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),
22 tcSplitDFunTy, mkFunTy )
23 import Inst ( newDictBndr, newDictBndrs, instToId, showLIE,
24 getOverlapFlag, tcExtendLocalInstEnv )
25 import InstEnv ( mkLocalInstance, instanceDFunId )
26 import FamInst ( tcExtendLocalFamInstEnv )
27 import FamInstEnv ( extractFamInsts )
28 import TcDeriv ( tcDeriving )
29 import TcEnv ( InstInfo(..), InstBindings(..),
30 newDFunName, tcExtendIdEnv, tcExtendGlobalEnv
32 import TcHsType ( kcHsSigType, tcHsKindedType )
33 import TcUnify ( checkSigTyVars )
34 import TcSimplify ( tcSimplifySuperClasses )
35 import Type ( zipOpenTvSubst, substTheta, mkTyConApp, mkTyVarTy,
36 TyThing(ATyCon), isTyVarTy, tcEqType,
37 substTys, emptyTvSubst, extendTvSubst )
38 import Coercion ( mkSymCoercion )
39 import TyCon ( TyCon, tyConName, newTyConCo_maybe, tyConTyVars,
40 isTyConAssoc, tyConFamInst_maybe, tyConDataCons,
41 assocTyConArgPoss_maybe )
42 import DataCon ( classDataCon, dataConInstArgTys )
43 import Class ( Class, classTyCon, classBigSig, classATs )
44 import Var ( TyVar, Id, idName, idType, tyVarName )
45 import MkId ( mkDictFunId )
46 import Name ( Name, getSrcLoc, nameOccName )
47 import NameSet ( addListToNameSet, emptyNameSet, minusNameSet,
49 import Maybe ( fromJust, catMaybes )
52 import DynFlags ( DynFlag(Opt_WarnMissingMethods) )
53 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart,
55 import ListSetOps ( minusList )
56 import Util ( snocView, dropList )
59 import BasicTypes ( Activation( AlwaysActive ), InlineSpec(..) )
60 import HscTypes ( implicitTyThings )
64 Typechecking instance declarations is done in two passes. The first
65 pass, made by @tcInstDecls1@, collects information to be used in the
68 This pre-processed info includes the as-yet-unprocessed bindings
69 inside the instance declaration. These are type-checked in the second
70 pass, when the class-instance envs and GVE contain all the info from
71 all the instance and value decls. Indeed that's the reason we need
72 two passes over the instance decls.
74 Here is the overall algorithm.
75 Assume that we have an instance declaration
77 instance c => k (t tvs) where b
81 $LIE_c$ is the LIE for the context of class $c$
83 $betas_bar$ is the free variables in the class method type, excluding the
86 $LIE_cop$ is the LIE constraining a particular class method
88 $tau_cop$ is the tau type of a class method
90 $LIE_i$ is the LIE for the context of instance $i$
92 $X$ is the instance constructor tycon
94 $gammas_bar$ is the set of type variables of the instance
96 $LIE_iop$ is the LIE for a particular class method instance
98 $tau_iop$ is the tau type for this instance of a class method
100 $alpha$ is the class variable
102 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
104 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
107 ToDo: Update the list above with names actually in the code.
111 First, make the LIEs for the class and instance contexts, which means
112 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
113 and make LIElistI and LIEI.
