2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcSimplify]{TcSimplify}
8 Inference (local definitions)
9 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
10 If the inst constrains a local type variable, then
11 [ReduceMe] if it's a literal or method inst, reduce it
13 [DontReduce] otherwise see whether the inst is just a constant
15 if not, add original to context
16 This check gets rid of constant dictionaries without
19 If the inst does not constrain a local type variable then
20 [Free] then throw it out as free.
22 Inference (top level definitions)
23 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
24 If the inst does not constrain a local type variable, then
25 [FreeIfTautological] try for tautology;
26 if so, throw it out as free
27 (discarding result of tautology check)
28 if not, make original inst part of the context
29 (eliminating superclasses as usual)
31 If the inst constrains a local type variable, then
32 as for inference (local defns)
35 Checking (local defns)
37 If the inst constrains a local type variable then
38 [ReduceMe] reduce (signal error on failure)
40 If the inst does not constrain a local type variable then
41 [Free] throw it out as free.
45 If the inst constrains a local type variable then
46 as for checking (local defns)
48 If the inst does not constrain a local type variable then
49 as for checking (local defns)
53 Checking once per module
54 ~~~~~~~~~~~~~~~~~~~~~~~~~
55 For dicts of the form (C a), where C is a std class
56 and "a" is a type variable,
57 [DontReduce] add to context
59 otherwise [ReduceMe] always reduce
61 [NB: we may generate one Tree [Int] dict per module, so
62 sharing is not complete.]
64 Sort out ambiguity at the end.
71 f x = let g y = op (y::Int) in True
73 Here the principal type of f is (forall a. a->a)
74 but we'll produce the non-principal type
75 f :: forall a. C Int => a -> a
82 instance C (T a) Int where ...
83 instance C (T a) Bool where ...
85 and suppose we infer a context
89 from some expression, where x and y are type varibles,
90 and x is ambiguous, and y is being quantified over.
91 Should we complain, or should we generate the type
93 forall x y. C (T x) y => <type not involving x>
95 The idea is that at the call of the function we might
96 know that y is Int (say), so the "x" isn't really ambiguous.
97 Notice that we have to add "x" to the type variables over
100 Something similar can happen even if C constrains only ambiguous
101 variables. Suppose we infer the context
105 where x is ambiguous. Then we could infer the type
107 forall x. C [x] => <type not involving x>
109 in the hope that at the call site there was an instance
112 instance Num a => C [a] where ...
114 and hence the default mechanism would resolve the "a".
119 tcSimplify, tcSimplifyAndCheck, tcSimplifyToDicts,
120 tcSimplifyTop, tcSimplifyThetas, tcSimplifyCheckThetas,
124 #include "HsVersions.h"
126 import CmdLineOpts ( opt_MaxContextReductionDepth, opt_GlasgowExts, opt_WarnTypeDefaults )
127 import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
128 import TcHsSyn ( TcExpr, TcId,
129 TcMonoBinds, TcDictBinds
133 import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
135 isDict, isStdClassTyVarDict, isMethodFor, notFunDep,
136 instToId, instBindingRequired, instCanBeGeneralised,
139 instLoc, pprInst, zonkInst, tidyInst, tidyInsts,
140 Inst, LIE, pprInsts, pprInstsInFull, mkLIE, emptyLIE,
143 import TcEnv ( tcGetGlobalTyVars )
144 import TcType ( TcType, TcTyVarSet, typeToTcType )
145 import TcUnify ( unifyTauTy )
147 import Bag ( bagToList )
148 import Class ( Class, classBigSig, classInstEnv )
149 import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
151 import Type ( Type, ThetaType, TauType, mkTyVarTy, getTyVar,
152 isTyVarTy, splitSigmaTy, tyVarsOfTypes
154 import InstEnv ( InstEnv )
155 import Subst ( mkTopTyVarSubst, substTheta )
156 import PprType ( pprConstraint )
157 import TysWiredIn ( unitTy )
160 import BasicTypes ( TopLevelFlag(..) )
161 import CmdLineOpts ( opt_GlasgowExts )
164 import List ( partition )
168 %************************************************************************
170 \subsection[tcSimplify-main]{Main entry function}
172 %************************************************************************
174 The main wrapper is @tcSimplify@. It just calls @tcSimpl@, but with
175 the ``don't-squash-consts'' flag set depending on top-level ness. For
176 top level defns we *do* squash constants, so that they stay local to a
177 single defn. This makes things which are inlined more likely to be
178 exportable, because their constants are "inside". Later passes will
179 float them out if poss, after inlinings are sorted out.
