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 and family instance declarations
151 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
152 ; local_info_tycons <- mappM tcLocalInstDecl1 inst_decls
153 ; idx_tycons <- mappM tcIdxTyInstDeclTL idxty_decls
155 ; let { (local_infos,
156 at_tycons) = unzip local_info_tycons
157 ; local_info = concat local_infos
158 ; at_idx_tycon = concat at_tycons ++ catMaybes idx_tycons
159 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
160 ; implicit_things = concatMap implicitTyThings at_idx_tycon
163 -- (2) Add the tycons of indexed types and their implicit
164 -- tythings to the global environment
165 ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do {
167 -- (3) Instances from generic class declarations
168 ; generic_inst_info <- getGenericInstances clas_decls
170 -- Next, construct the instance environment so far, consisting
172 -- a) local instance decls
173 -- b) generic instances
174 -- c) local family instance decls
175 ; addInsts local_info $ do {
176 ; addInsts generic_inst_info $ do {
177 ; addFamInsts at_idx_tycon $ do {
179 -- (4) Compute instances from "deriving" clauses;
180 -- This stuff computes a context for the derived instance
181 -- decl, so it needs to know about all the instances possible
182 -- NB: class instance declarations can contain derivings as
183 -- part of associated data type declarations
184 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls
186 ; addInsts deriv_inst_info $ do {
188 ; gbl_env <- getGblEnv
190 generic_inst_info ++ deriv_inst_info ++ local_info,
194 -- Make sure that toplevel type instance are not for associated types.
195 -- !!!TODO: Need to perform this check for the TyThing of type functions,
197 tcIdxTyInstDeclTL ldecl@(L loc decl) =
198 do { tything <- tcFamInstDecl ldecl
200 when (isAssocFamily tything) $
201 addErr $ assocInClassErr (tcdName decl)
204 isAssocFamily (Just (ATyCon tycon)) =
205 case tyConFamInst_maybe tycon of
206 Nothing -> panic "isAssocFamily: no family?!?"
207 Just (fam, _) -> isTyConAssoc fam
208 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
209 isAssocFamily Nothing = False
211 assocInClassErr name =
212 ptext SLIT("Associated type") <+> quotes (ppr name) <+>
213 ptext SLIT("must be inside a class instance")
215 addInsts :: [InstInfo] -> TcM a -> TcM a
216 addInsts infos thing_inside
217 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
219 addFamInsts :: [TyThing] -> TcM a -> TcM a
220 addFamInsts tycons thing_inside
221 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
223 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
224 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
229 tcLocalInstDecl1 :: LInstDecl Name
230 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
231 -- A source-file instance declaration
232 -- Type-check all the stuff before the "where"
234 -- We check for respectable instance type, and context
235 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
236 = -- Prime error recovery, set source location
237 recoverM (returnM ([], [])) $
239 addErrCtxt (instDeclCtxt1 poly_ty) $
241 do { is_boot <- tcIsHsBoot
242 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
245 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
247 -- Next, process any associated types.
248 ; idx_tycons <- mappM tcFamInstDecl ats
250 -- Now, check the validity of the instance.
251 ; (clas, inst_tys) <- checkValidInstHead tau
252 ; checkValidInstance tyvars theta clas inst_tys
253 ; checkValidAndMissingATs clas (tyvars, inst_tys)
256 -- Finally, construct the Core representation of the instance.
257 -- (This no longer includes the associated types.)
258 ; dfun_name <- newDFunName clas inst_tys loc
259 ; overlap_flag <- getOverlapFlag
260 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
261 ispec = mkLocalInstance dfun overlap_flag
263 ; return ([InstInfo { iSpec = ispec,
264 iBinds = VanillaInst binds uprags }],
265 catMaybes idx_tycons)
268 -- We pass in the source form and the type checked form of the ATs. We
269 -- really need the source form only to be able to produce more informative
271 checkValidAndMissingATs :: Class
272 -> ([TyVar], [TcType]) -- instance types
273 -> [(LTyClDecl Name, -- source form of AT
274 Maybe TyThing)] -- Core form of AT
276 checkValidAndMissingATs clas inst_tys ats
277 = do { -- Issue a warning for each class AT that is not defined in this
279 ; let class_ats = map tyConName (classATs clas)
280 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
281 omitted = filterOut (`elemNameSet` defined_ats) class_ats
282 ; warn <- doptM Opt_WarnMissingMethods
283 ; mapM_ (warnTc warn . omittedATWarn) omitted
285 -- Ensure that all AT indexes that correspond to class parameters
286 -- coincide with the types in the instance head. All remaining
287 -- AT arguments must be variables. Also raise an error for any
288 -- type instances that are not associated with this class.
