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,
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 -> TcM (TcGblEnv, -- The full inst env
150 [InstInfo], -- Source-code instance decls to process;
151 -- contains all dfuns for this module
152 HsValBinds Name) -- Supporting bindings for derived instances
154 tcInstDecls1 tycl_decls inst_decls
156 do { -- Stop if addInstInfos etc discovers any errors
157 -- (they recover, so that we get more than one error each
160 -- (1) Do class instance declarations and instances of indexed
162 ; let { idxty_decls = filter (isIdxTyDecl . unLoc) tycl_decls }
163 ; local_info_tycons <- mappM tcLocalInstDecl1 inst_decls
164 ; idx_tycons <- mappM tcIdxTyInstDeclTL idxty_decls
166 ; let { (local_infos,
167 at_tycons) = unzip local_info_tycons
168 ; local_info = concat local_infos
169 ; at_idx_tycon = concat at_tycons ++ catMaybes idx_tycons
170 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
171 ; implicit_things = concatMap implicitTyThings at_idx_tycon
174 -- (2) Add the tycons of indexed types and their implicit
175 -- tythings to the global environment
176 ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do {
178 -- (3) Instances from generic class declarations
179 ; generic_inst_info <- getGenericInstances clas_decls
181 -- Next, construct the instance environment so far, consisting
183 -- a) local instance decls
184 -- b) generic instances
185 -- c) local family instance decls
186 ; addInsts local_info $ do {
187 ; addInsts generic_inst_info $ do {
188 ; addFamInsts at_idx_tycon $ 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_info,
202 -- Make sure that toplevel type instance are not for associated types.
203 -- !!!TODO: Need to perform this check for the TyThing of type functions,
205 tcIdxTyInstDeclTL ldecl@(L loc decl) =
206 do { tything <- tcIdxTyInstDecl ldecl
208 when (isAssocFamily tything) $
209 addErr $ assocInClassErr (tcdName decl)
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
227 addFamInsts :: [TyThing] -> TcM a -> TcM a
228 addFamInsts tycons thing_inside
229 = tcExtendLocalFamInstEnv (extractFamInsts tycons) thing_inside
233 tcLocalInstDecl1 :: LInstDecl Name
234 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
235 -- A source-file instance declaration
236 -- Type-check all the stuff before the "where"
238 -- We check for respectable instance type, and context
239 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
240 = -- Prime error recovery, set source location
241 recoverM (returnM ([], [])) $
243 addErrCtxt (instDeclCtxt1 poly_ty) $
245 do { is_boot <- tcIsHsBoot
246 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
249 -- Typecheck the instance type itself. We can't use
250 -- tcHsSigType, because it's not a valid user type.
251 ; kinded_ty <- kcHsSigType poly_ty
252 ; poly_ty' <- tcHsKindedType kinded_ty
253 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
255 -- Next, process any associated types.
256 ; idx_tycons <- mappM tcIdxTyInstDecl ats
258 -- Now, check the validity of the instance.
259 ; (clas, inst_tys) <- checkValidInstHead tau
260 ; checkValidInstance tyvars theta clas inst_tys
261 ; checkValidAndMissingATs clas (tyvars, inst_tys)
264 -- Finally, construct the Core representation of the instance.
265 -- (This no longer includes the associated types.)
266 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
267 ; overlap_flag <- getOverlapFlag
268 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
269 ispec = mkLocalInstance dfun overlap_flag
271 ; return ([InstInfo { iSpec = ispec,
272 iBinds = VanillaInst binds uprags }],
273 catMaybes idx_tycons)
276 -- We pass in the source form and the type checked form of the ATs. We
277 -- really need the source form only to be able to produce more informative
279 checkValidAndMissingATs :: Class
280 -> ([TyVar], [TcType]) -- instance types
281 -> [(LTyClDecl Name, -- source form of AT
282 Maybe TyThing)] -- Core form of AT
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) =
301 return () -- skip, we already had an error here
302 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
303 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
304 checkIndexes' clas inst_tys hsAT
306 snd . fromJust . tyConFamInst_maybe $ tycon)
307 checkIndexes _ _ _ = panic "checkIndexes"
309 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
310 = let atName = tcdName . unLoc $ hsAT
312 setSrcSpan (getLoc hsAT) $
313 addErrCtxt (atInstCtxt atName) $
314 case find ((atName ==) . tyConName) (classATs clas) of
315 Nothing -> addErrTc $ badATErr clas atName -- not in this class
317 case assocTyConArgPoss_maybe atDecl of
318 Nothing -> panic "checkIndexes': AT has no args poss?!?"
321 -- The following is tricky! We need to deal with three
322 -- complications: (1) The AT possibly only uses a subset of
323 -- the class parameters as indexes and those it uses may be in
324 -- a different order; (2) the AT may have extra arguments,
325 -- which must be type variables; and (3) variables in AT and
326 -- instance head will be different `Name's even if their
327 -- source lexemes are identical.