185 -> TcTyVarSet -- ``Local'' type variables
186 -- ASSERT: this tyvar set is already zonked
188 -> TcM s (LIE, -- Free
189 TcDictBinds, -- Bindings
190 LIE) -- Remaining wanteds; no dups
192 tcSimplify str top_lvl local_tvs wanted_lie
193 | isEmptyVarSet local_tvs
194 = returnTc (wanted_lie, EmptyMonoBinds, emptyLIE)
197 = reduceContext str try_me [] wanteds `thenTc` \ (binds, frees, irreds) ->
199 -- Check for non-generalisable insts
201 cant_generalise = filter (not . instCanBeGeneralised) irreds
203 checkTc (null cant_generalise)
204 (genCantGenErr cant_generalise) `thenTc_`
206 -- Check for ambiguous insts.
207 -- You might think these can't happen (I did) because an ambiguous
208 -- inst like (Eq a) will get tossed out with "frees", and eventually
209 -- dealt with by tcSimplifyTop.
210 -- But we can get stuck with
212 -- where "a" is one of the local_tvs, but "b" is unconstrained.
213 -- Then we must yell about the ambiguous b
214 -- But we must only do so if "b" really is unconstrained; so
215 -- we must grab the global tyvars to answer that question
216 tcGetGlobalTyVars `thenNF_Tc` \ global_tvs ->
218 avail_tvs = local_tvs `unionVarSet` global_tvs
219 (irreds', bad_guys) = partition (isEmptyVarSet . ambig_tv_fn) irreds
220 ambig_tv_fn dict = tyVarsOfInst dict `minusVarSet` avail_tvs
222 addAmbigErrs ambig_tv_fn bad_guys `thenNF_Tc_`
226 returnTc (mkLIE frees, binds, mkLIE irreds')
228 -- the idea behind filtering out the dependencies here is that
229 -- they've already served their purpose, and can be reconstructed
230 -- at a later point from the retained class predicates.
231 -- however, there *is* the possibility that a dependency
232 -- out-lives the predicate from which it arose.
233 -- I don't have any examples of this, but if they show up,
234 -- we'd want to consider the possibility of saving the
235 -- dependencies as hidden constraints (i.e. they'd only
236 -- show up in interface files) -- or maybe they'd be useful
237 -- as first class predicates...
238 wanteds = filter notFunDep (bagToList wanted_lie)
241 -- Does not constrain a local tyvar
242 | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs)
243 = -- if is_top_level then
244 -- FreeIfTautological -- Special case for inference on
245 -- -- top-level defns
249 -- We're infering (not checking) the type, and
250 -- the inst constrains a local type variable
251 | isDict inst = DontReduce -- Dicts
252 | otherwise = ReduceMe AddToIrreds -- Lits and Methods
255 @tcSimplifyAndCheck@ is similar to the above, except that it checks
256 that there is an empty wanted-set at the end. It may still return
257 some of constant insts, which have to be resolved finally at the end.
262 -> TcTyVarSet -- ``Local'' type variables
263 -- ASSERT: this tyvar set is already zonked
264 -> LIE -- Given; constrain only local tyvars
266 -> TcM s (LIE, -- Free
267 TcDictBinds) -- Bindings
269 tcSimplifyAndCheck str local_tvs given_lie wanted_lie
270 | isEmptyVarSet local_tvs
271 -- This can happen quite legitimately; for example in
272 -- instance Num Int where ...
273 = returnTc (wanted_lie, EmptyMonoBinds)
276 = reduceContext str try_me givens wanteds `thenTc` \ (binds, frees, irreds) ->
278 -- Complain about any irreducible ones
279 mapNF_Tc complain irreds `thenNF_Tc_`
282 returnTc (mkLIE frees, binds)
284 givens = bagToList given_lie
285 -- see comment on wanteds in tcSimplify
286 wanteds = filter notFunDep (bagToList wanted_lie)
287 given_dicts = filter isDict givens
290 -- Does not constrain a local tyvar
291 | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs)
294 -- When checking against a given signature we always reduce
295 -- until we find a match against something given, or can't reduce
297 = ReduceMe AddToIrreds
299 complain dict = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens ->
300 addNoInstanceErr str given_dicts dict
303 On the LHS of transformation rules we only simplify methods and constants,
304 getting dictionaries. We want to keep all of them unsimplified, to serve
305 as the available stuff for the RHS of the rule.
