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"
50 import Control.Monad hiding (zipWithM_, mapAndUnzipM)
54 Typechecking instance declarations is done in two passes. The first
55 pass, made by @tcInstDecls1@, collects information to be used in the
58 This pre-processed info includes the as-yet-unprocessed bindings
59 inside the instance declaration. These are type-checked in the second
60 pass, when the class-instance envs and GVE contain all the info from
61 all the instance and value decls. Indeed that's the reason we need
62 two passes over the instance decls.
64 Here is the overall algorithm.
65 Assume that we have an instance declaration
67 instance c => k (t tvs) where b
71 $LIE_c$ is the LIE for the context of class $c$
73 $betas_bar$ is the free variables in the class method type, excluding the
76 $LIE_cop$ is the LIE constraining a particular class method
78 $tau_cop$ is the tau type of a class method
80 $LIE_i$ is the LIE for the context of instance $i$
82 $X$ is the instance constructor tycon
84 $gammas_bar$ is the set of type variables of the instance
86 $LIE_iop$ is the LIE for a particular class method instance
88 $tau_iop$ is the tau type for this instance of a class method
90 $alpha$ is the class variable
92 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
94 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
97 ToDo: Update the list above with names actually in the code.
101 First, make the LIEs for the class and instance contexts, which means
102 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
103 and make LIElistI and LIEI.
105 Then process each method in turn.
107 order the instance methods according to the ordering of the class methods
109 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
111 Create final dictionary function from bindings generated already
113 df = lambda inst_tyvars
120 in <op1,op2,...,opn,sd1,...,sdm>
122 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
123 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
127 %************************************************************************
129 \subsection{Extracting instance decls}
131 %************************************************************************
133 Gather up the instance declarations from their various sources
136 tcInstDecls1 -- Deal with both source-code and imported instance decls
137 :: [LTyClDecl Name] -- For deriving stuff
138 -> [LInstDecl Name] -- Source code instance decls
139 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
140 -> TcM (TcGblEnv, -- The full inst env
141 [InstInfo], -- Source-code instance decls to process;
142 -- contains all dfuns for this module
143 HsValBinds Name) -- Supporting bindings for derived instances
145 tcInstDecls1 tycl_decls inst_decls deriv_decls
147 do { -- Stop if addInstInfos etc discovers any errors
148 -- (they recover, so that we get more than one error each
151 -- (1) Do class and family instance declarations
152 ; let { idxty_decls = filter (isFamInstDecl . 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 -- NB: class instance declarations can contain derivings as
184 -- part of associated data type declarations
185 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls
187 ; addInsts deriv_inst_info $ do {
189 ; gbl_env <- getGblEnv
191 generic_inst_info ++ deriv_inst_info ++ local_info,
195 -- Make sure that toplevel type instance are not for associated types.
196 -- !!!TODO: Need to perform this check for the TyThing of type functions,
198 tcIdxTyInstDeclTL ldecl@(L loc decl) =
199 do { tything <- tcFamInstDecl ldecl
201 when (isAssocFamily tything) $
202 addErr $ assocInClassErr (tcdName decl)
205 isAssocFamily (Just (ATyCon tycon)) =
206 case tyConFamInst_maybe tycon of
207 Nothing -> panic "isAssocFamily: no family?!?"
208 Just (fam, _) -> isTyConAssoc fam
209 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
210 isAssocFamily Nothing = False
212 assocInClassErr name =
213 ptext SLIT("Associated type") <+> quotes (ppr name) <+>
214 ptext SLIT("must be inside a class instance")
216 addInsts :: [InstInfo] -> TcM a -> TcM a
217 addInsts infos thing_inside
218 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
220 addFamInsts :: [TyThing] -> TcM a -> TcM a
221 addFamInsts tycons thing_inside
222 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
224 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
225 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
230 tcLocalInstDecl1 :: LInstDecl Name
231 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
232 -- A source-file instance declaration
233 -- Type-check all the stuff before the "where"
235 -- We check for respectable instance type, and context
236 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
237 = -- Prime error recovery, set source location
238 recoverM (returnM ([], [])) $
240 addErrCtxt (instDeclCtxt1 poly_ty) $
242 do { is_boot <- tcIsHsBoot
243 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
246 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
248 -- Next, process any associated types.