115 Then process each method in turn.
117 order the instance methods according to the ordering of the class methods
119 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
121 Create final dictionary function from bindings generated already
123 df = lambda inst_tyvars
130 in <op1,op2,...,opn,sd1,...,sdm>
132 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
133 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
137 %************************************************************************
139 \subsection{Extracting instance decls}
141 %************************************************************************
143 Gather up the instance declarations from their various sources
146 tcInstDecls1 -- Deal with both source-code and imported instance decls
147 :: [LTyClDecl Name] -- For deriving stuff
148 -> [LInstDecl Name] -- Source code instance decls
149 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
150 -> TcM (TcGblEnv, -- The full inst env
151 [InstInfo], -- Source-code instance decls to process;
152 -- contains all dfuns for this module
153 HsValBinds Name) -- Supporting bindings for derived instances
155 tcInstDecls1 tycl_decls inst_decls deriv_decls
157 do { -- Stop if addInstInfos etc discovers any errors
158 -- (they recover, so that we get more than one error each
161 -- (1) Do class instance declarations and instances of indexed
163 ; let { idxty_decls = filter (isIdxTyDecl . unLoc) tycl_decls }
164 ; local_info_tycons <- mappM tcLocalInstDecl1 inst_decls
165 ; idx_tycons <- mappM tcIdxTyInstDeclTL idxty_decls
167 ; let { (local_infos,
168 at_tycons) = unzip local_info_tycons
169 ; local_info = concat local_infos
170 ; at_idx_tycon = concat at_tycons ++ catMaybes idx_tycons
171 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
172 ; implicit_things = concatMap implicitTyThings at_idx_tycon
175 -- (2) Add the tycons of indexed types and their implicit
176 -- tythings to the global environment
177 ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do {
179 -- (3) Instances from generic class declarations
180 ; generic_inst_info <- getGenericInstances clas_decls
182 -- (3) Compute instances from "deriving" clauses;
183 -- This stuff computes a context for the derived instance decl, so it
184 -- needs to know about all the instances possible; hence inst_env4
185 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
186 addInsts deriv_inst_info $
188 -- (4) Compute instances from "deriving" clauses;
189 -- This stuff computes a context for the derived instance
190 -- decl, so it needs to know about all the instances possible
191 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls
192 ; addInsts deriv_inst_info $ do {
194 ; gbl_env <- getGblEnv
196 generic_inst_info ++ deriv_inst_info ++ local_info,
200 -- Make sure that toplevel type instance are not for associated types.
201 -- !!!TODO: Need to perform this check for the TyThing of type functions,
203 tcIdxTyInstDeclTL ldecl@(L loc decl) =
204 do { tything <- tcIdxTyInstDecl ldecl
206 when (isAssocFamily tything) $
207 addErr $ assocInClassErr (tcdName decl)
210 isAssocFamily (Just (ATyCon tycon)) =
211 case tyConFamInst_maybe tycon of
212 Nothing -> panic "isAssocFamily: no family?!?"
213 Just (fam, _) -> isTyConAssoc fam
214 isAssocFamily (Just _ ) = panic "isAssocFamily: no tycon?!?"
215 isAssocFamily Nothing = False
217 assocInClassErr name =
218 ptext SLIT("Associated type") <+> quotes (ppr name) <+>
219 ptext SLIT("must be inside a class instance")
221 addInsts :: [InstInfo] -> TcM a -> TcM a
222 addInsts infos thing_inside
223 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
225 addFamInsts :: [TyThing] -> TcM a -> TcM a
226 addFamInsts tycons thing_inside
227 = tcExtendLocalFamInstEnv (extractFamInsts tycons) thing_inside
231 tcLocalInstDecl1 :: LInstDecl Name
232 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
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 decl@(L loc (InstDecl poly_ty binds uprags ats))
238 = -- Prime error recovery, set source location
239 recoverM (returnM ([], [])) $
241 addErrCtxt (instDeclCtxt1 poly_ty) $
243 do { is_boot <- tcIsHsBoot
244 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
247 -- Typecheck the instance type itself. We can't use
248 -- tcHsSigType, because it's not a valid user type.
249 ; kinded_ty <- kcHsSigType poly_ty
250 ; poly_ty' <- tcHsKindedType kinded_ty
251 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
253 -- Next, process any associated types.
254 ; idx_tycons <- mappM tcIdxTyInstDecl ats
256 -- Now, check the validity of the instance.
257 ; (clas, inst_tys) <- checkValidInstHead tau
258 ; checkValidInstance tyvars theta clas inst_tys
259 ; checkValidAndMissingATs clas (tyvars, inst_tys)
262 -- Finally, construct the Core representation of the instance.
263 -- (This no longer includes the associated types.)
