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 -- Typecheck the instance type itself. We can't use
244 -- tcHsSigType, because it's not a valid user type.
245 ; kinded_ty <- kcHsSigType poly_ty
246 ; poly_ty' <- tcHsKindedType kinded_ty
247 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
249 -- Next, process any associated types.
250 ; idx_tycons <- mappM tcIdxTyInstDecl ats
252 -- Now, check the validity of the instance.
253 ; (clas, inst_tys) <- checkValidInstHead tau
254 ; checkValidInstance tyvars theta clas inst_tys
255 ; checkValidAndMissingATs clas (tyvars, inst_tys)
258 -- Finally, construct the Core representation of the instance.
259 -- (This no longer includes the associated types.)
260 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
261 ; overlap_flag <- getOverlapFlag
262 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
263 ispec = mkLocalInstance dfun overlap_flag
265 ; return ([InstInfo { iSpec = ispec,
266 iBinds = VanillaInst binds uprags }],
267 catMaybes idx_tycons)
270 -- We pass in the source form and the type checked form of the ATs. We
271 -- really need the source form only to be able to produce more informative
273 checkValidAndMissingATs :: Class
274 -> ([TyVar], [TcType]) -- instance types
275 -> [(LTyClDecl Name, -- source form of AT
276 Maybe TyThing)] -- Core form of AT
278 checkValidAndMissingATs clas inst_tys ats
279 = do { -- Issue a warning for each class AT that is not defined in this
281 ; let classDefATs = listToNameSet . map tyConName . classATs $ clas
282 definedATs = listToNameSet . map (tcdName.unLoc.fst) $ ats
283 omitted = classDefATs `minusNameSet` definedATs
284 ; warn <- doptM Opt_WarnMissingMethods
285 ; mapM_ (warnTc warn . omittedATWarn) (nameSetToList omitted)
287 -- Ensure that all AT indexes that correspond to class parameters
288 -- coincide with the types in the instance head. All remaining
289 -- AT arguments must be variables. Also raise an error for any
290 -- type instances that are not associated with this class.
291 ; mapM_ (checkIndexes clas inst_tys) ats
294 checkIndexes _ _ (hsAT, Nothing) =
295 return () -- skip, we already had an error here
296 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
297 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
298 checkIndexes' clas inst_tys hsAT
300 snd . fromJust . tyConFamInst_maybe $ tycon)
301 checkIndexes _ _ _ = panic "checkIndexes"
303 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
304 = let atName = tcdName . unLoc $ hsAT
306 setSrcSpan (getLoc hsAT) $
307 addErrCtxt (atInstCtxt atName) $
308 case find ((atName ==) . tyConName) (classATs clas) of
309 Nothing -> addErrTc $ badATErr clas atName -- not in this class
311 case assocTyConArgPoss_maybe atDecl of
312 Nothing -> panic "checkIndexes': AT has no args poss?!?"
315 -- The following is tricky! We need to deal with three
316 -- complications: (1) The AT possibly only uses a subset of
317 -- the class parameters as indexes and those it uses may be in
318 -- a different order; (2) the AT may have extra arguments,
319 -- which must be type variables; and (3) variables in AT and
320 -- instance head will be different `Name's even if their
321 -- source lexemes are identical.
323 -- Re (1), `poss' contains a permutation vector to extract the
324 -- class parameters in the right order.
326 -- Re (2), we wrap the (permuted) class parameters in a Maybe
327 -- type and use Nothing for any extra AT arguments. (First
328 -- equation of `checkIndex' below.)
330 -- Re (3), we replace any type variable in the AT parameters
331 -- that has the same source lexeme as some variable in the
332 -- instance types with the instance type variable sharing its
335 let relevantInstTys = map (instTys !!) poss
336 instArgs = map Just relevantInstTys ++
337 repeat Nothing -- extra arguments
338 renaming = substSameTyVar atTvs instTvs
340 zipWithM_ checkIndex (substTys renaming atTys) instArgs
342 checkIndex ty Nothing
343 | isTyVarTy ty = return ()
344 | otherwise = addErrTc $ mustBeVarArgErr ty
345 checkIndex ty (Just instTy)
346 | ty `tcEqType` instTy = return ()
347 | otherwise = addErrTc $ wrongATArgErr ty instTy
349 listToNameSet = addListToNameSet emptyNameSet
351 substSameTyVar [] _ = emptyTvSubst
352 substSameTyVar (tv:tvs) replacingTvs =
353 let replacement = case find (tv `sameLexeme`) replacingTvs of
354 Nothing -> mkTyVarTy tv
355 Just rtv -> mkTyVarTy rtv
357 tv1 `sameLexeme` tv2 =
358 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
360 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
364 %************************************************************************
366 \subsection{Type-checking instance declarations, pass 2}
368 %************************************************************************
371 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
372 -> TcM (LHsBinds Id, TcLclEnv)
373 -- (a) From each class declaration,
374 -- generate any default-method bindings
375 -- (b) From each instance decl
376 -- generate the dfun binding
378 tcInstDecls2 tycl_decls inst_decls
379 = do { -- (a) Default methods from class decls
380 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
381 filter (isClassDecl.unLoc) tycl_decls
382 ; tcExtendIdEnv (concat dm_ids_s) $ do
384 -- (b) instance declarations
385 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
388 ; let binds = unionManyBags dm_binds_s `unionBags`
389 unionManyBags inst_binds_s
390 ; tcl_env <- getLclEnv -- Default method Ids in here
391 ; returnM (binds, tcl_env) }
394 ======= New documentation starts here (Sept 92) ==============
396 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
397 the dictionary function for this instance declaration. For example
399 instance Foo a => Foo [a] where
403 might generate something like
405 dfun.Foo.List dFoo_a = let op1 x = ...
