2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcInstDecls]{Typechecking instance declarations}
7 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
9 #include "HsVersions.h"
12 import TcBinds ( mkPragFun, tcPrags, badBootDeclErr )
13 import TcTyClsDecls ( tcIdxTyInstDecl )
14 import TcClassDcl ( tcMethodBind, mkMethodBind, badMethodErr, badATErr,
15 omittedATWarn, tcClassDecl2, getGenericInstances )
17 import TcMType ( tcSkolSigType, checkValidInstance,
19 import TcType ( TcType, mkClassPred, tcSplitSigmaTy,
20 tcSplitDFunHead, SkolemInfo(InstSkol),
21 tcSplitDFunTy, mkFunTy )
22 import Inst ( newDictBndr, newDictBndrs, instToId, showLIE,
23 getOverlapFlag, tcExtendLocalInstEnv )
24 import InstEnv ( mkLocalInstance, instanceDFunId )
25 import TcDeriv ( tcDeriving )
26 import TcEnv ( InstInfo(..), InstBindings(..),
27 newDFunName, tcExtendIdEnv, tcExtendGlobalEnv
29 import TcHsType ( kcHsSigType, tcHsKindedType )
30 import TcUnify ( checkSigTyVars )
31 import TcSimplify ( tcSimplifySuperClasses )
32 import Type ( zipOpenTvSubst, substTheta, mkTyConApp, mkTyVarTy,
33 splitFunTys, TyThing(ATyCon), isTyVarTy, tcEqType,
34 substTys, emptyTvSubst, extendTvSubst )
35 import Coercion ( mkSymCoercion )
36 import TyCon ( TyCon, tyConName, newTyConCo_maybe, tyConTyVars,
37 isTyConAssoc, tyConFamInst_maybe,
38 assocTyConArgPoss_maybe )
39 import DataCon ( classDataCon, dataConTyCon, dataConInstArgTys )
40 import Class ( Class, classBigSig, classATs )
41 import Var ( TyVar, Id, idName, idType, tyVarKind, tyVarName )
42 import VarEnv ( rnBndrs2, mkRnEnv2, emptyInScopeSet )
43 import Id ( mkSysLocal )
44 import UniqSupply ( uniqsFromSupply, splitUniqSupply )
45 import MkId ( mkDictFunId )
46 import Name ( Name, getSrcLoc, nameOccName )
47 import NameSet ( addListToNameSet, emptyNameSet, minusNameSet,
49 import Maybe ( isNothing, fromJust, catMaybes )
52 import DynFlags ( DynFlag(Opt_WarnMissingMethods) )
53 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart,
55 import ListSetOps ( minusList )
58 import BasicTypes ( Activation( AlwaysActive ), InlineSpec(..) )
59 import HscTypes ( implicitTyThings )
63 Typechecking instance declarations is done in two passes. The first
64 pass, made by @tcInstDecls1@, collects information to be used in the
67 This pre-processed info includes the as-yet-unprocessed bindings
68 inside the instance declaration. These are type-checked in the second
69 pass, when the class-instance envs and GVE contain all the info from
70 all the instance and value decls. Indeed that's the reason we need
71 two passes over the instance decls.
73 Here is the overall algorithm.
74 Assume that we have an instance declaration
76 instance c => k (t tvs) where b
80 $LIE_c$ is the LIE for the context of class $c$
82 $betas_bar$ is the free variables in the class method type, excluding the
85 $LIE_cop$ is the LIE constraining a particular class method
87 $tau_cop$ is the tau type of a class method
89 $LIE_i$ is the LIE for the context of instance $i$
91 $X$ is the instance constructor tycon
93 $gammas_bar$ is the set of type variables of the instance
95 $LIE_iop$ is the LIE for a particular class method instance
97 $tau_iop$ is the tau type for this instance of a class method
99 $alpha$ is the class variable
101 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
103 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
106 ToDo: Update the list above with names actually in the code.
110 First, make the LIEs for the class and instance contexts, which means
111 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
112 and make LIElistI and LIEI.