289 ; mapM_ (checkIndexes clas inst_tys) ats
292 checkIndexes _ _ (hsAT, Nothing) =
293 return () -- skip, we already had an error here
294 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
295 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
296 checkIndexes' clas inst_tys hsAT
298 snd . fromJust . tyConFamInst_maybe $ tycon)
299 checkIndexes _ _ _ = panic "checkIndexes"
301 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
302 = let atName = tcdName . unLoc $ hsAT
304 setSrcSpan (getLoc hsAT) $
305 addErrCtxt (atInstCtxt atName) $
306 case find ((atName ==) . tyConName) (classATs clas) of
307 Nothing -> addErrTc $ badATErr clas atName -- not in this class
309 case assocTyConArgPoss_maybe atDecl of
310 Nothing -> panic "checkIndexes': AT has no args poss?!?"
313 -- The following is tricky! We need to deal with three
314 -- complications: (1) The AT possibly only uses a subset of
315 -- the class parameters as indexes and those it uses may be in
316 -- a different order; (2) the AT may have extra arguments,
317 -- which must be type variables; and (3) variables in AT and
318 -- instance head will be different `Name's even if their
319 -- source lexemes are identical.
321 -- Re (1), `poss' contains a permutation vector to extract the
322 -- class parameters in the right order.
324 -- Re (2), we wrap the (permuted) class parameters in a Maybe
325 -- type and use Nothing for any extra AT arguments. (First
326 -- equation of `checkIndex' below.)
328 -- Re (3), we replace any type variable in the AT parameters
329 -- that has the same source lexeme as some variable in the
330 -- instance types with the instance type variable sharing its
333 let relevantInstTys = map (instTys !!) poss
334 instArgs = map Just relevantInstTys ++
335 repeat Nothing -- extra arguments
336 renaming = substSameTyVar atTvs instTvs
338 zipWithM_ checkIndex (substTys renaming atTys) instArgs
340 checkIndex ty Nothing
341 | isTyVarTy ty = return ()
342 | otherwise = addErrTc $ mustBeVarArgErr ty
343 checkIndex ty (Just instTy)
344 | ty `tcEqType` instTy = return ()
345 | otherwise = addErrTc $ wrongATArgErr ty instTy
347 listToNameSet = addListToNameSet emptyNameSet
349 substSameTyVar [] _ = emptyTvSubst
350 substSameTyVar (tv:tvs) replacingTvs =
351 let replacement = case find (tv `sameLexeme`) replacingTvs of
352 Nothing -> mkTyVarTy tv
353 Just rtv -> mkTyVarTy rtv
355 tv1 `sameLexeme` tv2 =
356 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
358 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
362 %************************************************************************
364 \subsection{Type-checking instance declarations, pass 2}
366 %************************************************************************
369 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
370 -> TcM (LHsBinds Id, TcLclEnv)
371 -- (a) From each class declaration,
372 -- generate any default-method bindings
373 -- (b) From each instance decl
374 -- generate the dfun binding
376 tcInstDecls2 tycl_decls inst_decls
377 = do { -- (a) Default methods from class decls
378 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
379 filter (isClassDecl.unLoc) tycl_decls
380 ; tcExtendIdEnv (concat dm_ids_s) $ do
382 -- (b) instance declarations
383 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
386 ; let binds = unionManyBags dm_binds_s `unionBags`
387 unionManyBags inst_binds_s
388 ; tcl_env <- getLclEnv -- Default method Ids in here
389 ; returnM (binds, tcl_env) }
392 ======= New documentation starts here (Sept 92) ==============
394 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
395 the dictionary function for this instance declaration. For example
397 instance Foo a => Foo [a] where
401 might generate something like
403 dfun.Foo.List dFoo_a = let op1 x = ...
409 HOWEVER, if the instance decl has no context, then it returns a
410 bigger @HsBinds@ with declarations for each method. For example
412 instance Foo [a] where
418 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
419 const.Foo.op1.List a x = ...
