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
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 Name], -- 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 <- mapM tcLocalInstDecl1 inst_decls
153 ; idx_tycons <- mapM 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 failIfErrsM -- If the addInsts stuff gave any errors, don't
185 -- try the deriving stuff, becuase that may give
187 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls
189 ; addInsts deriv_inst_info $ do {
191 ; gbl_env <- getGblEnv
193 generic_inst_info ++ deriv_inst_info ++ local_info,
197 -- Make sure that toplevel type instance are not for associated types.
198 -- !!!TODO: Need to perform this check for the TyThing of type functions,
200 tcIdxTyInstDeclTL ldecl@(L loc decl) =
201 do { tything <- tcFamInstDecl ldecl
203 when (isAssocFamily tything) $
204 addErr $ assocInClassErr (tcdName decl)
207 isAssocFamily (Just (ATyCon tycon)) =
208 case tyConFamInst_maybe tycon of
209 Nothing -> panic "isAssocFamily: no family?!?"
210 Just (fam, _) -> isTyConAssoc fam
211 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
212 isAssocFamily Nothing = False
214 assocInClassErr :: Name -> SDoc
215 assocInClassErr name =
216 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
217 ptext (sLit "must be inside a class instance")
219 addInsts :: [InstInfo Name] -> TcM a -> TcM a
220 addInsts infos thing_inside
221 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
223 addFamInsts :: [TyThing] -> TcM a -> TcM a
224 addFamInsts tycons thing_inside
225 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
227 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
228 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
233 tcLocalInstDecl1 :: LInstDecl Name
234 -> TcM ([InstInfo Name], [TyThing]) -- [] if there was an error
235 -- A source-file instance declaration
236 -- Type-check all the stuff before the "where"
238 -- We check for respectable instance type, and context
239 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
240 = -- Prime error recovery, set source location
241 recoverM (return ([], [])) $
243 addErrCtxt (instDeclCtxt1 poly_ty) $
245 do { is_boot <- tcIsHsBoot
246 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
249 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
251 -- Next, process any associated types.
252 ; idx_tycons <- mapM tcFamInstDecl ats
254 -- Now, check the validity of the instance.
255 ; (clas, inst_tys) <- checkValidInstHead tau
256 ; checkValidInstance tyvars theta clas inst_tys
257 ; checkValidAndMissingATs clas (tyvars, inst_tys)
260 -- Finally, construct the Core representation of the instance.
261 -- (This no longer includes the associated types.)
262 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
263 -- Dfun location is that of instance *header*
264 ; overlap_flag <- getOverlapFlag
265 ; let (eq_theta,dict_theta) = partition isEqPred theta
266 theta' = eq_theta ++ dict_theta
267 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
268 ispec = mkLocalInstance dfun overlap_flag
270 ; return ([InstInfo { iSpec = ispec,
271 iBinds = VanillaInst binds uprags }],
272 catMaybes idx_tycons)
275 -- We pass in the source form and the type checked form of the ATs. We
276 -- really need the source form only to be able to produce more informative
278 checkValidAndMissingATs :: Class
279 -> ([TyVar], [TcType]) -- instance types
280 -> [(LTyClDecl Name, -- source form of AT
281 Maybe TyThing)] -- Core form of AT
283 checkValidAndMissingATs clas inst_tys ats
284 = do { -- Issue a warning for each class AT that is not defined in this
286 ; let class_ats = map tyConName (classATs clas)
287 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
288 omitted = filterOut (`elemNameSet` defined_ats) class_ats
289 ; warn <- doptM Opt_WarnMissingMethods
290 ; mapM_ (warnTc warn . omittedATWarn) omitted
292 -- Ensure that all AT indexes that correspond to class parameters
293 -- coincide with the types in the instance head. All remaining
294 -- AT arguments must be variables. Also raise an error for any
295 -- type instances that are not associated with this class.
