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 ( mkLocalFamInst )
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 (map mkLocalFamInstTyThing tycons) thing_inside
232 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
233 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
238 tcLocalInstDecl1 :: LInstDecl Name
239 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
240 -- A source-file instance declaration
241 -- Type-check all the stuff before the "where"
243 -- We check for respectable instance type, and context
244 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
245 = -- Prime error recovery, set source location
246 recoverM (returnM ([], [])) $
248 addErrCtxt (instDeclCtxt1 poly_ty) $
250 do { is_boot <- tcIsHsBoot
251 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
254 -- Typecheck the instance type itself. We can't use
255 -- tcHsSigType, because it's not a valid user type.
256 ; kinded_ty <- kcHsSigType poly_ty
257 ; poly_ty' <- tcHsKindedType kinded_ty
258 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
260 -- Next, process any associated types.
261 ; idx_tycons <- mappM tcIdxTyInstDecl ats
263 -- Now, check the validity of the instance.
264 ; (clas, inst_tys) <- checkValidInstHead tau
265 ; checkValidInstance tyvars theta clas inst_tys
266 ; checkValidAndMissingATs clas (tyvars, inst_tys)
269 -- Finally, construct the Core representation of the instance.
270 -- (This no longer includes the associated types.)
271 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
272 ; overlap_flag <- getOverlapFlag
273 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
274 ispec = mkLocalInstance dfun overlap_flag
276 ; return ([InstInfo { iSpec = ispec,
277 iBinds = VanillaInst binds uprags }],
278 catMaybes idx_tycons)
281 -- We pass in the source form and the type checked form of the ATs. We
282 -- really need the source form only to be able to produce more informative
284 checkValidAndMissingATs :: Class
285 -> ([TyVar], [TcType]) -- instance types
286 -> [(LTyClDecl Name, -- source form of AT
287 Maybe TyThing)] -- Core form of AT
289 checkValidAndMissingATs clas inst_tys ats
290 = do { -- Issue a warning for each class AT that is not defined in this
292 ; let classDefATs = listToNameSet . map tyConName . classATs $ clas
293 definedATs = listToNameSet . map (tcdName.unLoc.fst) $ ats
294 omitted = classDefATs `minusNameSet` definedATs
295 ; warn <- doptM Opt_WarnMissingMethods
296 ; mapM_ (warnTc warn . omittedATWarn) (nameSetToList omitted)
298 -- Ensure that all AT indexes that correspond to class parameters
299 -- coincide with the types in the instance head. All remaining
300 -- AT arguments must be variables. Also raise an error for any
301 -- type instances that are not associated with this class.
302 ; mapM_ (checkIndexes clas inst_tys) ats
305 checkIndexes _ _ (hsAT, Nothing) =
306 return () -- skip, we already had an error here
307 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
308 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
309 checkIndexes' clas inst_tys hsAT
311 snd . fromJust . tyConFamInst_maybe $ tycon)
312 checkIndexes _ _ _ = panic "checkIndexes"
314 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
315 = let atName = tcdName . unLoc $ hsAT
317 setSrcSpan (getLoc hsAT) $
318 addErrCtxt (atInstCtxt atName) $
319 case find ((atName ==) . tyConName) (classATs clas) of
320 Nothing -> addErrTc $ badATErr clas atName -- not in this class
322 case assocTyConArgPoss_maybe atDecl of
323 Nothing -> panic "checkIndexes': AT has no args poss?!?"
326 -- The following is tricky! We need to deal with three
327 -- complications: (1) The AT possibly only uses a subset of
328 -- the class parameters as indexes and those it uses may be in
329 -- a different order; (2) the AT may have extra arguments,
330 -- which must be type variables; and (3) variables in AT and
331 -- instance head will be different `Name's even if their
332 -- source lexemes are identical.
334 -- Re (1), `poss' contains a permutation vector to extract the
335 -- class parameters in the right order.
337 -- Re (2), we wrap the (permuted) class parameters in a Maybe
338 -- type and use Nothing for any extra AT arguments. (First
339 -- equation of `checkIndex' below.)
