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(..),
134 tyVarsOfInst, tyVarsOfInsts,
135 isDict, isClassDict, isStdClassTyVarDict,
136 isMethodFor, notFunDep,
137 instToId, instBindingRequired, instCanBeGeneralised,
139 getDictClassTys, getIPs,
140 instLoc, pprInst, zonkInst, tidyInst, tidyInsts,
141 Inst, LIE, pprInsts, pprInstsInFull,
142 mkLIE, emptyLIE, plusLIE, lieToList
144 import TcEnv ( tcGetGlobalTyVars )
145 import TcType ( TcType, TcTyVarSet, typeToTcType )
146 import TcUnify ( unifyTauTy )
148 import Class ( Class, classBigSig, classInstEnv )
149 import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
151 import Type ( Type, ThetaType, TauType, ClassContext,
153 isTyVarTy, splitSigmaTy, tyVarsOfTypes
155 import InstEnv ( InstEnv )
156 import Subst ( mkTopTyVarSubst, substClasses )
157 import PprType ( pprConstraint )
158 import TysWiredIn ( unitTy )
161 import BasicTypes ( TopLevelFlag(..) )
162 import CmdLineOpts ( opt_GlasgowExts )
165 import List ( partition )
169 %************************************************************************
171 \subsection[tcSimplify-main]{Main entry function}
173 %************************************************************************
175 The main wrapper is @tcSimplify@. It just calls @tcSimpl@, but with
176 the ``don't-squash-consts'' flag set depending on top-level ness. For
177 top level defns we *do* squash constants, so that they stay local to a
178 single defn. This makes things which are inlined more likely to be
179 exportable, because their constants are "inside". Later passes will
180 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 local_tvs wanted_lie
194 | isEmptyVarSet local_tvs
195 = returnTc (wanted_lie, EmptyMonoBinds, emptyLIE)
199 = reduceContext str try_me [] wanteds `thenTc` \ (binds, frees, irreds) ->
201 -- Check for non-generalisable insts
203 cant_generalise = filter (not . instCanBeGeneralised) irreds
205 checkTc (null cant_generalise)
206 (genCantGenErr cant_generalise) `thenTc_`
208 -- Check for ambiguous insts.
209 -- You might think these can't happen (I did) because an ambiguous
210 -- inst like (Eq a) will get tossed out with "frees", and eventually
211 -- dealt with by tcSimplifyTop.
212 -- But we can get stuck with
214 -- where "a" is one of the local_tvs, but "b" is unconstrained.
215 -- Then we must yell about the ambiguous b
216 -- But we must only do so if "b" really is unconstrained; so
217 -- we must grab the global tyvars to answer that question
218 tcGetGlobalTyVars `thenNF_Tc` \ global_tvs ->
220 avail_tvs = local_tvs `unionVarSet` global_tvs
221 (irreds', bad_guys) = partition (isEmptyVarSet . ambig_tv_fn) irreds
222 ambig_tv_fn dict = tyVarsOfInst dict `minusVarSet` avail_tvs
224 addAmbigErrs ambig_tv_fn bad_guys `thenNF_Tc_`
228 returnTc (mkLIE frees, binds, mkLIE irreds')
230 -- the idea behind filtering out the dependencies here is that
231 -- they've already served their purpose, and can be reconstructed
232 -- at a later point from the retained class predicates.
233 -- however, there *is* the possibility that a dependency
234 -- out-lives the predicate from which it arose.
235 -- I don't have any examples of this, but if they show up,
236 -- we'd want to consider the possibility of saving the
237 -- dependencies as hidden constraints (i.e. they'd only
238 -- show up in interface files) -- or maybe they'd be useful
239 -- as first class predicates...
240 wanteds = filter notFunDep (lieToList wanted_lie)
243 -- Does not constrain a local tyvar
244 | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs)
245 && null (getIPs inst)
246 = -- if is_top_level then
247 -- FreeIfTautological -- Special case for inference on
248 -- -- top-level defns
252 -- We're infering (not checking) the type, and
253 -- the inst constrains a local type variable
254 | isDict inst = DontReduce -- Dicts
255 | otherwise = ReduceMe AddToIrreds -- Lits and Methods
258 @tcSimplifyAndCheck@ is similar to the above, except that it checks
259 that there is an empty wanted-set at the end. It may still return
260 some of constant insts, which have to be resolved finally at the end.