307 The same thing is used for specialise pragmas. Consider
310 {-# SPECIALISE f :: Int -> Int #-}
313 The type checker generates a binding like:
315 f_spec = (f :: Int -> Int)
317 and we want to end up with
319 f_spec = _inline_me_ (f Int dNumInt)
321 But that means that we must simplify the Method for f to (f Int dNumInt)!
322 So tcSimplifyToDicts squeezes out all Methods.
325 tcSimplifyToDicts :: LIE -> TcM s (LIE, TcDictBinds)
326 tcSimplifyToDicts wanted_lie
327 = reduceContext (text "tcSimplifyToDicts") try_me [] wanteds `thenTc` \ (binds, frees, irreds) ->
329 returnTc (mkLIE irreds, binds)
331 wanteds = bagToList wanted_lie
333 -- Reduce methods and lits only; stop as soon as we get a dictionary
334 try_me inst | isDict inst = DontReduce
335 | otherwise = ReduceMe AddToIrreds
340 %************************************************************************
342 \subsection{Data types for the reduction mechanism}
344 %************************************************************************
346 The main control over context reduction is here
350 = ReduceMe -- Try to reduce this
351 NoInstanceAction -- What to do if there's no such instance
353 | DontReduce -- Return as irreducible
355 | Free -- Return as free
357 | FreeIfTautological -- Return as free iff it's tautological;
358 -- if not, return as irreducible
359 -- The FreeIfTautological case is to allow the possibility
360 -- of generating functions with types like
361 -- f :: C Int => Int -> Int
362 -- Here, the C Int isn't a tautology presumably because Int
363 -- isn't an instance of C in this module; but perhaps it will
364 -- be at f's call site(s). Haskell doesn't allow this at
367 data NoInstanceAction
368 = Stop -- Fail; no error message
369 -- (Only used when tautology checking.)
371 | AddToIrreds -- Just add the inst to the irreductible ones; don't
372 -- produce an error message of any kind.
373 -- It might be quite legitimate such as (Eq a)!
380 = (Avails s, -- What's available
381 [Inst], -- Insts for which try_me returned Free
382 [Inst] -- Insts for which try_me returned DontReduce
385 type Avails s = FiniteMap Inst Avail
389 TcId -- The "main Id"; that is, the Id for the Inst that
390 -- caused this avail to be put into the finite map in the first place
391 -- It is this Id that is bound to the RHS.
393 RHS -- The RHS: an expression whose value is that Inst.
394 -- The main Id should be bound to this RHS
396 [TcId] -- Extra Ids that must all be bound to the main Id.
397 -- At the end we generate a list of bindings
398 -- { i1 = main_id; i2 = main_id; i3 = main_id; ... }
401 = NoRhs -- Used for irreducible dictionaries,
402 -- which are going to be lambda bound, or for those that are
403 -- suppplied as "given" when checking againgst a signature.
405 -- NoRhs is also used for Insts like (CCallable f)
406 -- where no witness is required.
408 | Rhs -- Used when there is a RHS
410 Bool -- True => the RHS simply selects a superclass dictionary
411 -- from a subclass dictionary.
413 -- This is useful info, because superclass selection
414 -- is cheaper than building the dictionary using its dfun,
415 -- and we can sometimes replace the latter with the former
417 | PassiveScSel -- Used for as-yet-unactivated RHSs. For example suppose we have
418 -- an (Ord t) dictionary; then we put an (Eq t) entry in
419 -- the finite map, with an PassiveScSel. Then if the
420 -- the (Eq t) binding is ever *needed* we make it an Rhs
422 [Inst] -- List of Insts that are free in the RHS.
423 -- If the main Id is subsequently needed, we toss this list into
424 -- the needed-inst pool so that we make sure their bindings
425 -- will actually be produced.
427 -- Invariant: these Insts are already in the finite mapping
430 pprAvails avails = vcat (map pp (eltsFM avails))
432 pp (Avail main_id rhs ids)
433 = ppr main_id <> colon <+> brackets (ppr ids) <+> pprRhs rhs
435 pprRhs NoRhs = text "<no rhs>"
436 pprRhs (Rhs rhs b) = ppr rhs
437 pprRhs (PassiveScSel rhs is) = text "passive" <+> ppr rhs
441 %************************************************************************
443 \subsection[reduce]{@reduce@}
445 %************************************************************************
447 The main entry point for context reduction is @reduceContext@:
450 reduceContext :: SDoc -> (Inst -> WhatToDo)
453 -> TcM s (TcDictBinds,
455 [Inst]) -- Irreducible
457 reduceContext str try_me givens wanteds
459 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens ->
460 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds ->
463 pprTrace "reduceContext" (vcat [
464 text "----------------------",
466 text "given" <+> ppr givens,
467 text "wanted" <+> ppr wanteds,
468 text "----------------------"
471 -- Build the Avail mapping from "givens"
472 foldlNF_Tc addGiven emptyFM givens `thenNF_Tc` \ avails ->
475 reduceList (0,[]) try_me wanteds (avails, [], []) `thenTc` \ (avails, frees, irreds) ->
477 -- Extract the bindings from avails
479 binds = foldFM add_bind EmptyMonoBinds avails
481 add_bind _ (Avail main_id rhs ids) binds
482 = foldr add_synonym (add_rhs_bind rhs binds) ids
484 add_rhs_bind (Rhs rhs _) binds = binds `AndMonoBinds` VarMonoBind main_id rhs
485 add_rhs_bind other binds = binds
487 -- Add the trivial {x = y} bindings
488 -- The main Id can end up in the list when it's first added passively
489 -- and then activated, so we have to filter it out. A bit of a hack.
