2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcInstDecls: Typechecking instance declarations
9 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
52 Typechecking instance declarations is done in two passes. The first
53 pass, made by @tcInstDecls1@, collects information to be used in the
56 This pre-processed info includes the as-yet-unprocessed bindings
57 inside the instance declaration. These are type-checked in the second
58 pass, when the class-instance envs and GVE contain all the info from
59 all the instance and value decls. Indeed that's the reason we need
60 two passes over the instance decls.
62 Here is the overall algorithm.
63 Assume that we have an instance declaration
65 instance c => k (t tvs) where b
69 $LIE_c$ is the LIE for the context of class $c$
71 $betas_bar$ is the free variables in the class method type, excluding the
74 $LIE_cop$ is the LIE constraining a particular class method
76 $tau_cop$ is the tau type of a class method
78 $LIE_i$ is the LIE for the context of instance $i$
80 $X$ is the instance constructor tycon
82 $gammas_bar$ is the set of type variables of the instance
84 $LIE_iop$ is the LIE for a particular class method instance
86 $tau_iop$ is the tau type for this instance of a class method
88 $alpha$ is the class variable
90 $LIE_cop' = LIE_cop [X gammas_bar \/ alpha, fresh betas_bar]$
92 $tau_cop' = tau_cop [X gammas_bar \/ alpha, fresh betas_bar]$
95 ToDo: Update the list above with names actually in the code.
99 First, make the LIEs for the class and instance contexts, which means
100 instantiate $thetaC [X inst_tyvars \/ alpha ]$, yielding LIElistC' and LIEC',
101 and make LIElistI and LIEI.
103 Then process each method in turn.
105 order the instance methods according to the ordering of the class methods
107 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
109 Create final dictionary function from bindings generated already
111 df = lambda inst_tyvars
118 in <op1,op2,...,opn,sd1,...,sdm>
120 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
121 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
125 %************************************************************************
127 \subsection{Extracting instance decls}
129 %************************************************************************
131 Gather up the instance declarations from their various sources
134 tcInstDecls1 -- Deal with both source-code and imported instance decls
135 :: [LTyClDecl Name] -- For deriving stuff
136 -> [LInstDecl Name] -- Source code instance decls
137 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
138 -> TcM (TcGblEnv, -- The full inst env
139 [InstInfo Name], -- Source-code instance decls to process;
140 -- contains all dfuns for this module
141 HsValBinds Name) -- Supporting bindings for derived instances
143 tcInstDecls1 tycl_decls inst_decls deriv_decls
145 do { -- Stop if addInstInfos etc discovers any errors
146 -- (they recover, so that we get more than one error each
149 -- (1) Do class and family instance declarations
150 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
151 ; local_info_tycons <- mapM tcLocalInstDecl1 inst_decls
152 ; idx_tycons <- mapM tcIdxTyInstDeclTL idxty_decls
154 ; let { (local_infos,
155 at_tycons) = unzip local_info_tycons
156 ; local_info = concat local_infos
157 ; at_idx_tycon = concat at_tycons ++ catMaybes idx_tycons
158 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
159 ; implicit_things = concatMap implicitTyThings at_idx_tycon
162 -- (2) Add the tycons of indexed types and their implicit
163 -- tythings to the global environment
164 ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do {
166 -- (3) Instances from generic class declarations
167 ; generic_inst_info <- getGenericInstances clas_decls
169 -- Next, construct the instance environment so far, consisting
171 -- a) local instance decls
172 -- b) generic instances
173 -- c) local family instance decls
174 ; addInsts local_info $ do {
175 ; addInsts generic_inst_info $ do {
176 ; addFamInsts at_idx_tycon $ do {
178 -- (4) Compute instances from "deriving" clauses;
179 -- This stuff computes a context for the derived instance
180 -- decl, so it needs to know about all the instances possible
181 -- NB: class instance declarations can contain derivings as
182 -- part of associated data type declarations
183 failIfErrsM -- If the addInsts stuff gave any errors, don't
184 -- try the deriving stuff, becuase that may give
186 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls
188 ; addInsts deriv_inst_info $ do {
190 ; gbl_env <- getGblEnv
192 generic_inst_info ++ deriv_inst_info ++ local_info,
196 -- Make sure that toplevel type instance are not for associated types.
