
1495 We need to be careful when adding "the constaint we are trying to prove".
1496 Suppose we are *given* d1:Ord a, and want to deduce (d2:C [a]) where
1498 class Ord a => C a where
1499 instance Ord [a] => C [a] where ...
1501 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
1502 superclasses of C [a] to avails. But we must not overwrite the binding
1503 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
1506 Here's another variant, immortalised in tcrun020
1507 class Monad m => C1 m
1508 class C1 m => C2 m x
1509 instance C2 Maybe Bool
1510 For the instance decl we need to build (C1 Maybe), and it's no good if
1511 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1512 before we search for C1 Maybe.
1514 Here's another example
1515 class Eq b => Foo a b
1516 instance Eq a => Foo [a] a
1520 we'll first deduce that it holds (via the instance decl). We must not
1521 then overwrite the Eq t constraint with a superclass selection!
1523 At first I had a gross hack, whereby I simply did not add superclass constraints
1524 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1525 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1526 I found a very obscure program (now tcrun021) in which improvement meant the
1527 simplifier got two bites a the cherry... so something seemed to be an Stop
1528 first time, but reducible next time.
1530 Now we implement the Right Solution, which is to check for loops directly
1531 when adding superclasses. It's a bit like the occurs check in unification.
1533 Note [Recursive instances and superclases]
1534 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1535 Consider this code, which arises in the context of "Scrap Your
1536 Boilerplate with Class".
1540 instance Sat (ctx Char) => Data ctx Char
1541 instance (Sat (ctx [a]), Data ctx a) => Data ctx [a]
1543 class Data Maybe a => Foo a
1545 instance Foo t => Sat (Maybe t)
1547 instance Data Maybe a => Foo a
1548 instance Foo a => Foo [a]
1551 In the instance for Foo [a], when generating evidence for the superclasses
1552 (ie in tcSimplifySuperClasses) we need a superclass (Data Maybe [a]).
1553 Using the instance for Data, we therefore need
1554 (Sat (Maybe [a], Data Maybe a)
1555 But we are given (Foo a), and hence its superclass (Data Maybe a).
1556 So that leaves (Sat (Maybe [a])). Using the instance for Sat means
1557 we need (Foo [a]). And that is the very dictionary we are bulding
1558 an instance for! So we must put that in the "givens". So in this
1560 Given: Foo a, Foo [a]
1561 Wanted: Data Maybe [a]
1563 BUT we must *not not not* put the *superclasses* of (Foo [a]) in
1564 the givens, which is what 'addGiven' would normally do. Why? Because
1565 (Data Maybe [a]) is the superclass, so we'd "satisfy" the wanted
1566 by selecting a superclass from Foo [a], which simply makes a loop.
1568 On the other hand we *must* put the superclasses of (Foo a) in
1569 the givens, as you can see from the derivation described above.
1571 Conclusion: in the very special case of tcSimplifySuperClasses
1572 we have one 'given' (namely the "this" dictionary) whose superclasses
1573 must not be added to 'givens' by addGiven.
1575 There is a complication though. Suppose there are equalities
1576 instance (Eq a, a~b) => Num (a,b)
1577 Then we normalise the 'givens' wrt the equalities, so the original
1578 given "this" dictionary is cast to one of a different type. So it's a
1579 bit trickier than before to identify the "special" dictionary whose
1580 superclasses must not be added. See test
1581 indexed-types/should_run/EqInInstance
1583 We need a persistent property of the dictionary to record this
1584 special-ness. Current I'm using the InstLocOrigin (a bit of a hack,
1585 but cool), which is maintained by dictionary normalisation.
1586 Specifically, the InstLocOrigin is
1588 then the no-superclass thing kicks in. WATCH OUT if you fiddle
1591 Note [MATCHING-SYNONYMS]
1592 ~~~~~~~~~~~~~~~~~~~~~~~~
1593 When trying to match a dictionary (D tau) to a top-level instance, or a
1594 type family equation (F taus_1 ~ tau_2) to a top-level family instance,
1595 we do *not* need to expand type synonyms because the matcher will do that for us.
