2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcInstDecls: Typechecking instance declarations
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
18 #include "HsVersions.h"
57 import Control.Monad hiding (zipWithM_, mapAndUnzipM)
61 Typechecking instance declarations is done in two passes. The first
62 pass, made by @tcInstDecls1@, collects information to be used in the
65 This pre-processed info includes the as-yet-unprocessed bindings
66 inside the instance declaration. These are type-checked in the second
67 pass, when the class-instance envs and GVE contain all the info from
68 all the instance and value decls. Indeed that's the reason we need
69 two passes over the instance decls.
71 Here is the overall algorithm.
72 Assume that we have an instance declaration
74 instance c => k (t tvs) where b
78 $LIE_c$ is the LIE for the context of class $c$
80 $betas_bar$ is the free variables in the class method type, excluding the
83 $LIE_cop$ is the LIE constraining a particular class method
85 $tau_cop$ is the tau type of a class method
87 $LIE_i$ is the LIE for the context of instance $i$
89 $X$ is the instance constructor tycon
91 $gammas_bar$ is the set of type variables of the instance
93 $LIE_iop$ is the LIE for a particular class method instance
95 $tau_iop$ is the tau type for this instance of a class method
97 $alpha$ is the class variable
99 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
101 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
104 ToDo: Update the list above with names actually in the code.
108 First, make the LIEs for the class and instance contexts, which means
109 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
110 and make LIElistI and LIEI.
112 Then process each method in turn.
114 order the instance methods according to the ordering of the class methods
116 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
118 Create final dictionary function from bindings generated already
120 df = lambda inst_tyvars
127 in <op1,op2,...,opn,sd1,...,sdm>
129 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
130 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
134 %************************************************************************
136 \subsection{Extracting instance decls}
138 %************************************************************************
140 Gather up the instance declarations from their various sources
143 tcInstDecls1 -- Deal with both source-code and imported instance decls
144 :: [LTyClDecl Name] -- For deriving stuff
145 -> [LInstDecl Name] -- Source code instance decls
146 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
147 -> TcM (TcGblEnv, -- The full inst env
148 [InstInfo], -- Source-code instance decls to process;
149 -- contains all dfuns for this module
150 HsValBinds Name) -- Supporting bindings for derived instances
152 tcInstDecls1 tycl_decls inst_decls deriv_decls
154 do { -- Stop if addInstInfos etc discovers any errors
155 -- (they recover, so that we get more than one error each
158 -- (1) Do class and family instance declarations
159 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
160 ; local_info_tycons <- mappM tcLocalInstDecl1 inst_decls
161 ; idx_tycons <- mappM tcIdxTyInstDeclTL idxty_decls
163 ; let { (local_infos,
164 at_tycons) = unzip local_info_tycons
165 ; local_info = concat local_infos
166 ; at_idx_tycon = concat at_tycons ++ catMaybes idx_tycons
167 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
168 ; implicit_things = concatMap implicitTyThings at_idx_tycon
171 -- (2) Add the tycons of indexed types and their implicit
172 -- tythings to the global environment
173 ; tcExtendGlobalEnv (at_idx_tycon ++ implicit_things) $ do {
175 -- (3) Instances from generic class declarations
176 ; generic_inst_info <- getGenericInstances clas_decls
178 -- Next, construct the instance environment so far, consisting
180 -- a) local instance decls
181 -- b) generic instances
182 -- c) local family instance decls
183 ; addInsts local_info $ do {
184 ; addInsts generic_inst_info $ do {
185 ; addFamInsts at_idx_tycon $ do {
187 -- (4) Compute instances from "deriving" clauses;
188 -- This stuff computes a context for the derived instance
189 -- decl, so it needs to know about all the instances possible
190 -- NB: class instance declarations can contain derivings as
191 -- part of associated data type declarations
192 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_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 <- tcFamInstDecl 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 (map mkLocalFamInstTyThing tycons) thing_inside
231 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
232 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
237 tcLocalInstDecl1 :: LInstDecl Name
238 -> TcM ([InstInfo], [TyThing]) -- [] if there was an error
239 -- A source-file instance declaration
240 -- Type-check all the stuff before the "where"
242 -- We check for respectable instance type, and context
243 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
244 = -- Prime error recovery, set source location
245 recoverM (returnM ([], [])) $
247 addErrCtxt (instDeclCtxt1 poly_ty) $
249 do { is_boot <- tcIsHsBoot
250 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
253 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
255 -- Next, process any associated types.