197 -- !!!TODO: Need to perform this check for the TyThing of type functions,
199 tcIdxTyInstDeclTL ldecl@(L loc decl) =
200 do { tything <- tcFamInstDecl ldecl
202 when (isAssocFamily tything) $
203 addErr $ assocInClassErr (tcdName decl)
206 isAssocFamily (Just (ATyCon tycon)) =
207 case tyConFamInst_maybe tycon of
208 Nothing -> panic "isAssocFamily: no family?!?"
209 Just (fam, _) -> isTyConAssoc fam
210 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
211 isAssocFamily Nothing = False
213 assocInClassErr :: Name -> SDoc
214 assocInClassErr name =
215 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
216 ptext (sLit "must be inside a class instance")
218 addInsts :: [InstInfo Name] -> TcM a -> TcM a
219 addInsts infos thing_inside
220 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
222 addFamInsts :: [TyThing] -> TcM a -> TcM a
223 addFamInsts tycons thing_inside
224 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
226 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
227 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
232 tcLocalInstDecl1 :: LInstDecl Name
233 -> TcM ([InstInfo Name], [TyThing]) -- [] if there was an error
234 -- A source-file instance declaration
235 -- Type-check all the stuff before the "where"
237 -- We check for respectable instance type, and context
238 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
239 = -- Prime error recovery, set source location
240 recoverM (return ([], [])) $
242 addErrCtxt (instDeclCtxt1 poly_ty) $
244 do { is_boot <- tcIsHsBoot
245 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
248 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
250 -- Next, process any associated types.
251 ; idx_tycons <- mapM tcFamInstDecl ats
253 -- Now, check the validity of the instance.
254 ; (clas, inst_tys) <- checkValidInstHead tau
255 ; checkValidInstance tyvars theta clas inst_tys
256 ; checkValidAndMissingATs clas (tyvars, inst_tys)
259 -- Finally, construct the Core representation of the instance.
260 -- (This no longer includes the associated types.)
261 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
262 -- Dfun location is that of instance *header*
263 ; overlap_flag <- getOverlapFlag
264 ; let (eq_theta,dict_theta) = partition isEqPred theta
265 theta' = eq_theta ++ dict_theta
266 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
267 ispec = mkLocalInstance dfun overlap_flag
269 ; return ([InstInfo { iSpec = ispec,
270 iBinds = VanillaInst binds uprags }],
271 catMaybes idx_tycons)
274 -- We pass in the source form and the type checked form of the ATs. We
275 -- really need the source form only to be able to produce more informative
277 checkValidAndMissingATs :: Class
278 -> ([TyVar], [TcType]) -- instance types
279 -> [(LTyClDecl Name, -- source form of AT
280 Maybe TyThing)] -- Core form of AT
282 checkValidAndMissingATs clas inst_tys ats
283 = do { -- Issue a warning for each class AT that is not defined in this
285 ; let class_ats = map tyConName (classATs clas)
286 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
287 omitted = filterOut (`elemNameSet` defined_ats) class_ats
288 ; warn <- doptM Opt_WarnMissingMethods
289 ; mapM_ (warnTc warn . omittedATWarn) omitted
291 -- Ensure that all AT indexes that correspond to class parameters
292 -- coincide with the types in the instance head. All remaining
293 -- AT arguments must be variables. Also raise an error for any
294 -- type instances that are not associated with this class.
295 ; mapM_ (checkIndexes clas inst_tys) ats
298 checkIndexes _ _ (_, Nothing) =
299 return () -- skip, we already had an error here
300 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
301 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
302 checkIndexes' clas inst_tys hsAT
304 snd . fromJust . tyConFamInst_maybe $ tycon)
305 checkIndexes _ _ _ = panic "checkIndexes"
307 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
308 = let atName = tcdName . unLoc $ hsAT
310 setSrcSpan (getLoc hsAT) $
311 addErrCtxt (atInstCtxt atName) $
312 case find ((atName ==) . tyConName) (classATs clas) of
313 Nothing -> addErrTc $ badATErr clas atName -- not in this class
315 case assocTyConArgPoss_maybe atDecl of
316 Nothing -> panic "checkIndexes': AT has no args poss?!?"
319 -- The following is tricky! We need to deal with three
320 -- complications: (1) The AT possibly only uses a subset of
321 -- the class parameters as indexes and those it uses may be in
322 -- a different order; (2) the AT may have extra arguments,
323 -- which must be type variables; and (3) variables in AT and
324 -- instance head will be different `Name's even if their
325 -- source lexemes are identical.
