2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcInstDecls: Typechecking instance declarations
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
59 Typechecking instance declarations is done in two passes. The first
60 pass, made by @tcInstDecls1@, collects information to be used in the
63 This pre-processed info includes the as-yet-unprocessed bindings
64 inside the instance declaration. These are type-checked in the second
65 pass, when the class-instance envs and GVE contain all the info from
66 all the instance and value decls. Indeed that's the reason we need
67 two passes over the instance decls.
69 Here is the overall algorithm.
70 Assume that we have an instance declaration
72 instance c => k (t tvs) where b
76 $LIE_c$ is the LIE for the context of class $c$
78 $betas_bar$ is the free variables in the class method type, excluding the
81 $LIE_cop$ is the LIE constraining a particular class method
83 $tau_cop$ is the tau type of a class method
85 $LIE_i$ is the LIE for the context of instance $i$
87 $X$ is the instance constructor tycon
89 $gammas_bar$ is the set of type variables of the instance
91 $LIE_iop$ is the LIE for a particular class method instance
93 $tau_iop$ is the tau type for this instance of a class method
95 $alpha$ is the class variable
97 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
99 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
102 ToDo: Update the list above with names actually in the code.
106 First, make the LIEs for the class and instance contexts, which means
107 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
108 and make LIElistI and LIEI.
110 Then process each method in turn.
112 order the instance methods according to the ordering of the class methods
114 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
116 Create final dictionary function from bindings generated already
118 df = lambda inst_tyvars
125 in <op1,op2,...,opn,sd1,...,sdm>
127 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
128 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
132 %************************************************************************
134 \subsection{Extracting instance decls}
136 %************************************************************************
138 Gather up the instance declarations from their various sources
141 tcInstDecls1 -- Deal with both source-code and imported instance decls
142 :: [LTyClDecl Name] -- For deriving stuff
143 -> [LInstDecl Name] -- Source code instance decls
144 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
145 -> TcM (TcGblEnv, -- The full inst env
146 [InstInfo], -- Source-code instance decls to process;
147 -- contains all dfuns for this module
148 HsValBinds Name) -- Supporting bindings for derived instances
150 tcInstDecls1 tycl_decls inst_decls deriv_decls
152 do { -- Stop if addInstInfos etc discovers any errors
153 -- (they recover, so that we get more than one error each
156 -- (1) Do class and family instance declarations
157 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
158 ; local_info_tycons <- mapM tcLocalInstDecl1 inst_decls
159 ; idx_tycons <- mapM tcIdxTyInstDeclTL idxty_decls
161 ; let { (local_infos,
162 at_tycons) = unzip local_info_tycons
163 ; local_info = concat local_infos
164 ; at_idx_tycon = concat at_tycons ++ catMaybes idx_tycons
165 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
166 ; implicit_things = concatMap implicitTyThings at_idx_tycon
169 -- (2) Add the tycons of indexed types and their implicit
170 -- tythings to the global environment
171 ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do {
173 -- (3) Instances from generic class declarations
174 ; generic_inst_info <- getGenericInstances clas_decls
176 -- Next, construct the instance environment so far, consisting
178 -- a) local instance decls
179 -- b) generic instances
180 -- c) local family instance decls
181 ; addInsts local_info $ do {
182 ; addInsts generic_inst_info $ do {
183 ; addFamInsts at_idx_tycon $ do {
185 -- (4) Compute instances from "deriving" clauses;
186 -- This stuff computes a context for the derived instance
187 -- decl, so it needs to know about all the instances possible
188 -- NB: class instance declarations can contain derivings as
189 -- part of associated data type declarations
190 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls
192 ; addInsts deriv_inst_info $ do {
194 ; gbl_env <- getGblEnv
196 generic_inst_info ++ deriv_inst_info ++ local_info,
200 -- Make sure that toplevel type instance are not for associated types.
201 -- !!!TODO: Need to perform this check for the TyThing of type functions,
203 tcIdxTyInstDeclTL ldecl@(L loc decl) =
204 do { tything <- tcFamInstDecl ldecl
206 when (isAssocFamily tything) $
207 addErr $ assocInClassErr (tcdName decl)
210 isAssocFamily (Just (ATyCon tycon)) =
211 case tyConFamInst_maybe tycon of
212 Nothing -> panic "isAssocFamily: no family?!?"
213 Just (fam, _) -> isTyConAssoc fam
214 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
215 isAssocFamily Nothing = False
217 assocInClassErr name =
218 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
219 ptext (sLit "must be inside a class instance")
221 addInsts :: [InstInfo] -> TcM a -> TcM a
222 addInsts infos thing_inside
223 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
225 addFamInsts :: [TyThing] -> TcM a -> TcM a
226 addFamInsts tycons thing_inside
227 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
229 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
230 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
235 tcLocalInstDecl1 :: LInstDecl Name
236 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
237 -- A source-file instance declaration
238 -- Type-check all the stuff before the "where"
240 -- We check for respectable instance type, and context
241 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
242 = -- Prime error recovery, set source location
243 recoverM (return ([], [])) $
245 addErrCtxt (instDeclCtxt1 poly_ty) $
247 do { is_boot <- tcIsHsBoot
248 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
251 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
253 -- Next, process any associated types.