264 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
265 ; overlap_flag <- getOverlapFlag
266 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
267 ispec = mkLocalInstance dfun overlap_flag
269 ; return ([InstInfo { iSpec = ispec,
270 iBinds = VanillaInst binds uprags }],
271 catMaybes idx_tycons)
274 -- We pass in the source form and the type checked form of the ATs. We
275 -- really need the source form only to be able to produce more informative
277 checkValidAndMissingATs :: Class
278 -> ([TyVar], [TcType]) -- instance types
279 -> [(LTyClDecl Name, -- source form of AT
280 Maybe TyThing)] -- Core form of AT
282 checkValidAndMissingATs clas inst_tys ats
283 = do { -- Issue a warning for each class AT that is not defined in this
285 ; let classDefATs = listToNameSet . map tyConName . classATs $ clas
286 definedATs = listToNameSet . map (tcdName.unLoc.fst) $ ats
287 omitted = classDefATs `minusNameSet` definedATs
288 ; warn <- doptM Opt_WarnMissingMethods
289 ; mapM_ (warnTc warn . omittedATWarn) (nameSetToList omitted)
291 -- Ensure that all AT indexes that correspond to class parameters
292 -- coincide with the types in the instance head. All remaining
293 -- AT arguments must be variables. Also raise an error for any
294 -- type instances that are not associated with this class.
295 ; mapM_ (checkIndexes clas inst_tys) ats
298 checkIndexes _ _ (hsAT, Nothing) =
299 return () -- skip, we already had an error here
300 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
301 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
302 checkIndexes' clas inst_tys hsAT
304 snd . fromJust . tyConFamInst_maybe $ tycon)
305 checkIndexes _ _ _ = panic "checkIndexes"
307 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
308 = let atName = tcdName . unLoc $ hsAT
310 setSrcSpan (getLoc hsAT) $
311 addErrCtxt (atInstCtxt atName) $
312 case find ((atName ==) . tyConName) (classATs clas) of
313 Nothing -> addErrTc $ badATErr clas atName -- not in this class
315 case assocTyConArgPoss_maybe atDecl of
316 Nothing -> panic "checkIndexes': AT has no args poss?!?"
319 -- The following is tricky! We need to deal with three
320 -- complications: (1) The AT possibly only uses a subset of
321 -- the class parameters as indexes and those it uses may be in
322 -- a different order; (2) the AT may have extra arguments,
323 -- which must be type variables; and (3) variables in AT and
324 -- instance head will be different `Name's even if their
325 -- source lexemes are identical.
327 -- Re (1), `poss' contains a permutation vector to extract the
328 -- class parameters in the right order.
330 -- Re (2), we wrap the (permuted) class parameters in a Maybe
331 -- type and use Nothing for any extra AT arguments. (First
332 -- equation of `checkIndex' below.)
334 -- Re (3), we replace any type variable in the AT parameters
335 -- that has the same source lexeme as some variable in the
336 -- instance types with the instance type variable sharing its
339 let relevantInstTys = map (instTys !!) poss
340 instArgs = map Just relevantInstTys ++
341 repeat Nothing -- extra arguments
342 renaming = substSameTyVar atTvs instTvs
344 zipWithM_ checkIndex (substTys renaming atTys) instArgs
346 checkIndex ty Nothing
347 | isTyVarTy ty = return ()
348 | otherwise = addErrTc $ mustBeVarArgErr ty
349 checkIndex ty (Just instTy)
350 | ty `tcEqType` instTy = return ()
351 | otherwise = addErrTc $ wrongATArgErr ty instTy
353 listToNameSet = addListToNameSet emptyNameSet
355 substSameTyVar [] _ = emptyTvSubst
356 substSameTyVar (tv:tvs) replacingTvs =
357 let replacement = case find (tv `sameLexeme`) replacingTvs of
358 Nothing -> mkTyVarTy tv
359 Just rtv -> mkTyVarTy rtv
361 tv1 `sameLexeme` tv2 =
362 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
364 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
368 %************************************************************************
370 \subsection{Type-checking instance declarations, pass 2}
372 %************************************************************************
375 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
376 -> TcM (LHsBinds Id, TcLclEnv)
377 -- (a) From each class declaration,
378 -- generate any default-method bindings
379 -- (b) From each instance decl
380 -- generate the dfun binding
382 tcInstDecls2 tycl_decls inst_decls
383 = do { -- (a) Default methods from class decls
384 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
385 filter (isClassDecl.unLoc) tycl_decls
386 ; tcExtendIdEnv (concat dm_ids_s) $ do
388 -- (b) instance declarations
389 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
392 ; let binds = unionManyBags dm_binds_s `unionBags`
393 unionManyBags inst_binds_s
394 ; tcl_env <- getLclEnv -- Default method Ids in here
395 ; returnM (binds, tcl_env) }
398 ======= New documentation starts here (Sept 92) ==============
400 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
401 the dictionary function for this instance declaration. For example
403 instance Foo a => Foo [a] where
407 might generate something like
409 dfun.Foo.List dFoo_a = let op1 x = ...