327 -- Re (1), `poss' contains a permutation vector to extract the
328 -- class parameters in the right order.
330 -- Re (2), we wrap the (permuted) class parameters in a Maybe
331 -- type and use Nothing for any extra AT arguments. (First
332 -- equation of `checkIndex' below.)
334 -- Re (3), we replace any type variable in the AT parameters
335 -- that has the same source lexeme as some variable in the
336 -- instance types with the instance type variable sharing its
339 let relevantInstTys = map (instTys !!) poss
340 instArgs = map Just relevantInstTys ++
341 repeat Nothing -- extra arguments
342 renaming = substSameTyVar atTvs instTvs
344 zipWithM_ checkIndex (substTys renaming atTys) instArgs
346 checkIndex ty Nothing
347 | isTyVarTy ty = return ()
348 | otherwise = addErrTc $ mustBeVarArgErr ty
349 checkIndex ty (Just instTy)
350 | ty `tcEqType` instTy = return ()
351 | otherwise = addErrTc $ wrongATArgErr ty instTy
353 listToNameSet = addListToNameSet emptyNameSet
355 substSameTyVar [] _ = emptyTvSubst
356 substSameTyVar (tv:tvs) replacingTvs =
357 let replacement = case find (tv `sameLexeme`) replacingTvs of
358 Nothing -> mkTyVarTy tv
359 Just rtv -> mkTyVarTy rtv
361 tv1 `sameLexeme` tv2 =
362 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
364 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
368 %************************************************************************
370 \subsection{Type-checking instance declarations, pass 2}
372 %************************************************************************
375 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
376 -> TcM (LHsBinds Id, TcLclEnv)
377 -- (a) From each class declaration,
378 -- generate any default-method bindings
379 -- (b) From each instance decl
380 -- generate the dfun binding
382 tcInstDecls2 tycl_decls inst_decls
383 = do { -- (a) Default methods from class decls
384 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
385 filter (isClassDecl.unLoc) tycl_decls
386 ; tcExtendIdEnv (concat dm_ids_s) $ do
388 -- (b) instance declarations
389 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
392 ; let binds = unionManyBags dm_binds_s `unionBags`
393 unionManyBags inst_binds_s
394 ; tcl_env <- getLclEnv -- Default method Ids in here
395 ; return (binds, tcl_env) }
398 ======= New documentation starts here (Sept 92) ==============
400 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
401 the dictionary function for this instance declaration. For example
403 instance Foo a => Foo [a] where
407 might generate something like
409 dfun.Foo.List dFoo_a = let op1 x = ...
414 HOWEVER, if the instance decl has no context, then it returns a
415 bigger @HsBinds@ with declarations for each method. For example
417 instance Foo [a] where
423 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
424 const.Foo.op1.List a x = ...
425 const.Foo.op2.List a y = ...
427 This group may be mutually recursive, because (for example) there may
428 be no method supplied for op2 in which case we'll get
430 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
432 that is, the default method applied to the dictionary at this type.
433 What we actually produce in either case is:
435 AbsBinds [a] [dfun_theta_dicts]
436 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
437 { d = (sd1,sd2, ..., op1, op2, ...)
442 The "maybe" says that we only ask AbsBinds to make global constant methods
443 if the dfun_theta is empty.
445 For an instance declaration, say,
447 instance (C1 a, C2 b) => C (T a b) where
450 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
451 function whose type is
453 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
455 Notice that we pass it the superclass dictionaries at the instance type; this
456 is the ``Mark Jones optimisation''. The stuff before the "=>" here
457 is the @dfun_theta@ below.
461 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
462 -- Returns a binding for the dfun
464 ------------------------
465 -- Derived newtype instances; surprisingly tricky!
467 -- class Show a => Foo a b where ...