411 HOWEVER, if the instance decl has no context, then it returns a
412 bigger @HsBinds@ with declarations for each method. For example
414 instance Foo [a] where
420 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
421 const.Foo.op1.List a x = ...
422 const.Foo.op2.List a y = ...
424 This group may be mutually recursive, because (for example) there may
425 be no method supplied for op2 in which case we'll get
427 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
429 that is, the default method applied to the dictionary at this type.
431 What we actually produce in either case is:
433 AbsBinds [a] [dfun_theta_dicts]
434 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
435 { d = (sd1,sd2, ..., op1, op2, ...)
440 The "maybe" says that we only ask AbsBinds to make global constant methods
441 if the dfun_theta is empty.
444 For an instance declaration, say,
446 instance (C1 a, C2 b) => C (T a b) where
449 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
450 function whose type is
452 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
454 Notice that we pass it the superclass dictionaries at the instance type; this
455 is the ``Mark Jones optimisation''. The stuff before the "=>" here
456 is the @dfun_theta@ below.
458 First comes the easy case of a non-local instance decl.
462 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
463 -- Returns a binding for the dfun
465 ------------------------
466 -- Derived newtype instances; surprisingly tricky!
468 -- In the case of a newtype, things are rather easy
469 -- class Show a => Foo a b where ...
470 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
471 -- The newtype gives an FC axiom looking like
472 -- axiom CoT a :: T a :=: Tree [a]
473 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
475 -- So all need is to generate a binding looking like:
476 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
477 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
478 -- case df `cast` (Foo Int (sym (CoT a))) of
479 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
481 -- If there are no superclasses, matters are simpler, because we don't need the case
482 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
484 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
485 = do { let dfun_id = instanceDFunId ispec
486 rigid_info = InstSkol
487 origin = SigOrigin rigid_info
488 inst_ty = idType dfun_id
489 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
490 -- inst_head_ty is a PredType
492 ; inst_loc <- getInstLoc origin
493 ; (rep_dict_id : sc_dict_ids, wrap_fn, sc_binds)
494 <- make_wrapper inst_loc tvs theta mb_preds
495 -- Here, we are relying on the order of dictionary
496 -- arguments built by NewTypeDerived in TcDeriv;
497 -- namely, that the rep_dict_id comes first
499 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
500 cls_tycon = classTyCon cls
501 the_coercion = make_coercion cls_tycon cls_inst_tys
502 coerced_rep_dict = mkHsWrap the_coercion (HsVar rep_dict_id)
504 ; body <- make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
506 ; return (sc_binds `snocBag` (noLoc $ VarBind dfun_id $ noLoc $ mkHsWrap wrap_fn body)) }
509 -----------------------
511 -- We distinguish two cases:
512 -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
513 -- and the new dict can just be a constant
514 -- (mb_preds = Just preds)
515 -- (b) there are tyvars, so we must make a dict *fun*
516 -- (mb_preds = Nothing)
517 -- See the defn of NewTypeDerived for the meaning of mb_preds
518 make_wrapper inst_loc tvs theta (Just preds) -- Case (a)
519 = ASSERT( null tvs && null theta )
520 do { dicts <- newDictBndrs inst_loc preds
521 ; sc_binds <- addErrCtxt superClassCtxt $
522 tcSimplifySuperClasses inst_loc [] dicts
523 -- Use tcSimplifySuperClasses to avoid creating loops, for the
524 -- same reason as Note [SUPERCLASS-LOOP 1] in TcSimplify
525 ; return (map instToId dicts, idHsWrapper, sc_binds) }
527 make_wrapper inst_loc tvs theta Nothing -- Case (b)
528 = do { dicts <- newDictBndrs inst_loc theta
529 ; let dict_ids = map instToId dicts
530 ; return (dict_ids, mkWpTyLams tvs <.> mkWpLams dict_ids, emptyBag) }
532 -----------------------
534 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
535 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
536 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
537 -- where rep_ty is the (eta-reduced) type rep of T
538 -- So we just replace T with CoT, and insert a 'sym'
539 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
541 make_coercion cls_tycon cls_inst_tys
542 | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
543 , (tycon, tc_args) <- tcSplitTyConApp last_ty -- Should not fail
544 , Just co_con <- newTyConCo_maybe tycon
545 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
