2 -- Type definitions for the constraint solver
5 -- Canonical constraints
6 CanonicalCts, emptyCCan, andCCan, andCCans,
7 singleCCan, extendCCans, isEmptyCCan,
8 CanonicalCt(..), Xi, tyVarsOfCanonical, tyVarsOfCanonicals,
9 mkWantedConstraints, deCanonicaliseWanted,
10 makeGivens, makeSolved,
12 CtFlavor (..), isWanted, isGiven, isDerived, canRewrite,
13 joinFlavors, mkGivenFlavor,
15 TcS, runTcS, failTcS, panicTcS, traceTcS, traceTcS0, -- Basic functionality
16 tryTcS, nestImplicTcS, wrapErrTcS, wrapWarnTcS,
17 SimplContext(..), isInteractive, simplEqsOnly, performDefaulting,
19 -- Creation of evidence variables
21 newWantedCoVar, newGivOrDerCoVar, newGivOrDerEvVar,
22 newIPVar, newDictVar, newKindConstraint,
24 -- Setting evidence variables
25 setWantedCoBind, setDerivedCoBind,
26 setIPBind, setDictBind, setEvBind,
32 getInstEnvs, getFamInstEnvs, -- Getting the environments
33 getTopEnv, getGblEnv, getTcEvBinds, getUntouchables,
34 getTcEvBindsBag, getTcSContext, getTcSTyBinds, getTcSTyBindsMap,
37 newFlattenSkolemTy, -- Flatten skolems
40 instDFunTypes, -- Instantiation
47 getDefaultInfo, getDynFlags,
49 matchClass, matchFam, MatchInstResult (..),
52 pprEq, -- Smaller utils, re-exported from TcM
53 -- TODO (DV): these are only really used in the
54 -- instance matcher in TcSimplify. I am wondering
55 -- if the whole instance matcher simply belongs
59 mkWantedFunDepEqns -- Instantiation of 'Equations' from FunDeps
63 #include "HsVersions.h"
73 import NameSet ( addOneToNameSet )
75 import qualified TcRnMonad as TcM
76 import qualified TcMType as TcM
77 import qualified TcEnv as TcM
78 ( checkWellStaged, topIdLvl, tcLookupFamInst, tcGetDefaultTys )
97 import HsBinds -- for TcEvBinds stuff
108 %************************************************************************
110 %* Canonical constraints *
112 %* These are the constraints the low-level simplifier works with *
114 %************************************************************************
117 -- Types without any type functions inside. However, note that xi
118 -- types CAN contain unexpanded type synonyms; however, the
119 -- (transitive) expansions of those type synonyms will not contain any
121 type Xi = Type -- In many comments, "xi" ranges over Xi
123 type CanonicalCts = Bag CanonicalCt
126 -- Atomic canonical constraints
127 = CDictCan { -- e.g. Num xi
129 cc_flavor :: CtFlavor,
134 | CIPCan { -- ?x::tau
135 -- See note [Canonical implicit parameter constraints].