256 ; idx_tycons <- mappM tcFamInstDecl 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 loc
267 ; overlap_flag <- getOverlapFlag
268 ; let (eq_theta,dict_theta) = partition isEqPred theta
269 theta' = eq_theta ++ dict_theta
270 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
271 ispec = mkLocalInstance dfun overlap_flag
273 ; return ([InstInfo { iSpec = ispec,
274 iBinds = VanillaInst binds uprags }],
275 catMaybes idx_tycons)
278 -- We pass in the source form and the type checked form of the ATs. We
279 -- really need the source form only to be able to produce more informative
281 checkValidAndMissingATs :: Class
282 -> ([TyVar], [TcType]) -- instance types
283 -> [(LTyClDecl Name, -- source form of AT
284 Maybe TyThing)] -- Core form of AT
286 checkValidAndMissingATs clas inst_tys ats
287 = do { -- Issue a warning for each class AT that is not defined in this
289 ; let class_ats = map tyConName (classATs clas)
290 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
291 omitted = filterOut (`elemNameSet` defined_ats) class_ats
292 ; warn <- doptM Opt_WarnMissingMethods
293 ; mapM_ (warnTc warn . omittedATWarn) omitted
295 -- Ensure that all AT indexes that correspond to class parameters
296 -- coincide with the types in the instance head. All remaining
297 -- AT arguments must be variables. Also raise an error for any
298 -- type instances that are not associated with this class.
299 ; mapM_ (checkIndexes clas inst_tys) ats
302 checkIndexes _ _ (hsAT, Nothing) =
303 return () -- skip, we already had an error here
304 checkIndexes clas inst_tys (hsAT, Just (ATyCon tycon)) =
305 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
306 checkIndexes' clas inst_tys hsAT
308 snd . fromJust . tyConFamInst_maybe $ tycon)
309 checkIndexes _ _ _ = panic "checkIndexes"
311 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
312 = let atName = tcdName . unLoc $ hsAT
314 setSrcSpan (getLoc hsAT) $
315 addErrCtxt (atInstCtxt atName) $
316 case find ((atName ==) . tyConName) (classATs clas) of
317 Nothing -> addErrTc $ badATErr clas atName -- not in this class
319 case assocTyConArgPoss_maybe atDecl of
320 Nothing -> panic "checkIndexes': AT has no args poss?!?"
323 -- The following is tricky! We need to deal with three
324 -- complications: (1) The AT possibly only uses a subset of
325 -- the class parameters as indexes and those it uses may be in
326 -- a different order; (2) the AT may have extra arguments,
327 -- which must be type variables; and (3) variables in AT and
328 -- instance head will be different `Name's even if their
329 -- source lexemes are identical.
331 -- Re (1), `poss' contains a permutation vector to extract the
332 -- class parameters in the right order.
334 -- Re (2), we wrap the (permuted) class parameters in a Maybe
335 -- type and use Nothing for any extra AT arguments. (First
336 -- equation of `checkIndex' below.)
338 -- Re (3), we replace any type variable in the AT parameters
339 -- that has the same source lexeme as some variable in the
340 -- instance types with the instance type variable sharing its
343 let relevantInstTys = map (instTys !!) poss
344 instArgs = map Just relevantInstTys ++
345 repeat Nothing -- extra arguments
346 renaming = substSameTyVar atTvs instTvs
348 zipWithM_ checkIndex (substTys renaming atTys) instArgs
350 checkIndex ty Nothing
351 | isTyVarTy ty = return ()
352 | otherwise = addErrTc $ mustBeVarArgErr ty
353 checkIndex ty (Just instTy)
354 | ty `tcEqType` instTy = return ()
355 | otherwise = addErrTc $ wrongATArgErr ty instTy
357 listToNameSet = addListToNameSet emptyNameSet
359 substSameTyVar [] _ = emptyTvSubst
360 substSameTyVar (tv:tvs) replacingTvs =
361 let replacement = case find (tv `sameLexeme`) replacingTvs of
362 Nothing -> mkTyVarTy tv
363 Just rtv -> mkTyVarTy rtv
365 tv1 `sameLexeme` tv2 =
366 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
368 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
372 %************************************************************************
374 \subsection{Type-checking instance declarations, pass 2}
376 %************************************************************************
379 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
380 -> TcM (LHsBinds Id, TcLclEnv)
381 -- (a) From each class declaration,
382 -- generate any default-method bindings
383 -- (b) From each instance decl
384 -- generate the dfun binding
386 tcInstDecls2 tycl_decls inst_decls
387 = do { -- (a) Default methods from class decls
388 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
389 filter (isClassDecl.unLoc) tycl_decls
390 ; tcExtendIdEnv (concat dm_ids_s) $ do
392 -- (b) instance declarations
393 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
396 ; let binds = unionManyBags dm_binds_s `unionBags`
397 unionManyBags inst_binds_s
398 ; tcl_env <- getLclEnv -- Default method Ids in here
399 ; returnM (binds, tcl_env) }
402 ======= New documentation starts here (Sept 92) ==============
404 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
405 the dictionary function for this instance declaration. For example
407 instance Foo a => Foo [a] where
411 might generate something like
413 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.