249 ; idx_tycons <- mappM tcFamInstDecl ats
251 -- Now, check the validity of the instance.
252 ; (clas, inst_tys) <- checkValidInstHead tau
253 ; checkValidInstance tyvars theta clas inst_tys
254 ; checkValidAndMissingATs clas (tyvars, inst_tys)
257 -- Finally, construct the Core representation of the instance.
258 -- (This no longer includes the associated types.)
259 ; dfun_name <- newDFunName clas inst_tys loc
260 ; overlap_flag <- getOverlapFlag
261 ; let (eq_theta,dict_theta) = partition isEqPred theta
262 theta' = eq_theta ++ dict_theta
263 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
264 ispec = mkLocalInstance dfun overlap_flag
266 ; return ([InstInfo { iSpec = ispec,
267 iBinds = VanillaInst binds uprags }],
268 catMaybes idx_tycons)
271 -- We pass in the source form and the type checked form of the ATs. We
272 -- really need the source form only to be able to produce more informative
274 checkValidAndMissingATs :: Class
275 -> ([TyVar], [TcType]) -- instance types
276 -> [(LTyClDecl Name, -- source form of AT
277 Maybe TyThing)] -- Core form of AT
279 checkValidAndMissingATs clas inst_tys ats
280 = do { -- Issue a warning for each class AT that is not defined in this
282 ; let class_ats = map tyConName (classATs clas)
283 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
284 omitted = filterOut (`elemNameSet` defined_ats) class_ats
285 ; warn <- doptM Opt_WarnMissingMethods
286 ; mapM_ (warnTc warn . omittedATWarn) omitted
288 -- Ensure that all AT indexes that correspond to class parameters
289 -- coincide with the types in the instance head. All remaining
290 -- AT arguments must be variables. Also raise an error for any
291 -- type instances that are not associated with this class.
292 ; mapM_ (checkIndexes clas inst_tys) ats
295 checkIndexes _ _ (hsAT, Nothing) =
296 return () -- skip, we already had an error here
297 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
298 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
299 checkIndexes' clas inst_tys hsAT
301 snd . fromJust . tyConFamInst_maybe $ tycon)
302 checkIndexes _ _ _ = panic "checkIndexes"
304 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
305 = let atName = tcdName . unLoc $ hsAT
307 setSrcSpan (getLoc hsAT) $
308 addErrCtxt (atInstCtxt atName) $
309 case find ((atName ==) . tyConName) (classATs clas) of
310 Nothing -> addErrTc $ badATErr clas atName -- not in this class
312 case assocTyConArgPoss_maybe atDecl of
313 Nothing -> panic "checkIndexes': AT has no args poss?!?"
316 -- The following is tricky! We need to deal with three
317 -- complications: (1) The AT possibly only uses a subset of
318 -- the class parameters as indexes and those it uses may be in
319 -- a different order; (2) the AT may have extra arguments,
320 -- which must be type variables; and (3) variables in AT and
321 -- instance head will be different `Name's even if their
322 -- source lexemes are identical.
324 -- Re (1), `poss' contains a permutation vector to extract the
325 -- class parameters in the right order.
327 -- Re (2), we wrap the (permuted) class parameters in a Maybe
328 -- type and use Nothing for any extra AT arguments. (First
329 -- equation of `checkIndex' below.)