137 cc_flavor :: CtFlavor,
138 cc_ip_nm :: IPName Name,
139 cc_ip_ty :: TcTauType
142 | CTyEqCan { -- tv ~ xi (recall xi means function free)
144 -- * tv not in tvs(xi) (occurs check)
145 -- * If tv is a MetaTyVar, then typeKind xi <: typeKind tv
146 -- a skolem, then typeKind xi = typeKind tv
148 cc_flavor :: CtFlavor,
153 | CFunEqCan { -- F xis ~ xi
154 -- Invariant: * isSynFamilyTyCon cc_fun
155 -- * cc_rhs is not a touchable unification variable
156 -- See Note [No touchables as FunEq RHS]
157 -- * typeKind (TyConApp cc_fun cc_tyargs) == typeKind cc_rhs
159 cc_flavor :: CtFlavor,
160 cc_fun :: TyCon, -- A type function
161 cc_tyargs :: [Xi], -- Either under-saturated or exactly saturated
162 cc_rhs :: Xi -- *never* over-saturated (because if so
163 -- we should have decomposed)
167 makeGivens :: CanonicalCts -> CanonicalCts
168 makeGivens = mapBag (\ct -> ct { cc_flavor = mkGivenFlavor (cc_flavor ct) UnkSkol })
169 -- The UnkSkol doesn't matter because these givens are
170 -- not contradictory (else we'd have rejected them already)
172 makeSolved :: CanonicalCt -> CanonicalCt
173 -- Record that a constraint is now solved
175 -- Given, Derived -> no-op
177 | Wanted loc <- cc_flavor ct = ct { cc_flavor = Derived loc }
180 mkWantedConstraints :: CanonicalCts -> Bag Implication -> WantedConstraints
181 mkWantedConstraints flats implics
182 = mapBag (WcEvVar . deCanonicaliseWanted) flats `unionBags` mapBag WcImplic implics
184 deCanonicaliseWanted :: CanonicalCt -> WantedEvVar
185 deCanonicaliseWanted ct
186 = WARN( not (isWanted $ cc_flavor ct), ppr ct )
187 let Wanted loc = cc_flavor ct
188 in WantedEvVar (cc_id ct) loc
190 tyVarsOfCanonical :: CanonicalCt -> TcTyVarSet
191 tyVarsOfCanonical (CTyEqCan { cc_tyvar = tv, cc_rhs = xi }) = extendVarSet (tyVarsOfType xi) tv
192 tyVarsOfCanonical (CFunEqCan { cc_tyargs = tys, cc_rhs = xi }) = tyVarsOfTypes (xi:tys)
193 tyVarsOfCanonical (CDictCan { cc_tyargs = tys }) = tyVarsOfTypes tys
194 tyVarsOfCanonical (CIPCan { cc_ip_ty = ty }) = tyVarsOfType ty
196 tyVarsOfCanonicals :: CanonicalCts -> TcTyVarSet
197 tyVarsOfCanonicals = foldrBag (unionVarSet . tyVarsOfCanonical) emptyVarSet
199 instance Outputable CanonicalCt where
200 ppr (CDictCan d fl cls tys)
201 = ppr fl <+> ppr d <+> dcolon <+> pprClassPred cls tys
202 ppr (CIPCan ip fl ip_nm ty)
203 = ppr fl <+> ppr ip <+> dcolon <+> parens (ppr ip_nm <> dcolon <> ppr ty)
204 ppr (CTyEqCan co fl tv ty)
205 = ppr fl <+> ppr co <+> dcolon <+> pprEqPred (mkTyVarTy tv, ty)
206 ppr (CFunEqCan co fl tc tys ty)
207 = ppr fl <+> ppr co <+> dcolon <+> pprEqPred (mkTyConApp tc tys, ty)
211 Note [No touchables as FunEq RHS]
212 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
213 Notice that (F xis ~ beta), where beta is an touchable unification
214 variable, is not canonical. Why?
215 * If (F xis ~ beta) was the only wanted constraint, we'd
216 definitely want to spontaneously-unify it
218 * But suppose we had an earlier wanted (beta ~ Int), and
219 have already spontaneously unified it. Then we have an
220 identity given (id : beta ~ Int) in the inert set.
222 * But (F xis ~ beta) does not react with that given (because we
223 don't subsitute on the RHS of a function equality). So there's a
224 serious danger that we'd spontaneously unify it a second time.
228 Note [Canonical implicit parameter constraints]
229 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
230 The type in a canonical implicit parameter constraint doesn't need to
231 be a xi (type-function-free type) since we can defer the flattening
232 until checking this type for equality with another type. If we
233 encounter two IP constraints with the same name, they MUST have the
234 same type, and at that point we can generate a flattened equality
235 constraint between the types. (On the other hand, the types in two
236 class constraints for the same class MAY be equal, so they need to be
237 flattened in the first place to facilitate comparing them.)