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.
449 For an instance declaration, say,
451 instance (C1 a, C2 b) => C (T a b) where
454 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
455 function whose type is
457 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
459 Notice that we pass it the superclass dictionaries at the instance type; this
460 is the ``Mark Jones optimisation''. The stuff before the "=>" here
461 is the @dfun_theta@ below.
465 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
466 -- Returns a binding for the dfun
468 ------------------------
469 -- Derived newtype instances; surprisingly tricky!
471 -- class Show a => Foo a b where ...
472 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
474 -- The newtype gives an FC axiom looking like
475 -- axiom CoN a :: N a :=: Tree [a]
476 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
478 -- So all need is to generate a binding looking like:
479 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
480 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
481 -- case df `cast` (Foo Int (sym (CoN a))) of
482 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
484 -- If there are no superclasses, matters are simpler, because we don't need the case
485 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
487 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived })
488 = do { let dfun_id = instanceDFunId ispec
489 rigid_info = InstSkol
490 origin = SigOrigin rigid_info
491 inst_ty = idType dfun_id
492 ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
493 -- inst_head_ty is a PredType
495 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
496 (class_tyvars, sc_theta, _, op_items) = classBigSig cls
497 cls_tycon = classTyCon cls
498 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
500 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
501 (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail
502 rep_ty = newTyConInstRhs nt_tycon tc_args
504 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
505 -- In our example, rep_pred is (Foo Int (Tree [a]))
506 the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
507 -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)
509 ; inst_loc <- getInstLoc origin
510 ; sc_loc <- getInstLoc InstScOrigin
511 ; dfun_dicts <- newDictBndrs inst_loc theta
512 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
513 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
514 ; rep_dict <- newDictBndr inst_loc rep_pred
516 -- Figure out bindings for the superclass context from dfun_dicts
517 -- Don't include this_dict in the 'givens', else
518 -- wanted_sc_insts get bound by just selecting from this_dict!!
519 ; sc_binds <- addErrCtxt superClassCtxt $
520 tcSimplifySuperClasses inst_loc dfun_dicts (rep_dict:sc_dicts)
522 ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict))
524 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
525 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
527 ; return (unitBag $ noLoc $
528 AbsBinds tvs (map instToVar dfun_dicts)
529 [(tvs, dfun_id, instToId this_dict, [])]
530 (dict_bind `consBag` sc_binds)) }
532 -----------------------
534 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
535 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
536 -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
537 -- where rep_ty is the (eta-reduced) type rep of T
538 -- So we just replace T with CoT, and insert a 'sym'
539 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
541 make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
542 | Just co_con <- newTyConCo_maybe nt_tycon
543 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
544 = WpCo (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))
545 | otherwise -- The newtype is transparent; no need for a cast
548 -----------------------
549 -- (make_body C tys scs coreced_rep_dict)
551 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
552 -- But if there are no superclasses, it returns just coerced_rep_dict
553 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
555 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
556 | null sc_dicts -- Case (a)
557 = return coerced_rep_dict
558 | otherwise -- Case (b)
559 = do { op_ids <- newSysLocalIds FSLIT("op") op_tys
560 ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
561 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
562 pat_dicts = dummy_sc_dict_ids,
563 pat_binds = emptyLHsBinds,
564 pat_args = PrefixCon (map nlVarPat op_ids),
566 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
567 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
568 map HsVar (sc_dict_ids ++ op_ids)
570 -- Warning: this HsCase scrutinises a value with a PredTy, which is
571 -- never otherwise seen in Haskell source code. It'd be
572 -- nicer to generate Core directly!