296 ; mapM_ (checkIndexes clas inst_tys) ats
299 checkIndexes _ _ (_, Nothing) =
300 return () -- skip, we already had an error here
301 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
302 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
303 checkIndexes' clas inst_tys hsAT
305 snd . fromJust . tyConFamInst_maybe $ tycon)
306 checkIndexes _ _ _ = panic "checkIndexes"
308 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
309 = let atName = tcdName . unLoc $ hsAT
311 setSrcSpan (getLoc hsAT) $
312 addErrCtxt (atInstCtxt atName) $
313 case find ((atName ==) . tyConName) (classATs clas) of
314 Nothing -> addErrTc $ badATErr clas atName -- not in this class
316 case assocTyConArgPoss_maybe atDecl of
317 Nothing -> panic "checkIndexes': AT has no args poss?!?"
320 -- The following is tricky! We need to deal with three
321 -- complications: (1) The AT possibly only uses a subset of
322 -- the class parameters as indexes and those it uses may be in
323 -- a different order; (2) the AT may have extra arguments,
324 -- which must be type variables; and (3) variables in AT and
325 -- instance head will be different `Name's even if their
326 -- source lexemes are identical.
328 -- Re (1), `poss' contains a permutation vector to extract the
329 -- class parameters in the right order.
331 -- Re (2), we wrap the (permuted) class parameters in a Maybe
332 -- type and use Nothing for any extra AT arguments. (First
333 -- equation of `checkIndex' below.)
335 -- Re (3), we replace any type variable in the AT parameters
336 -- that has the same source lexeme as some variable in the
337 -- instance types with the instance type variable sharing its
340 let relevantInstTys = map (instTys !!) poss
341 instArgs = map Just relevantInstTys ++
342 repeat Nothing -- extra arguments
343 renaming = substSameTyVar atTvs instTvs
345 zipWithM_ checkIndex (substTys renaming atTys) instArgs
347 checkIndex ty Nothing
348 | isTyVarTy ty = return ()
349 | otherwise = addErrTc $ mustBeVarArgErr ty
350 checkIndex ty (Just instTy)
351 | ty `tcEqType` instTy = return ()
352 | otherwise = addErrTc $ wrongATArgErr ty instTy
354 listToNameSet = addListToNameSet emptyNameSet
356 substSameTyVar [] _ = emptyTvSubst
357 substSameTyVar (tv:tvs) replacingTvs =
358 let replacement = case find (tv `sameLexeme`) replacingTvs of
359 Nothing -> mkTyVarTy tv
360 Just rtv -> mkTyVarTy rtv
362 tv1 `sameLexeme` tv2 =
363 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
365 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
369 %************************************************************************
371 \subsection{Type-checking instance declarations, pass 2}
373 %************************************************************************
376 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
377 -> TcM (LHsBinds Id, TcLclEnv)
378 -- (a) From each class declaration,
379 -- generate any default-method bindings
380 -- (b) From each instance decl
381 -- generate the dfun binding
383 tcInstDecls2 tycl_decls inst_decls
384 = do { -- (a) Default methods from class decls
385 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
386 filter (isClassDecl.unLoc) tycl_decls
387 ; tcExtendIdEnv (concat dm_ids_s) $ do
389 -- (b) instance declarations
390 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
393 ; let binds = unionManyBags dm_binds_s `unionBags`
394 unionManyBags inst_binds_s
395 ; tcl_env <- getLclEnv -- Default method Ids in here
396 ; return (binds, tcl_env) }
399 ======= New documentation starts here (Sept 92) ==============
401 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
402 the dictionary function for this instance declaration. For example
404 instance Foo a => Foo [a] where
408 might generate something like
410 dfun.Foo.List dFoo_a = let op1 x = ...
415 HOWEVER, if the instance decl has no context, then it returns a
416 bigger @HsBinds@ with declarations for each method. For example
418 instance Foo [a] where
424 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
425 const.Foo.op1.List a x = ...
426 const.Foo.op2.List a y = ...
428 This group may be mutually recursive, because (for example) there may
429 be no method supplied for op2 in which case we'll get
431 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
433 that is, the default method applied to the dictionary at this type.
434 What we actually produce in either case is:
436 AbsBinds [a] [dfun_theta_dicts]
437 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
438 { d = (sd1,sd2, ..., op1, op2, ...)
443 The "maybe" says that we only ask AbsBinds to make global constant methods
444 if the dfun_theta is empty.