114 Then process each method in turn.
116 order the instance methods according to the ordering of the class methods
118 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
120 Create final dictionary function from bindings generated already
122 df = lambda inst_tyvars
129 in <op1,op2,...,opn,sd1,...,sdm>
131 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
132 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
136 %************************************************************************
138 \subsection{Extracting instance decls}
140 %************************************************************************
142 Gather up the instance declarations from their various sources
145 tcInstDecls1 -- Deal with both source-code and imported instance decls
146 :: [LTyClDecl Name] -- For deriving stuff
147 -> [LInstDecl Name] -- Source code instance decls
148 -> TcM (TcGblEnv, -- The full inst env
149 [InstInfo], -- Source-code instance decls to process;
150 -- contains all dfuns for this module
151 HsValBinds Name) -- Supporting bindings for derived instances
153 tcInstDecls1 tycl_decls inst_decls
155 do { -- Stop if addInstInfos etc discovers any errors
156 -- (they recover, so that we get more than one error each
159 -- (1) Do class instance declarations and instances of indexed
161 ; let { idxty_decls = filter (isIdxTyDecl . unLoc) tycl_decls }
162 ; local_info_tycons <- mappM tcLocalInstDecl1 inst_decls
163 ; idxty_info_tycons <- mappM tcIdxTyInstDeclTL idxty_decls
165 ; let { (local_infos,
166 local_tycons) = unzip local_info_tycons
168 idxty_tycons) = unzip idxty_info_tycons
169 ; local_idxty_info = concat local_infos ++ catMaybes idxty_infos
170 ; local_idxty_tycon = concat local_tycons ++
171 catMaybes idxty_tycons
172 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
173 ; implicit_things = concatMap implicitTyThings local_idxty_tycon
176 -- (2) Add the tycons of associated types and their implicit
177 -- tythings to the global environment
178 ; tcExtendGlobalEnv (local_idxty_tycon ++ implicit_things) $ do {
180 -- (3) Instances from generic class declarations
181 ; generic_inst_info <- getGenericInstances clas_decls
183 -- Next, construct the instance environment so far, consisting
185 -- a) local instance decls
186 -- b) generic instances
187 ; addInsts local_idxty_info $ do {
188 ; addInsts generic_inst_info $ do {
190 -- (4) Compute instances from "deriving" clauses;
191 -- This stuff computes a context for the derived instance
192 -- decl, so it needs to know about all the instances possible
193 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls
194 ; addInsts deriv_inst_info $ do {
196 ; gbl_env <- getGblEnv
198 generic_inst_info ++ deriv_inst_info ++ local_idxty_info,
202 -- Make sure that toplevel type instance are not for associated types.
203 -- !!!TODO: Need to perform this check for the InstInfo structures of type
205 tcIdxTyInstDeclTL ldecl@(L loc decl) =
206 do { (info, tything) <- tcIdxTyInstDecl ldecl
208 when (isAssocFamily tything) $
209 addErr $ assocInClassErr (tcdName decl)
210 ; return (info, tything)
212 isAssocFamily (Just (ATyCon tycon)) =
213 case tyConFamInst_maybe tycon of
214 Nothing -> panic "isAssocFamily: no family?!?"
215 Just (fam, _) -> isTyConAssoc fam
216 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
217 isAssocFamily Nothing = False
219 assocInClassErr name =
220 ptext SLIT("Associated type") <+> quotes (ppr name) <+>
221 ptext SLIT("must be inside a class instance")
223 addInsts :: [InstInfo] -> TcM a -> TcM a
224 addInsts infos thing_inside
225 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
229 tcLocalInstDecl1 :: LInstDecl Name
230 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
231 -- A source-file instance declaration
232 -- Type-check all the stuff before the "where"
234 -- We check for respectable instance type, and context
235 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
236 = -- Prime error recovery, set source location
237 recoverM (returnM ([], [])) $
239 addErrCtxt (instDeclCtxt1 poly_ty) $
241 do { is_boot <- tcIsHsBoot
242 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
245 -- Typecheck the instance type itself. We can't use
246 -- tcHsSigType, because it's not a valid user type.
247 ; kinded_ty <- kcHsSigType poly_ty
248 ; poly_ty' <- tcHsKindedType kinded_ty
249 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
251 -- Next, process any associated types.
252 ; idxty_info_tycons <- mappM tcIdxTyInstDecl ats
254 -- Now, check the validity of the instance.
255 ; (clas, inst_tys) <- checkValidInstHead tau
256 ; checkValidInstance tyvars theta clas inst_tys
257 ; checkValidAndMissingATs clas (tyvars, inst_tys)
258 (zip ats idxty_info_tycons)
260 -- Finally, construct the Core representation of the instance.
