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 failIfErrsM -- If the addInsts stuff gave any errors, don't
191 -- try the deriving stuff, becuase that may give
193 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls
195 ; addInsts deriv_inst_info $ do {
197 ; gbl_env <- getGblEnv
199 generic_inst_info ++ deriv_inst_info ++ local_info,
203 -- Make sure that toplevel type instance are not for associated types.
204 -- !!!TODO: Need to perform this check for the TyThing of type functions,
206 tcIdxTyInstDeclTL ldecl@(L loc decl) =
207 do { tything <- tcFamInstDecl ldecl
209 when (isAssocFamily tything) $
210 addErr $ assocInClassErr (tcdName decl)
213 isAssocFamily (Just (ATyCon tycon)) =
214 case tyConFamInst_maybe tycon of
215 Nothing -> panic "isAssocFamily: no family?!?"
216 Just (fam, _) -> isTyConAssoc fam
217 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
218 isAssocFamily Nothing = False
220 assocInClassErr name =
221 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
222 ptext (sLit "must be inside a class instance")
224 addInsts :: [InstInfo] -> TcM a -> TcM a
225 addInsts infos thing_inside
226 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
228 addFamInsts :: [TyThing] -> TcM a -> TcM a
229 addFamInsts tycons thing_inside
230 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
232 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
233 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
238 tcLocalInstDecl1 :: LInstDecl Name
239 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
240 -- A source-file instance declaration
241 -- Type-check all the stuff before the "where"
243 -- We check for respectable instance type, and context
244 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
245 = -- Prime error recovery, set source location
246 recoverM (return ([], [])) $
248 addErrCtxt (instDeclCtxt1 poly_ty) $
250 do { is_boot <- tcIsHsBoot
251 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
254 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
256 -- Next, process any associated types.
257 ; idx_tycons <- mapM tcFamInstDecl ats
259 -- Now, check the validity of the instance.
260 ; (clas, inst_tys) <- checkValidInstHead tau
261 ; checkValidInstance tyvars theta clas inst_tys
262 ; checkValidAndMissingATs clas (tyvars, inst_tys)
265 -- Finally, construct the Core representation of the instance.
266 -- (This no longer includes the associated types.)
267 ; dfun_name <- newDFunName clas inst_tys loc
268 ; overlap_flag <- getOverlapFlag
269 ; let (eq_theta,dict_theta) = partition isEqPred theta
270 theta' = eq_theta ++ dict_theta
271 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
272 ispec = mkLocalInstance dfun overlap_flag
274 ; return ([InstInfo { iSpec = ispec,
275 iBinds = VanillaInst binds uprags }],
276 catMaybes idx_tycons)
279 -- We pass in the source form and the type checked form of the ATs. We
280 -- really need the source form only to be able to produce more informative
282 checkValidAndMissingATs :: Class
283 -> ([TyVar], [TcType]) -- instance types
284 -> [(LTyClDecl Name, -- source form of AT
285 Maybe TyThing)] -- Core form of AT
287 checkValidAndMissingATs clas inst_tys ats
288 = do { -- Issue a warning for each class AT that is not defined in this
290 ; let class_ats = map tyConName (classATs clas)
291 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
292 omitted = filterOut (`elemNameSet` defined_ats) class_ats
293 ; warn <- doptM Opt_WarnMissingMethods
294 ; mapM_ (warnTc warn . omittedATWarn) omitted
296 -- Ensure that all AT indexes that correspond to class parameters
297 -- coincide with the types in the instance head. All remaining
298 -- AT arguments must be variables. Also raise an error for any
299 -- type instances that are not associated with this class.
300 ; mapM_ (checkIndexes clas inst_tys) ats
303 checkIndexes _ _ (hsAT, Nothing) =
304 return () -- skip, we already had an error here
305 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
306 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
307 checkIndexes' clas inst_tys hsAT
309 snd . fromJust . tyConFamInst_maybe $ tycon)
310 checkIndexes _ _ _ = panic "checkIndexes"
312 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
313 = let atName = tcdName . unLoc $ hsAT
315 setSrcSpan (getLoc hsAT) $
316 addErrCtxt (atInstCtxt atName) $
317 case find ((atName ==) . tyConName) (classATs clas) of
318 Nothing -> addErrTc $ badATErr clas atName -- not in this class
320 case assocTyConArgPoss_maybe atDecl of
321 Nothing -> panic "checkIndexes': AT has no args poss?!?"
324 -- The following is tricky! We need to deal with three
325 -- complications: (1) The AT possibly only uses a subset of
326 -- the class parameters as indexes and those it uses may be in
327 -- a different order; (2) the AT may have extra arguments,
328 -- which must be type variables; and (3) variables in AT and
329 -- instance head will be different `Name's even if their
330 -- source lexemes are identical.