415 HOWEVER, if the instance decl has no context, then it returns a
416 bigger @HsBinds@ with declarations for each method. For example
418 instance Foo [a] where
424 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
425 const.Foo.op1.List a x = ...
426 const.Foo.op2.List a y = ...
428 This group may be mutually recursive, because (for example) there may
429 be no method supplied for op2 in which case we'll get
431 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
433 that is, the default method applied to the dictionary at this type.
435 What we actually produce in either case is:
437 AbsBinds [a] [dfun_theta_dicts]
438 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
439 { d = (sd1,sd2, ..., op1, op2, ...)
444 The "maybe" says that we only ask AbsBinds to make global constant methods
445 if the dfun_theta is empty.
448 For an instance declaration, say,
450 instance (C1 a, C2 b) => C (T a b) where
453 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
454 function whose type is
456 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
458 Notice that we pass it the superclass dictionaries at the instance type; this
459 is the ``Mark Jones optimisation''. The stuff before the "=>" here
460 is the @dfun_theta@ below.
462 First comes the easy case of a non-local instance decl.
466 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
467 -- Returns a binding for the dfun
469 ------------------------
470 -- Derived newtype instances; surprisingly tricky!
472 -- In the case of a newtype, things are rather easy
473 -- class Show a => Foo a b where ...
474 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
475 -- The newtype gives an FC axiom looking like
476 -- axiom CoT a :: T 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 (T a)) => Foo Int (T a)
481 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
482 -- case df `cast` (Foo Int (sym (CoT 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 mb_preds })
489 = do { let dfun_id = instanceDFunId ispec
490 rigid_info = InstSkol dfun_id
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 ; inst_loc <- getInstLoc origin
497 ; (rep_dict_id : sc_dict_ids, wrap_fn, sc_binds)
498 <- make_wrapper inst_loc tvs theta mb_preds
499 -- Here, we are relying on the order of dictionary
500 -- arguments built by NewTypeDerived in TcDeriv;
501 -- namely, that the rep_dict_id comes first
503 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
504 cls_tycon = classTyCon cls
505 the_coercion = make_coercion cls_tycon cls_inst_tys
506 coerced_rep_dict = mkHsWrap the_coercion (HsVar rep_dict_id)
508 ; body <- make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
510 ; return (sc_binds `snocBag` (noLoc $ VarBind dfun_id $ noLoc $ mkHsWrap wrap_fn body)) }
513 -----------------------
515 -- We distinguish two cases:
516 -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
517 -- and the new dict can just be a constant
518 -- (mb_preds = Just preds)
519 -- (b) there are tyvars, so we must make a dict *fun*
520 -- (mb_preds = Nothing)
521 -- See the defn of NewTypeDerived for the meaning of mb_preds
522 make_wrapper inst_loc tvs theta (Just preds) -- Case (a)
523 = ASSERT( null tvs && null theta )
524 do { dicts <- newDictBndrs inst_loc preds
525 ; sc_binds <- addErrCtxt superClassCtxt (tcSimplifySuperClasses [] [] dicts)
526 -- Use tcSimplifySuperClasses to avoid creating loops, for the
527 -- same reason as Note [SUPERCLASS-LOOP 1] in TcSimplify
528 ; return (map instToId dicts, idHsWrapper, sc_binds) }
530 make_wrapper inst_loc tvs theta Nothing -- Case (b)
531 = do { dicts <- newDictBndrs inst_loc theta
532 ; let dict_ids = map instToId dicts
533 ; return (dict_ids, mkWpTyLams tvs <.> mkWpLams dict_ids, emptyBag) }
535 -----------------------
537 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
538 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
539 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
540 -- where rep_ty is the (eta-reduced) type rep of T
541 -- So we just replace T with CoT, and insert a 'sym'
542 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
544 make_coercion cls_tycon cls_inst_tys
545 | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
546 , (tycon, tc_args) <- tcSplitTyConApp last_ty -- Should not fail
547 , Just co_con <- newTyConCo_maybe tycon
548 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
549 = WpCo (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
550 | otherwise -- The newtype is transparent; no need for a cast
553 -----------------------
555 -- Two cases; see Note [Newtype deriving superclasses] in TcDeriv.lhs
556 -- (a) no superclasses; then we can just use the coerced dict
557 -- (b) one or more superclasses; then new need to do the unpack/repack
559 make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
560 | null sc_dict_ids -- Case (a)
561 = return coerced_rep_dict
562 | otherwise -- Case (b)
563 = do { op_ids <- newSysLocalIds FSLIT("op") op_tys
564 ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
565 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
566 pat_dicts = dummy_sc_dict_ids,
567 pat_binds = emptyLHsBinds,
568 pat_args = PrefixCon (map nlVarPat op_ids),
570 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
571 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
572 map HsVar (sc_dict_ids ++ op_ids)
574 -- Warning: this HsCase scrutinises a value with a PredTy, which is
575 -- never otherwise seen in Haskell source code. It'd be
576 -- nicer to generate Core directly!
