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 -- Next, construct the instance environment so far, consisting
184 -- a) local instance decls
185 -- b) generic instances
186 -- c) local family instance decls
187 ; addInsts local_info $ do {
188 ; addInsts generic_inst_info $ do {
189 ; addFamInsts at_idx_tycon $ do {
191 -- (4) Compute instances from "deriving" clauses;
192 -- This stuff computes a context for the derived instance
193 -- decl, so it needs to know about all the instances possible
194 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls deriv_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 <- tcIdxTyInstDecl 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 (extractFamInsts tycons) thing_inside
234 tcLocalInstDecl1 :: LInstDecl Name
235 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
236 -- A source-file instance declaration
237 -- Type-check all the stuff before the "where"
239 -- We check for respectable instance type, and context
240 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
241 = -- Prime error recovery, set source location
242 recoverM (returnM ([], [])) $
244 addErrCtxt (instDeclCtxt1 poly_ty) $
246 do { is_boot <- tcIsHsBoot
247 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
250 -- Typecheck the instance type itself. We can't use
251 -- tcHsSigType, because it's not a valid user type.
252 ; kinded_ty <- kcHsSigType poly_ty
253 ; poly_ty' <- tcHsKindedType kinded_ty
254 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
256 -- Next, process any associated types.
257 ; idx_tycons <- mappM tcIdxTyInstDecl 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 (srcSpanStart loc)
268 ; overlap_flag <- getOverlapFlag
269 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
270 ispec = mkLocalInstance dfun overlap_flag
272 ; return ([InstInfo { iSpec = ispec,
273 iBinds = VanillaInst binds uprags }],
274 catMaybes idx_tycons)
277 -- We pass in the source form and the type checked form of the ATs. We
278 -- really need the source form only to be able to produce more informative
280 checkValidAndMissingATs :: Class
281 -> ([TyVar], [TcType]) -- instance types
282 -> [(LTyClDecl Name, -- source form of AT
283 Maybe TyThing)] -- Core form of AT
285 checkValidAndMissingATs clas inst_tys ats
286 = do { -- Issue a warning for each class AT that is not defined in this
288 ; let classDefATs = listToNameSet . map tyConName . classATs $ clas
289 definedATs = listToNameSet . map (tcdName.unLoc.fst) $ ats
290 omitted = classDefATs `minusNameSet` definedATs
291 ; warn <- doptM Opt_WarnMissingMethods
292 ; mapM_ (warnTc warn . omittedATWarn) (nameSetToList omitted)
294 -- Ensure that all AT indexes that correspond to class parameters
295 -- coincide with the types in the instance head. All remaining
296 -- AT arguments must be variables. Also raise an error for any
297 -- type instances that are not associated with this class.
298 ; mapM_ (checkIndexes clas inst_tys) ats
301 checkIndexes _ _ (hsAT, Nothing) =
302 return () -- skip, we already had an error here
303 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
304 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
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; surprisingly tricky!
475 -- In the case of a newtype, things are rather easy
476 -- class Show a => Foo a b where ...
477 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
478 -- The newtype gives an FC axiom looking like
479 -- axiom CoT a :: T a :=: Tree [a]
480 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
482 -- So all need is to generate a binding looking like:
483 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
484 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
485 -- case df `cast` (Foo Int (sym (CoT a))) of
486 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
488 -- If there are no superclasses, matters are simpler, because we don't need the case
489 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
491 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
492 = do { let dfun_id = instanceDFunId ispec
493 rigid_info = InstSkol dfun_id
494 origin = SigOrigin rigid_info
495 inst_ty = idType dfun_id
496 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
497 -- inst_head_ty is a PredType
499 ; inst_loc <- getInstLoc origin
500 ; (rep_dict_id : sc_dict_ids, wrap_fn, sc_binds)
501 <- make_wrapper inst_loc tvs theta mb_preds
502 -- Here, we are relying on the order of dictionary
503 -- arguments built by NewTypeDerived in TcDeriv;
504 -- namely, that the rep_dict_id comes first
506 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
507 cls_tycon = classTyCon cls
508 the_coercion = make_coercion cls_tycon cls_inst_tys
509 coerced_rep_dict = mkHsWrap the_coercion (HsVar rep_dict_id)
511 ; body <- make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
513 ; return (sc_binds `snocBag` (noLoc $ VarBind dfun_id $ noLoc $ mkHsWrap wrap_fn body)) }
516 -----------------------
518 -- We distinguish two cases:
519 -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
520 -- and the new dict can just be a constant
521 -- (mb_preds = Just preds)
522 -- (b) there are tyvars, so we must make a dict *fun*
523 -- (mb_preds = Nothing)
524 -- See the defn of NewTypeDerived for the meaning of mb_preds
525 make_wrapper inst_loc tvs theta (Just preds) -- Case (a)
