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
52 Typechecking instance declarations is done in two passes. The first
53 pass, made by @tcInstDecls1@, collects information to be used in the
56 This pre-processed info includes the as-yet-unprocessed bindings
57 inside the instance declaration. These are type-checked in the second
58 pass, when the class-instance envs and GVE contain all the info from
59 all the instance and value decls. Indeed that's the reason we need
60 two passes over the instance decls.
62 Here is the overall algorithm.
63 Assume that we have an instance declaration
65 instance c => k (t tvs) where b
69 $LIE_c$ is the LIE for the context of class $c$
71 $betas_bar$ is the free variables in the class method type, excluding the
74 $LIE_cop$ is the LIE constraining a particular class method
76 $tau_cop$ is the tau type of a class method
78 $LIE_i$ is the LIE for the context of instance $i$
80 $X$ is the instance constructor tycon
82 $gammas_bar$ is the set of type variables of the instance
84 $LIE_iop$ is the LIE for a particular class method instance
86 $tau_iop$ is the tau type for this instance of a class method
88 $alpha$ is the class variable
90 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
92 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
95 ToDo: Update the list above with names actually in the code.
99 First, make the LIEs for the class and instance contexts, which means
100 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
101 and make LIElistI and LIEI.
103 Then process each method in turn.
105 order the instance methods according to the ordering of the class methods
107 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
109 Create final dictionary function from bindings generated already
111 df = lambda inst_tyvars
118 in <op1,op2,...,opn,sd1,...,sdm>
120 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
121 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
125 %************************************************************************
127 \subsection{Extracting instance decls}
129 %************************************************************************
131 Gather up the instance declarations from their various sources
134 tcInstDecls1 -- Deal with both source-code and imported instance decls
135 :: [LTyClDecl Name] -- For deriving stuff
136 -> [LInstDecl Name] -- Source code instance decls
137 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
138 -> TcM (TcGblEnv, -- The full inst env
139 [InstInfo], -- Source-code instance decls to process;
140 -- contains all dfuns for this module
141 HsValBinds Name) -- Supporting bindings for derived instances
143 tcInstDecls1 tycl_decls inst_decls deriv_decls
145 do { -- Stop if addInstInfos etc discovers any errors
146 -- (they recover, so that we get more than one error each
149 -- (1) Do class and family instance declarations
150 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
151 ; local_info_tycons <- mapM tcLocalInstDecl1 inst_decls
152 ; idx_tycons <- mapM tcIdxTyInstDeclTL idxty_decls
154 ; let { (local_infos,
155 at_tycons) = unzip local_info_tycons
156 ; local_info = concat local_infos
157 ; at_idx_tycon = concat at_tycons ++ catMaybes 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 (Just (ATyCon tycon)) =
207 case tyConFamInst_maybe tycon of
208 Nothing -> panic "isAssocFamily: no family?!?"
209 Just (fam, _) -> isTyConAssoc fam
210 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
211 isAssocFamily Nothing = False
213 assocInClassErr :: Name -> SDoc
214 assocInClassErr name =
215 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
216 ptext (sLit "must be inside a class instance")
218 addInsts :: [InstInfo] -> TcM a -> TcM a
219 addInsts infos thing_inside
220 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
222 addFamInsts :: [TyThing] -> TcM a -> TcM a
223 addFamInsts tycons thing_inside
224 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
226 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
227 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
232 tcLocalInstDecl1 :: LInstDecl Name
233 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
234 -- A source-file instance declaration
235 -- Type-check all the stuff before the "where"
237 -- We check for respectable instance type, and context
238 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
239 = -- Prime error recovery, set source location
240 recoverM (return ([], [])) $
242 addErrCtxt (instDeclCtxt1 poly_ty) $
244 do { is_boot <- tcIsHsBoot
245 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
248 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
250 -- Next, process any associated types.
251 ; idx_tycons <- mapM tcFamInstDecl ats
253 -- Now, check the validity of the instance.
254 ; (clas, inst_tys) <- checkValidInstHead tau
255 ; checkValidInstance tyvars theta clas inst_tys
256 ; checkValidAndMissingATs clas (tyvars, inst_tys)
259 -- Finally, construct the Core representation of the instance.
260 -- (This no longer includes the associated types.)