240 singleCCan :: CanonicalCt -> CanonicalCts
243 andCCan :: CanonicalCts -> CanonicalCts -> CanonicalCts
246 extendCCans :: CanonicalCts -> CanonicalCt -> CanonicalCts
247 extendCCans = snocBag
249 andCCans :: [CanonicalCts] -> CanonicalCts
250 andCCans = unionManyBags
252 emptyCCan :: CanonicalCts
255 isEmptyCCan :: CanonicalCts -> Bool
256 isEmptyCCan = isEmptyBag
259 %************************************************************************
262 The "flavor" of a canonical constraint
264 %************************************************************************
268 = Given GivenLoc -- We have evidence for this constraint in TcEvBinds
269 | Derived WantedLoc -- We have evidence for this constraint in TcEvBinds;
270 -- *however* this evidence can contain wanteds, so
271 -- it's valid only provisionally to the solution of
273 | Wanted WantedLoc -- We have no evidence bindings for this constraint.
275 instance Outputable CtFlavor where
276 ppr (Given _) = ptext (sLit "[Given]")
277 ppr (Wanted _) = ptext (sLit "[Wanted]")
278 ppr (Derived _) = ptext (sLit "[Derived]")
280 isWanted :: CtFlavor -> Bool
281 isWanted (Wanted {}) = True
284 isGiven :: CtFlavor -> Bool
285 isGiven (Given {}) = True
288 isDerived :: CtFlavor -> Bool
289 isDerived (Derived {}) = True
292 canRewrite :: CtFlavor -> CtFlavor -> Bool
293 -- canRewrite ctid1 ctid2
294 -- The constraint ctid1 can be used to rewrite ctid2
295 canRewrite (Given {}) _ = True
296 canRewrite (Derived {}) (Wanted {}) = True
297 canRewrite (Derived {}) (Derived {}) = True
298 canRewrite (Wanted {}) (Wanted {}) = True
299 canRewrite _ _ = False
301 joinFlavors :: CtFlavor -> CtFlavor -> CtFlavor
302 joinFlavors (Wanted loc) _ = Wanted loc
303 joinFlavors _ (Wanted loc) = Wanted loc
304 joinFlavors (Derived loc) _ = Derived loc
305 joinFlavors _ (Derived loc) = Derived loc
306 joinFlavors (Given loc) _ = Given loc
308 mkGivenFlavor :: CtFlavor -> SkolemInfo -> CtFlavor
309 mkGivenFlavor (Wanted loc) sk = Given (setCtLocOrigin loc sk)
310 mkGivenFlavor (Derived loc) sk = Given (setCtLocOrigin loc sk)
311 mkGivenFlavor (Given loc) sk = Given (setCtLocOrigin loc sk)
315 %************************************************************************
317 %* The TcS solver monad *
319 %************************************************************************
323 The TcS monad is a weak form of the main Tc monad
327 * allocate new variables
328 * fill in evidence variables
330 Filling in a dictionary evidence variable means to create a binding
331 for it, so TcS carries a mutable location where the binding can be
332 added. This is initialised from the innermost implication constraint.