332 -- Re (1), `poss' contains a permutation vector to extract the
333 -- class parameters in the right order.
335 -- Re (2), we wrap the (permuted) class parameters in a Maybe
336 -- type and use Nothing for any extra AT arguments. (First
337 -- equation of `checkIndex' below.)
339 -- Re (3), we replace any type variable in the AT parameters
340 -- that has the same source lexeme as some variable in the
341 -- instance types with the instance type variable sharing its
344 let relevantInstTys = map (instTys !!) poss
345 instArgs = map Just relevantInstTys ++
346 repeat Nothing -- extra arguments
347 renaming = substSameTyVar atTvs instTvs
349 zipWithM_ checkIndex (substTys renaming atTys) instArgs
351 checkIndex ty Nothing
352 | isTyVarTy ty = return ()
353 | otherwise = addErrTc $ mustBeVarArgErr ty
354 checkIndex ty (Just instTy)
355 | ty `tcEqType` instTy = return ()
356 | otherwise = addErrTc $ wrongATArgErr ty instTy
358 listToNameSet = addListToNameSet emptyNameSet
360 substSameTyVar [] _ = emptyTvSubst
361 substSameTyVar (tv:tvs) replacingTvs =
362 let replacement = case find (tv `sameLexeme`) replacingTvs of
363 Nothing -> mkTyVarTy tv
364 Just rtv -> mkTyVarTy rtv
366 tv1 `sameLexeme` tv2 =
367 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
369 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
373 %************************************************************************
375 \subsection{Type-checking instance declarations, pass 2}
377 %************************************************************************
380 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
381 -> TcM (LHsBinds Id, TcLclEnv)
382 -- (a) From each class declaration,
383 -- generate any default-method bindings
384 -- (b) From each instance decl
385 -- generate the dfun binding
387 tcInstDecls2 tycl_decls inst_decls
388 = do { -- (a) Default methods from class decls
389 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
390 filter (isClassDecl.unLoc) tycl_decls
391 ; tcExtendIdEnv (concat dm_ids_s) $ do
393 -- (b) instance declarations
394 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
397 ; let binds = unionManyBags dm_binds_s `unionBags`
398 unionManyBags inst_binds_s
399 ; tcl_env <- getLclEnv -- Default method Ids in here
400 ; return (binds, tcl_env) }
403 ======= New documentation starts here (Sept 92) ==============
405 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
406 the dictionary function for this instance declaration. For example
408 instance Foo a => Foo [a] where
412 might generate something like
414 dfun.Foo.List dFoo_a = let op1 x = ...
419 HOWEVER, if the instance decl has no context, then it returns a
420 bigger @HsBinds@ with declarations for each method. For example
422 instance Foo [a] where
428 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
429 const.Foo.op1.List a x = ...
430 const.Foo.op2.List a y = ...
432 This group may be mutually recursive, because (for example) there may
433 be no method supplied for op2 in which case we'll get
435 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
437 that is, the default method applied to the dictionary at this type.
438 What we actually produce in either case is:
440 AbsBinds [a] [dfun_theta_dicts]
441 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
442 { d = (sd1,sd2, ..., op1, op2, ...)
447 The "maybe" says that we only ask AbsBinds to make global constant methods
448 if the dfun_theta is empty.
450 For an instance declaration, say,
452 instance (C1 a, C2 b) => C (T a b) where
455 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
456 function whose type is
458 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
460 Notice that we pass it the superclass dictionaries at the instance type; this
461 is the ``Mark Jones optimisation''. The stuff before the "=>" here
462 is the @dfun_theta@ below.
466 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
467 -- Returns a binding for the dfun
469 ------------------------
470 -- Derived newtype instances; surprisingly tricky!