329 -- Re (1), `poss' contains a permutation vector to extract the
330 -- class parameters in the right order.
332 -- Re (2), we wrap the (permuted) class parameters in a Maybe
333 -- type and use Nothing for any extra AT arguments. (First
334 -- equation of `checkIndex' below.)
336 -- Re (3), we replace any type variable in the AT parameters
337 -- that has the same source lexeme as some variable in the
338 -- instance types with the instance type variable sharing its
341 let relevantInstTys = map (instTys !!) poss
342 instArgs = map Just relevantInstTys ++
343 repeat Nothing -- extra arguments
344 renaming = substSameTyVar atTvs instTvs
346 zipWithM_ checkIndex (substTys renaming atTys) instArgs
348 checkIndex ty Nothing
349 | isTyVarTy ty = return ()
350 | otherwise = addErrTc $ mustBeVarArgErr ty
351 checkIndex ty (Just instTy)
352 | ty `tcEqType` instTy = return ()
353 | otherwise = addErrTc $ wrongATArgErr ty instTy
355 listToNameSet = addListToNameSet emptyNameSet
357 substSameTyVar [] _ = emptyTvSubst
358 substSameTyVar (tv:tvs) replacingTvs =
359 let replacement = case find (tv `sameLexeme`) replacingTvs of
360 Nothing -> mkTyVarTy tv
361 Just rtv -> mkTyVarTy rtv
363 tv1 `sameLexeme` tv2 =
364 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
366 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
370 %************************************************************************
372 \subsection{Type-checking instance declarations, pass 2}
374 %************************************************************************
377 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
378 -> TcM (LHsBinds Id, TcLclEnv)
379 -- (a) From each class declaration,
380 -- generate any default-method bindings
381 -- (b) From each instance decl
382 -- generate the dfun binding
384 tcInstDecls2 tycl_decls inst_decls
385 = do { -- (a) Default methods from class decls
386 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
387 filter (isClassDecl.unLoc) tycl_decls
388 ; tcExtendIdEnv (concat dm_ids_s) $ do
390 -- (b) instance declarations
391 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
394 ; let binds = unionManyBags dm_binds_s `unionBags`
395 unionManyBags inst_binds_s
396 ; tcl_env <- getLclEnv -- Default method Ids in here
397 ; returnM (binds, tcl_env) }
400 ======= New documentation starts here (Sept 92) ==============
402 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
403 the dictionary function for this instance declaration. For example
405 instance Foo a => Foo [a] where
409 might generate something like
411 dfun.Foo.List dFoo_a = let op1 x = ...
417 HOWEVER, if the instance decl has no context, then it returns a
418 bigger @HsBinds@ with declarations for each method. For example
420 instance Foo [a] where
426 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
427 const.Foo.op1.List a x = ...
428 const.Foo.op2.List a y = ...
430 This group may be mutually recursive, because (for example) there may
431 be no method supplied for op2 in which case we'll get
433 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
435 that is, the default method applied to the dictionary at this type.
437 What we actually produce in either case is:
439 AbsBinds [a] [dfun_theta_dicts]
440 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
441 { d = (sd1,sd2, ..., op1, op2, ...)
446 The "maybe" says that we only ask AbsBinds to make global constant methods
447 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.
464 First comes the easy case of a non-local instance decl.
468 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
469 -- Returns a binding for the dfun
471 ------------------------
472 -- Derived newtype instances
474 -- In the case of a newtype, things are rather easy
475 -- class Show a => Foo a b where ...