341 -- Re (3), we replace any type variable in the AT parameters
342 -- that has the same source lexeme as some variable in the
343 -- instance types with the instance type variable sharing its
346 let relevantInstTys = map (instTys !!) poss
347 instArgs = map Just relevantInstTys ++
348 repeat Nothing -- extra arguments
349 renaming = substSameTyVar atTvs instTvs
351 zipWithM_ checkIndex (substTys renaming atTys) instArgs
353 checkIndex ty Nothing
354 | isTyVarTy ty = return ()
355 | otherwise = addErrTc $ mustBeVarArgErr ty
356 checkIndex ty (Just instTy)
357 | ty `tcEqType` instTy = return ()
358 | otherwise = addErrTc $ wrongATArgErr ty instTy
360 listToNameSet = addListToNameSet emptyNameSet
362 substSameTyVar [] _ = emptyTvSubst
363 substSameTyVar (tv:tvs) replacingTvs =
364 let replacement = case find (tv `sameLexeme`) replacingTvs of
365 Nothing -> mkTyVarTy tv
366 Just rtv -> mkTyVarTy rtv
368 tv1 `sameLexeme` tv2 =
369 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
371 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
375 %************************************************************************
377 \subsection{Type-checking instance declarations, pass 2}
379 %************************************************************************
382 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
383 -> TcM (LHsBinds Id, TcLclEnv)
384 -- (a) From each class declaration,
385 -- generate any default-method bindings
386 -- (b) From each instance decl
387 -- generate the dfun binding
389 tcInstDecls2 tycl_decls inst_decls
390 = do { -- (a) Default methods from class decls
391 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
392 filter (isClassDecl.unLoc) tycl_decls
393 ; tcExtendIdEnv (concat dm_ids_s) $ do
395 -- (b) instance declarations
396 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
399 ; let binds = unionManyBags dm_binds_s `unionBags`
400 unionManyBags inst_binds_s
401 ; tcl_env <- getLclEnv -- Default method Ids in here
402 ; returnM (binds, tcl_env) }
405 ======= New documentation starts here (Sept 92) ==============
407 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
408 the dictionary function for this instance declaration. For example
410 instance Foo a => Foo [a] where
414 might generate something like
416 dfun.Foo.List dFoo_a = let op1 x = ...
422 HOWEVER, if the instance decl has no context, then it returns a
423 bigger @HsBinds@ with declarations for each method. For example
425 instance Foo [a] where
431 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
432 const.Foo.op1.List a x = ...
433 const.Foo.op2.List a y = ...
435 This group may be mutually recursive, because (for example) there may
436 be no method supplied for op2 in which case we'll get
438 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
440 that is, the default method applied to the dictionary at this type.
442 What we actually produce in either case is:
444 AbsBinds [a] [dfun_theta_dicts]
445 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
446 { d = (sd1,sd2, ..., op1, op2, ...)
451 The "maybe" says that we only ask AbsBinds to make global constant methods
452 if the dfun_theta is empty.
455 For an instance declaration, say,
457 instance (C1 a, C2 b) => C (T a b) where
460 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
461 function whose type is
463 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
465 Notice that we pass it the superclass dictionaries at the instance type; this
466 is the ``Mark Jones optimisation''. The stuff before the "=>" here
467 is the @dfun_theta@ below.
469 First comes the easy case of a non-local instance decl.
473 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
474 -- Returns a binding for the dfun
476 ------------------------
477 -- Derived newtype instances; surprisingly tricky!
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]
484 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
486 -- So all need is to generate a binding looking like:
487 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
488 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
489 -- case df `cast` (Foo Int (sym (CoT a))) of
490 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
492 -- If there are no superclasses, matters are simpler, because we don't need the case
493 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
495 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
496 = do { let dfun_id = instanceDFunId ispec
497 rigid_info = InstSkol dfun_id
498 origin = SigOrigin rigid_info
499 inst_ty = idType dfun_id
500 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
501 -- inst_head_ty is a PredType
503 ; inst_loc <- getInstLoc origin
504 ; (rep_dict_id : sc_dict_ids, wrap_fn, sc_binds)
505 <- make_wrapper inst_loc tvs theta mb_preds
506 -- Here, we are relying on the order of dictionary
507 -- arguments built by NewTypeDerived in TcDeriv;
508 -- namely, that the rep_dict_id comes first
510 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
511 cls_tycon = classTyCon cls
512 the_coercion = make_coercion cls_tycon cls_inst_tys
513 coerced_rep_dict = mkHsWrap the_coercion (HsVar rep_dict_id)
515 ; body <- make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
517 ; return (sc_binds `snocBag` (noLoc $ VarBind dfun_id $ noLoc $ mkHsWrap wrap_fn body)) }
520 -----------------------
522 -- We distinguish two cases:
523 -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
524 -- and the new dict can just be a constant
525 -- (mb_preds = Just preds)
526 -- (b) there are tyvars, so we must make a dict *fun*
527 -- (mb_preds = Nothing)
528 -- See the defn of NewTypeDerived for the meaning of mb_preds
529 make_wrapper inst_loc tvs theta (Just preds) -- Case (a)
530 = ASSERT( null tvs && null theta )
531 do { dicts <- newDictBndrs inst_loc preds
532 ; sc_binds <- addErrCtxt superClassCtxt (tcSimplifySuperClasses [] [] dicts)