337 tcs_ev_binds :: EvBindsVar,
340 tcs_ty_binds :: IORef (TyVarEnv (TcTyVar, TcType)),
341 -- Global type bindings
343 tcs_context :: SimplContext,
345 tcs_untch :: Untouchables
349 = SimplInfer -- Inferring type of a let-bound thing
350 | SimplRuleLhs -- Inferring type of a RULE lhs
351 | SimplInteractive -- Inferring type at GHCi prompt
352 | SimplCheck -- Checking a type signature or RULE rhs
354 instance Outputable SimplContext where
355 ppr SimplInfer = ptext (sLit "SimplInfer")
356 ppr SimplRuleLhs = ptext (sLit "SimplRuleLhs")
357 ppr SimplInteractive = ptext (sLit "SimplInteractive")
358 ppr SimplCheck = ptext (sLit "SimplCheck")
360 isInteractive :: SimplContext -> Bool
361 isInteractive SimplInteractive = True
362 isInteractive _ = False
364 simplEqsOnly :: SimplContext -> Bool
365 -- Simplify equalities only, not dictionaries
366 -- This is used for the LHS of rules; ee
367 -- Note [Simplifying RULE lhs constraints] in TcSimplify
368 simplEqsOnly SimplRuleLhs = True
369 simplEqsOnly _ = False
371 performDefaulting :: SimplContext -> Bool
372 performDefaulting SimplInfer = False
373 performDefaulting SimplRuleLhs = False
374 performDefaulting SimplInteractive = True
375 performDefaulting SimplCheck = True
378 newtype TcS a = TcS { unTcS :: TcSEnv -> TcM a }
380 instance Functor TcS where
381 fmap f m = TcS $ fmap f . unTcS m
383 instance Monad TcS where
384 return x = TcS (\_ -> return x)
385 fail err = TcS (\_ -> fail err)
386 m >>= k = TcS (\ebs -> unTcS m ebs >>= \r -> unTcS (k r) ebs)
388 -- Basic functionality
389 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
390 wrapTcS :: TcM a -> TcS a
391 -- Do not export wrapTcS, because it promotes an arbitrary TcM to TcS,
392 -- and TcS is supposed to have limited functionality
393 wrapTcS = TcS . const -- a TcM action will not use the TcEvBinds
395 wrapErrTcS :: TcM a -> TcS a
396 -- The thing wrapped should just fail
397 -- There's no static check; it's up to the user
398 -- Having a variant for each error message is too painful
401 wrapWarnTcS :: TcM a -> TcS a
402 -- The thing wrapped should just add a warning, or no-op
403 -- There's no static check; it's up to the user
404 wrapWarnTcS = wrapTcS
406 failTcS, panicTcS :: SDoc -> TcS a
407 failTcS = wrapTcS . TcM.failWith
408 panicTcS doc = pprPanic "TcCanonical" doc
410 traceTcS :: String -> SDoc -> TcS ()
411 traceTcS herald doc = TcS $ \_env -> TcM.traceTc herald doc
413 traceTcS0 :: String -> SDoc -> TcS ()
414 traceTcS0 herald doc = TcS $ \_env -> TcM.traceTcN 0 herald doc
416 runTcS :: SimplContext
417 -> Untouchables -- Untouchables
418 -> TcS a -- What to run
419 -> TcM (a, Bag EvBind)
420 runTcS context untouch tcs
421 = do { ty_binds_var <- TcM.newTcRef emptyVarEnv
422 ; ev_binds_var@(EvBindsVar evb_ref _) <- TcM.newTcEvBinds
423 ; let env = TcSEnv { tcs_ev_binds = ev_binds_var
424 , tcs_ty_binds = ty_binds_var
425 , tcs_context = context
426 , tcs_untch = untouch }
428 -- Run the computation
429 ; res <- unTcS tcs env
431 -- Perform the type unifications required
432 ; ty_binds <- TcM.readTcRef ty_binds_var
433 ; mapM_ do_unification (varEnvElts ty_binds)
436 ; ev_binds <- TcM.readTcRef evb_ref
437 ; return (res, evBindMapBinds ev_binds) }
439 do_unification (tv,ty) = TcM.writeMetaTyVar tv ty
442 nestImplicTcS :: EvBindsVar -> Untouchables -> TcS a -> TcS a
443 nestImplicTcS ref untch (TcS thing_inside)
444 = TcS $ \ TcSEnv { tcs_ty_binds = ty_binds, tcs_context = ctxt } ->
446 nest_env = TcSEnv { tcs_ev_binds = ref
447 , tcs_ty_binds = ty_binds
449 , tcs_context = ctxtUnderImplic ctxt }
451 thing_inside nest_env
453 ctxtUnderImplic :: SimplContext -> SimplContext
454 -- See Note [Simplifying RULE lhs constraints] in TcSimplify
455 ctxtUnderImplic SimplRuleLhs = SimplCheck
456 ctxtUnderImplic ctxt = ctxt
458 tryTcS :: TcS a -> TcS a
459 -- Like runTcS, but from within the TcS monad
460 -- Ignore all the evidence generated, and do not affect caller's evidence!