446 For an instance declaration, say,
448 instance (C1 a, C2 b) => C (T a b) where
451 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
452 function whose type is
454 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
456 Notice that we pass it the superclass dictionaries at the instance type; this
457 is the ``Mark Jones optimisation''. The stuff before the "=>" here
458 is the @dfun_theta@ below.
462 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
463 -- Returns a binding for the dfun
465 ------------------------
466 -- Derived newtype instances; surprisingly tricky!
468 -- class Show a => Foo a b where ...
469 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
471 -- The newtype gives an FC axiom looking like
472 -- axiom CoN a :: N 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 (N a)) => Foo Int (N a)
477 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
478 -- case df `cast` (Foo Int (sym (CoN 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 })
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 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
493 (class_tyvars, sc_theta, _, _) = classBigSig cls
494 cls_tycon = classTyCon cls
495 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
497 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
498 (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail
499 rep_ty = newTyConInstRhs nt_tycon tc_args
501 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
502 -- In our example, rep_pred is (Foo Int (Tree [a]))
503 the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
504 -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)
506 ; inst_loc <- getInstLoc origin
507 ; sc_loc <- getInstLoc InstScOrigin
508 ; dfun_dicts <- newDictBndrs inst_loc theta
509 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
510 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
511 ; rep_dict <- newDictBndr inst_loc rep_pred
513 -- Figure out bindings for the superclass context from dfun_dicts
514 -- Don't include this_dict in the 'givens', else
515 -- wanted_sc_insts get bound by just selecting from this_dict!!
516 ; sc_binds <- addErrCtxt superClassCtxt $
517 tcSimplifySuperClasses inst_loc dfun_dicts (rep_dict:sc_dicts)
519 ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict))
521 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
522 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
524 ; return (unitBag $ noLoc $
525 AbsBinds tvs (map instToVar dfun_dicts)
526 [(tvs, dfun_id, instToId this_dict, [])]
527 (dict_bind `consBag` sc_binds)) }
529 -----------------------
531 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
532 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
533 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
534 -- where rep_ty is the (eta-reduced) type rep of T
535 -- So we just replace T with CoT, and insert a 'sym'
536 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
538 make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
539 | Just co_con <- newTyConCo_maybe nt_tycon
540 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
541 = WpCast (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))
542 | otherwise -- The newtype is transparent; no need for a cast
545 -----------------------
546 -- (make_body C tys scs coreced_rep_dict)
548 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
549 -- But if there are no superclasses, it returns just coerced_rep_dict
550 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
552 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
553 | null sc_dicts -- 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 sc_dict_ids = map instToId sc_dicts
574 pat_ty = mkTyConApp cls_tycon cls_inst_tys
575 cls_data_con = head (tyConDataCons cls_tycon)
576 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
577 op_tys = dropList sc_dict_ids cls_arg_tys
579 ------------------------
580 -- Ordinary instances
582 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
584 dfun_id = instanceDFunId ispec
585 rigid_info = InstSkol
586 inst_ty = idType dfun_id
587 loc = getSrcSpan dfun_id
589 -- Prime error recovery
590 recoverM (return emptyLHsBinds) $
592 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do
594 -- Instantiate the instance decl with skolem constants
595 (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
596 -- These inst_tyvars' scope over the 'where' part
597 -- Those tyvars are inside the dfun_id's type, which is a bit
598 -- bizarre, but OK so long as you realise it!
600 (clas, inst_tys') = tcSplitDFunHead inst_head'
601 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
603 -- Instantiate the super-class context with inst_tys
604 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
605 (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
606 origin = SigOrigin rigid_info
607 (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
609 -- Create dictionary Ids from the specified instance contexts.
610 sc_loc <- getInstLoc InstScOrigin
611 sc_dicts <- newDictBndrs sc_loc dict_sc_theta'
612 inst_loc <- getInstLoc origin
613 sc_covars <- mkMetaCoVars eq_sc_theta'
614 wanted_sc_eqs <- mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars)
615 dfun_covars <- mkCoVars eq_dfun_theta'
616 dfun_eqs <- mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars)
617 dfun_dicts <- newDictBndrs inst_loc dict_dfun_theta'
618 this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
619 -- Default-method Ids may be mentioned in synthesised RHSs,
620 -- but they'll already be in the environment.