577 ; return (HsCase (noLoc coerced_rep_dict) $
578 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
580 pat_ty = mkTyConApp cls_tycon cls_inst_tys
581 cls_data_con = head (tyConDataCons cls_tycon)
582 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
583 op_tys = dropList sc_dict_ids cls_arg_tys
585 ------------------------
586 -- Ordinary instances
588 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
590 dfun_id = instanceDFunId ispec
591 rigid_info = InstSkol dfun_id
592 inst_ty = idType dfun_id
594 -- Prime error recovery
595 recoverM (returnM emptyLHsBinds) $
596 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
597 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
599 -- Instantiate the instance decl with skolem constants
600 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
601 -- These inst_tyvars' scope over the 'where' part
602 -- Those tyvars are inside the dfun_id's type, which is a bit
603 -- bizarre, but OK so long as you realise it!
605 (clas, inst_tys') = tcSplitDFunHead inst_head'
606 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
608 -- Instantiate the super-class context with inst_tys
609 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
610 origin = SigOrigin rigid_info
612 -- Create dictionary Ids from the specified instance contexts.
613 getInstLoc InstScOrigin `thenM` \ sc_loc ->
614 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
615 getInstLoc origin `thenM` \ inst_loc ->
616 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
617 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
618 -- Default-method Ids may be mentioned in synthesised RHSs,
619 -- but they'll already be in the environment.
621 -- Typecheck the methods
622 let -- These insts are in scope; quite a few, eh?
623 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
625 tcMethods origin clas inst_tyvars'
626 dfun_theta' inst_tys' avail_insts
627 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
629 -- Figure out bindings for the superclass context
630 -- Don't include this_dict in the 'givens', else
631 -- sc_dicts get bound by just selecting from this_dict!!
632 addErrCtxt superClassCtxt
633 (tcSimplifySuperClasses inst_tyvars'
635 sc_dicts) `thenM` \ sc_binds ->
637 -- It's possible that the superclass stuff might unified one
638 -- of the inst_tyavars' with something in the envt
639 checkSigTyVars inst_tyvars' `thenM_`
641 -- Deal with 'SPECIALISE instance' pragmas
642 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
644 -- Create the result bindings
646 dict_constr = classDataCon clas
647 scs_and_meths = map instToId sc_dicts ++ meth_ids
648 this_dict_id = instToId this_dict
649 inline_prag | null dfun_arg_dicts = []
650 | otherwise = [InlinePrag (Inline AlwaysActive True)]
651 -- Always inline the dfun; this is an experimental decision
652 -- because it makes a big performance difference sometimes.
653 -- Often it means we can do the method selection, and then
654 -- inline the method as well. Marcin's idea; see comments below.
656 -- BUT: don't inline it if it's a constant dictionary;
657 -- we'll get all the benefit without inlining, and we get
658 -- a **lot** of code duplication if we inline it
660 -- See Note [Inline dfuns] below
663 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
664 -- We don't produce a binding for the dict_constr; instead we
665 -- rely on the simplifier to unfold this saturated application
666 -- We do this rather than generate an HsCon directly, because
667 -- it means that the special cases (e.g. dictionary with only one
668 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
669 -- than needing to be repeated here.