533 -- Use tcSimplifySuperClasses to avoid creating loops, for the
534 -- same reason as Note [SUPERCLASS-LOOP 1] in TcSimplify
535 ; return (map instToId dicts, idHsWrapper, sc_binds) }
537 make_wrapper inst_loc tvs theta Nothing -- Case (b)
538 = do { dicts <- newDictBndrs inst_loc theta
539 ; let dict_ids = map instToId dicts
540 ; return (dict_ids, mkWpTyLams tvs <.> mkWpLams dict_ids, emptyBag) }
542 -----------------------
544 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
545 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
546 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
547 -- where rep_ty is the (eta-reduced) type rep of T
548 -- So we just replace T with CoT, and insert a 'sym'
549 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
551 make_coercion cls_tycon cls_inst_tys
552 | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
553 , (tycon, tc_args) <- tcSplitTyConApp last_ty -- Should not fail
554 , Just co_con <- newTyConCo_maybe tycon
555 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
556 = WpCo (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
557 | otherwise -- The newtype is transparent; no need for a cast
560 -----------------------
562 -- Two cases; see Note [Newtype deriving superclasses] in TcDeriv.lhs
563 -- (a) no superclasses; then we can just use the coerced dict
564 -- (b) one or more superclasses; then new need to do the unpack/repack
566 make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
567 | null sc_dict_ids -- Case (a)
568 = return coerced_rep_dict
569 | otherwise -- Case (b)
570 = do { op_ids <- newSysLocalIds FSLIT("op") op_tys
571 ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
572 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
573 pat_dicts = dummy_sc_dict_ids,
574 pat_binds = emptyLHsBinds,
575 pat_args = PrefixCon (map nlVarPat op_ids),
577 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
578 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
579 map HsVar (sc_dict_ids ++ op_ids)
581 -- Warning: this HsCase scrutinises a value with a PredTy, which is
582 -- never otherwise seen in Haskell source code. It'd be
583 -- nicer to generate Core directly!
584 ; return (HsCase (noLoc coerced_rep_dict) $
585 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
587 pat_ty = mkTyConApp cls_tycon cls_inst_tys
588 cls_data_con = head (tyConDataCons cls_tycon)
589 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
590 op_tys = dropList sc_dict_ids cls_arg_tys
592 ------------------------
593 -- Ordinary instances
595 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
597 dfun_id = instanceDFunId ispec
598 rigid_info = InstSkol dfun_id
599 inst_ty = idType dfun_id
601 -- Prime error recovery
602 recoverM (returnM emptyLHsBinds) $
603 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
604 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
606 -- Instantiate the instance decl with skolem constants
607 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
608 -- These inst_tyvars' scope over the 'where' part
609 -- Those tyvars are inside the dfun_id's type, which is a bit
610 -- bizarre, but OK so long as you realise it!
612 (clas, inst_tys') = tcSplitDFunHead inst_head'
613 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
615 -- Instantiate the super-class context with inst_tys
616 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
617 origin = SigOrigin rigid_info
619 -- Create dictionary Ids from the specified instance contexts.
620 getInstLoc InstScOrigin `thenM` \ sc_loc ->
621 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
622 getInstLoc origin `thenM` \ inst_loc ->
623 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
624 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
625 -- Default-method Ids may be mentioned in synthesised RHSs,
626 -- but they'll already be in the environment.
628 -- Typecheck the methods
629 let -- These insts are in scope; quite a few, eh?
630 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
632 tcMethods origin clas inst_tyvars'
633 dfun_theta' inst_tys' avail_insts
634 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
636 -- Figure out bindings for the superclass context
637 -- Don't include this_dict in the 'givens', else
638 -- sc_dicts get bound by just selecting from this_dict!!
639 addErrCtxt superClassCtxt
640 (tcSimplifySuperClasses inst_tyvars'
642 sc_dicts) `thenM` \ sc_binds ->
644 -- It's possible that the superclass stuff might unified one
645 -- of the inst_tyavars' with something in the envt
646 checkSigTyVars inst_tyvars' `thenM_`
648 -- Deal with 'SPECIALISE instance' pragmas
649 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
651 -- Create the result bindings
653 dict_constr = classDataCon clas
654 scs_and_meths = map instToId sc_dicts ++ meth_ids
655 this_dict_id = instToId this_dict
656 inline_prag | null dfun_arg_dicts = []
657 | otherwise = [InlinePrag (Inline AlwaysActive True)]
658 -- Always inline the dfun; this is an experimental decision
659 -- because it makes a big performance difference sometimes.
660 -- Often it means we can do the method selection, and then
661 -- inline the method as well. Marcin's idea; see comments below.
663 -- BUT: don't inline it if it's a constant dictionary;
664 -- we'll get all the benefit without inlining, and we get
665 -- a **lot** of code duplication if we inline it
667 -- See Note [Inline dfuns] below
670 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
671 -- We don't produce a binding for the dict_constr; instead we
672 -- rely on the simplifier to unfold this saturated application
673 -- We do this rather than generate an HsCon directly, because
674 -- it means that the special cases (e.g. dictionary with only one
675 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
676 -- than needing to be repeated here.