261 -- (This no longer includes the associated types.)
262 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
263 ; overlap_flag <- getOverlapFlag
264 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
265 ispec = mkLocalInstance dfun overlap_flag
267 idxty_tycons) = unzip idxty_info_tycons
269 ; return ([InstInfo { iSpec = ispec,
270 iBinds = VanillaInst binds uprags }] ++
271 catMaybes idxty_infos,
272 catMaybes idxty_tycons)
275 -- We pass in the source form and the type checked form of the ATs. We
276 -- really need the source form only to be able to produce more informative
278 checkValidAndMissingATs :: Class
279 -> ([TyVar], [TcType]) -- instance types
280 -> [(LTyClDecl Name, -- source form of AT
281 (Maybe InstInfo, -- Core form for type
282 Maybe TyThing))] -- Core form for data
284 checkValidAndMissingATs clas inst_tys ats
285 = do { -- Issue a warning for each class AT that is not defined in this
287 ; let classDefATs = listToNameSet . map tyConName . classATs $ clas
288 definedATs = listToNameSet . map (tcdName.unLoc.fst) $ ats
289 omitted = classDefATs `minusNameSet` definedATs
290 ; warn <- doptM Opt_WarnMissingMethods
291 ; mapM_ (warnTc warn . omittedATWarn) (nameSetToList 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, Nothing)) =
301 return () -- skip, we already had an error here
302 checkIndexes clas inst_tys (hsAT, (Just _ , Nothing )) =
303 panic "do impl for AT syns" -- !!!TODO: also call checkIndexes'
304 checkIndexes clas inst_tys (hsAT, (Nothing , Just (ATyCon tycon))) =
305 checkIndexes' clas inst_tys hsAT
307 snd . fromJust . tyConFamInst_maybe $ tycon)
308 checkIndexes _ _ _ = panic "checkIndexes"
310 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
311 = let atName = tcdName . unLoc $ hsAT
313 setSrcSpan (getLoc hsAT) $
314 addErrCtxt (atInstCtxt atName) $
315 case find ((atName ==) . tyConName) (classATs clas) of
316 Nothing -> addErrTc $ badATErr clas atName -- not in this class
318 case assocTyConArgPoss_maybe atDecl of
319 Nothing -> panic "checkIndexes': AT has no args poss?!?"
322 -- The following is tricky! We need to deal with three
323 -- complications: (1) The AT possibly only uses a subset of
324 -- the class parameters as indexes and those it uses may be in
325 -- a different order; (2) the AT may have extra arguments,
326 -- which must be type variables; and (3) variables in AT and
327 -- instance head will be different `Name's even if their
328 -- source lexemes are identical.
330 -- Re (1), `poss' contains a permutation vector to extract the
331 -- class parameters in the right order.
333 -- Re (2), we wrap the (permuted) class parameters in a Maybe
334 -- type and use Nothing for any extra AT arguments. (First
335 -- equation of `checkIndex' below.)
337 -- Re (3), we replace any type variable in the AT parameters
338 -- that has the same source lexeme as some variable in the
339 -- instance types with the instance type variable sharing its
342 let relevantInstTys = map (instTys !!) poss
343 instArgs = map Just relevantInstTys ++
344 repeat Nothing -- extra arguments
345 renaming = substSameTyVar atTvs instTvs
347 zipWithM_ checkIndex (substTys renaming atTys) instArgs
349 checkIndex ty Nothing
350 | isTyVarTy ty = return ()
351 | otherwise = addErrTc $ mustBeVarArgErr ty
352 checkIndex ty (Just instTy)
353 | ty `tcEqType` instTy = return ()
354 | otherwise = addErrTc $ wrongATArgErr ty instTy
356 listToNameSet = addListToNameSet emptyNameSet
358 substSameTyVar [] _ = emptyTvSubst
359 substSameTyVar (tv:tvs) replacingTvs =
360 let replacement = case find (tv `sameLexeme`) replacingTvs of
361 Nothing -> mkTyVarTy tv
362 Just rtv -> mkTyVarTy rtv
364 tv1 `sameLexeme` tv2 =
365 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
367 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
371 %************************************************************************
373 \subsection{Type-checking instance declarations, pass 2}
375 %************************************************************************
378 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
379 -> TcM (LHsBinds Id, TcLclEnv)
380 -- (a) From each class declaration,
381 -- generate any default-method bindings
382 -- (b) From each instance decl
383 -- generate the dfun binding
385 tcInstDecls2 tycl_decls inst_decls
386 = do { -- (a) Default methods from class decls
387 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
388 filter (isClassDecl.unLoc) tycl_decls
389 ; tcExtendIdEnv (concat dm_ids_s) $ do
391 -- (b) instance declarations
392 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
395 ; let binds = unionManyBags dm_binds_s `unionBags`
396 unionManyBags inst_binds_s
397 ; tcl_env <- getLclEnv -- Default method Ids in here
398 ; returnM (binds, tcl_env) }
401 ======= New documentation starts here (Sept 92) ==============
403 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
404 the dictionary function for this instance declaration. For example
406 instance Foo a => Foo [a] where
410 might generate something like
412 dfun.Foo.List dFoo_a = let op1 x = ...