491 | id /= main_id = binds `AndMonoBinds` VarMonoBind id (HsVar main_id)
495 pprTrace ("reduceContext end") (vcat [
496 text "----------------------",
498 text "given" <+> ppr givens,
499 text "wanted" <+> ppr wanteds,
501 text "avails" <+> pprAvails avails,
502 text "irreds" <+> ppr irreds,
503 text "----------------------"
506 returnTc (binds, frees, irreds)
509 The main context-reduction function is @reduce@. Here's its game plan.
512 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
513 -- along with its depth
514 -> (Inst -> WhatToDo)
517 -> TcM s (RedState s)
521 try_me: given an inst, this function returns
523 DontReduce return this in "irreds"
524 Free return this in "frees"
526 wanteds: The list of insts to reduce
527 state: An accumulating parameter of type RedState
528 that contains the state of the algorithm
530 It returns a RedState.
532 The (n,stack) pair is just used for error reporting.
533 n is always the depth of the stack.
534 The stack is the stack of Insts being reduced: to produce X
535 I had to produce Y, to produce Y I had to produce Z, and so on.
538 reduceList (n,stack) try_me wanteds state
539 | n > opt_MaxContextReductionDepth
540 = failWithTc (reduceDepthErr n stack)
546 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
551 go [] state = returnTc state
552 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
555 -- Base case: we're done!
556 reduce stack try_me wanted state@(avails, frees, irreds)
557 -- It's the same as an existing inst, or a superclass thereof
558 | wanted `elemFM` avails
559 = returnTc (activate avails wanted, frees, irreds)
562 = case try_me wanted of {
564 ReduceMe no_instance_action -> -- It should be reduced
565 lookupInst wanted `thenNF_Tc` \ lookup_result ->
566 case lookup_result of
567 GenInst wanteds' rhs -> use_instance wanteds' rhs
568 SimpleInst rhs -> use_instance [] rhs
570 NoInstance -> -- No such instance!
571 case no_instance_action of
573 AddToIrreds -> add_to_irreds
575 Free -> -- It's free and this isn't a top-level binding, so just chuck it upstairs
576 -- First, see if the inst can be reduced to a constant in one step
577 lookupInst wanted `thenNF_Tc` \ lookup_result ->
578 case lookup_result of
579 SimpleInst rhs -> use_instance [] rhs
580 other -> add_to_frees
585 FreeIfTautological -> -- It's free and this is a top level binding, so
586 -- check whether it's a tautology or not
588 add_to_irreds -- If tautology trial fails, add to irreds
590 -- If tautology succeeds, just add to frees
591 (reduce stack try_me_taut wanted (avails, [], []) `thenTc_`
592 returnTc (avails, wanted:frees, irreds))
596 DontReduce -> -- It's irreducible (or at least should not be reduced)
597 -- See if the inst can be reduced to a constant in one step
598 lookupInst wanted `thenNF_Tc` \ lookup_result ->
599 case lookup_result of
600 SimpleInst rhs -> use_instance [] rhs
601 other -> add_to_irreds
604 -- The three main actions
606 avails' = addFree avails wanted
607 -- Add the thing to the avails set so any identical Insts
608 -- will be commoned up with it right here
610 returnTc (avails', wanted:frees, irreds)
612 add_to_irreds = addGiven avails wanted `thenNF_Tc` \ avails' ->
613 returnTc (avails', frees, wanted:irreds)
615 use_instance wanteds' rhs = addWanted avails wanted rhs `thenNF_Tc` \ avails' ->
616 reduceList stack try_me wanteds' (avails', frees, irreds)
619 -- The try-me to use when trying to identify tautologies
620 -- It blunders on reducing as much as possible
621 try_me_taut inst = ReduceMe Stop -- No error recovery
626 activate :: Avails s -> Inst -> Avails s
627 -- Activate the binding for Inst, ensuring that a binding for the
628 -- wanted Inst will be generated.
629 -- (Activate its parent if necessary, recursively).
630 -- Precondition: the Inst is in Avails already
632 activate avails wanted
633 | not (instBindingRequired wanted)
637 = case lookupFM avails wanted of
639 Just (Avail main_id (PassiveScSel rhs insts) ids) ->
640 foldl activate avails' insts -- Activate anything it needs
642 avails' = addToFM avails wanted avail'
643 avail' = Avail main_id (Rhs rhs True) (wanted_id : ids) -- Activate it
645 Just (Avail main_id other_rhs ids) -> -- Just add to the synonyms list
646 addToFM avails wanted (Avail main_id other_rhs (wanted_id : ids))
648 Nothing -> panic "activate"
650 wanted_id = instToId wanted
652 addWanted avails wanted rhs_expr
653 = ASSERT( not (wanted `elemFM` avails) )
654 returnNF_Tc (addToFM avails wanted avail)
655 -- NB: we don't add the thing's superclasses too!