254 ; idx_tycons <- mapM tcFamInstDecl ats
256 -- Now, check the validity of the instance.
257 ; (clas, inst_tys) <- checkValidInstHead tau
258 ; checkValidInstance tyvars theta clas inst_tys
259 ; checkValidAndMissingATs clas (tyvars, inst_tys)
262 -- Finally, construct the Core representation of the instance.
263 -- (This no longer includes the associated types.)
264 ; dfun_name <- newDFunName clas inst_tys loc
265 ; overlap_flag <- getOverlapFlag
266 ; let (eq_theta,dict_theta) = partition isEqPred theta
267 theta' = eq_theta ++ dict_theta
268 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
269 ispec = mkLocalInstance dfun overlap_flag
271 ; return ([InstInfo { iSpec = ispec,
272 iBinds = VanillaInst binds uprags }],
273 catMaybes idx_tycons)
276 -- We pass in the source form and the type checked form of the ATs. We
277 -- really need the source form only to be able to produce more informative
279 checkValidAndMissingATs :: Class
280 -> ([TyVar], [TcType]) -- instance types
281 -> [(LTyClDecl Name, -- source form of AT
282 Maybe TyThing)] -- Core form of AT
284 checkValidAndMissingATs clas inst_tys ats
285 = do { -- Issue a warning for each class AT that is not defined in this
287 ; let class_ats = map tyConName (classATs clas)
288 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
289 omitted = filterOut (`elemNameSet` defined_ats) class_ats
290 ; warn <- doptM Opt_WarnMissingMethods
291 ; mapM_ (warnTc warn . omittedATWarn) omitted
293 -- Ensure that all AT indexes that correspond to class parameters
294 -- coincide with the types in the instance head. All remaining
295 -- AT arguments must be variables. Also raise an error for any
296 -- type instances that are not associated with this class.
297 ; mapM_ (checkIndexes clas inst_tys) ats
300 checkIndexes _ _ (hsAT, Nothing) =
301 return () -- skip, we already had an error here
302 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
303 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
304 checkIndexes' clas inst_tys hsAT
306 snd . fromJust . tyConFamInst_maybe $ tycon)
307 checkIndexes _ _ _ = panic "checkIndexes"
309 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
310 = let atName = tcdName . unLoc $ hsAT
312 setSrcSpan (getLoc hsAT) $
313 addErrCtxt (atInstCtxt atName) $
314 case find ((atName ==) . tyConName) (classATs clas) of
315 Nothing -> addErrTc $ badATErr clas atName -- not in this class
317 case assocTyConArgPoss_maybe atDecl of
318 Nothing -> panic "checkIndexes': AT has no args poss?!?"
321 -- The following is tricky! We need to deal with three
322 -- complications: (1) The AT possibly only uses a subset of
323 -- the class parameters as indexes and those it uses may be in
324 -- a different order; (2) the AT may have extra arguments,
325 -- which must be type variables; and (3) variables in AT and
326 -- instance head will be different `Name's even if their
327 -- source lexemes are identical.
329 -- Re (1), `poss' contains a permutation vector to extract the
330 -- class parameters in the right order.
332 -- Re (2), we wrap the (permuted) class parameters in a Maybe
333 -- type and use Nothing for any extra AT arguments. (First
334 -- equation of `checkIndex' below.)