331 -- Re (3), we replace any type variable in the AT parameters
332 -- that has the same source lexeme as some variable in the
333 -- instance types with the instance type variable sharing its
336 let relevantInstTys = map (instTys !!) poss
337 instArgs = map Just relevantInstTys ++
338 repeat Nothing -- extra arguments
339 renaming = substSameTyVar atTvs instTvs
341 zipWithM_ checkIndex (substTys renaming atTys) instArgs
343 checkIndex ty Nothing
344 | isTyVarTy ty = return ()
345 | otherwise = addErrTc $ mustBeVarArgErr ty
346 checkIndex ty (Just instTy)
347 | ty `tcEqType` instTy = return ()
348 | otherwise = addErrTc $ wrongATArgErr ty instTy
350 listToNameSet = addListToNameSet emptyNameSet
352 substSameTyVar [] _ = emptyTvSubst
353 substSameTyVar (tv:tvs) replacingTvs =
354 let replacement = case find (tv `sameLexeme`) replacingTvs of
355 Nothing -> mkTyVarTy tv
356 Just rtv -> mkTyVarTy rtv
358 tv1 `sameLexeme` tv2 =
359 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
361 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
365 %************************************************************************
367 \subsection{Type-checking instance declarations, pass 2}
369 %************************************************************************
372 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
373 -> TcM (LHsBinds Id, TcLclEnv)
374 -- (a) From each class declaration,
375 -- generate any default-method bindings
376 -- (b) From each instance decl
377 -- generate the dfun binding
379 tcInstDecls2 tycl_decls inst_decls
380 = do { -- (a) Default methods from class decls
381 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
382 filter (isClassDecl.unLoc) tycl_decls
383 ; tcExtendIdEnv (concat dm_ids_s) $ do
385 -- (b) instance declarations
386 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
389 ; let binds = unionManyBags dm_binds_s `unionBags`
390 unionManyBags inst_binds_s
391 ; tcl_env <- getLclEnv -- Default method Ids in here
392 ; returnM (binds, tcl_env) }
395 ======= New documentation starts here (Sept 92) ==============
397 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
398 the dictionary function for this instance declaration. For example
400 instance Foo a => Foo [a] where
404 might generate something like
406 dfun.Foo.List dFoo_a = let op1 x = ...
412 HOWEVER, if the instance decl has no context, then it returns a
413 bigger @HsBinds@ with declarations for each method. For example
415 instance Foo [a] where
421 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
422 const.Foo.op1.List a x = ...
423 const.Foo.op2.List a y = ...
425 This group may be mutually recursive, because (for example) there may
426 be no method supplied for op2 in which case we'll get
428 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
430 that is, the default method applied to the dictionary at this type.
432 What we actually produce in either case is:
434 AbsBinds [a] [dfun_theta_dicts]
435 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
436 { d = (sd1,sd2, ..., op1, op2, ...)
441 The "maybe" says that we only ask AbsBinds to make global constant methods
442 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.
459 First comes the easy case of a non-local instance decl.
463 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
464 -- Returns a binding for the dfun
466 ------------------------
467 -- Derived newtype instances; surprisingly tricky!
469 -- In the case of a newtype, things are rather easy
470 -- class Show a => Foo a b where ...
471 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
472 -- The newtype gives an FC axiom looking like
473 -- axiom CoT a :: T a :=: Tree [a]
474 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
476 -- So all need is to generate a binding looking like:
477 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
478 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
479 -- case df `cast` (Foo Int (sym (CoT a))) of
480 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
482 -- If there are no superclasses, matters are simpler, because we don't need the case
483 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
485 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
486 = do { let dfun_id = instanceDFunId ispec
487 rigid_info = InstSkol
488 origin = SigOrigin rigid_info
489 inst_ty = idType dfun_id
490 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
491 -- inst_head_ty is a PredType
493 ; inst_loc <- getInstLoc origin
494 ; (rep_dict_id : sc_dict_ids, wrap_fn, sc_binds)
495 <- make_wrapper inst_loc tvs theta mb_preds
496 -- Here, we are relying on the order of dictionary