418 HOWEVER, if the instance decl has no context, then it returns a
419 bigger @HsBinds@ with declarations for each method. For example
421 instance Foo [a] where
427 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
428 const.Foo.op1.List a x = ...
429 const.Foo.op2.List a y = ...
431 This group may be mutually recursive, because (for example) there may
432 be no method supplied for op2 in which case we'll get
434 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
436 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.
451 For an instance declaration, say,
453 instance (C1 a, C2 b) => C (T a b) where
456 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
457 function whose type is
459 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
461 Notice that we pass it the superclass dictionaries at the instance type; this
462 is the ``Mark Jones optimisation''. The stuff before the "=>" here
463 is the @dfun_theta@ below.
465 First comes the easy case of a non-local instance decl.
469 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
470 -- Returns a binding for the dfun
472 ------------------------
473 -- Derived newtype instances
475 -- We need to make a copy of the dictionary we are deriving from
476 -- because we may need to change some of the superclass dictionaries
477 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
479 -- In the case of a newtype, things are rather easy
480 -- class Show a => Foo a b where ...
481 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
482 -- The newtype gives an FC axiom looking like
483 -- axiom CoT a :: T a :=: Tree [a]
485 -- So all need is to generate a binding looking like
486 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
487 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
488 -- case df `cast` (Foo Int (sym (CoT a))) of
489 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
491 tcInstDecl2 (InstInfo { iSpec = ispec,
492 iBinds = NewTypeDerived tycon rep_tys })
493 = do { let dfun_id = instanceDFunId ispec
494 rigid_info = InstSkol dfun_id
495 origin = SigOrigin rigid_info
496 inst_ty = idType dfun_id
497 ; inst_loc <- getInstLoc origin
498 ; (tvs, theta, inst_head) <- tcSkolSigType rigid_info inst_ty
499 ; dicts <- newDictBndrs inst_loc theta
500 ; uniqs <- newUniqueSupply
501 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head
502 ; this_dict <- newDictBndr inst_loc (mkClassPred cls rep_tys)
503 ; let (rep_dict_id:sc_dict_ids)
504 | null dicts = [instToId this_dict]
505 | otherwise = map instToId dicts
507 -- (Here, we are relying on the order of dictionary
508 -- arguments built by NewTypeDerived in TcDeriv.)