336 -- Re (3), we replace any type variable in the AT parameters
337 -- that has the same source lexeme as some variable in the
338 -- instance types with the instance type variable sharing its
341 let relevantInstTys = map (instTys !!) poss
342 instArgs = map Just relevantInstTys ++
343 repeat Nothing -- extra arguments
344 renaming = substSameTyVar atTvs instTvs
346 zipWithM_ checkIndex (substTys renaming atTys) instArgs
348 checkIndex ty Nothing
349 | isTyVarTy ty = return ()
350 | otherwise = addErrTc $ mustBeVarArgErr ty
351 checkIndex ty (Just instTy)
352 | ty `tcEqType` instTy = return ()
353 | otherwise = addErrTc $ wrongATArgErr ty instTy
355 listToNameSet = addListToNameSet emptyNameSet
357 substSameTyVar [] _ = emptyTvSubst
358 substSameTyVar (tv:tvs) replacingTvs =
359 let replacement = case find (tv `sameLexeme`) replacingTvs of
360 Nothing -> mkTyVarTy tv
361 Just rtv -> mkTyVarTy rtv
363 tv1 `sameLexeme` tv2 =
364 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
366 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
370 %************************************************************************
372 \subsection{Type-checking instance declarations, pass 2}
374 %************************************************************************
377 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
378 -> TcM (LHsBinds Id, TcLclEnv)
379 -- (a) From each class declaration,
380 -- generate any default-method bindings
381 -- (b) From each instance decl
382 -- generate the dfun binding
384 tcInstDecls2 tycl_decls inst_decls
385 = do { -- (a) Default methods from class decls
386 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
387 filter (isClassDecl.unLoc) tycl_decls
388 ; tcExtendIdEnv (concat dm_ids_s) $ do
390 -- (b) instance declarations
391 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
394 ; let binds = unionManyBags dm_binds_s `unionBags`
395 unionManyBags inst_binds_s
396 ; tcl_env <- getLclEnv -- Default method Ids in here
397 ; return (binds, tcl_env) }
400 ======= New documentation starts here (Sept 92) ==============
402 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
403 the dictionary function for this instance declaration. For example
405 instance Foo a => Foo [a] where
409 might generate something like
411 dfun.Foo.List dFoo_a = let op1 x = ...
416 HOWEVER, if the instance decl has no context, then it returns a
417 bigger @HsBinds@ with declarations for each method. For example
419 instance Foo [a] where
425 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
426 const.Foo.op1.List a x = ...
427 const.Foo.op2.List a y = ...
429 This group may be mutually recursive, because (for example) there may
430 be no method supplied for op2 in which case we'll get
432 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
434 that is, the default method applied to the dictionary at this type.
435 What we actually produce in either case is:
437 AbsBinds [a] [dfun_theta_dicts]
438 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
439 { d = (sd1,sd2, ..., op1, op2, ...)
444 The "maybe" says that we only ask AbsBinds to make global constant methods
445 if the dfun_theta is empty.
447 For an instance declaration, say,
449 instance (C1 a, C2 b) => C (T a b) where
452 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
453 function whose type is
455 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
457 Notice that we pass it the superclass dictionaries at the instance type; this
458 is the ``Mark Jones optimisation''. The stuff before the "=>" here
459 is the @dfun_theta@ below.
463 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
464 -- Returns a binding for the dfun
466 ------------------------
467 -- Derived newtype instances; surprisingly tricky!
469 -- class Show a => Foo a b where ...
470 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
472 -- The newtype gives an FC axiom looking like
473 -- axiom CoN a :: N 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 (N a)) => Foo Int (N a)
478 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
479 -- case df `cast` (Foo Int (sym (CoN 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 })
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 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
494 (class_tyvars, sc_theta, _, op_items) = classBigSig cls
495 cls_tycon = classTyCon cls
496 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
498 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
499 (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail
500 rep_ty = newTyConInstRhs nt_tycon tc_args
502 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
503 -- In our example, rep_pred is (Foo Int (Tree [a]))
504 the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
505 -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)
507 ; inst_loc <- getInstLoc origin
508 ; sc_loc <- getInstLoc InstScOrigin
509 ; dfun_dicts <- newDictBndrs inst_loc theta
510 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
511 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
512 ; rep_dict <- newDictBndr inst_loc rep_pred
514 -- Figure out bindings for the superclass context from dfun_dicts
515 -- Don't include this_dict in the 'givens', else
516 -- wanted_sc_insts get bound by just selecting from this_dict!!
517 ; sc_binds <- addErrCtxt superClassCtxt $
518 tcSimplifySuperClasses inst_loc dfun_dicts (rep_dict:sc_dicts)
520 ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict))
522 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
523 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
525 ; return (unitBag $ noLoc $
526 AbsBinds tvs (map instToVar dfun_dicts)
527 [(tvs, dfun_id, instToId this_dict, [])]
528 (dict_bind `consBag` sc_binds)) }
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 initial_cls_inst_tys nt_tycon tc_args
540 | Just co_con <- newTyConCo_maybe nt_tycon
541 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
542 = WpCast (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))
543 | otherwise -- The newtype is transparent; no need for a cast
546 -----------------------
547 -- (make_body C tys scs coreced_rep_dict)
549 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
550 -- But if there are no superclasses, it returns just coerced_rep_dict
551 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
553 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
554 | null sc_dicts -- Case (a)
555 = return coerced_rep_dict
556 | otherwise -- Case (b)
557 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
558 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
559 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
560 pat_dicts = dummy_sc_dict_ids,
561 pat_binds = emptyLHsBinds,
562 pat_args = PrefixCon (map nlVarPat op_ids),
564 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
565 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
566 map HsVar (sc_dict_ids ++ op_ids)
568 -- Warning: this HsCase scrutinises a value with a PredTy, which is
569 -- never otherwise seen in Haskell source code. It'd be
570 -- nicer to generate Core directly!