420 const.Foo.op2.List a y = ...
422 This group may be mutually recursive, because (for example) there may
423 be no method supplied for op2 in which case we'll get
425 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
427 that is, the default method applied to the dictionary at this type.
429 What we actually produce in either case is:
431 AbsBinds [a] [dfun_theta_dicts]
432 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
433 { d = (sd1,sd2, ..., op1, op2, ...)
438 The "maybe" says that we only ask AbsBinds to make global constant methods
439 if the dfun_theta is empty.
442 For an instance declaration, say,
444 instance (C1 a, C2 b) => C (T a b) where
447 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
448 function whose type is
450 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
452 Notice that we pass it the superclass dictionaries at the instance type; this
453 is the ``Mark Jones optimisation''. The stuff before the "=>" here
454 is the @dfun_theta@ below.
456 First comes the easy case of a non-local instance decl.
460 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
461 -- Returns a binding for the dfun
463 ------------------------
464 -- Derived newtype instances; surprisingly tricky!
466 -- In the case of a newtype, things are rather easy
467 -- class Show a => Foo a b where ...
468 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
469 -- The newtype gives an FC axiom looking like
470 -- axiom CoT a :: T a :=: Tree [a]
471 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
473 -- So all need is to generate a binding looking like:
474 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
475 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
476 -- case df `cast` (Foo Int (sym (CoT a))) of
477 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
479 -- If there are no superclasses, matters are simpler, because we don't need the case
480 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
482 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
483 = do { let dfun_id = instanceDFunId ispec
484 rigid_info = InstSkol
485 origin = SigOrigin rigid_info
486 inst_ty = idType dfun_id
487 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
488 -- inst_head_ty is a PredType
490 ; inst_loc <- getInstLoc origin
491 ; (rep_dict_id : sc_dict_ids, wrap_fn, sc_binds)
492 <- make_wrapper inst_loc tvs theta mb_preds
493 -- Here, we are relying on the order of dictionary
494 -- arguments built by NewTypeDerived in TcDeriv;
495 -- namely, that the rep_dict_id comes first
497 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
498 cls_tycon = classTyCon cls
499 the_coercion = make_coercion cls_tycon cls_inst_tys
500 coerced_rep_dict = mkHsWrap the_coercion (HsVar rep_dict_id)
502 ; body <- make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
504 ; return (sc_binds `snocBag` (noLoc $ VarBind dfun_id $ noLoc $ mkHsWrap wrap_fn body)) }
507 -----------------------
509 -- We distinguish two cases:
510 -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
511 -- and the new dict can just be a constant
512 -- (mb_preds = Just preds)
513 -- (b) there are tyvars, so we must make a dict *fun*
514 -- (mb_preds = Nothing)
515 -- See the defn of NewTypeDerived for the meaning of mb_preds
516 make_wrapper inst_loc tvs theta (Just preds) -- Case (a)
517 = ASSERT( null tvs && null theta )
518 do { dicts <- newDictBndrs inst_loc preds
519 ; sc_binds <- addErrCtxt superClassCtxt $
520 tcSimplifySuperClasses inst_loc [] dicts
521 -- Use tcSimplifySuperClasses to avoid creating loops, for the
522 -- same reason as Note [SUPERCLASS-LOOP 1] in TcSimplify
523 ; return (map instToId dicts, idHsWrapper, sc_binds) }
525 make_wrapper inst_loc tvs theta Nothing -- Case (b)
526 = do { dicts <- newDictBndrs inst_loc theta
527 ; let dict_ids = map instToId dicts
528 ; return (dict_ids, mkWpTyLams tvs <.> mkWpLams dict_ids, emptyBag) }
530 -----------------------
532 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
533 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
534 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
535 -- where rep_ty is the (eta-reduced) type rep of T
536 -- So we just replace T with CoT, and insert a 'sym'
537 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
539 make_coercion cls_tycon cls_inst_tys
540 | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
541 , (tycon, tc_args) <- tcSplitTyConApp last_ty -- Should not fail
542 , Just co_con <- newTyConCo_maybe tycon
543 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
544 = WpCo (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
545 | otherwise -- The newtype is transparent; no need for a cast
548 -----------------------
550 -- Two cases; see Note [Newtype deriving superclasses] in TcDeriv.lhs
551 -- (a) no superclasses; then we can just use the coerced dict
552 -- (b) one or more superclasses; then new need to do the unpack/repack
554 make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
555 | null sc_dict_ids -- Case (a)
556 = return coerced_rep_dict
557 | otherwise -- Case (b)
558 = do { op_ids <- newSysLocalIds FSLIT("op") op_tys
559 ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
560 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
561 pat_dicts = dummy_sc_dict_ids,
562 pat_binds = emptyLHsBinds,
563 pat_args = PrefixCon (map nlVarPat op_ids),
565 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
566 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
567 map HsVar (sc_dict_ids ++ op_ids)
569 -- Warning: this HsCase scrutinises a value with a PredTy, which is
570 -- never otherwise seen in Haskell source code. It'd be
571 -- nicer to generate Core directly!
