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
11 #include "HsVersions.h"
49 import Control.Monad hiding (zipWithM_, mapAndUnzipM)
53 Typechecking instance declarations is done in two passes. The first
54 pass, made by @tcInstDecls1@, collects information to be used in the
57 This pre-processed info includes the as-yet-unprocessed bindings
58 inside the instance declaration. These are type-checked in the second
59 pass, when the class-instance envs and GVE contain all the info from
60 all the instance and value decls. Indeed that's the reason we need
61 two passes over the instance decls.
63 Here is the overall algorithm.
64 Assume that we have an instance declaration
66 instance c => k (t tvs) where b
70 $LIE_c$ is the LIE for the context of class $c$
72 $betas_bar$ is the free variables in the class method type, excluding the
75 $LIE_cop$ is the LIE constraining a particular class method
77 $tau_cop$ is the tau type of a class method
79 $LIE_i$ is the LIE for the context of instance $i$
81 $X$ is the instance constructor tycon
83 $gammas_bar$ is the set of type variables of the instance
85 $LIE_iop$ is the LIE for a particular class method instance
87 $tau_iop$ is the tau type for this instance of a class method
89 $alpha$ is the class variable
91 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
93 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
96 ToDo: Update the list above with names actually in the code.
100 First, make the LIEs for the class and instance contexts, which means
101 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
102 and make LIElistI and LIEI.
104 Then process each method in turn.
106 order the instance methods according to the ordering of the class methods
108 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
110 Create final dictionary function from bindings generated already
112 df = lambda inst_tyvars
119 in <op1,op2,...,opn,sd1,...,sdm>
121 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
122 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
126 %************************************************************************
128 \subsection{Extracting instance decls}
130 %************************************************************************
132 Gather up the instance declarations from their various sources
135 tcInstDecls1 -- Deal with both source-code and imported instance decls
136 :: [LTyClDecl Name] -- For deriving stuff
137 -> [LInstDecl Name] -- Source code instance decls
138 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
139 -> TcM (TcGblEnv, -- The full inst env
140 [InstInfo], -- Source-code instance decls to process;
141 -- contains all dfuns for this module
142 HsValBinds Name) -- Supporting bindings for derived instances
144 tcInstDecls1 tycl_decls inst_decls deriv_decls
146 do { -- Stop if addInstInfos etc discovers any errors
147 -- (they recover, so that we get more than one error each
150 -- (1) Do class instance declarations and instances of indexed
152 ; let { idxty_decls = filter (isIdxTyDecl . unLoc) tycl_decls }
153 ; local_info_tycons <- mappM tcLocalInstDecl1 inst_decls
154 ; idx_tycons <- mappM tcIdxTyInstDeclTL idxty_decls
156 ; let { (local_infos,
157 at_tycons) = unzip local_info_tycons
158 ; local_info = concat local_infos
159 ; at_idx_tycon = concat at_tycons ++ catMaybes idx_tycons
160 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
161 ; implicit_things = concatMap implicitTyThings at_idx_tycon
164 -- (2) Add the tycons of indexed types and their implicit
165 -- tythings to the global environment
166 ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do {
168 -- (3) Instances from generic class declarations
169 ; generic_inst_info <- getGenericInstances clas_decls
171 -- Next, construct the instance environment so far, consisting
173 -- a) local instance decls
174 -- b) generic instances
175 -- c) local family instance decls
176 ; addInsts local_info $ do {
177 ; addInsts generic_inst_info $ do {
178 ; addFamInsts at_idx_tycon $ do {
180 -- (4) Compute instances from "deriving" clauses;
181 -- This stuff computes a context for the derived instance
182 -- decl, so it needs to know about all the instances possible
183 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls deriv_decls
184 ; addInsts deriv_inst_info $ do {
186 ; gbl_env <- getGblEnv
188 generic_inst_info ++ deriv_inst_info ++ local_info,
192 -- Make sure that toplevel type instance are not for associated types.