462 = TcS (\env -> do { ty_binds_var <- TcM.newTcRef emptyVarEnv
463 ; ev_binds_var <- TcM.newTcEvBinds
464 ; let env1 = env { tcs_ev_binds = ev_binds_var
465 , tcs_ty_binds = ty_binds_var }
469 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
471 getDynFlags :: TcS DynFlags
472 getDynFlags = wrapTcS TcM.getDOpts
474 getTcSContext :: TcS SimplContext
475 getTcSContext = TcS (return . tcs_context)
477 getTcEvBinds :: TcS EvBindsVar
478 getTcEvBinds = TcS (return . tcs_ev_binds)
480 getUntouchables :: TcS Untouchables
481 getUntouchables = TcS (return . tcs_untch)
483 getTcSTyBinds :: TcS (IORef (TyVarEnv (TcTyVar, TcType)))
484 getTcSTyBinds = TcS (return . tcs_ty_binds)
486 getTcSTyBindsMap :: TcS (TyVarEnv (TcTyVar, TcType))
487 getTcSTyBindsMap = getTcSTyBinds >>= wrapTcS . (TcM.readTcRef)
490 getTcEvBindsBag :: TcS EvBindMap
492 = do { EvBindsVar ev_ref _ <- getTcEvBinds
493 ; wrapTcS $ TcM.readTcRef ev_ref }
495 setWantedCoBind :: CoVar -> Coercion -> TcS ()
496 setWantedCoBind cv co
497 = setEvBind cv (EvCoercion co)
498 -- Was: wrapTcS $ TcM.writeWantedCoVar cv co
500 setDerivedCoBind :: CoVar -> Coercion -> TcS ()
501 setDerivedCoBind cv co
502 = setEvBind cv (EvCoercion co)
504 setWantedTyBind :: TcTyVar -> TcType -> TcS ()
505 -- Add a type binding
506 setWantedTyBind tv ty
507 = do { ref <- getTcSTyBinds
509 do { ty_binds <- TcM.readTcRef ref
510 ; TcM.writeTcRef ref (extendVarEnv ty_binds tv (tv,ty)) } }
512 setIPBind :: EvVar -> EvTerm -> TcS ()
513 setIPBind = setEvBind
515 setDictBind :: EvVar -> EvTerm -> TcS ()
516 setDictBind = setEvBind
518 setEvBind :: EvVar -> EvTerm -> TcS ()
521 = do { tc_evbinds <- getTcEvBinds
522 ; wrapTcS (TcM.addTcEvBind tc_evbinds ev rhs) }
524 newTcEvBindsTcS :: TcS EvBindsVar
525 newTcEvBindsTcS = wrapTcS (TcM.newTcEvBinds)
527 warnTcS :: CtLoc orig -> Bool -> SDoc -> TcS ()
528 warnTcS loc warn_if doc
529 | warn_if = wrapTcS $ TcM.setCtLoc loc $ TcM.addWarnTc doc
530 | otherwise = return ()
532 getDefaultInfo :: TcS (SimplContext, [Type], (Bool, Bool))
534 = do { ctxt <- getTcSContext
535 ; (tys, flags) <- wrapTcS (TcM.tcGetDefaultTys (isInteractive ctxt))
536 ; return (ctxt, tys, flags) }
538 -- Just get some environments needed for instance looking up and matching
539 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
541 getInstEnvs :: TcS (InstEnv, InstEnv)
542 getInstEnvs = wrapTcS $ Inst.tcGetInstEnvs
544 getFamInstEnvs :: TcS (FamInstEnv, FamInstEnv)
545 getFamInstEnvs = wrapTcS $ FamInst.tcGetFamInstEnvs
547 getTopEnv :: TcS HscEnv
548 getTopEnv = wrapTcS $ TcM.getTopEnv
550 getGblEnv :: TcS TcGblEnv
551 getGblEnv = wrapTcS $ TcM.getGblEnv
553 -- Various smaller utilities [TODO, maybe will be absorbed in the instance matcher]
554 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
556 checkWellStagedDFun :: PredType -> DFunId -> WantedLoc -> TcS ()
557 checkWellStagedDFun pred dfun_id loc
558 = wrapTcS $ TcM.setCtLoc loc $
559 do { use_stage <- TcM.getStage
560 ; TcM.checkWellStaged pp_thing bind_lvl (thLevel use_stage) }
562 pp_thing = ptext (sLit "instance for") <+> quotes (ppr pred)
563 bind_lvl = TcM.topIdLvl dfun_id
565 pprEq :: TcType -> TcType -> SDoc
566 pprEq ty1 ty2 = pprPred $ mkEqPred (ty1,ty2)
568 isTouchableMetaTyVar :: TcTyVar -> TcS Bool
569 -- is touchable variable!