572 ; return (HsCase (noLoc coerced_rep_dict) $
573 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
575 pat_ty = mkTyConApp cls_tycon cls_inst_tys
576 cls_data_con = head (tyConDataCons cls_tycon)
577 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
578 op_tys = dropList sc_dict_ids cls_arg_tys
580 ------------------------
581 -- Ordinary instances
583 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
585 dfun_id = instanceDFunId ispec
586 rigid_info = InstSkol
587 inst_ty = idType dfun_id
588 loc = srcLocSpan (getSrcLoc dfun_id)
590 -- Prime error recovery
591 recoverM (returnM emptyLHsBinds) $
593 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
595 -- Instantiate the instance decl with skolem constants
596 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
597 -- These inst_tyvars' scope over the 'where' part
598 -- Those tyvars are inside the dfun_id's type, which is a bit
599 -- bizarre, but OK so long as you realise it!
601 (clas, inst_tys') = tcSplitDFunHead inst_head'
602 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
604 -- Instantiate the super-class context with inst_tys
605 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
606 origin = SigOrigin rigid_info
608 -- Create dictionary Ids from the specified instance contexts.
609 getInstLoc InstScOrigin `thenM` \ sc_loc ->
610 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
611 getInstLoc origin `thenM` \ inst_loc ->
612 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
613 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
614 -- Default-method Ids may be mentioned in synthesised RHSs,
615 -- but they'll already be in the environment.
617 -- Typecheck the methods
618 let -- These insts are in scope; quite a few, eh?
619 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
621 tcMethods origin clas inst_tyvars'
622 dfun_theta' inst_tys' avail_insts
623 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
625 -- Figure out bindings for the superclass context
626 -- Don't include this_dict in the 'givens', else
627 -- sc_dicts get bound by just selecting from this_dict!!
628 addErrCtxt superClassCtxt
629 (tcSimplifySuperClasses inst_loc
630 dfun_arg_dicts sc_dicts) `thenM` \ sc_binds ->
632 -- It's possible that the superclass stuff might unified one
633 -- of the inst_tyavars' with something in the envt
634 checkSigTyVars inst_tyvars' `thenM_`
636 -- Deal with 'SPECIALISE instance' pragmas
637 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
639 -- Create the result bindings
641 dict_constr = classDataCon clas
642 scs_and_meths = map instToId sc_dicts ++ meth_ids
643 this_dict_id = instToId this_dict
644 inline_prag | null dfun_arg_dicts = []
645 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
646 -- Always inline the dfun; this is an experimental decision
647 -- because it makes a big performance difference sometimes.
648 -- Often it means we can do the method selection, and then
649 -- inline the method as well. Marcin's idea; see comments below.
651 -- BUT: don't inline it if it's a constant dictionary;
652 -- we'll get all the benefit without inlining, and we get
653 -- a **lot** of code duplication if we inline it
655 -- See Note [Inline dfuns] below
658 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
659 -- We don't produce a binding for the dict_constr; instead we
660 -- rely on the simplifier to unfold this saturated application
661 -- We do this rather than generate an HsCon directly, because
662 -- it means that the special cases (e.g. dictionary with only one
663 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
664 -- than needing to be repeated here.