656 -- Why not? Because addWanted is used when we've successfully used an
657 -- instance decl to reduce something; e.g.
658 -- d:Ord [a] = dfunOrd (d1:Eq [a]) (d2:Ord a)
659 -- Note that we pass the superclasses to the dfun, so they will be "wanted".
660 -- If we put the superclasses of "d" in avails, then we might end up
661 -- expressing "d1" in terms of "d", which would be a disaster.
663 avail = Avail (instToId wanted) rhs []
665 rhs | instBindingRequired wanted = Rhs rhs_expr False -- Not superclass selection
668 addFree :: Avails s -> Inst -> (Avails s)
669 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
670 -- to avails, so that any other equal Insts will be commoned up right
671 -- here rather than also being tossed upstairs. This is really just
672 -- an optimisation, and perhaps it is more trouble that it is worth,
673 -- as the following comments show!
675 -- NB1: do *not* add superclasses. If we have
678 -- but a is not bound here, then we *don't* want to derive
679 -- dn from df here lest we lose sharing.
681 -- NB2: do *not* add the Inst to avails at all if it's a method.
682 -- The following situation shows why this is bad:
683 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
684 -- From an application (truncate f i) we get
685 -- t1 = truncate at f
687 -- If we have also have a secon occurrence of truncate, we get
688 -- t3 = truncate at f
690 -- When simplifying with i,f free, we might still notice that
691 -- t1=t3; but alas, the binding for t2 (which mentions t1)
692 -- will continue to float out!
693 -- Solution: never put methods in avail till they are captured
694 -- in which case addFree isn't used
696 | isDict free = addToFM avails free (Avail (instToId free) NoRhs [])
699 addGiven :: Avails s -> Inst -> NF_TcM s (Avails s)
700 addGiven avails given
701 = -- ASSERT( not (given `elemFM` avails) )
702 -- This assertion isn't necessarily true. It's permitted
703 -- to given a redundant context in a type signature (eg (Ord a, Eq a) => ...)
704 -- and when typechecking instance decls we generate redundant "givens" too.
705 addAvail avails given avail
707 avail = Avail (instToId given) NoRhs []
709 addAvail avails wanted avail
710 = addSuperClasses (addToFM avails wanted avail) wanted
712 addSuperClasses :: Avails s -> Inst -> NF_TcM s (Avails s)
713 -- Add all the superclasses of the Inst to Avails
714 -- Invariant: the Inst is already in Avails.
716 addSuperClasses avails dict
720 | otherwise -- It is a dictionary
721 = foldlNF_Tc add_sc avails (zipEqual "addSuperClasses" sc_theta' sc_sels)
723 (clas, tys) = getDictClassTys dict
725 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
726 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
728 add_sc avails ((super_clas, super_tys), sc_sel)
729 = newDictFromOld dict super_clas super_tys `thenNF_Tc` \ super_dict ->
731 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys)
734 case lookupFM avails super_dict of
736 Just (Avail main_id (Rhs rhs False {- not sc selection -}) ids) ->
737 -- Already there, but not as a superclass selector
738 -- No need to look at its superclasses; since it's there
739 -- already they must be already in avails
740 -- However, we must remember to activate the dictionary
741 -- from which it is (now) generated
742 returnNF_Tc (activate avails' dict)
744 avails' = addToFM avails super_dict avail
745 avail = Avail main_id (Rhs sc_sel_rhs True) ids -- Superclass selection
747 Just (Avail _ _ _) -> returnNF_Tc avails
748 -- Already there; no need to do anything
751 -- Not there at all, so add it, and its superclasses
752 addAvail avails super_dict avail
754 avail = Avail (instToId super_dict)
755 (PassiveScSel sc_sel_rhs [dict])
759 %************************************************************************
761 \subsection[simple]{@Simple@ versions}
763 %************************************************************************
765 Much simpler versions when there are no bindings to make!
767 @tcSimplifyThetas@ simplifies class-type constraints formed by
768 @deriving@ declarations and when specialising instances. We are
769 only interested in the simplified bunch of class/type constraints.
771 It simplifies to constraints of the form (C a b c) where
772 a,b,c are type variables. This is required for the context of
773 instance declarations.