476 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
477 -- The newtype gives an FC axiom looking like
478 -- axiom CoT a :: T a :=: Tree [a]
479 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
481 -- So all need is to generate a binding looking like:
482 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
483 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
484 -- case df `cast` (Foo Int (sym (CoT a))) of
485 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
487 -- If there are no superclasses, matters are simpler, because we don't need the case
488 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
490 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
491 = do { let dfun_id = instanceDFunId ispec
492 rigid_info = InstSkol dfun_id
493 origin = SigOrigin rigid_info
494 inst_ty = idType dfun_id
495 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
496 -- inst_head_ty is a PredType
498 ; inst_loc <- getInstLoc origin
499 ; (rep_dict_id : sc_dict_ids, wrap_fn)
500 <- make_wrapper inst_loc tvs theta mb_preds
501 -- Here, we are relying on the order of dictionary
502 -- arguments built by NewTypeDerived in TcDeriv;
503 -- namely, that the rep_dict_id comes first
505 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
506 the_coercion = make_coercion cls cls_inst_tys
507 coerced_rep_dict = mkHsCoerce the_coercion (HsVar rep_dict_id)
509 ; body <- make_body cls cls_inst_tys inst_head_ty sc_dict_ids coerced_rep_dict
511 ; return (unitBag (noLoc $ VarBind dfun_id $ noLoc $ mkHsCoerce wrap_fn body)) }
514 -----------------------
516 -- We distinguish two cases:
517 -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
518 -- and the new dict can just be a constant
519 -- (mb_preds = Just preds)
520 -- (b) there are tyvars, so we must make a dict *fun*
521 -- (mb_preds = Nothing)
522 -- See the defn of NewTypeDerived for the meaning of mb_preds
523 make_wrapper inst_loc tvs theta (Just preds) -- Case (a)
524 = ASSERT( null tvs && null theta )
525 do { dicts <- newDictBndrs inst_loc preds
527 ; return (map instToId dicts, idCoercion) }
528 make_wrapper inst_loc tvs theta Nothing -- Case (b)
529 = do { dicts <- newDictBndrs inst_loc theta
530 ; let dict_ids = map instToId dicts
531 ; return (dict_ids, mkCoTyLams tvs <.> mkCoLams dict_ids) }
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 cls_inst_tys
543 | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
544 , (tycon, tc_args) <- tcSplitTyConApp last_ty -- Should not fail
545 , Just co_con <- newTyConCo_maybe tycon
546 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
547 = ExprCoFn (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
548 | otherwise -- The newtype is transparent; no need for a cast
551 cls_tycon = classTyCon cls
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 cls_inst_tys inst_head_ty 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),
569 pat_ty = inst_head_ty}
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 ; return (HsCase (noLoc coerced_rep_dict) $
575 MatchGroup [the_match] (mkFunTy inst_head_ty inst_head_ty)) }
577 cls_data_con = classDataCon cls
578 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
579 op_tys = dropList sc_dict_ids cls_arg_tys
581 ------------------------
582 -- Ordinary instances
584 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
586 dfun_id = instanceDFunId ispec
587 rigid_info = InstSkol dfun_id
588 inst_ty = idType dfun_id
590 -- Prime error recovery
591 recoverM (returnM emptyLHsBinds) $
592 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
593 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
595 -- Instantiate the instance decl with skolem constants
596 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
597 -- These inst_tyvars' scope over the 'where' part
598 -- Those tyvars are inside the dfun_id's type, which is a bit
599 -- bizarre, but OK so long as you realise it!
601 (clas, inst_tys') = tcSplitDFunHead inst_head'
602 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
604 -- Instantiate the super-class context with inst_tys
605 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
606 origin = SigOrigin rigid_info
608 -- Create dictionary Ids from the specified instance contexts.
609 getInstLoc InstScOrigin `thenM` \ sc_loc ->
610 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
611 getInstLoc origin `thenM` \ inst_loc ->
612 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
613 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
614 -- Default-method Ids may be mentioned in synthesised RHSs,
615 -- but they'll already be in the environment.
617 -- Typecheck the methods
618 let -- These insts are in scope; quite a few, eh?
619 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
621 tcMethods origin clas inst_tyvars'
622 dfun_theta' inst_tys' avail_insts
623 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
625 -- Figure out bindings for the superclass context
626 -- Don't include this_dict in the 'givens', else
627 -- sc_dicts get bound by just selecting from this_dict!!
628 addErrCtxt superClassCtxt
629 (tcSimplifySuperClasses inst_tyvars'
631 sc_dicts) `thenM` \ sc_binds ->
633 -- It's possible that the superclass stuff might unified one
634 -- of the inst_tyavars' with something in the envt
635 checkSigTyVars inst_tyvars' `thenM_`
637 -- Deal with 'SPECIALISE instance' pragmas
638 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
640 -- Create the result bindings
642 dict_constr = classDataCon clas
643 scs_and_meths = map instToId sc_dicts ++ meth_ids
644 this_dict_id = instToId this_dict
645 inline_prag | null dfun_arg_dicts = []
646 | otherwise = [InlinePrag (Inline AlwaysActive True)]
647 -- Always inline the dfun; this is an experimental decision
648 -- because it makes a big performance difference sometimes.
649 -- Often it means we can do the method selection, and then
650 -- inline the method as well. Marcin's idea; see comments below.
652 -- BUT: don't inline it if it's a constant dictionary;
653 -- we'll get all the benefit without inlining, and we get
654 -- a **lot** of code duplication if we inline it
656 -- See Note [Inline dfuns] below
659 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
660 -- We don't produce a binding for the dict_constr; instead we
661 -- rely on the simplifier to unfold this saturated application
662 -- We do this rather than generate an HsCon directly, because
663 -- it means that the special cases (e.g. dictionary with only one
664 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
665 -- than needing to be repeated here.