193 -- !!!TODO: Need to perform this check for the TyThing of type functions,
195 tcIdxTyInstDeclTL ldecl@(L loc decl) =
196 do { tything <- tcIdxTyInstDecl ldecl
198 when (isAssocFamily tything) $
199 addErr $ assocInClassErr (tcdName decl)
202 isAssocFamily (Just (ATyCon tycon)) =
203 case tyConFamInst_maybe tycon of
204 Nothing -> panic "isAssocFamily: no family?!?"
205 Just (fam, _) -> isTyConAssoc fam
206 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
207 isAssocFamily Nothing = False
209 assocInClassErr name =
210 ptext SLIT("Associated type") <+> quotes (ppr name) <+>
211 ptext SLIT("must be inside a class instance")
213 addInsts :: [InstInfo] -> TcM a -> TcM a
214 addInsts infos thing_inside
215 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
217 addFamInsts :: [TyThing] -> TcM a -> TcM a
218 addFamInsts tycons thing_inside
219 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
221 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
222 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
227 tcLocalInstDecl1 :: LInstDecl Name
228 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
229 -- A source-file instance declaration
230 -- Type-check all the stuff before the "where"
232 -- We check for respectable instance type, and context
233 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
234 = -- Prime error recovery, set source location
235 recoverM (returnM ([], [])) $
237 addErrCtxt (instDeclCtxt1 poly_ty) $
239 do { is_boot <- tcIsHsBoot
240 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
243 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
245 -- Next, process any associated types.
246 ; idx_tycons <- mappM tcIdxTyInstDecl ats
248 -- Now, check the validity of the instance.
249 ; (clas, inst_tys) <- checkValidInstHead tau
250 ; checkValidInstance tyvars theta clas inst_tys
251 ; checkValidAndMissingATs clas (tyvars, inst_tys)
254 -- Finally, construct the Core representation of the instance.
255 -- (This no longer includes the associated types.)
256 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
257 ; overlap_flag <- getOverlapFlag
258 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
259 ispec = mkLocalInstance dfun overlap_flag
261 ; return ([InstInfo { iSpec = ispec,
262 iBinds = VanillaInst binds uprags }],
263 catMaybes idx_tycons)
266 -- We pass in the source form and the type checked form of the ATs. We
267 -- really need the source form only to be able to produce more informative
269 checkValidAndMissingATs :: Class
270 -> ([TyVar], [TcType]) -- instance types
271 -> [(LTyClDecl Name, -- source form of AT
272 Maybe TyThing)] -- Core form of AT
274 checkValidAndMissingATs clas inst_tys ats
275 = do { -- Issue a warning for each class AT that is not defined in this
277 ; let class_ats = map tyConName (classATs clas)
278 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
279 omitted = filterOut (`elemNameSet` defined_ats) class_ats
280 ; warn <- doptM Opt_WarnMissingMethods
281 ; mapM_ (warnTc warn . omittedATWarn) omitted
283 -- Ensure that all AT indexes that correspond to class parameters
284 -- coincide with the types in the instance head. All remaining
285 -- AT arguments must be variables. Also raise an error for any
286 -- type instances that are not associated with this class.
287 ; mapM_ (checkIndexes clas inst_tys) ats
290 checkIndexes _ _ (hsAT, Nothing) =
291 return () -- skip, we already had an error here
292 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
293 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
294 checkIndexes' clas inst_tys hsAT
296 snd . fromJust . tyConFamInst_maybe $ tycon)
297 checkIndexes _ _ _ = panic "checkIndexes"
299 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
300 = let atName = tcdName . unLoc $ hsAT
302 setSrcSpan (getLoc hsAT) $
303 addErrCtxt (atInstCtxt atName) $
304 case find ((atName ==) . tyConName) (classATs clas) of
305 Nothing -> addErrTc $ badATErr clas atName -- not in this class
307 case assocTyConArgPoss_maybe atDecl of
308 Nothing -> panic "checkIndexes': AT has no args poss?!?"
311 -- The following is tricky! We need to deal with three
312 -- complications: (1) The AT possibly only uses a subset of
313 -- the class parameters as indexes and those it uses may be in
314 -- a different order; (2) the AT may have extra arguments,
315 -- which must be type variables; and (3) variables in AT and
316 -- instance head will be different `Name's even if their
317 -- source lexemes are identical.