265 -> TcTyVarSet -- ``Local'' type variables
266 -- ASSERT: this tyvar set is already zonked
267 -> LIE -- Given; constrain only local tyvars
269 -> TcM s (LIE, -- Free
270 TcDictBinds) -- Bindings
272 tcSimplifyAndCheck str local_tvs given_lie wanted_lie
273 | isEmptyVarSet local_tvs
274 -- This can happen quite legitimately; for example in
275 -- instance Num Int where ...
276 = returnTc (wanted_lie, EmptyMonoBinds)
279 = reduceContext str try_me givens wanteds `thenTc` \ (binds, frees, irreds) ->
281 -- Complain about any irreducible ones
282 mapNF_Tc complain irreds `thenNF_Tc_`
285 returnTc (mkLIE frees, binds)
287 givens = lieToList given_lie
288 -- see comment on wanteds in tcSimplify
289 wanteds = filter notFunDep (lieToList wanted_lie)
290 given_dicts = filter isClassDict givens
293 -- Does not constrain a local tyvar
294 | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs)
297 -- When checking against a given signature we always reduce
298 -- until we find a match against something given, or can't reduce
300 = ReduceMe AddToIrreds
302 complain dict = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens ->
303 addNoInstanceErr str given_dicts dict
306 On the LHS of transformation rules we only simplify methods and constants,
307 getting dictionaries. We want to keep all of them unsimplified, to serve
308 as the available stuff for the RHS of the rule.
310 The same thing is used for specialise pragmas. Consider
313 {-# SPECIALISE f :: Int -> Int #-}
316 The type checker generates a binding like:
318 f_spec = (f :: Int -> Int)
320 and we want to end up with
322 f_spec = _inline_me_ (f Int dNumInt)
324 But that means that we must simplify the Method for f to (f Int dNumInt)!
325 So tcSimplifyToDicts squeezes out all Methods.
328 tcSimplifyToDicts :: LIE -> TcM s (LIE, TcDictBinds)
329 tcSimplifyToDicts wanted_lie
330 = reduceContext (text "tcSimplifyToDicts") try_me [] wanteds `thenTc` \ (binds, frees, irreds) ->
332 returnTc (mkLIE irreds, binds)
334 -- see comment on wanteds in tcSimplify
335 wanteds = filter notFunDep (lieToList wanted_lie)
337 -- Reduce methods and lits only; stop as soon as we get a dictionary
338 try_me inst | isDict inst = DontReduce
339 | otherwise = ReduceMe AddToIrreds
344 %************************************************************************
346 \subsection{Data types for the reduction mechanism}
348 %************************************************************************
350 The main control over context reduction is here
354 = ReduceMe -- Try to reduce this
355 NoInstanceAction -- What to do if there's no such instance
357 | DontReduce -- Return as irreducible
359 | Free -- Return as free
361 | FreeIfTautological -- Return as free iff it's tautological;
362 -- if not, return as irreducible
363 -- The FreeIfTautological case is to allow the possibility
364 -- of generating functions with types like
365 -- f :: C Int => Int -> Int
366 -- Here, the C Int isn't a tautology presumably because Int
367 -- isn't an instance of C in this module; but perhaps it will
368 -- be at f's call site(s). Haskell doesn't allow this at
371 data NoInstanceAction
372 = Stop -- Fail; no error message
373 -- (Only used when tautology checking.)
375 | AddToIrreds -- Just add the inst to the irreductible ones; don't
376 -- produce an error message of any kind.
377 -- It might be quite legitimate such as (Eq a)!
384 = (Avails s, -- What's available
385 [Inst], -- Insts for which try_me returned Free
386 [Inst] -- Insts for which try_me returned DontReduce
389 type Avails s = FiniteMap Inst Avail
393 TcId -- The "main Id"; that is, the Id for the Inst that
394 -- caused this avail to be put into the finite map in the first place
395 -- It is this Id that is bound to the RHS.
397 RHS -- The RHS: an expression whose value is that Inst.
398 -- The main Id should be bound to this RHS
400 [TcId] -- Extra Ids that must all be bound to the main Id.
401 -- At the end we generate a list of bindings
402 -- { i1 = main_id; i2 = main_id; i3 = main_id; ... }
405 = NoRhs -- Used for irreducible dictionaries,
406 -- which are going to be lambda bound, or for those that are
407 -- suppplied as "given" when checking againgst a signature.
409 -- NoRhs is also used for Insts like (CCallable f)
410 -- where no witness is required.
412 | Rhs -- Used when there is a RHS
414 Bool -- True => the RHS simply selects a superclass dictionary
415 -- from a subclass dictionary.