497 -- arguments built by NewTypeDerived in TcDeriv;
498 -- namely, that the rep_dict_id comes first
500 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
501 cls_tycon = classTyCon cls
502 the_coercion = make_coercion cls_tycon cls_inst_tys
503 coerced_rep_dict = mkHsWrap the_coercion (HsVar rep_dict_id)
505 ; body <- make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
507 ; return (sc_binds `snocBag` (noLoc $ VarBind dfun_id $ noLoc $ mkHsWrap wrap_fn body)) }
510 -----------------------
512 -- We distinguish two cases:
513 -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
514 -- and the new dict can just be a constant
515 -- (mb_preds = Just preds)
516 -- (b) there are tyvars, so we must make a dict *fun*
517 -- (mb_preds = Nothing)
518 -- See the defn of NewTypeDerived for the meaning of mb_preds
519 make_wrapper inst_loc tvs theta (Just preds) -- Case (a)
520 = ASSERT( null tvs && null theta )
521 do { dicts <- newDictBndrs inst_loc preds
522 ; sc_binds <- addErrCtxt superClassCtxt $
523 tcSimplifySuperClasses inst_loc [] dicts
524 -- Use tcSimplifySuperClasses to avoid creating loops, for the
525 -- same reason as Note [SUPERCLASS-LOOP 1] in TcSimplify
526 ; return (map instToId dicts, idHsWrapper, sc_binds) }
528 make_wrapper inst_loc tvs theta Nothing -- Case (b)
529 = do { dicts <- newDictBndrs inst_loc theta
530 ; let dict_ids = map instToId dicts
531 ; return (dict_ids, mkWpTyLams tvs <.> mkWpLams dict_ids, emptyBag) }
533 -----------------------
535 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
536 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
537 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
538 -- where rep_ty is the (eta-reduced) type rep of T
539 -- So we just replace T with CoT, and insert a 'sym'
540 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
542 make_coercion cls_tycon cls_inst_tys
543 | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
544 , (tycon, tc_args) <- tcSplitTyConApp last_ty -- Should not fail
545 , Just co_con <- newTyConCo_maybe tycon
546 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
547 = WpCo (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
548 | otherwise -- The newtype is transparent; no need for a cast
551 -----------------------
553 -- Two cases; see Note [Newtype deriving superclasses] in TcDeriv.lhs
554 -- (a) no superclasses; then we can just use the coerced dict
555 -- (b) one or more superclasses; then new need to do the unpack/repack
557 make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
558 | null sc_dict_ids -- Case (a)
559 = return coerced_rep_dict
560 | otherwise -- Case (b)
561 = do { op_ids <- newSysLocalIds FSLIT("op") op_tys
562 ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
563 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
564 pat_dicts = dummy_sc_dict_ids,
565 pat_binds = emptyLHsBinds,
566 pat_args = PrefixCon (map nlVarPat op_ids),
568 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
569 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
570 map HsVar (sc_dict_ids ++ op_ids)
572 -- Warning: this HsCase scrutinises a value with a PredTy, which is
573 -- never otherwise seen in Haskell source code. It'd be
574 -- nicer to generate Core directly!
575 ; return (HsCase (noLoc coerced_rep_dict) $
576 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
578 pat_ty = mkTyConApp cls_tycon cls_inst_tys
579 cls_data_con = head (tyConDataCons cls_tycon)
580 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
581 op_tys = dropList sc_dict_ids cls_arg_tys
583 ------------------------
584 -- Ordinary instances
586 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
588 dfun_id = instanceDFunId ispec
589 rigid_info = InstSkol
590 inst_ty = idType dfun_id
591 loc = srcLocSpan (getSrcLoc dfun_id)
593 -- Prime error recovery
594 recoverM (returnM emptyLHsBinds) $
596 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
598 -- Instantiate the instance decl with skolem constants
599 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
600 -- These inst_tyvars' scope over the 'where' part
601 -- Those tyvars are inside the dfun_id's type, which is a bit
602 -- bizarre, but OK so long as you realise it!
604 (clas, inst_tys') = tcSplitDFunHead inst_head'
605 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
607 -- Instantiate the super-class context with inst_tys
608 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
609 (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
610 origin = SigOrigin rigid_info
611 (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
613 -- Create dictionary Ids from the specified instance contexts.