510 wrap_fn = mkCoTyLams tvs <.> mkCoLams (rep_dict_id:sc_dict_ids)
512 -- we need to find the kind that this class applies to
513 -- and drop trailing tvs appropriately
514 cls_kind = tyVarKind (head (reverse (tyConTyVars cls_tycon)))
515 the_tvs = drop_tail (length (fst (splitFunTys cls_kind))) tvs
517 coerced_rep_dict = mkHsCoerce (co_fn the_tvs cls_tycon cls_inst_tys) (HsVar rep_dict_id)
519 body | null sc_dict_ids = coerced_rep_dict
520 | otherwise = HsCase (noLoc coerced_rep_dict) $
521 MatchGroup [the_match] (mkFunTy in_dict_ty inst_head)
522 in_dict_ty = mkTyConApp cls_tycon cls_inst_tys
524 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
525 the_rhs = mkHsConApp cls_data_con cls_inst_tys (map HsVar (sc_dict_ids ++ op_ids))
527 (uniqs1, uniqs2) = splitUniqSupply uniqs
529 op_ids = zipWith (mkSysLocal FSLIT("op"))
530 (uniqsFromSupply uniqs1) op_tys
532 dict_ids = zipWith (mkSysLocal FSLIT("dict"))
533 (uniqsFromSupply uniqs2) (map idType sc_dict_ids)
535 the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
536 pat_dicts = dict_ids,
537 pat_binds = emptyLHsBinds,
538 pat_args = PrefixCon (map nlVarPat op_ids),
541 cls_data_con = classDataCon cls
542 cls_tycon = dataConTyCon cls_data_con
543 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
545 n_dict_args = if length dicts == 0 then 0 else length dicts - 1
546 op_tys = drop n_dict_args cls_arg_tys
548 dict = mkHsCoerce wrap_fn body
549 ; return (unitBag (noLoc $ VarBind dfun_id (noLoc dict))) }
551 -- For newtype T a = MkT <ty>
552 -- The returned coercion has kind :: C (T a):=:C <ty>
553 co_fn tvs cls_tycon cls_inst_tys | Just co_con <- newTyConCo_maybe tycon
554 = ExprCoFn (mkTyConApp cls_tycon (drop_tail 1 cls_inst_tys ++
555 [mkSymCoercion (mkTyConApp co_con (map mkTyVarTy tvs))]))
558 drop_tail n l = take (length l - n) l
560 ------------------------
561 -- Ordinary instances
563 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
565 dfun_id = instanceDFunId ispec
566 rigid_info = InstSkol dfun_id
567 inst_ty = idType dfun_id
569 -- Prime error recovery
570 recoverM (returnM emptyLHsBinds) $
571 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
572 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
574 -- Instantiate the instance decl with skolem constants
575 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
576 -- These inst_tyvars' scope over the 'where' part
577 -- Those tyvars are inside the dfun_id's type, which is a bit
578 -- bizarre, but OK so long as you realise it!
580 (clas, inst_tys') = tcSplitDFunHead inst_head'
581 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
583 -- Instantiate the super-class context with inst_tys
584 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
585 origin = SigOrigin rigid_info
587 -- Create dictionary Ids from the specified instance contexts.
588 getInstLoc InstScOrigin `thenM` \ sc_loc ->
589 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
590 getInstLoc origin `thenM` \ inst_loc ->
591 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
592 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
593 -- Default-method Ids may be mentioned in synthesised RHSs,
594 -- but they'll already be in the environment.
596 -- Typecheck the methods
597 let -- These insts are in scope; quite a few, eh?
598 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
600 tcMethods origin clas inst_tyvars'
601 dfun_theta' inst_tys' avail_insts
602 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
604 -- Figure out bindings for the superclass context
605 -- Don't include this_dict in the 'givens', else
606 -- sc_dicts get bound by just selecting from this_dict!!
607 addErrCtxt superClassCtxt
608 (tcSimplifySuperClasses inst_tyvars'
610 sc_dicts) `thenM` \ sc_binds ->
612 -- It's possible that the superclass stuff might unified one
613 -- of the inst_tyavars' with something in the envt
614 checkSigTyVars inst_tyvars' `thenM_`
616 -- Deal with 'SPECIALISE instance' pragmas
617 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
619 -- Create the result bindings
621 dict_constr = classDataCon clas
622 scs_and_meths = map instToId sc_dicts ++ meth_ids
623 this_dict_id = instToId this_dict
624 inline_prag | null dfun_arg_dicts = []
625 | otherwise = [InlinePrag (Inline AlwaysActive True)]
626 -- Always inline the dfun; this is an experimental decision
627 -- because it makes a big performance difference sometimes.
628 -- Often it means we can do the method selection, and then
629 -- inline the method as well. Marcin's idea; see comments below.
631 -- BUT: don't inline it if it's a constant dictionary;
632 -- we'll get all the benefit without inlining, and we get
633 -- a **lot** of code duplication if we inline it
635 -- See Note [Inline dfuns] below
638 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
639 -- We don't produce a binding for the dict_constr; instead we
640 -- rely on the simplifier to unfold this saturated application
641 -- We do this rather than generate an HsCon directly, because
642 -- it means that the special cases (e.g. dictionary with only one
643 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
644 -- than needing to be repeated here.