666 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
667 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
669 main_bind = noLoc $ AbsBinds
671 (map instToId dfun_arg_dicts)
672 [(inst_tyvars', dfun_id, this_dict_id,
673 inline_prag ++ prags)]
676 showLIE (text "instance") `thenM_`
677 returnM (unitBag main_bind)
680 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
681 avail_insts op_items monobinds uprags
682 = -- Check that all the method bindings come from this class
684 sel_names = [idName sel_id | (sel_id, _) <- op_items]
685 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
687 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
689 -- Make the method bindings
691 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
693 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
695 -- And type check them
696 -- It's really worth making meth_insts available to the tcMethodBind
697 -- Consider instance Monad (ST s) where
698 -- {-# INLINE (>>) #-}
699 -- (>>) = ...(>>=)...
700 -- If we don't include meth_insts, we end up with bindings like this:
701 -- rec { dict = MkD then bind ...
702 -- then = inline_me (... (GHC.Base.>>= dict) ...)
704 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
705 -- and (b) the inline_me prevents us inlining the >>= selector, which
706 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
707 -- is not inlined across modules. Rather ironic since this does not
708 -- happen without the INLINE pragma!
710 -- Solution: make meth_insts available, so that 'then' refers directly
711 -- to the local 'bind' rather than going via the dictionary.
713 -- BUT WATCH OUT! If the method type mentions the class variable, then
714 -- this optimisation is not right. Consider
718 -- instance C Int where
720 -- The occurrence of 'op' on the rhs gives rise to a constraint
722 -- The trouble is that the 'meth_inst' for op, which is 'available', also
723 -- looks like 'op at Int'. But they are not the same.
725 prag_fn = mkPragFun uprags
726 all_insts = avail_insts ++ catMaybes meth_insts
727 sig_fn n = Just [] -- No scoped type variables, but every method has
728 -- a type signature, in effect, so that we check
729 -- the method has the right type
730 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
731 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
734 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
736 returnM (meth_ids, unionManyBags meth_binds_s)
740 ------------------------------
741 [Inline dfuns] Inlining dfuns unconditionally
742 ------------------------------
744 The code above unconditionally inlines dict funs. Here's why.
745 Consider this program:
747 test :: Int -> Int -> Bool
748 test x y = (x,y) == (y,x) || test y x
749 -- Recursive to avoid making it inline.
751 This needs the (Eq (Int,Int)) instance. If we inline that dfun
752 the code we end up with is good:
755 \r -> case ==# [ww ww1] of wild {
756 PrelBase.False -> Test.$wtest ww1 ww;
758 case ==# [ww1 ww] of wild1 {
759 PrelBase.False -> Test.$wtest ww1 ww;
760 PrelBase.True -> PrelBase.True [];
763 Test.test = \r [w w1]
766 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
769 If we don't inline the dfun, the code is not nearly as good:
771 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
772 PrelBase.:DEq tpl1 tpl2 -> tpl2;
777 let { y = PrelBase.I#! [ww1]; } in
778 let { x = PrelBase.I#! [ww]; } in
779 let { sat_slx = PrelTup.(,)! [y x]; } in
780 let { sat_sly = PrelTup.(,)! [x y];
782 case == sat_sly sat_slx of wild {
783 PrelBase.False -> Test.$wtest ww1 ww;
784 PrelBase.True -> PrelBase.True [];
791 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
794 Why doesn't GHC inline $fEq? Because it looks big:
796 PrelTup.zdfEqZ1T{-rcX-}
797 = \ @ a{-reT-} :: * @ b{-reS-} :: *
798 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
799 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
801 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
802 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
804 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
805 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
807 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
808 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
809 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
811 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
813 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
815 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
816 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
820 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
821 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
822 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
823 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
825 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
827 and it's not as bad as it seems, because it's further dramatically
828 simplified: only zeze2 is extracted and its body is simplified.
831 %************************************************************************
833 \subsection{Error messages}
835 %************************************************************************
838 instDeclCtxt1 hs_inst_ty
839 = inst_decl_ctxt (case unLoc hs_inst_ty of
840 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
841 HsPredTy pred -> ppr pred
842 other -> ppr hs_inst_ty) -- Don't expect this
843 instDeclCtxt2 dfun_ty
844 = inst_decl_ctxt (ppr (mkClassPred cls tys))
846 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
848 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
850 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
852 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
856 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
857 ptext SLIT("must be variables")
858 , ptext SLIT("Instead of a variable, found") <+> ppr ty
861 wrongATArgErr ty instTy =
862 sep [ ptext SLIT("Type indexes must match class instance head")
863 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>