1598 Note [RHS-FAMILY-SYNONYMS]
1599 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1600 The RHS of a family instance is represented as yet another constructor which is
1601 like a type synonym for the real RHS the programmer declared. Eg:
1602 type instance F (a,a) = [a]
1604 :R32 a = [a] -- internal type synonym introduced
1605 F (a,a) ~ :R32 a -- instance
1607 When we react a family instance with a type family equation in the work list
1608 we keep the synonym-using RHS without expansion.
1611 *********************************************************************************
1613 The top-reaction Stage
1615 *********************************************************************************
1618 -- If a work item has any form of interaction with top-level we get this
1619 data TopInteractResult
1620 = NoTopInt -- No top-level interaction
1621 -- Equivalent to (SomeTopInt emptyWorkList (ContinueWith work_item))
1623 { tir_new_work :: WorkList -- Sub-goals or new work (could be given,
1624 -- for superclasses)
1625 , tir_new_inert :: StopOrContinue -- The input work item, ready to become *inert* now:
1626 } -- NB: in ``given'' (solved) form if the
1627 -- original was wanted or given and instance match
1628 -- was found, but may also be in wanted form if we
1629 -- only reacted with functional dependencies
1630 -- arising from top-level instances.
1632 topReactionsStage :: SimplifierStage
1633 topReactionsStage depth workItem inerts
1634 = do { tir <- tryTopReact workItem
1637 return $ SR { sr_inerts = inerts
1638 , sr_new_work = emptyWorkList
1639 , sr_stop = ContinueWith workItem }
1640 SomeTopInt tir_new_work tir_new_inert ->
1641 do { bumpStepCountTcS
1642 ; traceFireTcS depth (ptext (sLit "Top react")
1643 <+> vcat [ ptext (sLit "Work =") <+> ppr workItem
1644 , ptext (sLit "New =") <+> ppr tir_new_work ])
1645 ; return $ SR { sr_inerts = inerts
1646 , sr_new_work = tir_new_work
1647 , sr_stop = tir_new_inert
1651 tryTopReact :: WorkItem -> TcS TopInteractResult
1652 tryTopReact workitem
1653 = do { -- A flag controls the amount of interaction allowed
1654 -- See Note [Simplifying RULE lhs constraints]
1655 ctxt <- getTcSContext
1656 ; if allowedTopReaction (simplEqsOnly ctxt) workitem
1657 then do { traceTcS "tryTopReact / calling doTopReact" (ppr workitem)
1658 ; doTopReact workitem }
1659 else return NoTopInt
1662 allowedTopReaction :: Bool -> WorkItem -> Bool
1663 allowedTopReaction eqs_only (CDictCan {}) = not eqs_only
1664 allowedTopReaction _ _ = True
1666 doTopReact :: WorkItem -> TcS TopInteractResult
1667 -- The work item does not react with the inert set, so try interaction with top-level instances
1668 -- NB: The place to add superclasses in *not* in doTopReact stage. Instead superclasses are
1669 -- added in the worklist as part of the canonicalisation process.
1670 -- See Note [Adding superclasses] in TcCanonical.
1673 -- See Note [Given constraint that matches an instance declaration]
1674 doTopReact (CDictCan { cc_flavor = Given {} })
1675 = return NoTopInt -- NB: Superclasses already added since it's canonical
1677 -- Derived dictionary: just look for functional dependencies
1678 doTopReact workItem@(CDictCan { cc_flavor = fl@(Derived loc)
1679 , cc_class = cls, cc_tyargs = xis })
1680 = do { instEnvs <- getInstEnvs
1681 ; let fd_eqns = improveFromInstEnv instEnvs
1682 (ClassP cls xis, pprArisingAt loc)
1683 ; m <- rewriteWithFunDeps fd_eqns xis fl
1685 Nothing -> return NoTopInt
1686 Just (xis',_,fd_work) ->
1687 let workItem' = workItem { cc_tyargs = xis' }
1688 -- Deriveds are not supposed to have identity (cc_id is unused!)