614 getInstLoc InstScOrigin `thenM` \ sc_loc ->
615 newDictBndrs sc_loc dict_sc_theta' `thenM` \ sc_dicts ->
616 getInstLoc origin `thenM` \ inst_loc ->
617 mkMetaCoVars eq_sc_theta' `thenM` \ sc_covars ->
618 mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars) `thenM` \ wanted_sc_eqs ->
619 mkCoVars eq_dfun_theta' `thenM` \ dfun_covars ->
620 mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars) `thenM` \ dfun_eqs ->
621 newDictBndrs inst_loc dict_dfun_theta' `thenM` \ dfun_dicts ->
622 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
623 -- Default-method Ids may be mentioned in synthesised RHSs,
624 -- but they'll already be in the environment.
626 -- Typecheck the methods
627 let -- These insts are in scope; quite a few, eh?
628 dfun_insts = dfun_eqs ++ dfun_dicts
629 wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
630 given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
631 given_sc_insts = given_sc_eqs ++ sc_dicts
632 avail_insts = [this_dict] ++ dfun_insts ++ given_sc_insts
634 tcMethods origin clas inst_tyvars'
635 dfun_theta' inst_tys' avail_insts
636 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
638 -- Figure out bindings for the superclass context
639 -- Don't include this_dict in the 'givens', else
640 -- wanted_sc_insts get bound by just selecting from this_dict!!
641 addErrCtxt superClassCtxt
642 (tcSimplifySuperClasses inst_loc
643 dfun_insts wanted_sc_insts) `thenM` \ sc_binds ->
645 -- It's possible that the superclass stuff might unified one
646 -- of the inst_tyavars' with something in the envt
647 checkSigTyVars inst_tyvars' `thenM_`
649 -- Deal with 'SPECIALISE instance' pragmas
650 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
652 -- Create the result bindings
654 dict_constr = classDataCon clas
655 scs_and_meths = map instToId sc_dicts ++ meth_ids
656 this_dict_id = instToId this_dict
657 inline_prag | null dfun_insts = []
658 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
659 -- Always inline the dfun; this is an experimental decision
660 -- because it makes a big performance difference sometimes.
661 -- Often it means we can do the method selection, and then
662 -- inline the method as well. Marcin's idea; see comments below.
664 -- BUT: don't inline it if it's a constant dictionary;
665 -- we'll get all the benefit without inlining, and we get
666 -- a **lot** of code duplication if we inline it
668 -- See Note [Inline dfuns] below
671 = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars) (map HsVar scs_and_meths)
672 -- We don't produce a binding for the dict_constr; instead we
673 -- rely on the simplifier to unfold this saturated application
674 -- We do this rather than generate an HsCon directly, because
675 -- it means that the special cases (e.g. dictionary with only one
676 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
677 -- than needing to be repeated here.
679 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
680 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
682 main_bind = noLoc $ AbsBinds
683 (inst_tyvars' ++ dfun_covars)
684 (map instToId dfun_dicts)
685 [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
688 showLIE (text "instance") `thenM_`
689 returnM (unitBag main_bind)
691 mkCoVars :: [PredType] -> TcM [TyVar]
692 mkCoVars [] = return []
693 mkCoVars (pred:preds) =
694 do { uniq <- newUnique
695 ; let name = mkSysTvName uniq FSLIT("mkCoVars")
696 ; let tv = mkCoVar name (PredTy pred)
697 ; tvs <- mkCoVars preds
701 mkMetaCoVars :: [PredType] -> TcM [TyVar]
702 mkMetaCoVars [] = return []
703 mkMetaCoVars (EqPred ty1 ty2:preds) =
704 do { tv <- newMetaTyVar TauTv (mkCoKind ty1 ty2)
705 ; tvs <- mkMetaCoVars preds
710 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
711 avail_insts op_items monobinds uprags
712 = -- Check that all the method bindings come from this class
714 sel_names = [idName sel_id | (sel_id, _) <- op_items]
715 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
717 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
719 -- Make the method bindings
721 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
723 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
725 -- And type check them
726 -- It's really worth making meth_insts available to the tcMethodBind
727 -- Consider instance Monad (ST s) where
728 -- {-# INLINE (>>) #-}
729 -- (>>) = ...(>>=)...