261 ; dfun_name <- newDFunName clas inst_tys loc
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 }],
270 catMaybes idx_tycons)
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 Maybe 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 _ _ (_, Nothing) =
298 return () -- skip, we already had an error here
299 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
300 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
301 checkIndexes' clas inst_tys hsAT
303 snd . fromJust . tyConFamInst_maybe $ tycon)
304 checkIndexes _ _ _ = panic "checkIndexes"
306 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
307 = let atName = tcdName . unLoc $ hsAT
309 setSrcSpan (getLoc hsAT) $
310 addErrCtxt (atInstCtxt atName) $
311 case find ((atName ==) . tyConName) (classATs clas) of
312 Nothing -> addErrTc $ badATErr clas atName -- not in this class
314 case assocTyConArgPoss_maybe atDecl of
315 Nothing -> panic "checkIndexes': AT has no args poss?!?"
318 -- The following is tricky! We need to deal with three
319 -- complications: (1) The AT possibly only uses a subset of
320 -- the class parameters as indexes and those it uses may be in
321 -- a different order; (2) the AT may have extra arguments,
322 -- which must be type variables; and (3) variables in AT and
323 -- instance head will be different `Name's even if their
324 -- source lexemes are identical.
326 -- Re (1), `poss' contains a permutation vector to extract the
327 -- class parameters in the right order.
329 -- Re (2), we wrap the (permuted) class parameters in a Maybe
330 -- type and use Nothing for any extra AT arguments. (First
331 -- equation of `checkIndex' below.)
333 -- Re (3), we replace any type variable in the AT parameters
334 -- that has the same source lexeme as some variable in the
335 -- instance types with the instance type variable sharing its
338 let relevantInstTys = map (instTys !!) poss
339 instArgs = map Just relevantInstTys ++
340 repeat Nothing -- extra arguments
341 renaming = substSameTyVar atTvs instTvs
343 zipWithM_ checkIndex (substTys renaming atTys) instArgs
345 checkIndex ty Nothing
346 | isTyVarTy ty = return ()
347 | otherwise = addErrTc $ mustBeVarArgErr ty
348 checkIndex ty (Just instTy)
349 | ty `tcEqType` instTy = return ()
350 | otherwise = addErrTc $ wrongATArgErr ty instTy
352 listToNameSet = addListToNameSet emptyNameSet
354 substSameTyVar [] _ = emptyTvSubst
355 substSameTyVar (tv:tvs) replacingTvs =
356 let replacement = case find (tv `sameLexeme`) replacingTvs of
357 Nothing -> mkTyVarTy tv
358 Just rtv -> mkTyVarTy rtv
360 tv1 `sameLexeme` tv2 =
361 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
363 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
367 %************************************************************************
369 \subsection{Type-checking instance declarations, pass 2}
371 %************************************************************************
374 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
375 -> TcM (LHsBinds Id, TcLclEnv)
376 -- (a) From each class declaration,
377 -- generate any default-method bindings
378 -- (b) From each instance decl
379 -- generate the dfun binding
381 tcInstDecls2 tycl_decls inst_decls
382 = do { -- (a) Default methods from class decls
383 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
384 filter (isClassDecl.unLoc) tycl_decls
385 ; tcExtendIdEnv (concat dm_ids_s) $ do
387 -- (b) instance declarations
388 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
391 ; let binds = unionManyBags dm_binds_s `unionBags`
392 unionManyBags inst_binds_s
393 ; tcl_env <- getLclEnv -- Default method Ids in here
394 ; return (binds, tcl_env) }
397 ======= New documentation starts here (Sept 92) ==============
399 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
400 the dictionary function for this instance declaration. For example
402 instance Foo a => Foo [a] where
406 might generate something like
408 dfun.Foo.List dFoo_a = let op1 x = ...
413 HOWEVER, if the instance decl has no context, then it returns a
414 bigger @HsBinds@ with declarations for each method. For example
416 instance Foo [a] where
422 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
423 const.Foo.op1.List a x = ...
424 const.Foo.op2.List a y = ...
426 This group may be mutually recursive, because (for example) there may
427 be no method supplied for op2 in which case we'll get
429 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
431 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.
444 For an instance declaration, say,
446 instance (C1 a, C2 b) => C (T a b) where
449 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
450 function whose type is
452 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
454 Notice that we pass it the superclass dictionaries at the instance type; this
455 is the ``Mark Jones optimisation''. The stuff before the "=>" here
456 is the @dfun_theta@ below.
460 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
461 -- Returns a binding for the dfun
463 ------------------------
464 -- Derived newtype instances; surprisingly tricky!