319 -- Re (1), `poss' contains a permutation vector to extract the
320 -- class parameters in the right order.
322 -- Re (2), we wrap the (permuted) class parameters in a Maybe
323 -- type and use Nothing for any extra AT arguments. (First
324 -- equation of `checkIndex' below.)
326 -- Re (3), we replace any type variable in the AT parameters
327 -- that has the same source lexeme as some variable in the
328 -- instance types with the instance type variable sharing its
331 let relevantInstTys = map (instTys !!) poss
332 instArgs = map Just relevantInstTys ++
333 repeat Nothing -- extra arguments
334 renaming = substSameTyVar atTvs instTvs
336 zipWithM_ checkIndex (substTys renaming atTys) instArgs
338 checkIndex ty Nothing
339 | isTyVarTy ty = return ()
340 | otherwise = addErrTc $ mustBeVarArgErr ty
341 checkIndex ty (Just instTy)
342 | ty `tcEqType` instTy = return ()
343 | otherwise = addErrTc $ wrongATArgErr ty instTy
345 listToNameSet = addListToNameSet emptyNameSet
347 substSameTyVar [] _ = emptyTvSubst
348 substSameTyVar (tv:tvs) replacingTvs =
349 let replacement = case find (tv `sameLexeme`) replacingTvs of
350 Nothing -> mkTyVarTy tv
351 Just rtv -> mkTyVarTy rtv
353 tv1 `sameLexeme` tv2 =
354 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
356 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
360 %************************************************************************
362 \subsection{Type-checking instance declarations, pass 2}
364 %************************************************************************
367 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
368 -> TcM (LHsBinds Id, TcLclEnv)
369 -- (a) From each class declaration,
370 -- generate any default-method bindings
371 -- (b) From each instance decl
372 -- generate the dfun binding
374 tcInstDecls2 tycl_decls inst_decls
375 = do { -- (a) Default methods from class decls
376 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
377 filter (isClassDecl.unLoc) tycl_decls
378 ; tcExtendIdEnv (concat dm_ids_s) $ do
380 -- (b) instance declarations
381 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
384 ; let binds = unionManyBags dm_binds_s `unionBags`
385 unionManyBags inst_binds_s
386 ; tcl_env <- getLclEnv -- Default method Ids in here
387 ; returnM (binds, tcl_env) }
390 ======= New documentation starts here (Sept 92) ==============
392 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
393 the dictionary function for this instance declaration. For example
395 instance Foo a => Foo [a] where
399 might generate something like
401 dfun.Foo.List dFoo_a = let op1 x = ...
407 HOWEVER, if the instance decl has no context, then it returns a
408 bigger @HsBinds@ with declarations for each method. For example
410 instance Foo [a] where
416 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
417 const.Foo.op1.List a x = ...
418 const.Foo.op2.List a y = ...
420 This group may be mutually recursive, because (for example) there may
421 be no method supplied for op2 in which case we'll get
423 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
425 that is, the default method applied to the dictionary at this type.
427 What we actually produce in either case is:
429 AbsBinds [a] [dfun_theta_dicts]
430 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
431 { d = (sd1,sd2, ..., op1, op2, ...)
436 The "maybe" says that we only ask AbsBinds to make global constant methods
437 if the dfun_theta is empty.