570 isTouchableMetaTyVar tv
571 | isMetaTyVar tv = do { untch <- getUntouchables
572 ; return (inTouchableRange untch tv) }
573 | otherwise = return False
577 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
579 newFlattenSkolemTy :: TcType -> TcS TcType
580 newFlattenSkolemTy ty = mkTyVarTy <$> newFlattenSkolemTyVar ty
581 where newFlattenSkolemTyVar :: TcType -> TcS TcTyVar
582 newFlattenSkolemTyVar ty
583 = wrapTcS $ do { uniq <- TcM.newUnique
584 ; let name = mkSysTvName uniq (fsLit "f")
586 mkTcTyVar name (typeKind ty) (FlatSkol ty)
590 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
592 instDFunTypes :: [Either TyVar TcType] -> TcS [TcType]
593 instDFunTypes mb_inst_tys =
594 let inst_tv :: Either TyVar TcType -> TcS Type
595 inst_tv (Left tv) = wrapTcS $ TcM.tcInstTyVar tv >>= return . mkTyVarTy
596 inst_tv (Right ty) = return ty
597 in mapM inst_tv mb_inst_tys
600 instDFunConstraints :: TcThetaType -> TcS [EvVar]
601 instDFunConstraints preds = wrapTcS $ TcM.newWantedEvVars preds
604 -- Superclasses and recursive dictionaries
605 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
607 newGivOrDerEvVar :: TcPredType -> EvTerm -> TcS EvVar
608 newGivOrDerEvVar pty evtrm
609 = do { ev <- wrapTcS $ TcM.newEvVar pty
613 newGivOrDerCoVar :: TcType -> TcType -> Coercion -> TcS EvVar
614 -- Note we create immutable variables for given or derived, since we
615 -- must bind them to TcEvBinds (because their evidence may involve
616 -- superclasses). However we should be able to override existing
617 -- 'derived' evidence, even in TcEvBinds
618 newGivOrDerCoVar ty1 ty2 co
619 = do { cv <- newCoVar ty1 ty2
620 ; setEvBind cv (EvCoercion co)
623 newWantedCoVar :: TcType -> TcType -> TcS EvVar
624 newWantedCoVar ty1 ty2 = wrapTcS $ TcM.newWantedCoVar ty1 ty2
626 newKindConstraint :: TcType -> Kind -> TcS (CoVar, TcType)
627 newKindConstraint ty kind = wrapTcS $ TcM.newKindConstraint ty kind
629 newCoVar :: TcType -> TcType -> TcS EvVar
630 newCoVar ty1 ty2 = wrapTcS $ TcM.newCoVar ty1 ty2
632 newIPVar :: IPName Name -> TcType -> TcS EvVar
633 newIPVar nm ty = wrapTcS $ TcM.newIP nm ty
635 newDictVar :: Class -> [TcType] -> TcS EvVar
636 newDictVar cl tys = wrapTcS $ TcM.newDict cl tys
641 isGoodRecEv :: EvVar -> WantedEvVar -> TcS Bool
642 -- In a call (isGoodRecEv ev wv), we are considering solving wv
643 -- using some term that involves ev, such as:
644 -- by setting wv = ev
645 -- or wv = EvCast x |> ev
647 -- But that would be Very Bad if the evidence for 'ev' mentions 'wv',
648 -- in an "unguarded" way. So isGoodRecEv looks at the evidence ev
649 -- recursively through the evidence binds, to see if uses of 'wv' are guarded.