468 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
470 -- The newtype gives an FC axiom looking like
471 -- axiom CoN a :: N a :=: Tree [a]
472 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
474 -- So all need is to generate a binding looking like:
475 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
476 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
477 -- case df `cast` (Foo Int (sym (CoN a))) of
478 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
480 -- If there are no superclasses, matters are simpler, because we don't need the case
481 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
483 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived })
484 = do { let dfun_id = instanceDFunId ispec
485 rigid_info = InstSkol
486 origin = SigOrigin rigid_info
487 inst_ty = idType dfun_id
488 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
489 -- inst_head_ty is a PredType
491 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
492 (class_tyvars, sc_theta, _, _) = classBigSig cls
493 cls_tycon = classTyCon cls
494 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
496 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
497 (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail
498 rep_ty = newTyConInstRhs nt_tycon tc_args
500 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
501 -- In our example, rep_pred is (Foo Int (Tree [a]))
502 the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
503 -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)
505 ; inst_loc <- getInstLoc origin
506 ; sc_loc <- getInstLoc InstScOrigin
507 ; dfun_dicts <- newDictBndrs inst_loc theta
508 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
509 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
510 ; rep_dict <- newDictBndr inst_loc rep_pred
512 -- Figure out bindings for the superclass context from dfun_dicts
513 -- Don't include this_dict in the 'givens', else
514 -- wanted_sc_insts get bound by just selecting from this_dict!!
515 ; sc_binds <- addErrCtxt superClassCtxt $
516 tcSimplifySuperClasses inst_loc dfun_dicts (rep_dict:sc_dicts)
518 ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict))
520 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
521 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
523 ; return (unitBag $ noLoc $
524 AbsBinds tvs (map instToVar dfun_dicts)
525 [(tvs, dfun_id, instToId this_dict, [])]
526 (dict_bind `consBag` sc_binds)) }
528 -----------------------
530 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
531 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
532 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
533 -- where rep_ty is the (eta-reduced) type rep of T
534 -- So we just replace T with CoT, and insert a 'sym'
535 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
537 make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
538 | Just co_con <- newTyConCo_maybe nt_tycon
539 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
540 = WpCast (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))
541 | otherwise -- The newtype is transparent; no need for a cast
544 -----------------------
545 -- (make_body C tys scs coreced_rep_dict)
547 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
548 -- But if there are no superclasses, it returns just coerced_rep_dict
549 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
551 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
552 | null sc_dicts -- Case (a)
553 = return coerced_rep_dict
554 | otherwise -- Case (b)
555 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
556 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
557 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
558 pat_dicts = dummy_sc_dict_ids,
559 pat_binds = emptyLHsBinds,
560 pat_args = PrefixCon (map nlVarPat op_ids),
562 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
563 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
564 map HsVar (sc_dict_ids ++ op_ids)
566 -- Warning: this HsCase scrutinises a value with a PredTy, which is
567 -- never otherwise seen in Haskell source code. It'd be
568 -- nicer to generate Core directly!
569 ; return (HsCase (noLoc coerced_rep_dict) $
570 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
572 sc_dict_ids = map instToId sc_dicts
573 pat_ty = mkTyConApp cls_tycon cls_inst_tys
574 cls_data_con = head (tyConDataCons cls_tycon)
575 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
576 op_tys = dropList sc_dict_ids cls_arg_tys
578 ------------------------
579 -- Ordinary instances
581 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
583 dfun_id = instanceDFunId ispec
584 rigid_info = InstSkol
585 inst_ty = idType dfun_id
586 loc = getSrcSpan dfun_id
588 -- Prime error recovery
589 recoverM (return emptyLHsBinds) $
591 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do
593 -- Instantiate the instance decl with skolem constants
594 (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
595 -- These inst_tyvars' scope over the 'where' part
596 -- Those tyvars are inside the dfun_id's type, which is a bit
597 -- bizarre, but OK so long as you realise it!
599 (clas, inst_tys') = tcSplitDFunHead inst_head'
600 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
602 -- Instantiate the super-class context with inst_tys
603 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
604 (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
605 origin = SigOrigin rigid_info
606 (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
608 -- Create dictionary Ids from the specified instance contexts.
609 sc_loc <- getInstLoc InstScOrigin
610 sc_dicts <- newDictBndrs sc_loc dict_sc_theta'
611 inst_loc <- getInstLoc origin
612 sc_covars <- mkMetaCoVars eq_sc_theta'
613 wanted_sc_eqs <- mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars)
614 dfun_covars <- mkCoVars eq_dfun_theta'
615 dfun_eqs <- mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars)
616 dfun_dicts <- newDictBndrs inst_loc dict_dfun_theta'
617 this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
618 -- Default-method Ids may be mentioned in synthesised RHSs,
619 -- but they'll already be in the environment.