571 ; return (HsCase (noLoc coerced_rep_dict) $
572 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
574 sc_dict_ids = map instToId sc_dicts
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 (return emptyLHsBinds) $
593 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do
595 -- Instantiate the instance decl with skolem constants
596 (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
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 (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
607 origin = SigOrigin rigid_info
608 (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
610 -- Create dictionary Ids from the specified instance contexts.
611 sc_loc <- getInstLoc InstScOrigin
612 sc_dicts <- newDictBndrs sc_loc dict_sc_theta'
613 inst_loc <- getInstLoc origin
614 sc_covars <- mkMetaCoVars eq_sc_theta'
615 wanted_sc_eqs <- mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars)
616 dfun_covars <- mkCoVars eq_dfun_theta'
617 dfun_eqs <- mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars)
618 dfun_dicts <- newDictBndrs inst_loc dict_dfun_theta'
619 this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
620 -- Default-method Ids may be mentioned in synthesised RHSs,
621 -- but they'll already be in the environment.
623 -- Typecheck the methods
624 let -- These insts are in scope; quite a few, eh?
625 dfun_insts = dfun_eqs ++ dfun_dicts
626 wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
627 given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
628 given_sc_insts = given_sc_eqs ++ sc_dicts
629 avail_insts = dfun_insts ++ given_sc_insts
631 (meth_ids, meth_binds) <- tcMethods origin clas inst_tyvars'
632 dfun_theta' inst_tys' this_dict avail_insts
633 op_items monobinds uprags
635 -- Figure out bindings for the superclass context
636 -- Don't include this_dict in the 'givens', else
637 -- wanted_sc_insts get bound by just selecting from this_dict!!
638 sc_binds <- addErrCtxt superClassCtxt
639 (tcSimplifySuperClasses inst_loc dfun_insts wanted_sc_insts)
641 -- It's possible that the superclass stuff might unified one
642 -- of the inst_tyavars' with something in the envt
643 checkSigTyVars inst_tyvars'
645 -- Deal with 'SPECIALISE instance' pragmas
646 prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
648 -- Create the result bindings
650 dict_constr = classDataCon clas
651 scs_and_meths = map instToId sc_dicts ++ meth_ids
652 this_dict_id = instToId this_dict
653 inline_prag | null dfun_insts = []
654 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
655 -- Always inline the dfun; this is an experimental decision
656 -- because it makes a big performance difference sometimes.
657 -- Often it means we can do the method selection, and then
658 -- inline the method as well. Marcin's idea; see comments below.
660 -- BUT: don't inline it if it's a constant dictionary;
661 -- we'll get all the benefit without inlining, and we get
662 -- a **lot** of code duplication if we inline it
664 -- See Note [Inline dfuns] below
666 dict_rhs = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars)
667 (map HsVar scs_and_meths)
668 -- We don't produce a binding for the dict_constr; instead we
669 -- rely on the simplifier to unfold this saturated application
670 -- We do this rather than generate an HsCon directly, because
671 -- it means that the special cases (e.g. dictionary with only one
672 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
673 -- than needing to be repeated here.
675 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
676 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
678 main_bind = noLoc $ AbsBinds
679 (inst_tyvars' ++ dfun_covars)
680 (map instToId dfun_dicts)
681 [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
684 showLIE (text "instance")
685 return (unitBag main_bind)
687 mkCoVars :: [PredType] -> TcM [TyVar]
688 mkCoVars = newCoVars . map unEqPred
690 unEqPred (EqPred ty1 ty2) = (ty1, ty2)
691 unEqPred _ = panic "TcInstDcls.mkCoVars"
693 mkMetaCoVars :: [PredType] -> TcM [TyVar]
694 mkMetaCoVars = mapM eqPredToCoVar
696 eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2
697 eqPredToCoVar _ = panic "TcInstDcls.mkMetaCoVars"
699 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
700 this_dict extra_insts op_items monobinds uprags = do
701 -- Check that all the method bindings come from this class
703 sel_names = [idName sel_id | (sel_id, _) <- op_items]
704 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
706 mapM (addErrTc . badMethodErr clas) bad_bndrs
708 -- Make the method bindings
710 mk_method_id (sel_id, _) = mkMethId origin clas sel_id inst_tys'
712 (meth_insts, meth_ids) <- mapAndUnzipM mk_method_id op_items
714 -- And type check them
715 -- It's really worth making meth_insts available to the tcMethodBind
716 -- Consider instance Monad (ST s) where
717 -- {-# INLINE (>>) #-}
718 -- (>>) = ...(>>=)...