440 For an instance declaration, say,
442 instance (C1 a, C2 b) => C (T a b) where
445 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
446 function whose type is
448 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
450 Notice that we pass it the superclass dictionaries at the instance type; this
451 is the ``Mark Jones optimisation''. The stuff before the "=>" here
452 is the @dfun_theta@ below.
454 First comes the easy case of a non-local instance decl.
458 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
459 -- Returns a binding for the dfun
461 ------------------------
462 -- Derived newtype instances; surprisingly tricky!
464 -- In the case of a newtype, things are rather easy
465 -- class Show a => Foo a b where ...
466 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
467 -- The newtype gives an FC axiom looking like
468 -- axiom CoT a :: T a :=: Tree [a]
469 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
471 -- So all need is to generate a binding looking like:
472 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
473 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
474 -- case df `cast` (Foo Int (sym (CoT a))) of
475 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
477 -- If there are no superclasses, matters are simpler, because we don't need the case
478 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
480 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
481 = do { let dfun_id = instanceDFunId ispec
482 rigid_info = InstSkol
483 origin = SigOrigin rigid_info
484 inst_ty = idType dfun_id
485 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
486 -- inst_head_ty is a PredType
488 ; inst_loc <- getInstLoc origin
489 ; (rep_dict_id : sc_dict_ids, wrap_fn, sc_binds)
490 <- make_wrapper inst_loc tvs theta mb_preds
491 -- Here, we are relying on the order of dictionary
492 -- arguments built by NewTypeDerived in TcDeriv;
493 -- namely, that the rep_dict_id comes first
495 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
496 cls_tycon = classTyCon cls
497 the_coercion = make_coercion cls_tycon cls_inst_tys
498 coerced_rep_dict = mkHsWrap the_coercion (HsVar rep_dict_id)
500 ; body <- make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
502 ; return (sc_binds `snocBag` (noLoc $ VarBind dfun_id $ noLoc $ mkHsWrap wrap_fn body)) }
505 -----------------------
507 -- We distinguish two cases:
508 -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
509 -- and the new dict can just be a constant
510 -- (mb_preds = Just preds)
511 -- (b) there are tyvars, so we must make a dict *fun*
512 -- (mb_preds = Nothing)
513 -- See the defn of NewTypeDerived for the meaning of mb_preds
514 make_wrapper inst_loc tvs theta (Just preds) -- Case (a)
515 = ASSERT( null tvs && null theta )
516 do { dicts <- newDictBndrs inst_loc preds
517 ; sc_binds <- addErrCtxt superClassCtxt $
518 tcSimplifySuperClasses inst_loc [] dicts
519 -- Use tcSimplifySuperClasses to avoid creating loops, for the
520 -- same reason as Note [SUPERCLASS-LOOP 1] in TcSimplify
521 ; return (map instToId dicts, idHsWrapper, sc_binds) }
523 make_wrapper inst_loc tvs theta Nothing -- Case (b)
524 = do { dicts <- newDictBndrs inst_loc theta
525 ; let dict_ids = map instToId dicts
526 ; return (dict_ids, mkWpTyLams tvs <.> mkWpLams dict_ids, emptyBag) }
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 cls_inst_tys
538 | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
539 , (tycon, tc_args) <- tcSplitTyConApp last_ty -- Should not fail
540 , Just co_con <- newTyConCo_maybe tycon
541 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
542 = WpCo (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
543 | otherwise -- The newtype is transparent; no need for a cast
546 -----------------------
548 -- Two cases; see Note [Newtype deriving superclasses] in TcDeriv.lhs
549 -- (a) no superclasses; then we can just use the coerced dict
550 -- (b) one or more superclasses; then new need to do the unpack/repack
552 make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
553 | null sc_dict_ids -- Case (a)
554 = return coerced_rep_dict
555 | otherwise -- Case (b)
556 = do { op_ids <- newSysLocalIds FSLIT("op") op_tys
557 ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
558 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
559 pat_dicts = dummy_sc_dict_ids,
560 pat_binds = emptyLHsBinds,
561 pat_args = PrefixCon (map nlVarPat op_ids),
563 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
564 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
565 map HsVar (sc_dict_ids ++ op_ids)
567 -- Warning: this HsCase scrutinises a value with a PredTy, which is
568 -- never otherwise seen in Haskell source code. It'd be
569 -- nicer to generate Core directly!