466 -- class Show a => Foo a b where ...
467 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
469 -- The newtype gives an FC axiom looking like
470 -- axiom CoN a :: N a :=: Tree [a]
471 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
473 -- So all need is to generate a binding looking like:
474 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
475 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
476 -- case df `cast` (Foo Int (sym (CoN a))) of
477 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
479 -- If there are no superclasses, matters are simpler, because we don't need the case
480 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
482 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived })
483 = do { let dfun_id = instanceDFunId ispec
484 rigid_info = InstSkol
485 origin = SigOrigin rigid_info
486 inst_ty = idType dfun_id
487 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
488 -- inst_head_ty is a PredType
490 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
491 (class_tyvars, sc_theta, _, _) = classBigSig cls
492 cls_tycon = classTyCon cls
493 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
495 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
496 (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail
497 rep_ty = newTyConInstRhs nt_tycon tc_args
499 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
500 -- In our example, rep_pred is (Foo Int (Tree [a]))
501 the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
502 -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)
504 ; inst_loc <- getInstLoc origin
505 ; sc_loc <- getInstLoc InstScOrigin
506 ; dfun_dicts <- newDictBndrs inst_loc theta
507 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
508 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
509 ; rep_dict <- newDictBndr inst_loc rep_pred
511 -- Figure out bindings for the superclass context from dfun_dicts
512 -- Don't include this_dict in the 'givens', else
513 -- wanted_sc_insts get bound by just selecting from this_dict!!
514 ; sc_binds <- addErrCtxt superClassCtxt $
515 tcSimplifySuperClasses inst_loc dfun_dicts (rep_dict:sc_dicts)
517 ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict))
519 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
520 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
522 ; return (unitBag $ noLoc $
523 AbsBinds tvs (map instToVar dfun_dicts)
524 [(tvs, dfun_id, instToId this_dict, [])]
525 (dict_bind `consBag` sc_binds)) }
527 -----------------------
529 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
530 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
531 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
532 -- where rep_ty is the (eta-reduced) type rep of T
533 -- So we just replace T with CoT, and insert a 'sym'
534 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
536 make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
537 | Just co_con <- newTyConCo_maybe nt_tycon
538 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
539 = WpCast (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))
540 | otherwise -- The newtype is transparent; no need for a cast
543 -----------------------
544 -- (make_body C tys scs coreced_rep_dict)
546 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
547 -- But if there are no superclasses, it returns just coerced_rep_dict
548 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
550 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
551 | null sc_dicts -- Case (a)
552 = return coerced_rep_dict
553 | otherwise -- Case (b)
554 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
555 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
556 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
557 pat_dicts = dummy_sc_dict_ids,
558 pat_binds = emptyLHsBinds,
559 pat_args = PrefixCon (map nlVarPat op_ids),
561 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
562 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
563 map HsVar (sc_dict_ids ++ op_ids)
565 -- Warning: this HsCase scrutinises a value with a PredTy, which is
566 -- never otherwise seen in Haskell source code. It'd be
567 -- nicer to generate Core directly!
568 ; return (HsCase (noLoc coerced_rep_dict) $
569 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
571 sc_dict_ids = map instToId sc_dicts
572 pat_ty = mkTyConApp cls_tycon cls_inst_tys
573 cls_data_con = head (tyConDataCons cls_tycon)
574 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
575 op_tys = dropList sc_dict_ids cls_arg_tys
577 ------------------------
578 -- Ordinary instances
580 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
582 dfun_id = instanceDFunId ispec
583 rigid_info = InstSkol
584 inst_ty = idType dfun_id
585 loc = srcLocSpan (getSrcLoc dfun_id)
587 -- Prime error recovery
588 recoverM (return emptyLHsBinds) $
590 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do
592 -- Instantiate the instance decl with skolem constants
593 (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
594 -- These inst_tyvars' scope over the 'where' part
595 -- Those tyvars are inside the dfun_id's type, which is a bit
596 -- bizarre, but OK so long as you realise it!
598 (clas, inst_tys') = tcSplitDFunHead inst_head'
599 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
601 -- Instantiate the super-class context with inst_tys
602 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
603 (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
604 origin = SigOrigin rigid_info
605 (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
607 -- Create dictionary Ids from the specified instance contexts.