730 -- If we don't include meth_insts, we end up with bindings like this:
731 -- rec { dict = MkD then bind ...
732 -- then = inline_me (... (GHC.Base.>>= dict) ...)
734 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
735 -- and (b) the inline_me prevents us inlining the >>= selector, which
736 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
737 -- is not inlined across modules. Rather ironic since this does not
738 -- happen without the INLINE pragma!
740 -- Solution: make meth_insts available, so that 'then' refers directly
741 -- to the local 'bind' rather than going via the dictionary.
743 -- BUT WATCH OUT! If the method type mentions the class variable, then
744 -- this optimisation is not right. Consider
748 -- instance C Int where
750 -- The occurrence of 'op' on the rhs gives rise to a constraint
752 -- The trouble is that the 'meth_inst' for op, which is 'available', also
753 -- looks like 'op at Int'. But they are not the same.
755 prag_fn = mkPragFun uprags
756 all_insts = avail_insts ++ catMaybes meth_insts
757 sig_fn n = Just [] -- No scoped type variables, but every method has
758 -- a type signature, in effect, so that we check
759 -- the method has the right type
760 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
761 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
764 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
766 returnM (meth_ids, unionManyBags meth_binds_s)
770 ------------------------------
771 [Inline dfuns] Inlining dfuns unconditionally
772 ------------------------------
774 The code above unconditionally inlines dict funs. Here's why.
775 Consider this program:
777 test :: Int -> Int -> Bool
778 test x y = (x,y) == (y,x) || test y x
779 -- Recursive to avoid making it inline.
781 This needs the (Eq (Int,Int)) instance. If we inline that dfun
782 the code we end up with is good:
785 \r -> case ==# [ww ww1] of wild {
786 PrelBase.False -> Test.$wtest ww1 ww;
788 case ==# [ww1 ww] of wild1 {
789 PrelBase.False -> Test.$wtest ww1 ww;
790 PrelBase.True -> PrelBase.True [];
793 Test.test = \r [w w1]
796 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
799 If we don't inline the dfun, the code is not nearly as good:
801 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
802 PrelBase.:DEq tpl1 tpl2 -> tpl2;
807 let { y = PrelBase.I#! [ww1]; } in
808 let { x = PrelBase.I#! [ww]; } in
809 let { sat_slx = PrelTup.(,)! [y x]; } in
810 let { sat_sly = PrelTup.(,)! [x y];
812 case == sat_sly sat_slx of wild {
813 PrelBase.False -> Test.$wtest ww1 ww;
814 PrelBase.True -> PrelBase.True [];
821 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
824 Why doesn't GHC inline $fEq? Because it looks big:
826 PrelTup.zdfEqZ1T{-rcX-}
827 = \ @ a{-reT-} :: * @ b{-reS-} :: *
828 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
829 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
831 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
832 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
834 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
835 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
837 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
838 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
839 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
841 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
843 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
845 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
846 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
850 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
851 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
852 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
853 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
855 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
857 and it's not as bad as it seems, because it's further dramatically
858 simplified: only zeze2 is extracted and its body is simplified.
861 %************************************************************************
863 \subsection{Error messages}
865 %************************************************************************
868 instDeclCtxt1 hs_inst_ty
869 = inst_decl_ctxt (case unLoc hs_inst_ty of
870 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
871 HsPredTy pred -> ppr pred
872 other -> ppr hs_inst_ty) -- Don't expect this
873 instDeclCtxt2 dfun_ty
874 = inst_decl_ctxt (ppr (mkClassPred cls tys))
876 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
878 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
880 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
882 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
886 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
887 ptext SLIT("must be variables")
888 , ptext SLIT("Instead of a variable, found") <+> ppr ty
891 wrongATArgErr ty instTy =
892 sep [ ptext SLIT("Type indexes must match class instance head")
893 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>