573 ; return (HsCase (noLoc coerced_rep_dict) $
574 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
576 sc_dict_ids = map instToId sc_dicts
577 pat_ty = mkTyConApp cls_tycon cls_inst_tys
578 cls_data_con = head (tyConDataCons cls_tycon)
579 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
580 op_tys = dropList sc_dict_ids cls_arg_tys
582 ------------------------
583 -- Ordinary instances
585 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
587 dfun_id = instanceDFunId ispec
588 rigid_info = InstSkol
589 inst_ty = idType dfun_id
590 loc = srcLocSpan (getSrcLoc dfun_id)
592 -- Prime error recovery
593 recoverM (returnM emptyLHsBinds) $
595 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
597 -- Instantiate the instance decl with skolem constants
598 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
599 -- These inst_tyvars' scope over the 'where' part
600 -- Those tyvars are inside the dfun_id's type, which is a bit
601 -- bizarre, but OK so long as you realise it!
603 (clas, inst_tys') = tcSplitDFunHead inst_head'
604 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
606 -- Instantiate the super-class context with inst_tys
607 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
608 (eq_sc_theta',dict_sc_theta') = partition isEqPred sc_theta'
609 origin = SigOrigin rigid_info
610 (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'
612 -- Create dictionary Ids from the specified instance contexts.
613 getInstLoc InstScOrigin `thenM` \ sc_loc ->
614 newDictBndrs sc_loc dict_sc_theta' `thenM` \ sc_dicts ->
615 getInstLoc origin `thenM` \ inst_loc ->
616 mkMetaCoVars eq_sc_theta' `thenM` \ sc_covars ->
617 mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars) `thenM` \ wanted_sc_eqs ->
618 mkCoVars eq_dfun_theta' `thenM` \ dfun_covars ->
619 mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars) `thenM` \ dfun_eqs ->
620 newDictBndrs inst_loc dict_dfun_theta' `thenM` \ dfun_dicts ->
621 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
622 -- Default-method Ids may be mentioned in synthesised RHSs,
623 -- but they'll already be in the environment.
625 -- Typecheck the methods
626 let -- These insts are in scope; quite a few, eh?
627 dfun_insts = dfun_eqs ++ dfun_dicts
628 wanted_sc_insts = wanted_sc_eqs ++ sc_dicts
629 given_sc_eqs = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
630 given_sc_insts = given_sc_eqs ++ sc_dicts
631 avail_insts = [this_dict] ++ dfun_insts ++ given_sc_insts
633 tcMethods origin clas inst_tyvars'
634 dfun_theta' inst_tys' avail_insts
635 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
637 -- Figure out bindings for the superclass context
638 -- Don't include this_dict in the 'givens', else
639 -- wanted_sc_insts get bound by just selecting from this_dict!!
640 addErrCtxt superClassCtxt
641 (tcSimplifySuperClasses inst_loc
642 dfun_insts wanted_sc_insts) `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_insts = []
657 | otherwise = [L loc (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' ++ mkTyVarTys sc_covars) (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
682 (inst_tyvars' ++ dfun_covars)
683 (map instToId dfun_dicts)
684 [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)]
687 showLIE (text "instance") `thenM_`
688 returnM (unitBag main_bind)
690 mkCoVars :: [PredType] -> TcM [TyVar]
691 mkCoVars = newCoVars . map unEqPred
693 unEqPred (EqPred ty1 ty2) = (ty1, ty2)
694 unEqPred _ = panic "TcInstDcls.mkCoVars"
696 mkMetaCoVars :: [PredType] -> TcM [TyVar]
697 mkMetaCoVars = mappM eqPredToCoVar
699 eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2
700 eqPredToCoVar _ = panic "TcInstDcls.mkMetaCoVars"
702 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
703 avail_insts op_items monobinds uprags
704 = -- Check that all the method bindings come from this class
706 sel_names = [idName sel_id | (sel_id, _) <- op_items]
707 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
709 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
711 -- Make the method bindings
713 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
715 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
717 -- And type check them
718 -- It's really worth making meth_insts available to the tcMethodBind
719 -- Consider instance Monad (ST s) where
720 -- {-# INLINE (>>) #-}
721 -- (>>) = ...(>>=)...