651 -- Guarded means: more instance calls than superclass selections. We
652 -- compute this by chasing the evidence, adding +1 for every instance
653 -- call (constructor) and -1 for every superclass selection (destructor).
655 -- See Note [Superclasses and recursive dictionaries] in TcInteract
656 isGoodRecEv ev_var (WantedEvVar wv _)
657 = do { tc_evbinds <- getTcEvBindsBag
658 ; mb <- chase_ev_var tc_evbinds wv 0 [] ev_var
659 ; return $ case mb of
661 Just min_guardedness -> min_guardedness > 0
664 where chase_ev_var :: EvBindMap -- Evidence binds
665 -> EvVar -- Target variable whose gravity we want to return
666 -> Int -- Current gravity
667 -> [EvVar] -- Visited nodes
668 -> EvVar -- Current node
670 chase_ev_var assocs trg curr_grav visited orig
671 | trg == orig = return $ Just curr_grav
672 | orig `elem` visited = return $ Nothing
673 | Just (EvBind _ ev_trm) <- lookupEvBind assocs orig
674 = chase_ev assocs trg curr_grav (orig:visited) ev_trm
676 {- No longer needed: evidence is in the EvBinds
677 | isTcTyVar orig && isMetaTyVar orig
678 = do { meta_details <- wrapTcS $ TcM.readWantedCoVar orig
679 ; case meta_details of
680 Flexi -> return Nothing
681 Indirect tyco -> chase_ev assocs trg curr_grav
682 (orig:visited) (EvCoercion tyco)
685 | otherwise = return Nothing
687 chase_ev assocs trg curr_grav visited (EvId v)
688 = chase_ev_var assocs trg curr_grav visited v
689 chase_ev assocs trg curr_grav visited (EvSuperClass d_id _)
690 = chase_ev_var assocs trg (curr_grav-1) visited d_id
691 chase_ev assocs trg curr_grav visited (EvCast v co)
692 = do { m1 <- chase_ev_var assocs trg curr_grav visited v
693 ; m2 <- chase_co assocs trg curr_grav visited co
694 ; return (comb_chase_res Nothing [m1,m2]) }
696 chase_ev assocs trg curr_grav visited (EvCoercion co)
697 = chase_co assocs trg curr_grav visited co
698 chase_ev assocs trg curr_grav visited (EvDFunApp _ _ ev_vars)
699 = do { chase_results <- mapM (chase_ev_var assocs trg (curr_grav+1) visited) ev_vars
700 ; return (comb_chase_res Nothing chase_results) }
702 chase_co assocs trg curr_grav visited co
703 = -- Look for all the coercion variables in the coercion
704 -- chase them, and combine the results. This is OK since the
705 -- coercion will not contain any superclass terms -- anything
706 -- that involves dictionaries will be bound in assocs.