671 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
672 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
674 main_bind = noLoc $ AbsBinds
676 (map instToId dfun_arg_dicts)
677 [(inst_tyvars', dfun_id, this_dict_id,
678 inline_prag ++ prags)]
681 showLIE (text "instance") `thenM_`
682 returnM (unitBag main_bind)
685 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
686 avail_insts op_items monobinds uprags
687 = -- Check that all the method bindings come from this class
689 sel_names = [idName sel_id | (sel_id, _) <- op_items]
690 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
692 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
694 -- Make the method bindings
696 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
698 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
700 -- And type check them
701 -- It's really worth making meth_insts available to the tcMethodBind
702 -- Consider instance Monad (ST s) where
703 -- {-# INLINE (>>) #-}
704 -- (>>) = ...(>>=)...
705 -- If we don't include meth_insts, we end up with bindings like this:
706 -- rec { dict = MkD then bind ...
707 -- then = inline_me (... (GHC.Base.>>= dict) ...)
709 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
710 -- and (b) the inline_me prevents us inlining the >>= selector, which
711 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
712 -- is not inlined across modules. Rather ironic since this does not
713 -- happen without the INLINE pragma!
715 -- Solution: make meth_insts available, so that 'then' refers directly
716 -- to the local 'bind' rather than going via the dictionary.
718 -- BUT WATCH OUT! If the method type mentions the class variable, then
719 -- this optimisation is not right. Consider
723 -- instance C Int where
725 -- The occurrence of 'op' on the rhs gives rise to a constraint
727 -- The trouble is that the 'meth_inst' for op, which is 'available', also
728 -- looks like 'op at Int'. But they are not the same.
730 prag_fn = mkPragFun uprags
731 all_insts = avail_insts ++ catMaybes meth_insts
732 sig_fn n = Just [] -- No scoped type variables, but every method has
733 -- a type signature, in effect, so that we check
734 -- the method has the right type
735 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
736 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
739 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
741 returnM (meth_ids, unionManyBags meth_binds_s)
745 ------------------------------
746 [Inline dfuns] Inlining dfuns unconditionally
747 ------------------------------
749 The code above unconditionally inlines dict funs. Here's why.
750 Consider this program:
752 test :: Int -> Int -> Bool
753 test x y = (x,y) == (y,x) || test y x
754 -- Recursive to avoid making it inline.
756 This needs the (Eq (Int,Int)) instance. If we inline that dfun
757 the code we end up with is good:
760 \r -> case ==# [ww ww1] of wild {
761 PrelBase.False -> Test.$wtest ww1 ww;
763 case ==# [ww1 ww] of wild1 {
764 PrelBase.False -> Test.$wtest ww1 ww;
765 PrelBase.True -> PrelBase.True [];
768 Test.test = \r [w w1]
771 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
774 If we don't inline the dfun, the code is not nearly as good:
776 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
777 PrelBase.:DEq tpl1 tpl2 -> tpl2;
782 let { y = PrelBase.I#! [ww1]; } in
783 let { x = PrelBase.I#! [ww]; } in
784 let { sat_slx = PrelTup.(,)! [y x]; } in
785 let { sat_sly = PrelTup.(,)! [x y];
787 case == sat_sly sat_slx of wild {
788 PrelBase.False -> Test.$wtest ww1 ww;
789 PrelBase.True -> PrelBase.True [];
796 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
799 Why doesn't GHC inline $fEq? Because it looks big:
801 PrelTup.zdfEqZ1T{-rcX-}
802 = \ @ a{-reT-} :: * @ b{-reS-} :: *
803 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
804 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
806 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
807 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
809 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
810 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
812 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
813 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
814 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
816 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
818 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
820 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
821 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
825 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
826 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
827 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
828 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
830 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
832 and it's not as bad as it seems, because it's further dramatically
833 simplified: only zeze2 is extracted and its body is simplified.
836 %************************************************************************
838 \subsection{Error messages}
840 %************************************************************************
843 instDeclCtxt1 hs_inst_ty
844 = inst_decl_ctxt (case unLoc hs_inst_ty of
845 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
846 HsPredTy pred -> ppr pred
847 other -> ppr hs_inst_ty) -- Don't expect this
848 instDeclCtxt2 dfun_ty
849 = inst_decl_ctxt (ppr (mkClassPred cls tys))
851 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
853 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
855 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
857 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
861 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
862 ptext SLIT("must be variables")
863 , ptext SLIT("Instead of a variable, found") <+> ppr ty
866 wrongATArgErr ty instTy =
867 sep [ ptext SLIT("Type indexes must match class instance head")
868 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>