417 -- This is useful info, because superclass selection
418 -- is cheaper than building the dictionary using its dfun,
419 -- and we can sometimes replace the latter with the former
421 | PassiveScSel -- Used for as-yet-unactivated RHSs. For example suppose we have
422 -- an (Ord t) dictionary; then we put an (Eq t) entry in
423 -- the finite map, with an PassiveScSel. Then if the
424 -- the (Eq t) binding is ever *needed* we make it an Rhs
426 [Inst] -- List of Insts that are free in the RHS.
427 -- If the main Id is subsequently needed, we toss this list into
428 -- the needed-inst pool so that we make sure their bindings
429 -- will actually be produced.
431 -- Invariant: these Insts are already in the finite mapping
434 pprAvails avails = vcat (map pprAvail (eltsFM avails))
436 pprAvail (Avail main_id rhs ids)
437 = ppr main_id <> colon <+> brackets (ppr ids) <+> pprRhs rhs
439 pprRhs NoRhs = text "<no rhs>"
440 pprRhs (Rhs rhs b) = ppr rhs
441 pprRhs (PassiveScSel rhs is) = text "passive" <+> ppr rhs
445 %************************************************************************
447 \subsection[reduce]{@reduce@}
449 %************************************************************************
451 The main entry point for context reduction is @reduceContext@:
454 reduceContext :: SDoc -> (Inst -> WhatToDo)
457 -> TcM s (TcDictBinds,
459 [Inst]) -- Irreducible
461 reduceContext str try_me givens wanteds
463 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens ->
464 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds ->
467 pprTrace "reduceContext" (vcat [
468 text "----------------------",
470 text "given" <+> ppr givens,
471 text "wanted" <+> ppr wanteds,
472 text "----------------------"
475 -- Build the Avail mapping from "givens"
476 foldlNF_Tc addGiven emptyFM givens `thenNF_Tc` \ avails ->
479 reduceList (0,[]) try_me wanteds (avails, [], []) `thenTc` \ (avails, frees, irreds) ->
481 -- Extract the bindings from avails
483 binds = foldFM add_bind EmptyMonoBinds avails
485 add_bind _ (Avail main_id rhs ids) binds
486 = foldr add_synonym (add_rhs_bind rhs binds) ids
488 add_rhs_bind (Rhs rhs _) binds = binds `AndMonoBinds` VarMonoBind main_id rhs
489 add_rhs_bind other binds = binds
491 -- Add the trivial {x = y} bindings
492 -- The main Id can end up in the list when it's first added passively
493 -- and then activated, so we have to filter it out. A bit of a hack.
495 | id /= main_id = binds `AndMonoBinds` VarMonoBind id (HsVar main_id)
499 pprTrace ("reduceContext end") (vcat [
500 text "----------------------",
502 text "given" <+> ppr givens,
503 text "wanted" <+> ppr wanteds,
505 text "avails" <+> pprAvails avails,
506 text "irreds" <+> ppr irreds,
507 text "----------------------"
510 returnTc (binds, frees, irreds)
513 The main context-reduction function is @reduce@. Here's its game plan.
516 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
517 -- along with its depth
518 -> (Inst -> WhatToDo)
521 -> TcM s (RedState s)
525 try_me: given an inst, this function returns
527 DontReduce return this in "irreds"
528 Free return this in "frees"
530 wanteds: The list of insts to reduce
531 state: An accumulating parameter of type RedState
532 that contains the state of the algorithm
534 It returns a RedState.
536 The (n,stack) pair is just used for error reporting.
537 n is always the depth of the stack.
538 The stack is the stack of Insts being reduced: to produce X
539 I had to produce Y, to produce Y I had to produce Z, and so on.
542 reduceList (n,stack) try_me wanteds state
543 | n > opt_MaxContextReductionDepth
544 = failWithTc (reduceDepthErr n stack)
550 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
555 go [] state = returnTc state
556 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
559 -- Base case: we're done!