570 ; return (HsCase (noLoc coerced_rep_dict) $
571 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
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
587 -- Prime error recovery
588 recoverM (returnM emptyLHsBinds) $
589 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
590 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
592 -- Instantiate the instance decl with skolem constants
593 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
594 -- These inst_tyvars' scope over the 'where' part
595 -- Those tyvars are inside the dfun_id's type, which is a bit
596 -- bizarre, but OK so long as you realise it!
598 (clas, inst_tys') = tcSplitDFunHead inst_head'
599 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
601 -- Instantiate the super-class context with inst_tys
602 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
603 origin = SigOrigin rigid_info
605 -- Create dictionary Ids from the specified instance contexts.
606 getInstLoc InstScOrigin `thenM` \ sc_loc ->
607 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
608 getInstLoc origin `thenM` \ inst_loc ->
609 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
610 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
611 -- Default-method Ids may be mentioned in synthesised RHSs,
612 -- but they'll already be in the environment.
614 -- Typecheck the methods
615 let -- These insts are in scope; quite a few, eh?
616 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
618 tcMethods origin clas inst_tyvars'
619 dfun_theta' inst_tys' avail_insts
620 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
622 -- Figure out bindings for the superclass context
623 -- Don't include this_dict in the 'givens', else
624 -- sc_dicts get bound by just selecting from this_dict!!
625 addErrCtxt superClassCtxt
626 (tcSimplifySuperClasses inst_loc
627 dfun_arg_dicts sc_dicts) `thenM` \ sc_binds ->
629 -- It's possible that the superclass stuff might unified one
630 -- of the inst_tyavars' with something in the envt
631 checkSigTyVars inst_tyvars' `thenM_`
633 -- Deal with 'SPECIALISE instance' pragmas
634 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
636 -- Create the result bindings
638 dict_constr = classDataCon clas
639 scs_and_meths = map instToId sc_dicts ++ meth_ids
640 this_dict_id = instToId this_dict
641 inline_prag | null dfun_arg_dicts = []
642 | otherwise = [InlinePrag (Inline AlwaysActive True)]
643 -- Always inline the dfun; this is an experimental decision
644 -- because it makes a big performance difference sometimes.
645 -- Often it means we can do the method selection, and then
646 -- inline the method as well. Marcin's idea; see comments below.
648 -- BUT: don't inline it if it's a constant dictionary;
649 -- we'll get all the benefit without inlining, and we get
650 -- a **lot** of code duplication if we inline it
652 -- See Note [Inline dfuns] below
655 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
656 -- We don't produce a binding for the dict_constr; instead we
657 -- rely on the simplifier to unfold this saturated application
658 -- We do this rather than generate an HsCon directly, because
659 -- it means that the special cases (e.g. dictionary with only one
660 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
661 -- than needing to be repeated here.
663 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
664 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
666 main_bind = noLoc $ AbsBinds
668 (map instToId dfun_arg_dicts)
669 [(inst_tyvars', dfun_id, this_dict_id,
670 inline_prag ++ prags)]
673 showLIE (text "instance") `thenM_`
674 returnM (unitBag main_bind)
677 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
678 avail_insts op_items monobinds uprags
679 = -- Check that all the method bindings come from this class
681 sel_names = [idName sel_id | (sel_id, _) <- op_items]
682 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
684 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
686 -- Make the method bindings
688 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
690 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
692 -- And type check them
693 -- It's really worth making meth_insts available to the tcMethodBind
694 -- Consider instance Monad (ST s) where
695 -- {-# INLINE (>>) #-}
696 -- (>>) = ...(>>=)...