667 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
668 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
670 main_bind = noLoc $ AbsBinds
672 (map instToId dfun_arg_dicts)
673 [(inst_tyvars', dfun_id, this_dict_id,
674 inline_prag ++ prags)]
677 showLIE (text "instance") `thenM_`
678 returnM (unitBag main_bind)
681 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
682 avail_insts op_items monobinds uprags
683 = -- Check that all the method bindings come from this class
685 sel_names = [idName sel_id | (sel_id, _) <- op_items]
686 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
688 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
690 -- Make the method bindings
692 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
694 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
696 -- And type check them
697 -- It's really worth making meth_insts available to the tcMethodBind
698 -- Consider instance Monad (ST s) where
699 -- {-# INLINE (>>) #-}
700 -- (>>) = ...(>>=)...
701 -- If we don't include meth_insts, we end up with bindings like this:
702 -- rec { dict = MkD then bind ...
703 -- then = inline_me (... (GHC.Base.>>= dict) ...)
705 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
706 -- and (b) the inline_me prevents us inlining the >>= selector, which
707 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
708 -- is not inlined across modules. Rather ironic since this does not
709 -- happen without the INLINE pragma!
711 -- Solution: make meth_insts available, so that 'then' refers directly
712 -- to the local 'bind' rather than going via the dictionary.
714 -- BUT WATCH OUT! If the method type mentions the class variable, then
715 -- this optimisation is not right. Consider
719 -- instance C Int where
721 -- The occurrence of 'op' on the rhs gives rise to a constraint
723 -- The trouble is that the 'meth_inst' for op, which is 'available', also
724 -- looks like 'op at Int'. But they are not the same.
726 prag_fn = mkPragFun uprags
727 all_insts = avail_insts ++ catMaybes meth_insts
728 sig_fn n = Just [] -- No scoped type variables, but every method has
729 -- a type signature, in effect, so that we check
730 -- the method has the right type
731 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
732 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
735 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
737 returnM (meth_ids, unionManyBags meth_binds_s)
741 ------------------------------
742 [Inline dfuns] Inlining dfuns unconditionally
743 ------------------------------
745 The code above unconditionally inlines dict funs. Here's why.
746 Consider this program:
748 test :: Int -> Int -> Bool
749 test x y = (x,y) == (y,x) || test y x
750 -- Recursive to avoid making it inline.
752 This needs the (Eq (Int,Int)) instance. If we inline that dfun
753 the code we end up with is good:
756 \r -> case ==# [ww ww1] of wild {
757 PrelBase.False -> Test.$wtest ww1 ww;
759 case ==# [ww1 ww] of wild1 {
760 PrelBase.False -> Test.$wtest ww1 ww;
761 PrelBase.True -> PrelBase.True [];
764 Test.test = \r [w w1]
767 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
770 If we don't inline the dfun, the code is not nearly as good:
772 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
773 PrelBase.:DEq tpl1 tpl2 -> tpl2;
778 let { y = PrelBase.I#! [ww1]; } in
779 let { x = PrelBase.I#! [ww]; } in
780 let { sat_slx = PrelTup.(,)! [y x]; } in
781 let { sat_sly = PrelTup.(,)! [x y];
783 case == sat_sly sat_slx of wild {
784 PrelBase.False -> Test.$wtest ww1 ww;
785 PrelBase.True -> PrelBase.True [];
792 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
795 Why doesn't GHC inline $fEq? Because it looks big:
797 PrelTup.zdfEqZ1T{-rcX-}
798 = \ @ a{-reT-} :: * @ b{-reS-} :: *
799 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
800 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
802 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
803 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
805 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
806 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
808 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
809 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
810 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
812 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
814 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
816 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
817 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
821 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
822 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
823 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
824 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
826 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
828 and it's not as bad as it seems, because it's further dramatically
829 simplified: only zeze2 is extracted and its body is simplified.
832 %************************************************************************
834 \subsection{Error messages}
836 %************************************************************************
839 instDeclCtxt1 hs_inst_ty
840 = inst_decl_ctxt (case unLoc hs_inst_ty of
841 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
842 HsPredTy pred -> ppr pred
843 other -> ppr hs_inst_ty) -- Don't expect this
844 instDeclCtxt2 dfun_ty
845 = inst_decl_ctxt (ppr (mkClassPred cls tys))
847 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
849 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
851 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
853 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
857 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
858 ptext SLIT("must be variables")
859 , ptext SLIT("Instead of a variable, found") <+> ppr ty
862 wrongATArgErr ty instTy =
863 sep [ ptext SLIT("Type indexes must match class instance head")
864 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>