560 reduce stack try_me wanted state@(avails, frees, irreds)
561 -- It's the same as an existing inst, or a superclass thereof
562 | wanted `elemFM` avails
563 = returnTc (activate avails wanted, frees, irreds)
566 = case try_me wanted of {
568 ReduceMe no_instance_action -> -- It should be reduced
569 lookupInst wanted `thenNF_Tc` \ lookup_result ->
570 case lookup_result of
571 GenInst wanteds' rhs -> use_instance wanteds' rhs
572 SimpleInst rhs -> use_instance [] rhs
574 NoInstance -> -- No such instance!
575 case no_instance_action of
577 AddToIrreds -> add_to_irreds
579 Free -> -- It's free and this isn't a top-level binding, so just chuck it upstairs
580 -- First, see if the inst can be reduced to a constant in one step
581 lookupInst wanted `thenNF_Tc` \ lookup_result ->
582 case lookup_result of
583 SimpleInst rhs -> use_instance [] rhs
584 other -> add_to_frees
589 FreeIfTautological -> -- It's free and this is a top level binding, so
590 -- check whether it's a tautology or not
592 add_to_irreds -- If tautology trial fails, add to irreds
594 -- If tautology succeeds, just add to frees
595 (reduce stack try_me_taut wanted (avails, [], []) `thenTc_`
596 returnTc (avails, wanted:frees, irreds))
600 DontReduce -> -- It's irreducible (or at least should not be reduced)
601 -- See if the inst can be reduced to a constant in one step
602 lookupInst wanted `thenNF_Tc` \ lookup_result ->
603 case lookup_result of
604 SimpleInst rhs -> use_instance [] rhs
605 other -> add_to_irreds
608 -- The three main actions
610 avails' = addFree avails wanted
611 -- Add the thing to the avails set so any identical Insts
612 -- will be commoned up with it right here
614 returnTc (avails', wanted:frees, irreds)
616 add_to_irreds = addGiven avails wanted `thenNF_Tc` \ avails' ->
617 returnTc (avails', frees, wanted:irreds)
619 use_instance wanteds' rhs = addWanted avails wanted rhs `thenNF_Tc` \ avails' ->
620 reduceList stack try_me wanteds' (avails', frees, irreds)
623 -- The try-me to use when trying to identify tautologies
624 -- It blunders on reducing as much as possible
625 try_me_taut inst = ReduceMe Stop -- No error recovery
630 activate :: Avails s -> Inst -> Avails s
631 -- Activate the binding for Inst, ensuring that a binding for the
632 -- wanted Inst will be generated.
633 -- (Activate its parent if necessary, recursively).
634 -- Precondition: the Inst is in Avails already
636 activate avails wanted
637 | not (instBindingRequired wanted)
641 = case lookupFM avails wanted of
643 Just (Avail main_id (PassiveScSel rhs insts) ids) ->
644 foldl activate avails' insts -- Activate anything it needs
646 avails' = addToFM avails wanted avail'
647 avail' = Avail main_id (Rhs rhs True) (wanted_id : ids) -- Activate it
649 Just (Avail main_id other_rhs ids) -> -- Just add to the synonyms list
650 addToFM avails wanted (Avail main_id other_rhs (wanted_id : ids))
652 Nothing -> panic "activate"
654 wanted_id = instToId wanted
656 addWanted avails wanted rhs_expr
657 = ASSERT( not (wanted `elemFM` avails) )
658 returnNF_Tc (addToFM avails wanted avail)
659 -- NB: we don't add the thing's superclasses too!
660 -- Why not? Because addWanted is used when we've successfully used an
661 -- instance decl to reduce something; e.g.
662 -- d:Ord [a] = dfunOrd (d1:Eq [a]) (d2:Ord a)
663 -- Note that we pass the superclasses to the dfun, so they will be "wanted".
664 -- If we put the superclasses of "d" in avails, then we might end up
665 -- expressing "d1" in terms of "d", which would be a disaster.
667 avail = Avail (instToId wanted) rhs []
669 rhs | instBindingRequired wanted = Rhs rhs_expr False -- Not superclass selection
672 addFree :: Avails s -> Inst -> (Avails s)
673 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
674 -- to avails, so that any other equal Insts will be commoned up right
675 -- here rather than also being tossed upstairs. This is really just
676 -- an optimisation, and perhaps it is more trouble that it is worth,
677 -- as the following comments show!
679 -- NB1: do *not* add superclasses. If we have
682 -- but a is not bound here, then we *don't* want to derive
683 -- dn from df here lest we lose sharing.
685 -- NB2: do *not* add the Inst to avails at all if it's a method.
686 -- The following situation shows why this is bad:
687 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
688 -- From an application (truncate f i) we get
689 -- t1 = truncate at f
691 -- If we have also have a secon occurrence of truncate, we get
692 -- t3 = truncate at f
694 -- When simplifying with i,f free, we might still notice that
695 -- t1=t3; but alas, the binding for t2 (which mentions t1)
696 -- will continue to float out!