472 -- class Show a => Foo a b where ...
473 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
475 -- The newtype gives an FC axiom looking like
476 -- axiom CoN a :: N a :=: Tree [a]
477 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
479 -- So all need is to generate a binding looking like:
480 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
481 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
482 -- case df `cast` (Foo Int (sym (CoN a))) of
483 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
485 -- If there are no superclasses, matters are simpler, because we don't need the case
486 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
488 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived })
489 = do { let dfun_id = instanceDFunId ispec
490 rigid_info = InstSkol
491 origin = SigOrigin rigid_info
492 inst_ty = idType dfun_id
493 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
494 -- inst_head_ty is a PredType
496 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
497 (class_tyvars, sc_theta, _, op_items) = classBigSig cls
498 cls_tycon = classTyCon cls
499 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
501 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
502 (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail
503 rep_ty = newTyConInstRhs nt_tycon tc_args
505 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
506 -- In our example, rep_pred is (Foo Int (Tree [a]))
507 the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
508 -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)
510 ; inst_loc <- getInstLoc origin
511 ; sc_loc <- getInstLoc InstScOrigin
512 ; dfun_dicts <- newDictBndrs inst_loc theta
513 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
514 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
515 ; rep_dict <- newDictBndr inst_loc rep_pred
517 -- Figure out bindings for the superclass context from dfun_dicts
518 -- Don't include this_dict in the 'givens', else
519 -- wanted_sc_insts get bound by just selecting from this_dict!!
520 ; sc_binds <- addErrCtxt superClassCtxt $
521 tcSimplifySuperClasses inst_loc dfun_dicts (rep_dict:sc_dicts)
523 ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict))
525 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
526 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
528 ; return (unitBag $ noLoc $
529 AbsBinds tvs (map instToVar dfun_dicts)
530 [(tvs, dfun_id, instToId this_dict, [])]
531 (dict_bind `consBag` sc_binds)) }
533 -----------------------
535 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
536 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
537 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
538 -- where rep_ty is the (eta-reduced) type rep of T
539 -- So we just replace T with CoT, and insert a 'sym'
540 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
542 make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
543 | Just co_con <- newTyConCo_maybe nt_tycon
544 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
545 = WpCast (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))
546 | otherwise -- The newtype is transparent; no need for a cast
549 -----------------------
550 -- (make_body C tys scs coreced_rep_dict)
552 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
553 -- But if there are no superclasses, it returns just coerced_rep_dict
554 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
556 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
557 | null sc_dicts -- Case (a)
558 = return coerced_rep_dict
559 | otherwise -- Case (b)
560 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
561 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
562 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
563 pat_dicts = dummy_sc_dict_ids,
564 pat_binds = emptyLHsBinds,
565 pat_args = PrefixCon (map nlVarPat op_ids),
567 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
568 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
569 map HsVar (sc_dict_ids ++ op_ids)
571 -- Warning: this HsCase scrutinises a value with a PredTy, which is
572 -- never otherwise seen in Haskell source code. It'd be
573 -- nicer to generate Core directly!
574 ; return (HsCase (noLoc coerced_rep_dict) $
575 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
577 sc_dict_ids = map instToId sc_dicts
578 pat_ty = mkTyConApp cls_tycon cls_inst_tys
579 cls_data_con = head (tyConDataCons cls_tycon)
580 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
581 op_tys = dropList sc_dict_ids cls_arg_tys
583 ------------------------
584 -- Ordinary instances
586 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
588 dfun_id = instanceDFunId ispec
589 rigid_info = InstSkol
590 inst_ty = idType dfun_id
591 loc = srcLocSpan (getSrcLoc dfun_id)
593 -- Prime error recovery
594 recoverM (return emptyLHsBinds) $
596 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do
598 -- Instantiate the instance decl with skolem constants
599 (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
600 -- These inst_tyvars' scope over the 'where' part
601 -- Those tyvars are inside the dfun_id's type, which is a bit
602 -- bizarre, but OK so long as you realise it!
604 (clas, inst_tys') = tcSplitDFunHead inst_head'
605 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
607 -- Instantiate the super-class context with inst_tys
608 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
609 (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
610 origin = SigOrigin rigid_info
611 (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
613 -- Create dictionary Ids from the specified instance contexts.