776 tcSimplifyThetas :: (Class -> InstEnv) -- How to find the InstEnv
777 -> ThetaType -- Wanted
778 -> TcM s ThetaType -- Needed
780 tcSimplifyThetas inst_mapper wanteds
781 = reduceSimple inst_mapper [] wanteds `thenNF_Tc` \ irreds ->
783 -- For multi-param Haskell, check that the returned dictionaries
784 -- don't have any of the form (C Int Bool) for which
785 -- we expect an instance here
786 -- For Haskell 98, check that all the constraints are of the form C a,
787 -- where a is a type variable
788 bad_guys | opt_GlasgowExts = [ct | ct@(clas,tys) <- irreds,
789 isEmptyVarSet (tyVarsOfTypes tys)]
790 | otherwise = [ct | ct@(clas,tys) <- irreds,
791 not (all isTyVarTy tys)]
793 if null bad_guys then
796 mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
800 @tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
801 used with \tr{default} declarations. We are only interested in
802 whether it worked or not.
805 tcSimplifyCheckThetas :: ThetaType -- Given
806 -> ThetaType -- Wanted
809 tcSimplifyCheckThetas givens wanteds
810 = reduceSimple classInstEnv givens wanteds `thenNF_Tc` \ irreds ->
814 mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
820 type AvailsSimple = FiniteMap (Class, [TauType]) Bool
821 -- True => irreducible
822 -- False => given, or can be derived from a given or from an irreducible
824 reduceSimple :: (Class -> InstEnv)
825 -> ThetaType -- Given
826 -> ThetaType -- Wanted
827 -> NF_TcM s ThetaType -- Irreducible
829 reduceSimple inst_mapper givens wanteds
830 = reduce_simple (0,[]) inst_mapper givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
831 returnNF_Tc [ct | (ct,True) <- fmToList givens_fm']
833 givens_fm = foldl addNonIrred emptyFM givens
835 reduce_simple :: (Int,ThetaType) -- Stack
836 -> (Class -> InstEnv)
839 -> NF_TcM s AvailsSimple
841 reduce_simple (n,stack) inst_mapper avails wanteds
844 go avails [] = returnNF_Tc avails
845 go avails (w:ws) = reduce_simple_help (n+1,w:stack) inst_mapper avails w `thenNF_Tc` \ avails' ->
848 reduce_simple_help stack inst_mapper givens wanted@(clas,tys)
849 | wanted `elemFM` givens
853 = lookupSimpleInst (inst_mapper clas) clas tys `thenNF_Tc` \ maybe_theta ->
856 Nothing -> returnNF_Tc (addIrred givens wanted)
857 Just theta -> reduce_simple stack inst_mapper (addNonIrred givens wanted) theta
859 addIrred :: AvailsSimple -> (Class, [TauType]) -> AvailsSimple
861 = addSCs (addToFM givens ct True) ct
863 addNonIrred :: AvailsSimple -> (Class, [TauType]) -> AvailsSimple
864 addNonIrred givens ct
865 = addSCs (addToFM givens ct False) ct
867 addSCs givens ct@(clas,tys)
868 = foldl add givens sc_theta
870 (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
871 sc_theta = substTheta (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
873 add givens ct = case lookupFM givens ct of
874 Nothing -> -- Add it and its superclasses
875 addSCs (addToFM givens ct False) ct
877 Just True -> -- Set its flag to False; superclasses already done
878 addToFM givens ct False
880 Just False -> -- Already done
885 %************************************************************************
887 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
889 %************************************************************************
891 When doing a binding group, we may have @Insts@ of local functions.
892 For example, we might have...
894 let f x = x + 1 -- orig local function (overloaded)
895 f.1 = f Int -- two instances of f
900 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
901 where @f@ is in scope; those @Insts@ must certainly not be passed
902 upwards towards the top-level. If the @Insts@ were binding-ified up
903 there, they would have unresolvable references to @f@.
905 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
906 For each method @Inst@ in the @init_lie@ that mentions one of the
907 @Ids@, we create a binding. We return the remaining @Insts@ (in an
908 @LIE@), as well as the @HsBinds@ generated.
911 bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM s (LIE, TcMonoBinds)
913 bindInstsOfLocalFuns init_lie local_ids
914 | null overloaded_ids || null lie_for_here
916 = returnTc (init_lie, EmptyMonoBinds)
919 = reduceContext (text "bindInsts" <+> ppr local_ids)
920 try_me [] lie_for_here `thenTc` \ (binds, frees, irreds) ->
921 ASSERT( null irreds )
922 returnTc (mkLIE frees `plusLIE` mkLIE lie_not_for_here, binds)
924 overloaded_ids = filter is_overloaded local_ids
925 is_overloaded id = case splitSigmaTy (idType id) of
926 (_, theta, _) -> not (null theta)
928 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
929 -- so it's worth building a set, so that
930 -- lookup (in isMethodFor) is faster
932 -- No sense in repeatedly zonking lots of
933 -- constant constraints so filter them out here
934 (lie_for_here, lie_not_for_here) = partition (isMethodFor overloaded_set)
936 try_me inst | isMethodFor overloaded_set inst = ReduceMe AddToIrreds
941 %************************************************************************
943 \section[Disambig]{Disambiguation of overloading}
945 %************************************************************************
948 If a dictionary constrains a type variable which is
951 not mentioned in the environment
953 and not mentioned in the type of the expression
955 then it is ambiguous. No further information will arise to instantiate
956 the type variable; nor will it be generalised and turned into an extra
957 parameter to a function.