697 -- Solution: never put methods in avail till they are captured
698 -- in which case addFree isn't used
700 | isDict free = addToFM avails free (Avail (instToId free) NoRhs [])
703 addGiven :: Avails s -> Inst -> NF_TcM s (Avails s)
704 addGiven avails given
705 = -- ASSERT( not (given `elemFM` avails) )
706 -- This assertion isn't necessarily true. It's permitted
707 -- to given a redundant context in a type signature (eg (Ord a, Eq a) => ...)
708 -- and when typechecking instance decls we generate redundant "givens" too.
709 -- addAvail avails given avail
710 addAvail avails given avail `thenNF_Tc` \av ->
711 zonkInst given `thenNF_Tc` \given' ->
714 avail = Avail (instToId given) NoRhs []
716 addAvail avails wanted avail
717 = addSuperClasses (addToFM avails wanted avail) wanted
719 addSuperClasses :: Avails s -> Inst -> NF_TcM s (Avails s)
720 -- Add all the superclasses of the Inst to Avails
721 -- Invariant: the Inst is already in Avails.
723 addSuperClasses avails dict
724 | not (isClassDict dict)
727 | otherwise -- It is a dictionary
728 = foldlNF_Tc add_sc avails (zipEqual "addSuperClasses" sc_theta' sc_sels)
730 (clas, tys) = getDictClassTys dict
732 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
733 sc_theta' = substClasses (mkTopTyVarSubst tyvars tys) sc_theta
735 add_sc avails ((super_clas, super_tys), sc_sel)
736 = newDictFromOld dict super_clas super_tys `thenNF_Tc` \ super_dict ->
738 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys)
741 case lookupFM avails super_dict of
743 Just (Avail main_id (Rhs rhs False {- not sc selection -}) ids) ->
744 -- Already there, but not as a superclass selector
745 -- No need to look at its superclasses; since it's there
746 -- already they must be already in avails
747 -- However, we must remember to activate the dictionary
748 -- from which it is (now) generated
749 returnNF_Tc (activate avails' dict)
751 avails' = addToFM avails super_dict avail
752 avail = Avail main_id (Rhs sc_sel_rhs True) ids -- Superclass selection
754 Just (Avail _ _ _) -> returnNF_Tc avails
755 -- Already there; no need to do anything
758 -- Not there at all, so add it, and its superclasses
759 addAvail avails super_dict avail
761 avail = Avail (instToId super_dict)
762 (PassiveScSel sc_sel_rhs [dict])
766 %************************************************************************
768 \subsection[simple]{@Simple@ versions}
770 %************************************************************************
772 Much simpler versions when there are no bindings to make!
774 @tcSimplifyThetas@ simplifies class-type constraints formed by
775 @deriving@ declarations and when specialising instances. We are
776 only interested in the simplified bunch of class/type constraints.
778 It simplifies to constraints of the form (C a b c) where
779 a,b,c are type variables. This is required for the context of
780 instance declarations.
783 tcSimplifyThetas :: (Class -> InstEnv) -- How to find the InstEnv
784 -> ClassContext -- Wanted
785 -> TcM s ClassContext -- Needed
787 tcSimplifyThetas inst_mapper wanteds
788 = reduceSimple inst_mapper [] wanteds `thenNF_Tc` \ irreds ->
790 -- For multi-param Haskell, check that the returned dictionaries
791 -- don't have any of the form (C Int Bool) for which
792 -- we expect an instance here
793 -- For Haskell 98, check that all the constraints are of the form C a,
794 -- where a is a type variable
795 bad_guys | opt_GlasgowExts = [ct | ct@(clas,tys) <- irreds,
796 isEmptyVarSet (tyVarsOfTypes tys)]
797 | otherwise = [ct | ct@(clas,tys) <- irreds,
798 not (all isTyVarTy tys)]
800 if null bad_guys then
803 mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
807 @tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
808 used with \tr{default} declarations. We are only interested in
809 whether it worked or not.