1689 in return $ SomeTopInt { tir_new_work = fd_work
1690 , tir_new_inert = ContinueWith workItem' } }
1692 -- Wanted dictionary
1693 doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = fl@(Wanted loc)
1694 , cc_class = cls, cc_tyargs = xis })
1695 = do { -- See Note [MATCHING-SYNONYMS]
1696 ; lkp_inst_res <- matchClassInst cls xis loc
1697 ; case lkp_inst_res of
1699 do { traceTcS "doTopReact/ no class instance for" (ppr dv)
1701 ; instEnvs <- getInstEnvs
1702 ; let fd_eqns = improveFromInstEnv instEnvs
1703 (ClassP cls xis, pprArisingAt loc)
1704 ; m <- rewriteWithFunDeps fd_eqns xis fl
1706 Nothing -> return NoTopInt
1707 Just (xis',cos,fd_work) ->
1708 do { let dict_co = mkTyConCoercion (classTyCon cls) cos
1709 ; dv'<- newDictVar cls xis'
1710 ; setDictBind dv (EvCast dv' dict_co)
1711 ; let workItem' = CDictCan { cc_id = dv', cc_flavor = fl,
1712 cc_class = cls, cc_tyargs = xis' }
1714 SomeTopInt { tir_new_work = singleCCan workItem' `andCCan` fd_work
1715 , tir_new_inert = Stop } } }
1717 GenInst wtvs ev_term -- Solved
1718 -- No need to do fundeps stuff here; the instance
1719 -- matches already so we won't get any more info
1720 -- from functional dependencies
1722 -> do { traceTcS "doTopReact/ found nullary class instance for" (ppr dv)
1723 ; setDictBind dv ev_term
1724 -- Solved in one step and no new wanted work produced.
1725 -- i.e we directly matched a top-level instance
1726 -- No point in caching this in 'inert'; hence Stop
1727 ; return $ SomeTopInt { tir_new_work = emptyWorkList
1728 , tir_new_inert = Stop } }
1731 -> do { traceTcS "doTopReact/ found nullary class instance for" (ppr dv)
1732 ; setDictBind dv ev_term
1733 -- Solved and new wanted work produced, you may cache the
1734 -- (tentatively solved) dictionary as Given! (used to be: Derived)
1735 ; let solved = workItem { cc_flavor = given_fl }
1736 given_fl = Given (setCtLocOrigin loc UnkSkol)
1737 ; inst_work <- canWanteds wtvs
1738 ; return $ SomeTopInt { tir_new_work = inst_work
1739 , tir_new_inert = ContinueWith solved } }
1743 doTopReact (CFunEqCan { cc_id = cv, cc_flavor = fl
1744 , cc_fun = tc, cc_tyargs = args, cc_rhs = xi })
1745 = ASSERT (isSynFamilyTyCon tc) -- No associated data families have reached that far
1746 do { match_res <- matchFam tc args -- See Note [MATCHING-SYNONYMS]
1750 MatchInstSingle (rep_tc, rep_tys)
1751 -> do { let Just coe_tc = tyConFamilyCoercion_maybe rep_tc
1752 Just rhs_ty = tcView (mkTyConApp rep_tc rep_tys)
1753 -- Eagerly expand away the type synonym on the
1754 -- RHS of a type function, so that it never
1755 -- appears in an error message
1756 -- See Note [Type synonym families] in TyCon
1757 coe = mkTyConApp coe_tc rep_tys
1759 Wanted {} -> do { cv' <- newWantedCoVar rhs_ty xi
1760 ; setWantedCoBind cv $
1761 coe `mkTransCoercion`
1764 Given {} -> newGivenCoVar xi rhs_ty $
1765 mkSymCoercion (mkCoVarCoercion cv) `mkTransCoercion` coe
1766 Derived {} -> newDerivedId (EqPred xi rhs_ty)
1767 ; can_cts <- mkCanonical fl cv'
1768 ; return $ SomeTopInt can_cts Stop }
1770 -> panicTcS $ text "TcSMonad.matchFam returned multiple instances!"
1774 -- Any other work item does not react with any top-level equations
1775 doTopReact _workItem = return NoTopInt
1779 Note [FunDep and implicit parameter reactions]
1780 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1781 Currently, our story of interacting two dictionaries (or a dictionary
1782 and top-level instances) for functional dependencies, and implicit
1783 paramters, is that we simply produce new wanted equalities. So for example
1785 class D a b | a -> b where ...