707 let co_vars = foldVarSet (\v vrs -> if isCoVar v then (v:vrs) else vrs) []
709 in do { chase_results <- mapM (chase_ev_var assocs trg curr_grav visited) co_vars
710 ; return (comb_chase_res Nothing chase_results) }
712 comb_chase_res f [] = f
713 comb_chase_res f (Nothing:rest) = comb_chase_res f rest
714 comb_chase_res Nothing (Just n:rest) = comb_chase_res (Just n) rest
715 comb_chase_res (Just m) (Just n:rest) = comb_chase_res (Just (min n m)) rest
718 -- Matching and looking up classes and family instances
719 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
721 data MatchInstResult mi
722 = MatchInstNo -- No matching instance
723 | MatchInstSingle mi -- Single matching instance
724 | MatchInstMany -- Multiple matching instances
727 matchClass :: Class -> [Type] -> TcS (MatchInstResult (DFunId, [Either TyVar TcType]))
728 -- Look up a class constraint in the instance environment
730 = do { let pred = mkClassPred clas tys
731 ; instEnvs <- getInstEnvs
732 ; case lookupInstEnv instEnvs clas tys of {
733 ([], unifs) -- Nothing matches
734 -> do { traceTcS "matchClass not matching"
735 (vcat [ text "dict" <+> ppr pred,
736 text "unifs" <+> ppr unifs ])
739 ([(ispec, inst_tys)], []) -- A single match
740 -> do { let dfun_id = is_dfun ispec
741 ; traceTcS "matchClass success"
742 (vcat [text "dict" <+> ppr pred,
743 text "witness" <+> ppr dfun_id
744 <+> ppr (idType dfun_id) ])
745 -- Record that this dfun is needed
746 ; record_dfun_usage dfun_id
747 ; return $ MatchInstSingle (dfun_id, inst_tys)
749 (matches, unifs) -- More than one matches
750 -> do { traceTcS "matchClass multiple matches, deferring choice"
751 (vcat [text "dict" <+> ppr pred,
752 text "matches" <+> ppr matches,
753 text "unifs" <+> ppr unifs])
754 ; return MatchInstMany
758 where record_dfun_usage :: Id -> TcS ()
759 record_dfun_usage dfun_id
760 = do { hsc_env <- getTopEnv
761 ; let dfun_name = idName dfun_id
762 dfun_mod = ASSERT( isExternalName dfun_name )
764 ; if isInternalName dfun_name || -- Internal name => defined in this module
765 modulePackageId dfun_mod /= thisPackage (hsc_dflags hsc_env)
766 then return () -- internal, or in another package
767 else do updInstUses dfun_id
770 updInstUses :: Id -> TcS ()
772 = do { tcg_env <- getGblEnv
773 ; wrapTcS $ TcM.updMutVar (tcg_inst_uses tcg_env)
774 (`addOneToNameSet` idName dfun_id)
779 -> TcS (MatchInstResult (TyCon, [Type]))
781 = do { mb <- wrapTcS $ TcM.tcLookupFamInst tycon args
783 Nothing -> return MatchInstNo
784 Just res -> return $ MatchInstSingle res
785 -- DV: We never return MatchInstMany, since tcLookupFamInst never returns
786 -- multiple matches. Check.
790 -- Functional dependencies, instantiation of equations
791 -------------------------------------------------------
793 mkWantedFunDepEqns :: WantedLoc -> [(Equation, (PredType, SDoc), (PredType, SDoc))]
795 mkWantedFunDepEqns _ [] = return []
796 mkWantedFunDepEqns loc eqns
797 = do { traceTcS "Improve:" (vcat (map pprEquationDoc eqns))
798 ; wevvars <- mapM to_work_item eqns
799 ; return $ concat wevvars }
801 to_work_item :: (Equation, (PredType,SDoc), (PredType,SDoc)) -> TcS [WantedEvVar]
802 to_work_item ((qtvs, pairs), _, _)
803 = do { (_, _, tenv) <- wrapTcS $ TcM.tcInstTyVars (varSetElems qtvs)
804 ; mapM (do_one tenv) pairs }
806 do_one tenv (ty1, ty2) = do { let sty1 = substTy tenv ty1
807 sty2 = substTy tenv ty2
808 ; ev <- newWantedCoVar sty1 sty2
809 ; return (WantedEvVar ev loc) }
811 pprEquationDoc :: (Equation, (PredType, SDoc), (PredType, SDoc)) -> SDoc
812 pprEquationDoc (eqn, (p1, _), (p2, _))
813 = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]