614 sc_loc <- getInstLoc InstScOrigin
615 sc_dicts <- newDictBndrs sc_loc dict_sc_theta'
616 inst_loc <- getInstLoc origin
617 sc_covars <- mkMetaCoVars eq_sc_theta'
618 wanted_sc_eqs <- mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars)
619 dfun_covars <- mkCoVars eq_dfun_theta'
620 dfun_eqs <- mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars)
621 dfun_dicts <- newDictBndrs inst_loc dict_dfun_theta'
622 this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
623 -- Default-method Ids may be mentioned in synthesised RHSs,
624 -- but they'll already be in the environment.
626 -- Typecheck the methods
627 let -- These insts are in scope; quite a few, eh?
628 dfun_insts = dfun_eqs ++ dfun_dicts
629 wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
630 given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
631 given_sc_insts = given_sc_eqs ++ sc_dicts
632 avail_insts = dfun_insts ++ given_sc_insts
634 (meth_ids, meth_binds) <- tcMethods origin clas inst_tyvars'
635 dfun_theta' inst_tys' this_dict avail_insts
636 op_items monobinds uprags
638 -- Figure out bindings for the superclass context
639 -- Don't include this_dict in the 'givens', else
640 -- wanted_sc_insts get bound by just selecting from this_dict!!
641 sc_binds <- addErrCtxt superClassCtxt
642 (tcSimplifySuperClasses inst_loc dfun_insts wanted_sc_insts)
644 -- It's possible that the superclass stuff might unified one
645 -- of the inst_tyavars' with something in the envt
646 checkSigTyVars inst_tyvars'
648 -- Deal with 'SPECIALISE instance' pragmas
649 prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
651 -- Create the result bindings
653 dict_constr = classDataCon clas
654 scs_and_meths = map instToId sc_dicts ++ meth_ids
655 this_dict_id = instToId this_dict
656 inline_prag | null dfun_insts = []
657 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
658 -- Always inline the dfun; this is an experimental decision
659 -- because it makes a big performance difference sometimes.
660 -- Often it means we can do the method selection, and then
661 -- inline the method as well. Marcin's idea; see comments below.
663 -- BUT: don't inline it if it's a constant dictionary;
664 -- we'll get all the benefit without inlining, and we get
665 -- a **lot** of code duplication if we inline it
667 -- See Note [Inline dfuns] below
669 dict_rhs = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars)
670 (map HsVar scs_and_meths)
671 -- We don't produce a binding for the dict_constr; instead we
672 -- rely on the simplifier to unfold this saturated application
673 -- We do this rather than generate an HsCon directly, because
674 -- it means that the special cases (e.g. dictionary with only one
675 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
676 -- than needing to be repeated here.
678 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
679 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
681 main_bind = noLoc $ AbsBinds
682 (inst_tyvars' ++ dfun_covars)
683 (map instToId dfun_dicts)
684 [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
687 showLIE (text "instance")
688 return (unitBag main_bind)
690 mkCoVars :: [PredType] -> TcM [TyVar]
691 mkCoVars = newCoVars . map unEqPred
693 unEqPred (EqPred ty1 ty2) = (ty1, ty2)
694 unEqPred _ = panic "TcInstDcls.mkCoVars"
696 mkMetaCoVars :: [PredType] -> TcM [TyVar]
697 mkMetaCoVars = mapM eqPredToCoVar
699 eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2
700 eqPredToCoVar _ = panic "TcInstDcls.mkMetaCoVars"
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 n = 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 hs_inst_ty
860 = inst_decl_ctxt (case unLoc hs_inst_ty of
861 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
862 HsPredTy pred -> ppr pred
863 other -> ppr hs_inst_ty) -- Don't expect this
864 instDeclCtxt2 dfun_ty
865 = inst_decl_ctxt (ppr (mkClassPred cls tys))
867 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
869 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
871 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
873 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
877 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
878 ptext (sLit "must be variables")
879 , ptext (sLit "Instead of a variable, found") <+> ppr ty
882 wrongATArgErr ty instTy =
883 sep [ ptext (sLit "Type indexes must match class instance head")
884 , ptext (sLit "Found") <+> ppr ty <+> ptext (sLit "but expected") <+>