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 <- mapAndRecoverM tcLocalInstDecl1 inst_decls
153 ; idx_tycons <- mapAndRecoverM tcIdxTyInstDeclTL idxty_decls
156 at_tycons_s) = unzip local_info_tycons
157 ; at_idx_tycon = concat at_tycons_s ++ idx_tycons
158 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
159 ; implicit_things = concatMap implicitTyThings at_idx_tycon
162 -- (2) Add the tycons of indexed types and their implicit
163 -- tythings to the global environment
164 ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do {
166 -- (3) Instances from generic class declarations
167 ; generic_inst_info <- getGenericInstances clas_decls
169 -- Next, construct the instance environment so far, consisting
171 -- a) local instance decls
172 -- b) generic instances
173 -- c) local family instance decls
174 ; addInsts local_info $ do {
175 ; addInsts generic_inst_info $ do {
176 ; addFamInsts at_idx_tycon $ do {
178 -- (4) Compute instances from "deriving" clauses;
179 -- This stuff computes a context for the derived instance
180 -- decl, so it needs to know about all the instances possible
181 -- NB: class instance declarations can contain derivings as
182 -- part of associated data type declarations
183 failIfErrsM -- If the addInsts stuff gave any errors, don't
184 -- try the deriving stuff, becuase that may give
186 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls
188 ; addInsts deriv_inst_info $ do {
190 ; gbl_env <- getGblEnv
192 generic_inst_info ++ deriv_inst_info ++ local_info,
196 -- Make sure that toplevel type instance are not for associated types.
197 -- !!!TODO: Need to perform this check for the TyThing of type functions,
199 tcIdxTyInstDeclTL ldecl@(L loc decl) =
200 do { tything <- tcFamInstDecl ldecl
202 when (isAssocFamily tything) $
203 addErr $ assocInClassErr (tcdName decl)
206 isAssocFamily (ATyCon tycon) =
207 case tyConFamInst_maybe tycon of
208 Nothing -> panic "isAssocFamily: no family?!?"
209 Just (fam, _) -> isTyConAssoc fam
210 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
212 assocInClassErr :: Name -> SDoc
213 assocInClassErr name =
214 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
215 ptext (sLit "must be inside a class instance")
217 addInsts :: [InstInfo Name] -> TcM a -> TcM a
218 addInsts infos thing_inside
219 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
221 addFamInsts :: [TyThing] -> TcM a -> TcM a
222 addFamInsts tycons thing_inside
223 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
225 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
226 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
231 tcLocalInstDecl1 :: LInstDecl Name
232 -> TcM (InstInfo Name, [TyThing])
233 -- A source-file instance declaration
234 -- Type-check all the stuff before the "where"
236 -- We check for respectable instance type, and context
237 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
239 addErrCtxt (instDeclCtxt1 poly_ty) $
241 do { is_boot <- tcIsHsBoot
242 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
245 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
247 -- Now, check the validity of the instance.
248 ; (clas, inst_tys) <- checkValidInstHead tau
249 ; checkValidInstance tyvars theta clas inst_tys
251 -- Next, process any associated types.
252 ; idx_tycons <- recoverM (return []) $
253 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
254 ; checkValidAndMissingATs clas (tyvars, inst_tys)
256 ; return idx_tycons }
258 -- Finally, construct the Core representation of the instance.
259 -- (This no longer includes the associated types.)
260 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
261 -- Dfun location is that of instance *header*
262 ; overlap_flag <- getOverlapFlag
263 ; let (eq_theta,dict_theta) = partition isEqPred theta
264 theta' = eq_theta ++ dict_theta
265 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
266 ispec = mkLocalInstance dfun overlap_flag
268 ; return (InstInfo { iSpec = ispec,
269 iBinds = VanillaInst binds uprags },
273 -- We pass in the source form and the type checked form of the ATs. We
274 -- really need the source form only to be able to produce more informative
276 checkValidAndMissingATs :: Class
277 -> ([TyVar], [TcType]) -- instance types
278 -> [(LTyClDecl Name, -- source form of AT
279 TyThing)] -- Core form of AT
281 checkValidAndMissingATs clas inst_tys ats
282 = do { -- Issue a warning for each class AT that is not defined in this
284 ; let class_ats = map tyConName (classATs clas)
285 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
286 omitted = filterOut (`elemNameSet` defined_ats) class_ats
287 ; warn <- doptM Opt_WarnMissingMethods
288 ; mapM_ (warnTc warn . omittedATWarn) omitted
290 -- Ensure that all AT indexes that correspond to class parameters
291 -- coincide with the types in the instance head. All remaining
292 -- AT arguments must be variables. Also raise an error for any
293 -- type instances that are not associated with this class.