812 tcSimplifyCheckThetas :: ClassContext -- Given
813 -> ClassContext -- Wanted
816 tcSimplifyCheckThetas givens wanteds
817 = reduceSimple classInstEnv givens wanteds `thenNF_Tc` \ irreds ->
821 mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
827 type AvailsSimple = FiniteMap (Class,[Type]) Bool
828 -- True => irreducible
829 -- False => given, or can be derived from a given or from an irreducible
831 reduceSimple :: (Class -> InstEnv)
832 -> ClassContext -- Given
833 -> ClassContext -- Wanted
834 -> NF_TcM s ClassContext -- Irreducible
836 reduceSimple inst_mapper givens wanteds
837 = reduce_simple (0,[]) inst_mapper givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
838 returnNF_Tc [ct | (ct,True) <- fmToList givens_fm']
840 givens_fm = foldl addNonIrred emptyFM givens
842 reduce_simple :: (Int,ClassContext) -- Stack
843 -> (Class -> InstEnv)
846 -> NF_TcM s AvailsSimple
848 reduce_simple (n,stack) inst_mapper avails wanteds
851 go avails [] = returnNF_Tc avails
852 go avails (w:ws) = reduce_simple_help (n+1,w:stack) inst_mapper avails w `thenNF_Tc` \ avails' ->
855 reduce_simple_help stack inst_mapper givens wanted@(clas,tys)
856 | wanted `elemFM` givens
860 = lookupSimpleInst (inst_mapper clas) clas tys `thenNF_Tc` \ maybe_theta ->
863 Nothing -> returnNF_Tc (addIrred givens wanted)
864 Just theta -> reduce_simple stack inst_mapper (addNonIrred givens wanted) theta
866 addIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
867 addIrred givens ct@(clas,tys)
868 = addSCs (addToFM givens ct True) ct
870 addNonIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
871 addNonIrred givens ct@(clas,tys)
872 = addSCs (addToFM givens ct False) ct
874 addSCs givens ct@(clas,tys)
875 = foldl add givens sc_theta
877 (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
878 sc_theta = substClasses (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
880 add givens ct@(clas, tys)
881 = case lookupFM givens ct of
882 Nothing -> -- Add it and its superclasses
883 addSCs (addToFM givens ct False) ct
885 Just True -> -- Set its flag to False; superclasses already done
886 addToFM givens ct False
888 Just False -> -- Already done
893 %************************************************************************
895 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
897 %************************************************************************
899 When doing a binding group, we may have @Insts@ of local functions.
900 For example, we might have...
902 let f x = x + 1 -- orig local function (overloaded)
903 f.1 = f Int -- two instances of f
908 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
909 where @f@ is in scope; those @Insts@ must certainly not be passed
910 upwards towards the top-level. If the @Insts@ were binding-ified up
911 there, they would have unresolvable references to @f@.
913 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
914 For each method @Inst@ in the @init_lie@ that mentions one of the
915 @Ids@, we create a binding. We return the remaining @Insts@ (in an
916 @LIE@), as well as the @HsBinds@ generated.
919 bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM s (LIE, TcMonoBinds)
921 bindInstsOfLocalFuns init_lie local_ids
922 | null overloaded_ids || null lie_for_here
924 = returnTc (init_lie, EmptyMonoBinds)
927 = reduceContext (text "bindInsts" <+> ppr local_ids)
928 try_me [] lie_for_here `thenTc` \ (binds, frees, irreds) ->
929 ASSERT( null irreds )
930 returnTc (mkLIE frees `plusLIE` mkLIE lie_not_for_here, binds)
932 overloaded_ids = filter is_overloaded local_ids
933 is_overloaded id = case splitSigmaTy (idType id) of
934 (_, theta, _) -> not (null theta)
936 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
937 -- so it's worth building a set, so that
938 -- lookup (in isMethodFor) is faster
940 -- No sense in repeatedly zonking lots of
941 -- constant constraints so filter them out here
942 (lie_for_here, lie_not_for_here) = partition (isMethodFor overloaded_set)
944 try_me inst | isMethodFor overloaded_set inst = ReduceMe AddToIrreds
949 %************************************************************************
951 \section[Disambig]{Disambiguation of overloading}
953 %************************************************************************
956 If a dictionary constrains a type variable which is
959 not mentioned in the environment
961 and not mentioned in the type of the expression
963 then it is ambiguous. No further information will arise to instantiate
964 the type variable; nor will it be generalised and turned into an extra
965 parameter to a function.
967 It is an error for this to occur, except that Haskell provided for
968 certain rules to be applied in the special case of numeric types.
973 at least one of its classes is a numeric class, and
975 all of its classes are numeric or standard
977 then the type variable can be defaulted to the first type in the
978 default-type list which is an instance of all the offending classes.
980 So here is the function which does the work. It takes the ambiguous
981 dictionaries and either resolves them (producing bindings) or
982 complains. It works by splitting the dictionary list by type
983 variable, and using @disambigOne@ to do the real business.