719 -- If we don't include meth_insts, we end up with bindings like this:
720 -- rec { dict = MkD then bind ...
721 -- then = inline_me (... (GHC.Base.>>= dict) ...)
723 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
724 -- and (b) the inline_me prevents us inlining the >>= selector, which
725 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
726 -- is not inlined across modules. Rather ironic since this does not
727 -- happen without the INLINE pragma!
729 -- Solution: make meth_insts available, so that 'then' refers directly
730 -- to the local 'bind' rather than going via the dictionary.
732 -- BUT WATCH OUT! If the method type mentions the class variable, then
733 -- this optimisation is not right. Consider
737 -- instance C Int where
739 -- The occurrence of 'op' on the rhs gives rise to a constraint
741 -- The trouble is that the 'meth_inst' for op, which is 'available', also
742 -- looks like 'op at Int'. But they are not the same.
744 prag_fn = mkPragFun uprags
745 all_insts = extra_insts ++ catMaybes meth_insts
746 sig_fn n = Just [] -- No scoped type variables, but every method has
747 -- a type signature, in effect, so that we check
748 -- the method has the right type
749 tc_method_bind = tcMethodBind origin inst_tyvars' dfun_theta' this_dict
750 all_insts sig_fn prag_fn monobinds
752 meth_binds_s <- zipWithM tc_method_bind op_items meth_ids
754 return (meth_ids, unionManyBags meth_binds_s)
758 ------------------------------
759 [Inline dfuns] Inlining dfuns unconditionally
760 ------------------------------
762 The code above unconditionally inlines dict funs. Here's why.
763 Consider this program:
765 test :: Int -> Int -> Bool
766 test x y = (x,y) == (y,x) || test y x
767 -- Recursive to avoid making it inline.
769 This needs the (Eq (Int,Int)) instance. If we inline that dfun
770 the code we end up with is good:
773 \r -> case ==# [ww ww1] of wild {
774 PrelBase.False -> Test.$wtest ww1 ww;
776 case ==# [ww1 ww] of wild1 {
777 PrelBase.False -> Test.$wtest ww1 ww;
778 PrelBase.True -> PrelBase.True [];
781 Test.test = \r [w w1]
784 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
787 If we don't inline the dfun, the code is not nearly as good:
789 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
790 PrelBase.:DEq tpl1 tpl2 -> tpl2;
795 let { y = PrelBase.I#! [ww1]; } in
796 let { x = PrelBase.I#! [ww]; } in
797 let { sat_slx = PrelTup.(,)! [y x]; } in
798 let { sat_sly = PrelTup.(,)! [x y];
800 case == sat_sly sat_slx of wild {
801 PrelBase.False -> Test.$wtest ww1 ww;
802 PrelBase.True -> PrelBase.True [];
809 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
812 Why doesn't GHC inline $fEq? Because it looks big:
814 PrelTup.zdfEqZ1T{-rcX-}
815 = \ @ a{-reT-} :: * @ b{-reS-} :: *
816 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
817 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
819 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
820 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
822 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
823 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
825 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
826 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
827 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
829 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
831 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
833 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
834 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
838 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
839 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
840 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
841 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
843 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
845 and it's not as bad as it seems, because it's further dramatically
846 simplified: only zeze2 is extracted and its body is simplified.
849 %************************************************************************
851 \subsection{Error messages}
853 %************************************************************************
856 instDeclCtxt1 hs_inst_ty
857 = inst_decl_ctxt (case unLoc hs_inst_ty of
858 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
859 HsPredTy pred -> ppr pred
860 other -> ppr hs_inst_ty) -- Don't expect this
861 instDeclCtxt2 dfun_ty
862 = inst_decl_ctxt (ppr (mkClassPred cls tys))
864 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
866 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
868 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
870 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
874 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
875 ptext (sLit "must be variables")
876 , ptext (sLit "Instead of a variable, found") <+> ppr ty
879 wrongATArgErr ty instTy =
880 sep [ ptext (sLit "Type indexes must match class instance head")
881 , ptext (sLit "Found") <+> ppr ty <+> ptext (sLit "but expected") <+>