546 = WpCo (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
547 | otherwise -- The newtype is transparent; no need for a cast
550 -----------------------
552 -- Two cases; see Note [Newtype deriving superclasses] in TcDeriv.lhs
553 -- (a) no superclasses; then we can just use the coerced dict
554 -- (b) one or more superclasses; then new need to do the unpack/repack
556 make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
557 | null sc_dict_ids -- Case (a)
558 = return coerced_rep_dict
559 | otherwise -- Case (b)
560 = do { op_ids <- newSysLocalIds FSLIT("op") op_tys
561 ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
562 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
563 pat_dicts = dummy_sc_dict_ids,
564 pat_binds = emptyLHsBinds,
565 pat_args = PrefixCon (map nlVarPat op_ids),
567 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
568 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
569 map HsVar (sc_dict_ids ++ op_ids)
571 -- Warning: this HsCase scrutinises a value with a PredTy, which is
572 -- never otherwise seen in Haskell source code. It'd be
573 -- nicer to generate Core directly!
574 ; return (HsCase (noLoc coerced_rep_dict) $
575 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
577 pat_ty = mkTyConApp cls_tycon cls_inst_tys
578 cls_data_con = head (tyConDataCons cls_tycon)
579 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
580 op_tys = dropList sc_dict_ids cls_arg_tys
582 ------------------------
583 -- Ordinary instances
585 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
587 dfun_id = instanceDFunId ispec
588 rigid_info = InstSkol
589 inst_ty = idType dfun_id
591 -- Prime error recovery
592 recoverM (returnM emptyLHsBinds) $
593 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
594 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
596 -- Instantiate the instance decl with skolem constants
597 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
598 -- These inst_tyvars' scope over the 'where' part
599 -- Those tyvars are inside the dfun_id's type, which is a bit
600 -- bizarre, but OK so long as you realise it!
602 (clas, inst_tys') = tcSplitDFunHead inst_head'
603 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
605 -- Instantiate the super-class context with inst_tys
606 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
607 origin = SigOrigin rigid_info
609 -- Create dictionary Ids from the specified instance contexts.
610 getInstLoc InstScOrigin `thenM` \ sc_loc ->
611 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
612 getInstLoc origin `thenM` \ inst_loc ->
613 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
614 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
615 -- Default-method Ids may be mentioned in synthesised RHSs,
616 -- but they'll already be in the environment.
618 -- Typecheck the methods
619 let -- These insts are in scope; quite a few, eh?
620 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
622 tcMethods origin clas inst_tyvars'
623 dfun_theta' inst_tys' avail_insts
624 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
626 -- Figure out bindings for the superclass context
627 -- Don't include this_dict in the 'givens', else
628 -- sc_dicts get bound by just selecting from this_dict!!
629 addErrCtxt superClassCtxt
630 (tcSimplifySuperClasses inst_loc
631 dfun_arg_dicts sc_dicts) `thenM` \ sc_binds ->
633 -- It's possible that the superclass stuff might unified one
634 -- of the inst_tyavars' with something in the envt
635 checkSigTyVars inst_tyvars' `thenM_`
637 -- Deal with 'SPECIALISE instance' pragmas
638 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
640 -- Create the result bindings
642 dict_constr = classDataCon clas
643 scs_and_meths = map instToId sc_dicts ++ meth_ids
644 this_dict_id = instToId this_dict
645 inline_prag | null dfun_arg_dicts = []
646 | otherwise = [InlinePrag (Inline AlwaysActive True)]
647 -- Always inline the dfun; this is an experimental decision
648 -- because it makes a big performance difference sometimes.
649 -- Often it means we can do the method selection, and then
650 -- inline the method as well. Marcin's idea; see comments below.
652 -- BUT: don't inline it if it's a constant dictionary;
653 -- we'll get all the benefit without inlining, and we get
654 -- a **lot** of code duplication if we inline it
656 -- See Note [Inline dfuns] below
659 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
660 -- We don't produce a binding for the dict_constr; instead we
661 -- rely on the simplifier to unfold this saturated application
662 -- We do this rather than generate an HsCon directly, because
663 -- it means that the special cases (e.g. dictionary with only one
664 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
665 -- than needing to be repeated here.