722 -- If we don't include meth_insts, we end up with bindings like this:
723 -- rec { dict = MkD then bind ...
724 -- then = inline_me (... (GHC.Base.>>= dict) ...)
726 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
727 -- and (b) the inline_me prevents us inlining the >>= selector, which
728 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
729 -- is not inlined across modules. Rather ironic since this does not
730 -- happen without the INLINE pragma!
732 -- Solution: make meth_insts available, so that 'then' refers directly
733 -- to the local 'bind' rather than going via the dictionary.
735 -- BUT WATCH OUT! If the method type mentions the class variable, then
736 -- this optimisation is not right. Consider
740 -- instance C Int where
742 -- The occurrence of 'op' on the rhs gives rise to a constraint
744 -- The trouble is that the 'meth_inst' for op, which is 'available', also
745 -- looks like 'op at Int'. But they are not the same.
747 prag_fn = mkPragFun uprags
748 all_insts = avail_insts ++ catMaybes meth_insts
749 sig_fn n = Just [] -- No scoped type variables, but every method has
750 -- a type signature, in effect, so that we check
751 -- the method has the right type
752 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
753 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
756 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
758 returnM (meth_ids, unionManyBags meth_binds_s)
762 ------------------------------
763 [Inline dfuns] Inlining dfuns unconditionally
764 ------------------------------
766 The code above unconditionally inlines dict funs. Here's why.
767 Consider this program:
769 test :: Int -> Int -> Bool
770 test x y = (x,y) == (y,x) || test y x
771 -- Recursive to avoid making it inline.
773 This needs the (Eq (Int,Int)) instance. If we inline that dfun
774 the code we end up with is good:
777 \r -> case ==# [ww ww1] of wild {
778 PrelBase.False -> Test.$wtest ww1 ww;
780 case ==# [ww1 ww] of wild1 {
781 PrelBase.False -> Test.$wtest ww1 ww;
782 PrelBase.True -> PrelBase.True [];
785 Test.test = \r [w w1]
788 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
791 If we don't inline the dfun, the code is not nearly as good:
793 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
794 PrelBase.:DEq tpl1 tpl2 -> tpl2;
799 let { y = PrelBase.I#! [ww1]; } in
800 let { x = PrelBase.I#! [ww]; } in
801 let { sat_slx = PrelTup.(,)! [y x]; } in
802 let { sat_sly = PrelTup.(,)! [x y];
804 case == sat_sly sat_slx of wild {
805 PrelBase.False -> Test.$wtest ww1 ww;
806 PrelBase.True -> PrelBase.True [];
813 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
816 Why doesn't GHC inline $fEq? Because it looks big:
818 PrelTup.zdfEqZ1T{-rcX-}
819 = \ @ a{-reT-} :: * @ b{-reS-} :: *
820 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
821 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
823 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
824 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
826 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
827 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
829 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
830 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
831 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
833 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
835 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
837 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
838 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
842 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
843 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
844 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
845 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
847 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
849 and it's not as bad as it seems, because it's further dramatically
850 simplified: only zeze2 is extracted and its body is simplified.
853 %************************************************************************
855 \subsection{Error messages}
857 %************************************************************************
860 instDeclCtxt1 hs_inst_ty
861 = inst_decl_ctxt (case unLoc hs_inst_ty of
862 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
863 HsPredTy pred -> ppr pred
864 other -> ppr hs_inst_ty) -- Don't expect this
865 instDeclCtxt2 dfun_ty
866 = inst_decl_ctxt (ppr (mkClassPred cls tys))
868 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
870 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
872 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")
874 atInstCtxt name = ptext SLIT("In the associated type instance for") <+>
878 sep [ ptext SLIT("Arguments that do not correspond to a class parameter") <+>
879 ptext SLIT("must be variables")
880 , ptext SLIT("Instead of a variable, found") <+> ppr ty
883 wrongATArgErr ty instTy =
884 sep [ ptext SLIT("Type indexes must match class instance head")
885 , ptext SLIT("Found") <+> ppr ty <+> ptext SLIT("but expected") <+>