608 sc_loc <- getInstLoc InstScOrigin
609 sc_dicts <- newDictBndrs sc_loc dict_sc_theta'
610 inst_loc <- getInstLoc origin
611 sc_covars <- mkMetaCoVars eq_sc_theta'
612 wanted_sc_eqs <- mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars)
613 dfun_covars <- mkCoVars eq_dfun_theta'
614 dfun_eqs <- mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars)
615 dfun_dicts <- newDictBndrs inst_loc dict_dfun_theta'
616 this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
617 -- Default-method Ids may be mentioned in synthesised RHSs,
618 -- but they'll already be in the environment.
620 -- Typecheck the methods
621 let -- These insts are in scope; quite a few, eh?
622 dfun_insts = dfun_eqs ++ dfun_dicts
623 wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
624 given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
625 given_sc_insts = given_sc_eqs ++ sc_dicts
626 avail_insts = dfun_insts ++ given_sc_insts
628 (meth_ids, meth_binds) <- tcMethods origin clas inst_tyvars'
629 dfun_theta' inst_tys' this_dict avail_insts
630 op_items monobinds uprags
632 -- Figure out bindings for the superclass context
633 -- Don't include this_dict in the 'givens', else
634 -- wanted_sc_insts get bound by just selecting from this_dict!!
635 sc_binds <- addErrCtxt superClassCtxt
636 (tcSimplifySuperClasses inst_loc dfun_insts wanted_sc_insts)
638 -- It's possible that the superclass stuff might unified one
639 -- of the inst_tyavars' with something in the envt
640 checkSigTyVars inst_tyvars'
642 -- Deal with 'SPECIALISE instance' pragmas
643 prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
645 -- Create the result bindings
647 dict_constr = classDataCon clas
648 scs_and_meths = map instToId sc_dicts ++ meth_ids
649 this_dict_id = instToId this_dict
650 inline_prag | null dfun_insts = []
651 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
652 -- Always inline the dfun; this is an experimental decision
653 -- because it makes a big performance difference sometimes.
654 -- Often it means we can do the method selection, and then
655 -- inline the method as well. Marcin's idea; see comments below.
657 -- BUT: don't inline it if it's a constant dictionary;
658 -- we'll get all the benefit without inlining, and we get
659 -- a **lot** of code duplication if we inline it
661 -- See Note [Inline dfuns] below
663 dict_rhs = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars)
664 (map HsVar scs_and_meths)
665 -- We don't produce a binding for the dict_constr; instead we
666 -- rely on the simplifier to unfold this saturated application
667 -- We do this rather than generate an HsCon directly, because
668 -- it means that the special cases (e.g. dictionary with only one
669 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
670 -- than needing to be repeated here.
672 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
673 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
675 main_bind = noLoc $ AbsBinds
676 (inst_tyvars' ++ dfun_covars)
677 (map instToId dfun_dicts)
678 [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
681 showLIE (text "instance")
682 return (unitBag main_bind)
684 mkCoVars :: [PredType] -> TcM [TyVar]
685 mkCoVars = newCoVars . map unEqPred
687 unEqPred (EqPred ty1 ty2) = (ty1, ty2)
688 unEqPred _ = panic "TcInstDcls.mkCoVars"
690 mkMetaCoVars :: [PredType] -> TcM [TyVar]
691 mkMetaCoVars = mapM eqPredToCoVar
693 eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2
694 eqPredToCoVar _ = panic "TcInstDcls.mkMetaCoVars"
696 tcMethods :: InstOrigin -> Class -> [TcTyVar] -> TcThetaType -> [TcType]
697 -> Inst -> [Inst] -> [(Id, DefMeth)] -> LHsBindsLR Name Name
699 -> TcM ([Id], Bag (LHsBind Id))
700 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
701 this_dict extra_insts op_items monobinds uprags = do
702 -- Check that all the method bindings come from this class
704 sel_names = [idName sel_id | (sel_id, _) <- op_items]
705 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
707 mapM (addErrTc . badMethodErr clas) bad_bndrs
709 -- Make the method bindings
711 mk_method_id (sel_id, _) = mkMethId origin clas sel_id inst_tys'
713 (meth_insts, meth_ids) <- mapAndUnzipM mk_method_id op_items
715 -- And type check them
716 -- It's really worth making meth_insts available to the tcMethodBind
717 -- Consider instance Monad (ST s) where
718 -- {-# INLINE (>>) #-}
719 -- (>>) = ...(>>=)...
720 -- If we don't include meth_insts, we end up with bindings like this:
721 -- rec { dict = MkD then bind ...