621 -- Typecheck the methods
622 let -- These insts are in scope; quite a few, eh?
623 dfun_insts = dfun_eqs ++ dfun_dicts
624 wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
625 given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
626 given_sc_insts = given_sc_eqs ++ sc_dicts
627 avail_insts = dfun_insts ++ given_sc_insts
629 (meth_ids, meth_binds) <- tcMethods origin clas inst_tyvars'
630 dfun_theta' inst_tys' this_dict avail_insts
631 op_items monobinds uprags
633 -- Figure out bindings for the superclass context
634 -- Don't include this_dict in the 'givens', else
635 -- wanted_sc_insts get bound by just selecting from this_dict!!
636 sc_binds <- addErrCtxt superClassCtxt
637 (tcSimplifySuperClasses inst_loc dfun_insts wanted_sc_insts)
639 -- It's possible that the superclass stuff might unified one
640 -- of the inst_tyavars' with something in the envt
641 checkSigTyVars inst_tyvars'
643 -- Deal with 'SPECIALISE instance' pragmas
644 prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
646 -- Create the result bindings
648 dict_constr = classDataCon clas
649 scs_and_meths = map instToId sc_dicts ++ meth_ids
650 this_dict_id = instToId this_dict
651 inline_prag | null dfun_insts = []
652 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
653 -- Always inline the dfun; this is an experimental decision
654 -- because it makes a big performance difference sometimes.
655 -- Often it means we can do the method selection, and then
656 -- inline the method as well. Marcin's idea; see comments below.
658 -- BUT: don't inline it if it's a constant dictionary;
659 -- we'll get all the benefit without inlining, and we get
660 -- a **lot** of code duplication if we inline it
662 -- See Note [Inline dfuns] below
664 dict_rhs = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars)
665 (map HsVar scs_and_meths)
666 -- We don't produce a binding for the dict_constr; instead we
667 -- rely on the simplifier to unfold this saturated application
668 -- We do this rather than generate an HsCon directly, because
669 -- it means that the special cases (e.g. dictionary with only one
670 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
671 -- than needing to be repeated here.
673 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
674 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
676 main_bind = noLoc $ AbsBinds
677 (inst_tyvars' ++ dfun_covars)
678 (map instToId dfun_dicts)
679 [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
682 showLIE (text "instance")
683 return (unitBag main_bind)
685 mkCoVars :: [PredType] -> TcM [TyVar]
686 mkCoVars = newCoVars . map unEqPred
688 unEqPred (EqPred ty1 ty2) = (ty1, ty2)
689 unEqPred _ = panic "TcInstDcls.mkCoVars"
691 mkMetaCoVars :: [PredType] -> TcM [TyVar]
692 mkMetaCoVars = mapM eqPredToCoVar
694 eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2
695 eqPredToCoVar _ = panic "TcInstDcls.mkMetaCoVars"
697 tcMethods :: InstOrigin -> Class -> [TcTyVar] -> TcThetaType -> [TcType]
698 -> Inst -> [Inst] -> [(Id, DefMeth)] -> LHsBindsLR Name Name
700 -> TcM ([Id], Bag (LHsBind Id))
701 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
702 this_dict extra_insts op_items monobinds uprags = do
703 -- Check that all the method bindings come from this class
705 sel_names = [idName sel_id | (sel_id, _) <- op_items]
706 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
708 mapM (addErrTc . badMethodErr clas) bad_bndrs
710 -- Make the method bindings
712 mk_method_id (sel_id, _) = mkMethId origin clas sel_id inst_tys'
714 (meth_insts, meth_ids) <- mapAndUnzipM mk_method_id op_items
716 -- And type check them
717 -- It's really worth making meth_insts available to the tcMethodBind
718 -- Consider instance Monad (ST s) where
719 -- {-# INLINE (>>) #-}
720 -- (>>) = ...(>>=)...
721 -- If we don't include meth_insts, we end up with bindings like this:
722 -- rec { dict = MkD then bind ...
723 -- then = inline_me (... (GHC.Base.>>= dict) ...)