1791 We generate the extra work item
1793 where 'cv' is currently unused. However, this new item reacts with d2,
1794 discharging it in favour of a new constraint d2' thus:
1796 d2 := d2' |> D Int cv
1797 Now d2' can be discharged from d1
1799 We could be more aggressive and try to *immediately* solve the dictionary
1800 using those extra equalities. With the same inert set and work item we
1801 might dischard d2 directly:
1804 d2 := d1 |> D Int cv
1806 But in general it's a bit painful to figure out the necessary coercion,
1807 so we just take the first approach. Here is a better example. Consider:
1808 class C a b c | a -> b
1810 [Given] d1 : C T Int Char
1811 [Wanted] d2 : C T beta Int
1812 In this case, it's *not even possible* to solve the wanted immediately.
1813 So we should simply output the functional dependency and add this guy
1814 [but NOT its superclasses] back in the worklist. Even worse:
1815 [Given] d1 : C T Int beta
1816 [Wanted] d2: C T beta Int
1817 Then it is solvable, but its very hard to detect this on the spot.
1819 It's exactly the same with implicit parameters, except that the
1820 "aggressive" approach would be much easier to implement.
1822 Note [When improvement happens]
1823 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1824 We fire an improvement rule when
1826 * Two constraints match (modulo the fundep)
1827 e.g. C t1 t2, C t1 t3 where C a b | a->b
1828 The two match because the first arg is identical
1830 * At least one is not Given. If they are both given, we don't fire
1831 the reaction because we have no way of constructing evidence for a
1832 new equality nor does it seem right to create a new wanted goal
1833 (because the goal will most likely contain untouchables, which
1834 can't be solved anyway)!
1836 Note that we *do* fire the improvement if one is Given and one is Derived.
1837 The latter can be a superclass of a wanted goal. Example (tcfail138)
1838 class L a b | a -> b
1839 class (G a, L a b) => C a b
1841 instance C a b' => G (Maybe a)
1842 instance C a b => C (Maybe a) a
1843 instance L (Maybe a) a
1845 When solving the superclasses of the (C (Maybe a) a) instance, we get
1846 Given: C a b ... and hance by superclasses, (G a, L a b)
1848 Use the instance decl to get
1850 The (C a b') is inert, so we generate its Derived superclasses (L a b'),
1851 and now we need improvement between that derived superclass an the Given (L a b)
1853 Note [Overriding implicit parameters]
1854 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1856 f :: (?x::a) -> Bool -> a
1858 g v = let ?x::Int = 3
1859 in (f v, let ?x::Bool = True in f v)
1861 This should probably be well typed, with
1862 g :: Bool -> (Int, Bool)
1864 So the inner binding for ?x::Bool *overrides* the outer one.
1865 Hence a work-item Given overrides an inert-item Given.
1867 Note [Given constraint that matches an instance declaration]
1868 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1869 What should we do when we discover that one (or more) top-level
1870 instances match a given (or solved) class constraint? We have
1873 1. Reject the program. The reason is that there may not be a unique
1874 best strategy for the solver. Example, from the OutsideIn(X) paper:
1875 instance P x => Q [x]
1876 instance (x ~ y) => R [x] y
1878 wob :: forall a b. (Q [b], R b a) => a -> Int
1880 g :: forall a. Q [a] => [a] -> Int
1883 will generate the impliation constraint:
1884 Q [a] => (Q [beta], R beta [a])
1885 If we react (Q [beta]) with its top-level axiom, we end up with a
1886 (P beta), which we have no way of discharging. On the other hand,
1887 if we react R beta [a] with the top-level we get (beta ~ a), which
1888 is solvable and can help us rewrite (Q [beta]) to (Q [a]) which is
1889 now solvable by the given Q [a].
1891 However, this option is restrictive, for instance [Example 3] from
1892 Note [Recursive dictionaries] will fail to work.
1894 2. Ignore the problem, hoping that the situations where there exist indeed
1895 such multiple strategies are rare: Indeed the cause of the previous
1896 problem is that (R [x] y) yields the new work (x ~ y) which can be
1897 *spontaneously* solved, not using the givens.