646 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
647 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
649 main_bind = noLoc $ AbsBinds
651 (map instToId dfun_arg_dicts)
652 [(inst_tyvars', dfun_id, this_dict_id,
653 inline_prag ++ prags)]
656 showLIE (text "instance") `thenM_`
657 returnM (unitBag main_bind)
660 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
661 avail_insts op_items monobinds uprags
662 = -- Check that all the method bindings come from this class
664 sel_names = [idName sel_id | (sel_id, _) <- op_items]
665 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
667 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
669 -- Make the method bindings
671 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
673 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
675 -- And type check them
676 -- It's really worth making meth_insts available to the tcMethodBind
677 -- Consider instance Monad (ST s) where
678 -- {-# INLINE (>>) #-}
679 -- (>>) = ...(>>=)...
680 -- If we don't include meth_insts, we end up with bindings like this:
681 -- rec { dict = MkD then bind ...
682 -- then = inline_me (... (GHC.Base.>>= dict) ...)
684 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
685 -- and (b) the inline_me prevents us inlining the >>= selector, which
686 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
687 -- is not inlined across modules. Rather ironic since this does not
688 -- happen without the INLINE pragma!
690 -- Solution: make meth_insts available, so that 'then' refers directly
691 -- to the local 'bind' rather than going via the dictionary.
693 -- BUT WATCH OUT! If the method type mentions the class variable, then
694 -- this optimisation is not right. Consider
698 -- instance C Int where
700 -- The occurrence of 'op' on the rhs gives rise to a constraint
702 -- The trouble is that the 'meth_inst' for op, which is 'available', also
703 -- looks like 'op at Int'. But they are not the same.
705 prag_fn = mkPragFun uprags
706 all_insts = avail_insts ++ catMaybes meth_insts
707 sig_fn n = Just [] -- No scoped type variables, but every method has
708 -- a type signature, in effect, so that we check
709 -- the method has the right type
710 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
711 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
714 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
716 returnM (meth_ids, unionManyBags meth_binds_s)
720 ------------------------------
721 [Inline dfuns] Inlining dfuns unconditionally
722 ------------------------------
724 The code above unconditionally inlines dict funs. Here's why.
725 Consider this program:
727 test :: Int -> Int -> Bool
728 test x y = (x,y) == (y,x) || test y x
729 -- Recursive to avoid making it inline.
731 This needs the (Eq (Int,Int)) instance. If we inline that dfun
732 the code we end up with is good:
735 \r -> case ==# [ww ww1] of wild {
736 PrelBase.False -> Test.$wtest ww1 ww;
738 case ==# [ww1 ww] of wild1 {
739 PrelBase.False -> Test.$wtest ww1 ww;
740 PrelBase.True -> PrelBase.True [];
743 Test.test = \r [w w1]
746 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
749 If we don't inline the dfun, the code is not nearly as good:
751 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
752 PrelBase.:DEq tpl1 tpl2 -> tpl2;
757 let { y = PrelBase.I#! [ww1]; } in
758 let { x = PrelBase.I#! [ww]; } in
759 let { sat_slx = PrelTup.(,)! [y x]; } in
760 let { sat_sly = PrelTup.(,)! [x y];
762 case == sat_sly sat_slx of wild {
763 PrelBase.False -> Test.$wtest ww1 ww;
764 PrelBase.True -> PrelBase.True [];
771 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
774 Why doesn't GHC inline $fEq? Because it looks big:
776 PrelTup.zdfEqZ1T{-rcX-}
777 = \ @ a{-reT-} :: * @ b{-reS-} :: *
778 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
779 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
781 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
782 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
784 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
785 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
787 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
788 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
789 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
791 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
793 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
795 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
796 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
800 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
801 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
802 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
803 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
805 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
807 and it's not as bad as it seems, because it's further dramatically
808 simplified: only zeze2 is extracted and its body is simplified.
811 %************************************************************************
813 \subsection{Error messages}
815 %************************************************************************
818 instDeclCtxt1 hs_inst_ty
819 = inst_decl_ctxt (case unLoc hs_inst_ty of
820 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
821 HsPredTy pred -> ppr pred
822 other -> ppr hs_inst_ty) -- Don't expect this
823 instDeclCtxt2 dfun_ty
824 = inst_decl_ctxt (ppr (mkClassPred cls tys))
826 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
828 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
830 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
832 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
836 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
837 ptext SLIT("must be variables")
838 , ptext SLIT("Instead of a variable, found") <+> ppr ty
841 wrongATArgErr ty instTy =
842 sep [ ptext SLIT("Type indexes must match class instance head")
843 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>