294 ; mapM_ (checkIndexes clas inst_tys) ats
297 checkIndexes clas inst_tys (hsAT, 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 Name]
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 <- mapM 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 ; return (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 = ...
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.
430 What we actually produce in either case is:
432 AbsBinds [a] [dfun_theta_dicts]
433 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
434 { d = (sd1,sd2, ..., op1, op2, ...)
439 The "maybe" says that we only ask AbsBinds to make global constant methods
440 if the dfun_theta is empty.
442 For an instance declaration, say,
444 instance (C1 a, C2 b) => C (T a b) where
447 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
448 function whose type is
450 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
452 Notice that we pass it the superclass dictionaries at the instance type; this
453 is the ``Mark Jones optimisation''. The stuff before the "=>" here
454 is the @dfun_theta@ below.
458 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
459 -- Returns a binding for the dfun
461 ------------------------
462 -- Derived newtype instances; surprisingly tricky!
464 -- class Show a => Foo a b where ...
465 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
467 -- The newtype gives an FC axiom looking like
468 -- axiom CoN a :: N a :=: Tree [a]
469 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
471 -- So all need is to generate a binding looking like:
472 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
473 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
474 -- case df `cast` (Foo Int (sym (CoN a))) of
475 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
477 -- If there are no superclasses, matters are simpler, because we don't need the case
478 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
480 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived })
481 = do { let dfun_id = instanceDFunId ispec
482 rigid_info = InstSkol
483 origin = SigOrigin rigid_info
484 inst_ty = idType dfun_id
485 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
486 -- inst_head_ty is a PredType
488 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
489 (class_tyvars, sc_theta, _, _) = classBigSig cls
490 cls_tycon = classTyCon cls
491 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
493 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
494 (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail
495 rep_ty = newTyConInstRhs nt_tycon tc_args
497 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
498 -- In our example, rep_pred is (Foo Int (Tree [a]))
499 the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
500 -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)
502 ; inst_loc <- getInstLoc origin
503 ; sc_loc <- getInstLoc InstScOrigin
504 ; dfun_dicts <- newDictBndrs inst_loc theta
505 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
506 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
507 ; rep_dict <- newDictBndr inst_loc rep_pred
509 -- Figure out bindings for the superclass context from dfun_dicts
510 -- Don't include this_dict in the 'givens', else
511 -- wanted_sc_insts get bound by just selecting from this_dict!!
512 ; sc_binds <- addErrCtxt superClassCtxt $
513 tcSimplifySuperClasses inst_loc dfun_dicts (rep_dict:sc_dicts)
515 ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict))
517 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
518 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
520 ; return (unitBag $ noLoc $
521 AbsBinds tvs (map instToVar dfun_dicts)
522 [(tvs, dfun_id, instToId this_dict, [])]
523 (dict_bind `consBag` sc_binds)) }
525 -----------------------
527 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
528 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
529 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
530 -- where rep_ty is the (eta-reduced) type rep of T
531 -- So we just replace T with CoT, and insert a 'sym'
532 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
534 make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
535 | Just co_con <- newTyConCo_maybe nt_tycon
536 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
537 = WpCast (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))
538 | otherwise -- The newtype is transparent; no need for a cast
541 -----------------------
542 -- (make_body C tys scs coreced_rep_dict)
544 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
545 -- But if there are no superclasses, it returns just coerced_rep_dict
546 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
548 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
549 | null sc_dicts -- Case (a)
550 = return coerced_rep_dict
551 | otherwise -- Case (b)
552 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
553 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
554 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
555 pat_dicts = dummy_sc_dict_ids,
556 pat_binds = emptyLHsBinds,
557 pat_args = PrefixCon (map nlVarPat op_ids),
559 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
560 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
561 map HsVar (sc_dict_ids ++ op_ids)
563 -- Warning: this HsCase scrutinises a value with a PredTy, which is
564 -- never otherwise seen in Haskell source code. It'd be
565 -- nicer to generate Core directly!
566 ; return (HsCase (noLoc coerced_rep_dict) $
567 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
569 sc_dict_ids = map instToId sc_dicts
570 pat_ty = mkTyConApp cls_tycon cls_inst_tys
571 cls_data_con = head (tyConDataCons cls_tycon)
572 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
573 op_tys = dropList sc_dict_ids cls_arg_tys
575 ------------------------
576 -- Ordinary instances
578 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
580 dfun_id = instanceDFunId ispec
581 rigid_info = InstSkol
582 inst_ty = idType dfun_id
583 loc = getSrcSpan dfun_id
585 -- Prime error recovery
586 recoverM (return emptyLHsBinds) $
588 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do
590 -- Instantiate the instance decl with skolem constants
591 (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
592 -- These inst_tyvars' scope over the 'where' part
593 -- Those tyvars are inside the dfun_id's type, which is a bit
594 -- bizarre, but OK so long as you realise it!