1899 We are choosing option 2 below but we might consider having a flag as well.
1902 Note [New Wanted Superclass Work]
1903 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1904 Even in the case of wanted constraints, we may add some superclasses
1905 as new given work. The reason is:
1907 To allow FD-like improvement for type families. Assume that
1909 class C a b | a -> b
1910 and we have to solve the implication constraint:
1912 Then, FD improvement can help us to produce a new wanted (beta ~ b)
1914 We want to have the same effect with the type family encoding of
1915 functional dependencies. Namely, consider:
1916 class (F a ~ b) => C a b
1917 Now suppose that we have:
1920 By interacting the given we will get given (F a ~ b) which is not
1921 enough by itself to make us discharge (C a beta). However, we
1922 may create a new derived equality from the super-class of the
1923 wanted constraint (C a beta), namely derived (F a ~ beta).
1924 Now we may interact this with given (F a ~ b) to get:
1926 But 'beta' is a touchable unification variable, and hence OK to
1927 unify it with 'b', replacing the derived evidence with the identity.
1929 This requires trySpontaneousSolve to solve *derived*
1930 equalities that have a touchable in their RHS, *in addition*
1931 to solving wanted equalities.
1933 We also need to somehow use the superclasses to quantify over a minimal,
1934 constraint see note [Minimize by Superclasses] in TcSimplify.
1937 Finally, here is another example where this is useful.
1941 class (F a ~ b) => C a b
1942 And we are given the wanteds:
1946 We surely do *not* want to quantify over (b ~ c), since if someone provides
1947 dictionaries for (C a b) and (C a c), these dictionaries can provide a proof
1948 of (b ~ c), hence no extra evidence is necessary. Here is what will happen:
1950 Step 1: We will get new *given* superclass work,
1951 provisionally to our solving of w1 and w2
1953 g1: F a ~ b, g2 : F a ~ c,
1954 w1 : C a b, w2 : C a c, w3 : b ~ c
1956 The evidence for g1 and g2 is a superclass evidence term:
1958 g1 := sc w1, g2 := sc w2
1960 Step 2: The givens will solve the wanted w3, so that
1961 w3 := sym (sc w1) ; sc w2
1963 Step 3: Now, one may naively assume that then w2 can be solve from w1
1964 after rewriting with the (now solved equality) (b ~ c).
1966 But this rewriting is ruled out by the isGoodRectDict!
1968 Conclusion, we will (correctly) end up with the unsolved goals
1971 NB: The desugarer needs be more clever to deal with equalities
1972 that participate in recursive dictionary bindings.
1975 data LookupInstResult
1977 | GenInst [WantedEvVar] EvTerm
1979 matchClassInst :: Class -> [Type] -> WantedLoc -> TcS LookupInstResult
1980 matchClassInst clas tys loc
1981 = do { let pred = mkClassPred clas tys
1982 ; mb_result <- matchClass clas tys
1984 MatchInstNo -> return NoInstance
1985 MatchInstMany -> return NoInstance -- defer any reactions of a multitude until
1986 -- we learn more about the reagent
1987 MatchInstSingle (dfun_id, mb_inst_tys) ->
1988 do { checkWellStagedDFun pred dfun_id loc
1990 -- It's possible that not all the tyvars are in
1991 -- the substitution, tenv. For example:
1992 -- instance C X a => D X where ...
1993 -- (presumably there's a functional dependency in class C)
1994 -- Hence mb_inst_tys :: Either TyVar TcType
1996 ; tys <- instDFunTypes mb_inst_tys
1997 ; let (theta, _) = tcSplitPhiTy (applyTys (idType dfun_id) tys)
1998 ; if null theta then
1999 return (GenInst [] (EvDFunApp dfun_id tys []))
2001 { ev_vars <- instDFunConstraints theta
2002 ; let wevs = [EvVarX w loc | w <- ev_vars]
2003 ; return $ GenInst wevs (EvDFunApp dfun_id tys ev_vars) }