986 @tcSimplifyTop@ is called once per module to simplify
987 all the constant and ambiguous Insts.
990 tcSimplifyTop :: LIE -> TcM s TcDictBinds
991 tcSimplifyTop wanted_lie
992 = reduceContext (text "tcSimplTop") try_me [] wanteds `thenTc` \ (binds1, frees, irreds) ->
996 -- All the non-std ones are definite errors
997 (stds, non_stds) = partition isStdClassTyVarDict irreds
1000 -- Group by type variable
1001 std_groups = equivClasses cmp_by_tyvar stds
1003 -- Pick the ones which its worth trying to disambiguate
1004 (std_oks, std_bads) = partition worth_a_try std_groups
1005 -- Have a try at disambiguation
1006 -- if the type variable isn't bound
1007 -- up with one of the non-standard classes
1008 worth_a_try group@(d:_) = isEmptyVarSet (tyVarsOfInst d `intersectVarSet` non_std_tyvars)
1009 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1011 -- Collect together all the bad guys
1012 bad_guys = non_stds ++ concat std_bads
1015 -- Disambiguate the ones that look feasible
1016 mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
1018 -- And complain about the ones that don't
1019 mapNF_Tc complain bad_guys `thenNF_Tc_`
1021 returnTc (binds1 `andMonoBinds` andMonoBindList binds_ambig)
1023 -- see comment on wanteds in tcSimplify
1024 wanteds = filter notFunDep (lieToList wanted_lie)
1025 try_me inst = ReduceMe AddToIrreds
1027 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1029 complain d | not (null (getIPs d)) = addTopIPErr d
1030 | isEmptyVarSet (tyVarsOfInst d) = addTopInstanceErr d
1031 | otherwise = addAmbigErr tyVarsOfInst d
1033 get_tv d = case getDictClassTys d of
1034 (clas, [ty]) -> getTyVar "tcSimplifyTop" ty
1035 get_clas d = case getDictClassTys d of
1036 (clas, [ty]) -> clas
1039 @disambigOne@ assumes that its arguments dictionaries constrain all
1040 the same type variable.
1042 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1043 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1044 the most common use of defaulting is code like:
1046 _ccall_ foo `seqPrimIO` bar
1048 Since we're not using the result of @foo@, the result if (presumably)
1052 disambigGroup :: [Inst] -- All standard classes of form (C a)
1053 -> TcM s TcDictBinds
1056 | any isNumericClass classes -- Guaranteed all standard classes
1057 -- see comment at the end of function for reasons as to
1058 -- why the defaulting mechanism doesn't apply to groups that
1059 -- include CCallable or CReturnable dicts.
1060 && not (any isCcallishClass classes)
1061 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1062 -- SO, TRY DEFAULT TYPES IN ORDER
1064 -- Failure here is caused by there being no type in the
1065 -- default list which can satisfy all the ambiguous classes.
1066 -- For example, if Real a is reqd, but the only type in the
1067 -- default list is Int.
1068 tcGetDefaultTys `thenNF_Tc` \ default_tys ->
1070 try_default [] -- No defaults work, so fail
1073 try_default (default_ty : default_tys)
1074 = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
1075 -- default_tys instead
1076 tcSimplifyCheckThetas [] thetas `thenTc` \ _ ->
1079 thetas = classes `zip` repeat [default_ty]
1081 -- See if any default works, and if so bind the type variable to it
1082 -- If not, add an AmbigErr
1083 recoverTc (complain dicts `thenNF_Tc_` returnTc EmptyMonoBinds) $
1085 try_default default_tys `thenTc` \ chosen_default_ty ->
1087 -- Bind the type variable and reduce the context, for real this time
1089 chosen_default_tc_ty = typeToTcType chosen_default_ty -- Tiresome!
1091 unifyTauTy chosen_default_tc_ty (mkTyVarTy tyvar) `thenTc_`
1092 reduceContext (text "disambig" <+> ppr dicts)
1093 try_me [] dicts `thenTc` \ (binds, frees, ambigs) ->
1094 ASSERT( null frees && null ambigs )
1095 warnDefault dicts chosen_default_ty `thenTc_`
1098 | all isCreturnableClass classes
1099 = -- Default CCall stuff to (); we don't even both to check that () is an
1100 -- instance of CReturnable, because we know it is.