596 (clas, inst_tys') = tcSplitDFunHead inst_head'
597 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
599 -- Instantiate the super-class context with inst_tys
600 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
601 (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
602 origin = SigOrigin rigid_info
603 (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
605 -- Create dictionary Ids from the specified instance contexts.
606 sc_loc <- getInstLoc InstScOrigin
607 sc_dicts <- newDictBndrs sc_loc dict_sc_theta'
608 inst_loc <- getInstLoc origin
609 sc_covars <- mkMetaCoVars eq_sc_theta'
610 wanted_sc_eqs <- mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars)
611 dfun_covars <- mkCoVars eq_dfun_theta'
612 dfun_eqs <- mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars)
613 dfun_dicts <- newDictBndrs inst_loc dict_dfun_theta'
614 this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
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 dfun_insts = dfun_eqs ++ dfun_dicts
621 wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
622 given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
623 given_sc_insts = given_sc_eqs ++ sc_dicts
624 avail_insts = dfun_insts ++ given_sc_insts
626 (meth_ids, meth_binds) <- tcMethods origin clas inst_tyvars'
627 dfun_theta' inst_tys' this_dict avail_insts
628 op_items monobinds uprags
630 -- Figure out bindings for the superclass context
631 -- Don't include this_dict in the 'givens', else
632 -- wanted_sc_insts get bound by just selecting from this_dict!!
633 sc_binds <- addErrCtxt superClassCtxt
634 (tcSimplifySuperClasses inst_loc dfun_insts wanted_sc_insts)
636 -- It's possible that the superclass stuff might unified one
637 -- of the inst_tyavars' with something in the envt
638 checkSigTyVars inst_tyvars'
640 -- Deal with 'SPECIALISE instance' pragmas
641 prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
643 -- Create the result bindings
645 dict_constr = classDataCon clas
646 scs_and_meths = map instToId sc_dicts ++ meth_ids
647 this_dict_id = instToId this_dict
648 inline_prag | null dfun_insts = []
649 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
650 -- Always inline the dfun; this is an experimental decision
651 -- because it makes a big performance difference sometimes.
652 -- Often it means we can do the method selection, and then
653 -- inline the method as well. Marcin's idea; see comments below.
655 -- BUT: don't inline it if it's a constant dictionary;
656 -- we'll get all the benefit without inlining, and we get
657 -- a **lot** of code duplication if we inline it
659 -- See Note [Inline dfuns] below
661 dict_rhs = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars)
662 (map HsVar scs_and_meths)
663 -- We don't produce a binding for the dict_constr; instead we
664 -- rely on the simplifier to unfold this saturated application
665 -- We do this rather than generate an HsCon directly, because
666 -- it means that the special cases (e.g. dictionary with only one
667 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
668 -- than needing to be repeated here.
670 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
671 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
673 main_bind = noLoc $ AbsBinds
674 (inst_tyvars' ++ dfun_covars)
675 (map instToId dfun_dicts)
676 [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
679 showLIE (text "instance")
680 return (unitBag main_bind)
682 mkCoVars :: [PredType] -> TcM [TyVar]
683 mkCoVars = newCoVars . map unEqPred
685 unEqPred (EqPred ty1 ty2) = (ty1, ty2)
686 unEqPred _ = panic "TcInstDcls.mkCoVars"
688 mkMetaCoVars :: [PredType] -> TcM [TyVar]
689 mkMetaCoVars = mapM eqPredToCoVar
691 eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2
692 eqPredToCoVar _ = panic "TcInstDcls.mkMetaCoVars"
694 tcMethods :: InstOrigin -> Class -> [TcTyVar] -> TcThetaType -> [TcType]
695 -> Inst -> [Inst] -> [(Id, DefMeth)] -> LHsBindsLR Name Name
697 -> TcM ([Id], Bag (LHsBind Id))
698 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
699 this_dict extra_insts op_items monobinds uprags = do
700 -- Check that all the method bindings come from this class
702 sel_names = [idName sel_id | (sel_id, _) <- op_items]
703 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
705 mapM (addErrTc . badMethodErr clas) bad_bndrs
707 -- Make the method bindings
709 mk_method_id (sel_id, _) = mkMethId origin clas sel_id inst_tys'
711 (meth_insts, meth_ids) <- mapAndUnzipM mk_method_id op_items
713 -- And type check them
714 -- It's really worth making meth_insts available to the tcMethodBind
715 -- Consider instance Monad (ST s) where
716 -- {-# INLINE (>>) #-}
717 -- (>>) = ...(>>=)...