622 -- Typecheck the methods
623 let -- These insts are in scope; quite a few, eh?
624 dfun_insts = dfun_eqs ++ dfun_dicts
625 wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
626 given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
627 given_sc_insts = given_sc_eqs ++ sc_dicts
628 avail_insts = dfun_insts ++ given_sc_insts
630 (meth_ids, meth_binds) <- tcMethods origin clas inst_tyvars'
631 dfun_theta' inst_tys' this_dict avail_insts
632 op_items monobinds uprags
634 -- Figure out bindings for the superclass context
635 -- Don't include this_dict in the 'givens', else
636 -- wanted_sc_insts get bound by just selecting from this_dict!!
637 sc_binds <- addErrCtxt superClassCtxt
638 (tcSimplifySuperClasses inst_loc dfun_insts wanted_sc_insts)
640 -- It's possible that the superclass stuff might unified one
641 -- of the inst_tyavars' with something in the envt
642 checkSigTyVars inst_tyvars'
644 -- Deal with 'SPECIALISE instance' pragmas
645 prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
647 -- Create the result bindings
649 dict_constr = classDataCon clas
650 scs_and_meths = map instToId sc_dicts ++ meth_ids
651 this_dict_id = instToId this_dict
652 inline_prag | null dfun_insts = []
653 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
654 -- Always inline the dfun; this is an experimental decision
655 -- because it makes a big performance difference sometimes.
656 -- Often it means we can do the method selection, and then
657 -- inline the method as well. Marcin's idea; see comments below.
659 -- BUT: don't inline it if it's a constant dictionary;
660 -- we'll get all the benefit without inlining, and we get
661 -- a **lot** of code duplication if we inline it
663 -- See Note [Inline dfuns] below
665 dict_rhs = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars)
666 (map HsVar scs_and_meths)
667 -- We don't produce a binding for the dict_constr; instead we
668 -- rely on the simplifier to unfold this saturated application
669 -- We do this rather than generate an HsCon directly, because
670 -- it means that the special cases (e.g. dictionary with only one
671 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
672 -- than needing to be repeated here.
674 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
675 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
677 main_bind = noLoc $ AbsBinds
678 (inst_tyvars' ++ dfun_covars)
679 (map instToId dfun_dicts)
680 [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
683 showLIE (text "instance")
684 return (unitBag main_bind)
686 mkCoVars :: [PredType] -> TcM [TyVar]
687 mkCoVars = newCoVars . map unEqPred
689 unEqPred (EqPred ty1 ty2) = (ty1, ty2)
690 unEqPred _ = panic "TcInstDcls.mkCoVars"
692 mkMetaCoVars :: [PredType] -> TcM [TyVar]
693 mkMetaCoVars = mapM eqPredToCoVar
695 eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2
696 eqPredToCoVar _ = panic "TcInstDcls.mkMetaCoVars"
698 tcMethods :: InstOrigin -> Class -> [TcTyVar] -> TcThetaType -> [TcType]
699 -> Inst -> [Inst] -> [(Id, DefMeth)] -> LHsBindsLR Name Name
701 -> TcM ([Id], Bag (LHsBind Id))
702 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
703 this_dict extra_insts op_items monobinds uprags = do
704 -- Check that all the method bindings come from this class
706 sel_names = [idName sel_id | (sel_id, _) <- op_items]
707 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
709 mapM (addErrTc . badMethodErr clas) bad_bndrs
711 -- Make the method bindings
713 mk_method_id (sel_id, _) = mkMethId origin clas sel_id inst_tys'
715 (meth_insts, meth_ids) <- mapAndUnzipM mk_method_id op_items
717 -- And type check them
718 -- It's really worth making meth_insts available to the tcMethodBind
719 -- Consider instance Monad (ST s) where
720 -- {-# INLINE (>>) #-}
721 -- (>>) = ...(>>=)...
722 -- If we don't include meth_insts, we end up with bindings like this:
723 -- rec { dict = MkD then bind ...
724 -- then = inline_me (... (GHC.Base.>>= dict) ...)
726 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
727 -- and (b) the inline_me prevents us inlining the >>= selector, which
728 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
729 -- is not inlined across modules. Rather ironic since this does not
730 -- happen without the INLINE pragma!