722 -- then = inline_me (... (GHC.Base.>>= dict) ...)
724 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
725 -- and (b) the inline_me prevents us inlining the >>= selector, which
726 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
727 -- is not inlined across modules. Rather ironic since this does not
728 -- happen without the INLINE pragma!
730 -- Solution: make meth_insts available, so that 'then' refers directly
731 -- to the local 'bind' rather than going via the dictionary.
733 -- BUT WATCH OUT! If the method type mentions the class variable, then
734 -- this optimisation is not right. Consider
738 -- instance C Int where
740 -- The occurrence of 'op' on the rhs gives rise to a constraint
742 -- The trouble is that the 'meth_inst' for op, which is 'available', also
743 -- looks like 'op at Int'. But they are not the same.
745 prag_fn = mkPragFun uprags
746 all_insts = extra_insts ++ catMaybes meth_insts
747 sig_fn _ = Just [] -- No scoped type variables, but every method has
748 -- a type signature, in effect, so that we check
749 -- the method has the right type
750 tc_method_bind = tcMethodBind origin inst_tyvars' dfun_theta' this_dict
751 all_insts sig_fn prag_fn monobinds
753 meth_binds_s <- zipWithM tc_method_bind op_items meth_ids
755 return (meth_ids, unionManyBags meth_binds_s)
759 ------------------------------
760 [Inline dfuns] Inlining dfuns unconditionally
761 ------------------------------
763 The code above unconditionally inlines dict funs. Here's why.
764 Consider this program:
766 test :: Int -> Int -> Bool
767 test x y = (x,y) == (y,x) || test y x
768 -- Recursive to avoid making it inline.
770 This needs the (Eq (Int,Int)) instance. If we inline that dfun
771 the code we end up with is good:
774 \r -> case ==# [ww ww1] of wild {
775 PrelBase.False -> Test.$wtest ww1 ww;
777 case ==# [ww1 ww] of wild1 {
778 PrelBase.False -> Test.$wtest ww1 ww;
779 PrelBase.True -> PrelBase.True [];
782 Test.test = \r [w w1]
785 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
788 If we don't inline the dfun, the code is not nearly as good:
790 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
791 PrelBase.:DEq tpl1 tpl2 -> tpl2;
796 let { y = PrelBase.I#! [ww1]; } in
797 let { x = PrelBase.I#! [ww]; } in
798 let { sat_slx = PrelTup.(,)! [y x]; } in
799 let { sat_sly = PrelTup.(,)! [x y];
801 case == sat_sly sat_slx of wild {
802 PrelBase.False -> Test.$wtest ww1 ww;
803 PrelBase.True -> PrelBase.True [];
810 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
813 Why doesn't GHC inline $fEq? Because it looks big:
815 PrelTup.zdfEqZ1T{-rcX-}
816 = \ @ a{-reT-} :: * @ b{-reS-} :: *
817 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
818 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
820 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
821 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
823 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
824 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
826 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
827 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
828 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
830 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
832 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
834 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
835 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
839 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
840 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
841 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
842 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
844 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
846 and it's not as bad as it seems, because it's further dramatically
847 simplified: only zeze2 is extracted and its body is simplified.
850 %************************************************************************
852 \subsection{Error messages}
854 %************************************************************************
857 instDeclCtxt1 :: LHsType Name -> SDoc
858 instDeclCtxt1 hs_inst_ty
859 = inst_decl_ctxt (case unLoc hs_inst_ty of
860 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
861 HsPredTy pred -> ppr pred
862 _ -> ppr hs_inst_ty) -- Don't expect this
863 instDeclCtxt2 :: Type -> SDoc
864 instDeclCtxt2 dfun_ty
865 = inst_decl_ctxt (ppr (mkClassPred cls tys))
867 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
869 inst_decl_ctxt :: SDoc -> SDoc
870 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
872 superClassCtxt :: SDoc
873 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
875 atInstCtxt :: Name -> SDoc
876 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
879 mustBeVarArgErr :: Type -> SDoc
881 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
882 ptext (sLit "must be variables")
883 , ptext (sLit "Instead of a variable, found") <+> ppr ty
886 wrongATArgErr :: Type -> Type -> SDoc
887 wrongATArgErr ty instTy =
888 sep [ ptext (sLit "Type indexes must match class instance head")
889 , ptext (sLit "Found") <+> ppr ty <+> ptext (sLit "but expected") <+>