718 -- If we don't include meth_insts, we end up with bindings like this:
719 -- rec { dict = MkD then bind ...
720 -- then = inline_me (... (GHC.Base.>>= dict) ...)
722 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
723 -- and (b) the inline_me prevents us inlining the >>= selector, which
724 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
725 -- is not inlined across modules. Rather ironic since this does not
726 -- happen without the INLINE pragma!
728 -- Solution: make meth_insts available, so that 'then' refers directly
729 -- to the local 'bind' rather than going via the dictionary.
731 -- BUT WATCH OUT! If the method type mentions the class variable, then
732 -- this optimisation is not right. Consider
736 -- instance C Int where
738 -- The occurrence of 'op' on the rhs gives rise to a constraint
740 -- The trouble is that the 'meth_inst' for op, which is 'available', also
741 -- looks like 'op at Int'. But they are not the same.
743 prag_fn = mkPragFun uprags
744 all_insts = extra_insts ++ catMaybes meth_insts
745 sig_fn _ = Just [] -- No scoped type variables, but every method has
746 -- a type signature, in effect, so that we check
747 -- the method has the right type
748 tc_method_bind = tcMethodBind origin inst_tyvars' dfun_theta' this_dict
749 all_insts sig_fn prag_fn monobinds
751 meth_binds_s <- zipWithM tc_method_bind op_items meth_ids
753 return (meth_ids, unionManyBags meth_binds_s)
757 ------------------------------
758 [Inline dfuns] Inlining dfuns unconditionally
759 ------------------------------
761 The code above unconditionally inlines dict funs. Here's why.
762 Consider this program:
764 test :: Int -> Int -> Bool
765 test x y = (x,y) == (y,x) || test y x
766 -- Recursive to avoid making it inline.
768 This needs the (Eq (Int,Int)) instance. If we inline that dfun
769 the code we end up with is good:
772 \r -> case ==# [ww ww1] of wild {
773 PrelBase.False -> Test.$wtest ww1 ww;
775 case ==# [ww1 ww] of wild1 {
776 PrelBase.False -> Test.$wtest ww1 ww;
777 PrelBase.True -> PrelBase.True [];
780 Test.test = \r [w w1]
783 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
786 If we don't inline the dfun, the code is not nearly as good:
788 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
789 PrelBase.:DEq tpl1 tpl2 -> tpl2;
794 let { y = PrelBase.I#! [ww1]; } in
795 let { x = PrelBase.I#! [ww]; } in
796 let { sat_slx = PrelTup.(,)! [y x]; } in
797 let { sat_sly = PrelTup.(,)! [x y];
799 case == sat_sly sat_slx of wild {
800 PrelBase.False -> Test.$wtest ww1 ww;
801 PrelBase.True -> PrelBase.True [];
808 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
811 Why doesn't GHC inline $fEq? Because it looks big:
813 PrelTup.zdfEqZ1T{-rcX-}
814 = \ @ a{-reT-} :: * @ b{-reS-} :: *
815 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
816 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
818 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
819 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
821 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
822 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
824 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
825 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
826 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
828 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
830 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
832 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
833 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
837 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
838 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
839 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
840 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
842 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
844 and it's not as bad as it seems, because it's further dramatically
845 simplified: only zeze2 is extracted and its body is simplified.
848 %************************************************************************
850 \subsection{Error messages}
852 %************************************************************************
855 instDeclCtxt1 :: LHsType Name -> SDoc
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 _ -> ppr hs_inst_ty) -- Don't expect this
861 instDeclCtxt2 :: Type -> SDoc
862 instDeclCtxt2 dfun_ty
863 = inst_decl_ctxt (ppr (mkClassPred cls tys))
865 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
867 inst_decl_ctxt :: SDoc -> SDoc
868 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
870 superClassCtxt :: SDoc
871 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
873 atInstCtxt :: Name -> SDoc
874 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
877 mustBeVarArgErr :: Type -> SDoc
879 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
880 ptext (sLit "must be variables")
881 , ptext (sLit "Instead of a variable, found") <+> ppr ty
884 wrongATArgErr :: Type -> Type -> SDoc
885 wrongATArgErr ty instTy =
886 sep [ ptext (sLit "Type indexes must match class instance head")
887 , ptext (sLit "Found") <+> ppr ty <+> ptext (sLit "but expected") <+>