732 -- Solution: make meth_insts available, so that 'then' refers directly
733 -- to the local 'bind' rather than going via the dictionary.
735 -- BUT WATCH OUT! If the method type mentions the class variable, then
736 -- this optimisation is not right. Consider
740 -- instance C Int where
742 -- The occurrence of 'op' on the rhs gives rise to a constraint
744 -- The trouble is that the 'meth_inst' for op, which is 'available', also
745 -- looks like 'op at Int'. But they are not the same.
747 prag_fn = mkPragFun uprags
748 all_insts = extra_insts ++ catMaybes meth_insts
749 sig_fn _ = Just [] -- No scoped type variables, but every method has
750 -- a type signature, in effect, so that we check
751 -- the method has the right type
752 tc_method_bind = tcMethodBind origin inst_tyvars' dfun_theta' this_dict
753 all_insts sig_fn prag_fn monobinds
755 meth_binds_s <- zipWithM tc_method_bind op_items meth_ids
757 return (meth_ids, unionManyBags meth_binds_s)
761 ------------------------------
762 [Inline dfuns] Inlining dfuns unconditionally
763 ------------------------------
765 The code above unconditionally inlines dict funs. Here's why.
766 Consider this program:
768 test :: Int -> Int -> Bool
769 test x y = (x,y) == (y,x) || test y x
770 -- Recursive to avoid making it inline.
772 This needs the (Eq (Int,Int)) instance. If we inline that dfun
773 the code we end up with is good:
776 \r -> case ==# [ww ww1] of wild {
777 PrelBase.False -> Test.$wtest ww1 ww;
779 case ==# [ww1 ww] of wild1 {
780 PrelBase.False -> Test.$wtest ww1 ww;
781 PrelBase.True -> PrelBase.True [];
784 Test.test = \r [w w1]
787 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
790 If we don't inline the dfun, the code is not nearly as good:
792 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
793 PrelBase.:DEq tpl1 tpl2 -> tpl2;
798 let { y = PrelBase.I#! [ww1]; } in
799 let { x = PrelBase.I#! [ww]; } in
800 let { sat_slx = PrelTup.(,)! [y x]; } in
801 let { sat_sly = PrelTup.(,)! [x y];
803 case == sat_sly sat_slx of wild {
804 PrelBase.False -> Test.$wtest ww1 ww;
805 PrelBase.True -> PrelBase.True [];
812 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
815 Why doesn't GHC inline $fEq? Because it looks big:
817 PrelTup.zdfEqZ1T{-rcX-}
818 = \ @ a{-reT-} :: * @ b{-reS-} :: *
819 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
820 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
822 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
823 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
825 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
826 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
828 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
829 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
830 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
832 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
834 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
836 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
837 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
841 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
842 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
843 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
844 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
846 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
848 and it's not as bad as it seems, because it's further dramatically
849 simplified: only zeze2 is extracted and its body is simplified.
852 %************************************************************************
854 \subsection{Error messages}
856 %************************************************************************
859 instDeclCtxt1 :: LHsType Name -> SDoc
860 instDeclCtxt1 hs_inst_ty
861 = inst_decl_ctxt (case unLoc hs_inst_ty of
862 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
863 HsPredTy pred -> ppr pred
864 _ -> ppr hs_inst_ty) -- Don't expect this
865 instDeclCtxt2 :: Type -> SDoc
866 instDeclCtxt2 dfun_ty
867 = inst_decl_ctxt (ppr (mkClassPred cls tys))
869 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
871 inst_decl_ctxt :: SDoc -> SDoc
872 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
874 superClassCtxt :: SDoc
875 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
877 atInstCtxt :: Name -> SDoc
878 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
881 mustBeVarArgErr :: Type -> SDoc
883 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
884 ptext (sLit "must be variables")
885 , ptext (sLit "Instead of a variable, found") <+> ppr ty
888 wrongATArgErr :: Type -> Type -> SDoc
889 wrongATArgErr ty instTy =
890 sep [ ptext (sLit "Type indexes must match class instance head")
891 , ptext (sLit "Found") <+> ppr ty <+> ptext (sLit "but expected") <+>