667 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
668 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
670 main_bind = noLoc $ AbsBinds
672 (map instToId dfun_arg_dicts)
673 [(inst_tyvars', dfun_id, this_dict_id,
674 inline_prag ++ prags)]
677 showLIE (text "instance") `thenM_`
678 returnM (unitBag main_bind)
681 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
682 avail_insts op_items monobinds uprags
683 = -- Check that all the method bindings come from this class
685 sel_names = [idName sel_id | (sel_id, _) <- op_items]
686 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
688 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
690 -- Make the method bindings
692 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
694 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
696 -- And type check them
697 -- It's really worth making meth_insts available to the tcMethodBind
698 -- Consider instance Monad (ST s) where
699 -- {-# INLINE (>>) #-}
700 -- (>>) = ...(>>=)...
701 -- If we don't include meth_insts, we end up with bindings like this:
702 -- rec { dict = MkD then bind ...
703 -- then = inline_me (... (GHC.Base.>>= dict) ...)
705 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
706 -- and (b) the inline_me prevents us inlining the >>= selector, which
707 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
708 -- is not inlined across modules. Rather ironic since this does not
709 -- happen without the INLINE pragma!
711 -- Solution: make meth_insts available, so that 'then' refers directly
712 -- to the local 'bind' rather than going via the dictionary.
714 -- BUT WATCH OUT! If the method type mentions the class variable, then
715 -- this optimisation is not right. Consider
719 -- instance C Int where
721 -- The occurrence of 'op' on the rhs gives rise to a constraint
723 -- The trouble is that the 'meth_inst' for op, which is 'available', also
724 -- looks like 'op at Int'. But they are not the same.
726 prag_fn = mkPragFun uprags
727 all_insts = avail_insts ++ catMaybes meth_insts
728 sig_fn n = Just [] -- No scoped type variables, but every method has
729 -- a type signature, in effect, so that we check
730 -- the method has the right type
731 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
732 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
735 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
737 returnM (meth_ids, unionManyBags meth_binds_s)
741 ------------------------------
742 [Inline dfuns] Inlining dfuns unconditionally
743 ------------------------------
745 The code above unconditionally inlines dict funs. Here's why.
746 Consider this program:
748 test :: Int -> Int -> Bool
749 test x y = (x,y) == (y,x) || test y x
750 -- Recursive to avoid making it inline.
752 This needs the (Eq (Int,Int)) instance. If we inline that dfun
753 the code we end up with is good:
756 \r -> case ==# [ww ww1] of wild {
757 PrelBase.False -> Test.$wtest ww1 ww;
759 case ==# [ww1 ww] of wild1 {
760 PrelBase.False -> Test.$wtest ww1 ww;
761 PrelBase.True -> PrelBase.True [];
764 Test.test = \r [w w1]
767 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
770 If we don't inline the dfun, the code is not nearly as good:
772 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
773 PrelBase.:DEq tpl1 tpl2 -> tpl2;
778 let { y = PrelBase.I#! [ww1]; } in
779 let { x = PrelBase.I#! [ww]; } in
780 let { sat_slx = PrelTup.(,)! [y x]; } in
781 let { sat_sly = PrelTup.(,)! [x y];
783 case == sat_sly sat_slx of wild {
784 PrelBase.False -> Test.$wtest ww1 ww;
785 PrelBase.True -> PrelBase.True [];
792 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
795 Why doesn't GHC inline $fEq? Because it looks big:
797 PrelTup.zdfEqZ1T{-rcX-}
798 = \ @ a{-reT-} :: * @ b{-reS-} :: *
799 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
800 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
802 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
803 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
805 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
806 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
808 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
809 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
810 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
812 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
814 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
816 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
817 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
821 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
822 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
823 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
824 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
826 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
828 and it's not as bad as it seems, because it's further dramatically
829 simplified: only zeze2 is extracted and its body is simplified.
832 %************************************************************************
834 \subsection{Error messages}
836 %************************************************************************
839 instDeclCtxt1 hs_inst_ty
840 = inst_decl_ctxt (case unLoc hs_inst_ty of
841 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
842 HsPredTy pred -> ppr pred
843 other -> ppr hs_inst_ty) -- Don't expect this
844 instDeclCtxt2 dfun_ty
845 = inst_decl_ctxt (ppr (mkClassPred cls tys))
847 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
849 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
851 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
853 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
857 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
858 ptext SLIT("must be variables")
859 , ptext SLIT("Instead of a variable, found") <+> ppr ty
862 wrongATArgErr ty instTy =
863 sep [ ptext SLIT("Type indexes must match class instance head")
864 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>