1101 unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
1102 returnTc EmptyMonoBinds
1104 | otherwise -- No defaults
1105 = complain dicts `thenNF_Tc_`
1106 returnTc EmptyMonoBinds
1109 complain = addAmbigErrs tyVarsOfInst
1110 try_me inst = ReduceMe AddToIrreds -- This reduce should not fail
1111 tyvar = get_tv (head dicts) -- Should be non-empty
1112 classes = map get_clas dicts
1115 [Aside - why the defaulting mechanism is turned off when
1116 dealing with arguments and results to ccalls.
1118 When typechecking _ccall_s, TcExpr ensures that the external
1119 function is only passed arguments (and in the other direction,
1120 results) of a restricted set of 'native' types. This is
1121 implemented via the help of the pseudo-type classes,
1122 @CReturnable@ (CR) and @CCallable@ (CC.)
1124 The interaction between the defaulting mechanism for numeric
1125 values and CC & CR can be a bit puzzling to the user at times.
1134 What type has 'x' got here? That depends on the default list
1135 in operation, if it is equal to Haskell 98's default-default
1136 of (Integer, Double), 'x' has type Double, since Integer
1137 is not an instance of CR. If the default list is equal to
1138 Haskell 1.4's default-default of (Int, Double), 'x' has type
1141 To try to minimise the potential for surprises here, the
1142 defaulting mechanism is turned off in the presence of
1143 CCallable and CReturnable.
1149 ToDo: for these error messages, should we note the location as coming
1150 from the insts, or just whatever seems to be around in the monad just
1154 genCantGenErr insts -- Can't generalise these Insts
1155 = sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
1156 nest 4 (pprInstsInFull insts)
1159 addAmbigErrs ambig_tv_fn dicts = mapNF_Tc (addAmbigErr ambig_tv_fn) dicts
1161 addAmbigErr ambig_tv_fn dict
1162 = addInstErrTcM (instLoc dict)
1164 sep [text "Ambiguous type variable(s)" <+>
1165 hsep (punctuate comma (map (quotes . ppr) ambig_tvs)),
1166 nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
1168 ambig_tvs = varSetElems (ambig_tv_fn tidy_dict)
1169 (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
1171 warnDefault dicts default_ty
1172 | not opt_WarnTypeDefaults
1178 msg | length dicts > 1
1179 = (ptext SLIT("Defaulting the following constraint(s) to type") <+> quotes (ppr default_ty))
1180 $$ pprInstsInFull tidy_dicts
1182 = ptext SLIT("Defaulting") <+> quotes (pprInst (head tidy_dicts)) <+>
1183 ptext SLIT("to type") <+> quotes (ppr default_ty)
1185 (_, tidy_dicts) = mapAccumL tidyInst emptyTidyEnv dicts
1188 = addInstErrTcM (instLoc dict)
1190 vcat [ptext SLIT("Could not deduce") <+> quotes (pprInst tidy_dict),
1191 nest 4 (ptext SLIT("LHS of a rule must have no overloading"))])
1193 (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
1196 = addInstErrTcM (instLoc dict)
1198 ptext SLIT("Unbound implicit parameter") <+> quotes (pprInst tidy_dict))
1200 (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
1202 -- Used for top-level irreducibles
1203 addTopInstanceErr dict
1204 = addInstErrTcM (instLoc dict)
1206 ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
1208 (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
1210 addNoInstanceErr str givens dict
1211 = addInstErrTcM (instLoc dict)
1213 sep [ptext SLIT("Could not deduce") <+> quotes (pprInst tidy_dict),
1214 nest 4 $ ptext SLIT("from the context:") <+> pprInsts tidy_givens]
1216 ptext SLIT("Probable cause:") <+>
1217 vcat [sep [ptext SLIT("missing") <+> quotes (pprInst tidy_dict),
1218 ptext SLIT("in") <+> str],
1219 if isClassDict dict && all_tyvars then empty else
1220 ptext SLIT("or missing instance declaration for") <+> quotes (pprInst tidy_dict)]
1223 all_tyvars = all isTyVarTy tys
1224 (_, tys) = getDictClassTys dict
1225 (tidy_env, tidy_dict:tidy_givens) = tidyInsts emptyTidyEnv (dict:givens)
1227 -- Used for the ...Thetas variants; all top level
1229 = addErrTc (ptext SLIT("No instance for") <+> quotes (pprConstraint c ts))
1231 reduceDepthErr n stack
1232 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
1233 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
1234 nest 4 (pprInstsInFull stack)]
1236 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)