526 = ASSERT( null tvs && null theta )
527 do { dicts <- newDictBndrs inst_loc preds
528 ; sc_binds <- addErrCtxt superClassCtxt (tcSimplifySuperClasses [] [] dicts)
529 -- Use tcSimplifySuperClasses to avoid creating loops, for the
530 -- same reason as Note [SUPERCLASS-LOOP 1] in TcSimplify
531 ; return (map instToId dicts, idHsWrapper, sc_binds) }
533 make_wrapper inst_loc tvs theta Nothing -- Case (b)
534 = do { dicts <- newDictBndrs inst_loc theta
535 ; let dict_ids = map instToId dicts
536 ; return (dict_ids, mkWpTyLams tvs <.> mkWpLams dict_ids, emptyBag) }
538 -----------------------
540 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
541 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
542 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
543 -- where rep_ty is the (eta-reduced) type rep of T
544 -- So we just replace T with CoT, and insert a 'sym'
545 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
547 make_coercion cls_tycon cls_inst_tys
548 | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
549 , (tycon, tc_args) <- tcSplitTyConApp last_ty -- Should not fail
550 , Just co_con <- newTyConCo_maybe tycon
551 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
552 = WpCo (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
553 | otherwise -- The newtype is transparent; no need for a cast
556 -----------------------
558 -- Two cases; see Note [Newtype deriving superclasses] in TcDeriv.lhs
559 -- (a) no superclasses; then we can just use the coerced dict
560 -- (b) one or more superclasses; then new need to do the unpack/repack
562 make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
563 | null sc_dict_ids -- Case (a)
564 = return coerced_rep_dict
565 | otherwise -- Case (b)
566 = do { op_ids <- newSysLocalIds FSLIT("op") op_tys
567 ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
568 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
569 pat_dicts = dummy_sc_dict_ids,
570 pat_binds = emptyLHsBinds,
571 pat_args = PrefixCon (map nlVarPat op_ids),
573 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
574 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
575 map HsVar (sc_dict_ids ++ op_ids)
577 -- Warning: this HsCase scrutinises a value with a PredTy, which is
578 -- never otherwise seen in Haskell source code. It'd be
579 -- nicer to generate Core directly!
580 ; return (HsCase (noLoc coerced_rep_dict) $
581 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
583 pat_ty = mkTyConApp cls_tycon cls_inst_tys
584 cls_data_con = head (tyConDataCons cls_tycon)
585 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
586 op_tys = dropList sc_dict_ids cls_arg_tys
588 ------------------------
589 -- Ordinary instances
591 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
593 dfun_id = instanceDFunId ispec
594 rigid_info = InstSkol dfun_id
595 inst_ty = idType dfun_id
597 -- Prime error recovery
598 recoverM (returnM emptyLHsBinds) $
599 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
600 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
602 -- Instantiate the instance decl with skolem constants
603 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
604 -- These inst_tyvars' scope over the 'where' part
605 -- Those tyvars are inside the dfun_id's type, which is a bit
606 -- bizarre, but OK so long as you realise it!
608 (clas, inst_tys') = tcSplitDFunHead inst_head'
609 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
611 -- Instantiate the super-class context with inst_tys
612 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
613 origin = SigOrigin rigid_info
615 -- Create dictionary Ids from the specified instance contexts.
616 getInstLoc InstScOrigin `thenM` \ sc_loc ->
617 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
618 getInstLoc origin `thenM` \ inst_loc ->
619 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
620 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
621 -- Default-method Ids may be mentioned in synthesised RHSs,
622 -- but they'll already be in the environment.
624 -- Typecheck the methods
625 let -- These insts are in scope; quite a few, eh?
626 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
628 tcMethods origin clas inst_tyvars'
629 dfun_theta' inst_tys' avail_insts
630 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
632 -- Figure out bindings for the superclass context
633 -- Don't include this_dict in the 'givens', else
634 -- sc_dicts get bound by just selecting from this_dict!!
635 addErrCtxt superClassCtxt
636 (tcSimplifySuperClasses inst_tyvars'
638 sc_dicts) `thenM` \ sc_binds ->
640 -- It's possible that the superclass stuff might unified one
641 -- of the inst_tyavars' with something in the envt
642 checkSigTyVars inst_tyvars' `thenM_`
644 -- Deal with 'SPECIALISE instance' pragmas
645 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
647 -- Create the result bindings
649 dict_constr = classDataCon clas
650 scs_and_meths = map instToId sc_dicts ++ meth_ids
651 this_dict_id = instToId this_dict
652 inline_prag | null dfun_arg_dicts = []
653 | otherwise = [InlinePrag (Inline AlwaysActive True)]
654 -- Always inline the dfun; this is an experimental decision
655 -- because it makes a big performance difference sometimes.
656 -- Often it means we can do the method selection, and then
657 -- inline the method as well. Marcin's idea; see comments below.