725 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
726 -- and (b) the inline_me prevents us inlining the >>= selector, which
727 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
728 -- is not inlined across modules. Rather ironic since this does not
729 -- happen without the INLINE pragma!
731 -- Solution: make meth_insts available, so that 'then' refers directly
732 -- to the local 'bind' rather than going via the dictionary.
734 -- BUT WATCH OUT! If the method type mentions the class variable, then
735 -- this optimisation is not right. Consider
739 -- instance C Int where
741 -- The occurrence of 'op' on the rhs gives rise to a constraint
743 -- The trouble is that the 'meth_inst' for op, which is 'available', also
744 -- looks like 'op at Int'. But they are not the same.
746 prag_fn = mkPragFun uprags
747 all_insts = extra_insts ++ catMaybes meth_insts
748 sig_fn _ = Just [] -- No scoped type variables, but every method has
749 -- a type signature, in effect, so that we check
750 -- the method has the right type
751 tc_method_bind = tcMethodBind origin inst_tyvars' dfun_theta' this_dict
752 all_insts sig_fn prag_fn monobinds
754 meth_binds_s <- zipWithM tc_method_bind op_items meth_ids
756 return (meth_ids, unionManyBags meth_binds_s)
760 ------------------------------
761 [Inline dfuns] Inlining dfuns unconditionally
762 ------------------------------
764 The code above unconditionally inlines dict funs. Here's why.
765 Consider this program:
767 test :: Int -> Int -> Bool
768 test x y = (x,y) == (y,x) || test y x
769 -- Recursive to avoid making it inline.
771 This needs the (Eq (Int,Int)) instance. If we inline that dfun
772 the code we end up with is good:
775 \r -> case ==# [ww ww1] of wild {
776 PrelBase.False -> Test.$wtest ww1 ww;
778 case ==# [ww1 ww] of wild1 {
779 PrelBase.False -> Test.$wtest ww1 ww;
780 PrelBase.True -> PrelBase.True [];
783 Test.test = \r [w w1]
786 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
789 If we don't inline the dfun, the code is not nearly as good:
791 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
792 PrelBase.:DEq tpl1 tpl2 -> tpl2;
797 let { y = PrelBase.I#! [ww1]; } in
798 let { x = PrelBase.I#! [ww]; } in
799 let { sat_slx = PrelTup.(,)! [y x]; } in
800 let { sat_sly = PrelTup.(,)! [x y];
802 case == sat_sly sat_slx of wild {
803 PrelBase.False -> Test.$wtest ww1 ww;
804 PrelBase.True -> PrelBase.True [];
811 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
814 Why doesn't GHC inline $fEq? Because it looks big:
816 PrelTup.zdfEqZ1T{-rcX-}
817 = \ @ a{-reT-} :: * @ b{-reS-} :: *
818 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
819 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
821 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
822 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
824 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
825 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
827 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
828 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
829 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
831 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
833 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
835 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
836 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
840 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
841 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
842 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
843 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
845 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
847 and it's not as bad as it seems, because it's further dramatically
848 simplified: only zeze2 is extracted and its body is simplified.
851 %************************************************************************
853 \subsection{Error messages}
855 %************************************************************************
858 instDeclCtxt1 :: LHsType Name -> SDoc
859 instDeclCtxt1 hs_inst_ty
860 = inst_decl_ctxt (case unLoc hs_inst_ty of
861 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
862 HsPredTy pred -> ppr pred
863 _ -> ppr hs_inst_ty) -- Don't expect this
864 instDeclCtxt2 :: Type -> SDoc
865 instDeclCtxt2 dfun_ty
866 = inst_decl_ctxt (ppr (mkClassPred cls tys))
868 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
870 inst_decl_ctxt :: SDoc -> SDoc
871 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
873 superClassCtxt :: SDoc
874 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
876 atInstCtxt :: Name -> SDoc
877 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
880 mustBeVarArgErr :: Type -> SDoc
882 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
883 ptext (sLit "must be variables")
884 , ptext (sLit "Instead of a variable, found") <+> ppr ty
887 wrongATArgErr :: Type -> Type -> SDoc
888 wrongATArgErr ty instTy =
889 sep [ ptext (sLit "Type indexes must match class instance head")
890 , ptext (sLit "Found") <+> ppr ty <+> ptext (sLit "but expected") <+>