959 It is an error for this to occur, except that Haskell provided for
960 certain rules to be applied in the special case of numeric types.
965 at least one of its classes is a numeric class, and
967 all of its classes are numeric or standard
969 then the type variable can be defaulted to the first type in the
970 default-type list which is an instance of all the offending classes.
972 So here is the function which does the work. It takes the ambiguous
973 dictionaries and either resolves them (producing bindings) or
974 complains. It works by splitting the dictionary list by type
975 variable, and using @disambigOne@ to do the real business.
978 @tcSimplifyTop@ is called once per module to simplify
979 all the constant and ambiguous Insts.
982 tcSimplifyTop :: LIE -> TcM s TcDictBinds
983 tcSimplifyTop wanted_lie
984 = reduceContext (text "tcSimplTop") try_me [] wanteds `thenTc` \ (binds1, frees, irreds) ->
988 -- All the non-std ones are definite errors
989 (stds, non_stds) = partition isStdClassTyVarDict irreds
992 -- Group by type variable
993 std_groups = equivClasses cmp_by_tyvar stds
995 -- Pick the ones which its worth trying to disambiguate
996 (std_oks, std_bads) = partition worth_a_try std_groups
997 -- Have a try at disambiguation
998 -- if the type variable isn't bound
999 -- up with one of the non-standard classes
1000 worth_a_try group@(d:_) = isEmptyVarSet (tyVarsOfInst d `intersectVarSet` non_std_tyvars)
1001 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1003 -- Collect together all the bad guys
1004 bad_guys = non_stds ++ concat std_bads
1007 -- Disambiguate the ones that look feasible
1008 mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
1010 -- And complain about the ones that don't
1011 mapNF_Tc complain bad_guys `thenNF_Tc_`
1013 returnTc (binds1 `andMonoBinds` andMonoBindList binds_ambig)
1015 -- see comment on wanteds in tcSimplify
1016 wanteds = filter notFunDep (bagToList wanted_lie)
1017 try_me inst = ReduceMe AddToIrreds
1019 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1021 complain d | isEmptyVarSet (tyVarsOfInst d) = addTopInstanceErr d
1022 | otherwise = addAmbigErr tyVarsOfInst d
1024 get_tv d = case getDictClassTys d of
1025 (clas, [ty]) -> getTyVar "tcSimplifyTop" ty
1026 get_clas d = case getDictClassTys d of
1027 (clas, [ty]) -> clas
1030 @disambigOne@ assumes that its arguments dictionaries constrain all
1031 the same type variable.
1033 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1034 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1035 the most common use of defaulting is code like:
1037 _ccall_ foo `seqPrimIO` bar
1039 Since we're not using the result of @foo@, the result if (presumably)
1043 disambigGroup :: [Inst] -- All standard classes of form (C a)
1044 -> TcM s TcDictBinds
1047 | any isNumericClass classes -- Guaranteed all standard classes
1048 -- see comment at the end of function for reasons as to
1049 -- why the defaulting mechanism doesn't apply to groups that
1050 -- include CCallable or CReturnable dicts.
1051 && not (any isCcallishClass classes)
1052 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1053 -- SO, TRY DEFAULT TYPES IN ORDER
1055 -- Failure here is caused by there being no type in the
1056 -- default list which can satisfy all the ambiguous classes.
1057 -- For example, if Real a is reqd, but the only type in the
1058 -- default list is Int.
1059 tcGetDefaultTys `thenNF_Tc` \ default_tys ->
1061 try_default [] -- No defaults work, so fail
1064 try_default (default_ty : default_tys)
1065 = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
1066 -- default_tys instead
1067 tcSimplifyCheckThetas [] thetas `thenTc` \ _ ->
1070 thetas = classes `zip` repeat [default_ty]
1072 -- See if any default works, and if so bind the type variable to it
1073 -- If not, add an AmbigErr
1074 recoverTc (complain dicts `thenNF_Tc_` returnTc EmptyMonoBinds) $
1076 try_default default_tys `thenTc` \ chosen_default_ty ->
1078 -- Bind the type variable and reduce the context, for real this time
1080 chosen_default_tc_ty = typeToTcType chosen_default_ty -- Tiresome!