659 -- BUT: don't inline it if it's a constant dictionary;
660 -- we'll get all the benefit without inlining, and we get
661 -- a **lot** of code duplication if we inline it
663 -- See Note [Inline dfuns] below
666 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
667 -- We don't produce a binding for the dict_constr; instead we
668 -- rely on the simplifier to unfold this saturated application
669 -- We do this rather than generate an HsCon directly, because
670 -- it means that the special cases (e.g. dictionary with only one
671 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
672 -- than needing to be repeated here.
674 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
675 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
677 main_bind = noLoc $ AbsBinds
679 (map instToId dfun_arg_dicts)
680 [(inst_tyvars', dfun_id, this_dict_id,
681 inline_prag ++ prags)]
684 showLIE (text "instance") `thenM_`
685 returnM (unitBag main_bind)
688 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
689 avail_insts op_items monobinds uprags
690 = -- Check that all the method bindings come from this class
692 sel_names = [idName sel_id | (sel_id, _) <- op_items]
693 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
695 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
697 -- Make the method bindings
699 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
701 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
703 -- And type check them
704 -- It's really worth making meth_insts available to the tcMethodBind
705 -- Consider instance Monad (ST s) where
706 -- {-# INLINE (>>) #-}
707 -- (>>) = ...(>>=)...
708 -- If we don't include meth_insts, we end up with bindings like this:
709 -- rec { dict = MkD then bind ...
710 -- then = inline_me (... (GHC.Base.>>= dict) ...)
712 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
713 -- and (b) the inline_me prevents us inlining the >>= selector, which
714 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
715 -- is not inlined across modules. Rather ironic since this does not
716 -- happen without the INLINE pragma!
718 -- Solution: make meth_insts available, so that 'then' refers directly
719 -- to the local 'bind' rather than going via the dictionary.
721 -- BUT WATCH OUT! If the method type mentions the class variable, then
722 -- this optimisation is not right. Consider
726 -- instance C Int where
728 -- The occurrence of 'op' on the rhs gives rise to a constraint
730 -- The trouble is that the 'meth_inst' for op, which is 'available', also
731 -- looks like 'op at Int'. But they are not the same.
733 prag_fn = mkPragFun uprags
734 all_insts = avail_insts ++ catMaybes meth_insts
735 sig_fn n = Just [] -- No scoped type variables, but every method has
736 -- a type signature, in effect, so that we check
737 -- the method has the right type
738 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
739 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
742 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
744 returnM (meth_ids, unionManyBags meth_binds_s)
748 ------------------------------
749 [Inline dfuns] Inlining dfuns unconditionally
750 ------------------------------
752 The code above unconditionally inlines dict funs. Here's why.
753 Consider this program:
755 test :: Int -> Int -> Bool
756 test x y = (x,y) == (y,x) || test y x
757 -- Recursive to avoid making it inline.
759 This needs the (Eq (Int,Int)) instance. If we inline that dfun
760 the code we end up with is good:
763 \r -> case ==# [ww ww1] of wild {
764 PrelBase.False -> Test.$wtest ww1 ww;
766 case ==# [ww1 ww] of wild1 {
767 PrelBase.False -> Test.$wtest ww1 ww;
768 PrelBase.True -> PrelBase.True [];
771 Test.test = \r [w w1]
774 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
777 If we don't inline the dfun, the code is not nearly as good:
779 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
780 PrelBase.:DEq tpl1 tpl2 -> tpl2;
785 let { y = PrelBase.I#! [ww1]; } in
786 let { x = PrelBase.I#! [ww]; } in
787 let { sat_slx = PrelTup.(,)! [y x]; } in
788 let { sat_sly = PrelTup.(,)! [x y];
790 case == sat_sly sat_slx of wild {
791 PrelBase.False -> Test.$wtest ww1 ww;
792 PrelBase.True -> PrelBase.True [];
799 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
802 Why doesn't GHC inline $fEq? Because it looks big:
804 PrelTup.zdfEqZ1T{-rcX-}
805 = \ @ a{-reT-} :: * @ b{-reS-} :: *
806 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
807 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
809 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
810 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
812 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
813 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
815 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
816 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
817 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
819 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
821 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
823 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
824 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
828 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
829 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
830 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
831 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
833 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
835 and it's not as bad as it seems, because it's further dramatically
836 simplified: only zeze2 is extracted and its body is simplified.
839 %************************************************************************
841 \subsection{Error messages}
843 %************************************************************************
846 instDeclCtxt1 hs_inst_ty
847 = inst_decl_ctxt (case unLoc hs_inst_ty of
848 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
849 HsPredTy pred -> ppr pred
850 other -> ppr hs_inst_ty) -- Don't expect this
851 instDeclCtxt2 dfun_ty
852 = inst_decl_ctxt (ppr (mkClassPred cls tys))
854 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
856 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
858 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
860 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
864 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
865 ptext SLIT("must be variables")
866 , ptext SLIT("Instead of a variable, found") <+> ppr ty
869 wrongATArgErr ty instTy =
870 sep [ ptext SLIT("Type indexes must match class instance head")
871 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>