678 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
679 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
681 main_bind = noLoc $ AbsBinds
683 (map instToId dfun_arg_dicts)
684 [(inst_tyvars', dfun_id, this_dict_id,
685 inline_prag ++ prags)]
688 showLIE (text "instance") `thenM_`
689 returnM (unitBag main_bind)
692 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
693 avail_insts op_items monobinds uprags
694 = -- Check that all the method bindings come from this class
696 sel_names = [idName sel_id | (sel_id, _) <- op_items]
697 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
699 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
701 -- Make the method bindings
703 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
705 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
707 -- And type check them
708 -- It's really worth making meth_insts available to the tcMethodBind
709 -- Consider instance Monad (ST s) where
710 -- {-# INLINE (>>) #-}
711 -- (>>) = ...(>>=)...
712 -- If we don't include meth_insts, we end up with bindings like this:
713 -- rec { dict = MkD then bind ...
714 -- then = inline_me (... (GHC.Base.>>= dict) ...)
716 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
717 -- and (b) the inline_me prevents us inlining the >>= selector, which
718 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
719 -- is not inlined across modules. Rather ironic since this does not
720 -- happen without the INLINE pragma!
722 -- Solution: make meth_insts available, so that 'then' refers directly
723 -- to the local 'bind' rather than going via the dictionary.
725 -- BUT WATCH OUT! If the method type mentions the class variable, then
726 -- this optimisation is not right. Consider
730 -- instance C Int where
732 -- The occurrence of 'op' on the rhs gives rise to a constraint
734 -- The trouble is that the 'meth_inst' for op, which is 'available', also
735 -- looks like 'op at Int'. But they are not the same.
737 prag_fn = mkPragFun uprags
738 all_insts = avail_insts ++ catMaybes meth_insts
739 sig_fn n = Just [] -- No scoped type variables, but every method has
740 -- a type signature, in effect, so that we check
741 -- the method has the right type
742 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
743 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
746 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
748 returnM (meth_ids, unionManyBags meth_binds_s)
752 ------------------------------
753 [Inline dfuns] Inlining dfuns unconditionally
754 ------------------------------
756 The code above unconditionally inlines dict funs. Here's why.
757 Consider this program:
759 test :: Int -> Int -> Bool
760 test x y = (x,y) == (y,x) || test y x
761 -- Recursive to avoid making it inline.
763 This needs the (Eq (Int,Int)) instance. If we inline that dfun
764 the code we end up with is good:
767 \r -> case ==# [ww ww1] of wild {
768 PrelBase.False -> Test.$wtest ww1 ww;
770 case ==# [ww1 ww] of wild1 {
771 PrelBase.False -> Test.$wtest ww1 ww;
772 PrelBase.True -> PrelBase.True [];
775 Test.test = \r [w w1]
778 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
781 If we don't inline the dfun, the code is not nearly as good:
783 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
784 PrelBase.:DEq tpl1 tpl2 -> tpl2;
789 let { y = PrelBase.I#! [ww1]; } in
790 let { x = PrelBase.I#! [ww]; } in
791 let { sat_slx = PrelTup.(,)! [y x]; } in
792 let { sat_sly = PrelTup.(,)! [x y];
794 case == sat_sly sat_slx of wild {
795 PrelBase.False -> Test.$wtest ww1 ww;
796 PrelBase.True -> PrelBase.True [];
803 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
806 Why doesn't GHC inline $fEq? Because it looks big:
808 PrelTup.zdfEqZ1T{-rcX-}
809 = \ @ a{-reT-} :: * @ b{-reS-} :: *
810 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
811 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
813 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
814 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
816 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
817 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
819 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
820 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
821 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
823 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
825 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
827 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
828 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
832 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
833 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
834 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
835 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
837 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
839 and it's not as bad as it seems, because it's further dramatically
840 simplified: only zeze2 is extracted and its body is simplified.
843 %************************************************************************
845 \subsection{Error messages}
847 %************************************************************************
850 instDeclCtxt1 hs_inst_ty
851 = inst_decl_ctxt (case unLoc hs_inst_ty of
852 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
853 HsPredTy pred -> ppr pred
854 other -> ppr hs_inst_ty) -- Don't expect this
855 instDeclCtxt2 dfun_ty
856 = inst_decl_ctxt (ppr (mkClassPred cls tys))
858 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
860 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
862 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
864 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
868 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
869 ptext SLIT("must be variables")
870 , ptext SLIT("Instead of a variable, found") <+> ppr ty
873 wrongATArgErr ty instTy =
874 sep [ ptext SLIT("Type indexes must match class instance head")
875 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>