1082 unifyTauTy chosen_default_tc_ty (mkTyVarTy tyvar) `thenTc_`
1083 reduceContext (text "disambig" <+> ppr dicts)
1084 try_me [] dicts `thenTc` \ (binds, frees, ambigs) ->
1085 ASSERT( null frees && null ambigs )
1086 warnDefault dicts chosen_default_ty `thenTc_`
1089 | all isCreturnableClass classes
1090 = -- Default CCall stuff to (); we don't even both to check that () is an
1091 -- instance of CReturnable, because we know it is.
1092 unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
1093 returnTc EmptyMonoBinds
1095 | otherwise -- No defaults
1096 = complain dicts `thenNF_Tc_`
1097 returnTc EmptyMonoBinds
1100 complain = addAmbigErrs tyVarsOfInst
1101 try_me inst = ReduceMe AddToIrreds -- This reduce should not fail
1102 tyvar = get_tv (head dicts) -- Should be non-empty
1103 classes = map get_clas dicts
1106 [Aside - why the defaulting mechanism is turned off when
1107 dealing with arguments and results to ccalls.
1109 When typechecking _ccall_s, TcExpr ensures that the external
1110 function is only passed arguments (and in the other direction,
1111 results) of a restricted set of 'native' types. This is
1112 implemented via the help of the pseudo-type classes,
1113 @CReturnable@ (CR) and @CCallable@ (CC.)
1115 The interaction between the defaulting mechanism for numeric
1116 values and CC & CR can be a bit puzzling to the user at times.
1125 What type has 'x' got here? That depends on the default list
1126 in operation, if it is equal to Haskell 98's default-default
1127 of (Integer, Double), 'x' has type Double, since Integer
1128 is not an instance of CR. If the default list is equal to
1129 Haskell 1.4's default-default of (Int, Double), 'x' has type
1132 To try to minimise the potential for surprises here, the
1133 defaulting mechanism is turned off in the presence of
1134 CCallable and CReturnable.
1140 ToDo: for these error messages, should we note the location as coming
1141 from the insts, or just whatever seems to be around in the monad just
1145 genCantGenErr insts -- Can't generalise these Insts
1146 = sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
1147 nest 4 (pprInstsInFull insts)
1150 addAmbigErrs ambig_tv_fn dicts = mapNF_Tc (addAmbigErr ambig_tv_fn) dicts
1152 addAmbigErr ambig_tv_fn dict
1153 = addInstErrTcM (instLoc dict)
1155 sep [text "Ambiguous type variable(s)" <+>
1156 hsep (punctuate comma (map (quotes . ppr) ambig_tvs)),
1157 nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
1159 ambig_tvs = varSetElems (ambig_tv_fn tidy_dict)
1160 (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
1162 warnDefault dicts default_ty
1163 | not opt_WarnTypeDefaults
1169 msg | length dicts > 1
1170 = (ptext SLIT("Defaulting the following constraint(s) to type") <+> quotes (ppr default_ty))
1171 $$ pprInstsInFull tidy_dicts
1173 = ptext SLIT("Defaulting") <+> quotes (pprInst (head tidy_dicts)) <+>
1174 ptext SLIT("to type") <+> quotes (ppr default_ty)
1176 (_, tidy_dicts) = mapAccumL tidyInst emptyTidyEnv dicts
1179 = addInstErrTcM (instLoc dict)
1181 vcat [ptext SLIT("Could not deduce") <+> quotes (pprInst tidy_dict),
1182 nest 4 (ptext SLIT("LHS of a rule must have no overloading"))])
1184 (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
1186 -- Used for top-level irreducibles
1187 addTopInstanceErr dict
1188 = addInstErrTcM (instLoc dict)
1190 ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
1192 (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
1194 addNoInstanceErr str givens dict
1195 = addInstErrTcM (instLoc dict)
1197 sep [ptext SLIT("Could not deduce") <+> quotes (pprInst tidy_dict),
1198 nest 4 $ ptext SLIT("from the context:") <+> pprInsts tidy_givens]
1200 ptext SLIT("Probable cause:") <+>
1201 vcat [sep [ptext SLIT("missing") <+> quotes (pprInst tidy_dict),
1202 ptext SLIT("in") <+> str],
1203 if all_tyvars then empty else
1204 ptext SLIT("or missing instance declaration for") <+> quotes (pprInst tidy_dict)]
1207 all_tyvars = all isTyVarTy tys
1208 (_, tys) = getDictClassTys dict
1209 (tidy_env, tidy_dict:tidy_givens) = tidyInsts emptyTidyEnv (dict:givens)
1211 -- Used for the ...Thetas variants; all top level
1213 = addErrTc (ptext SLIT("No instance for") <+> quotes (pprConstraint c ts))
1215 reduceDepthErr n stack
1216 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
1217 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
1218 nest 4 (pprInstsInFull stack)]
1220 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)