697 -- If we don't include meth_insts, we end up with bindings like this:
698 -- rec { dict = MkD then bind ...
699 -- then = inline_me (... (GHC.Base.>>= dict) ...)
701 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
702 -- and (b) the inline_me prevents us inlining the >>= selector, which
703 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
704 -- is not inlined across modules. Rather ironic since this does not
705 -- happen without the INLINE pragma!
707 -- Solution: make meth_insts available, so that 'then' refers directly
708 -- to the local 'bind' rather than going via the dictionary.
710 -- BUT WATCH OUT! If the method type mentions the class variable, then
711 -- this optimisation is not right. Consider
715 -- instance C Int where
717 -- The occurrence of 'op' on the rhs gives rise to a constraint
719 -- The trouble is that the 'meth_inst' for op, which is 'available', also
720 -- looks like 'op at Int'. But they are not the same.
722 prag_fn = mkPragFun uprags
723 all_insts = avail_insts ++ catMaybes meth_insts
724 sig_fn n = Just [] -- No scoped type variables, but every method has
725 -- a type signature, in effect, so that we check
726 -- the method has the right type
727 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
728 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
731 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
733 returnM (meth_ids, unionManyBags meth_binds_s)
737 ------------------------------
738 [Inline dfuns] Inlining dfuns unconditionally
739 ------------------------------
741 The code above unconditionally inlines dict funs. Here's why.
742 Consider this program:
744 test :: Int -> Int -> Bool
745 test x y = (x,y) == (y,x) || test y x
746 -- Recursive to avoid making it inline.
748 This needs the (Eq (Int,Int)) instance. If we inline that dfun
749 the code we end up with is good:
752 \r -> case ==# [ww ww1] of wild {
753 PrelBase.False -> Test.$wtest ww1 ww;
755 case ==# [ww1 ww] of wild1 {
756 PrelBase.False -> Test.$wtest ww1 ww;
757 PrelBase.True -> PrelBase.True [];
760 Test.test = \r [w w1]
763 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
766 If we don't inline the dfun, the code is not nearly as good:
768 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
769 PrelBase.:DEq tpl1 tpl2 -> tpl2;
774 let { y = PrelBase.I#! [ww1]; } in
775 let { x = PrelBase.I#! [ww]; } in
776 let { sat_slx = PrelTup.(,)! [y x]; } in
777 let { sat_sly = PrelTup.(,)! [x y];
779 case == sat_sly sat_slx of wild {
780 PrelBase.False -> Test.$wtest ww1 ww;
781 PrelBase.True -> PrelBase.True [];
788 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
791 Why doesn't GHC inline $fEq? Because it looks big:
793 PrelTup.zdfEqZ1T{-rcX-}
794 = \ @ a{-reT-} :: * @ b{-reS-} :: *
795 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
796 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
798 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
799 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
801 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
802 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
804 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
805 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
806 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
808 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
810 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
812 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
813 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
817 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
818 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
819 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
820 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
822 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
824 and it's not as bad as it seems, because it's further dramatically
825 simplified: only zeze2 is extracted and its body is simplified.
828 %************************************************************************
830 \subsection{Error messages}
832 %************************************************************************
835 instDeclCtxt1 hs_inst_ty
836 = inst_decl_ctxt (case unLoc hs_inst_ty of
837 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
838 HsPredTy pred -> ppr pred
839 other -> ppr hs_inst_ty) -- Don't expect this
840 instDeclCtxt2 dfun_ty
841 = inst_decl_ctxt (ppr (mkClassPred cls tys))
843 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
845 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
847 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
849 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
853 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
854 ptext SLIT("must be variables")
855 , ptext SLIT("Instead of a variable, found") <+> ppr ty
858 wrongATArgErr ty instTy =
859 sep [ ptext SLIT("Type indexes must match class instance head")
860 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>