2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcInstDecls]{Typechecking instance declarations}
7 module TcInstDcls ( tcInstDecls1, tcIfaceInstDecls,
8 tcInstDecls2, tcAddDeclCtxt ) where
10 #include "HsVersions.h"
13 import CmdLineOpts ( DynFlag(..) )
15 import HsSyn ( InstDecl(..), TyClDecl(..), HsType(..),
16 MonoBinds(..), HsExpr(..), HsLit(..), Sig(..), HsTyVarBndr(..),
17 andMonoBindList, collectMonoBinders,
18 isClassDecl, isSourceInstDecl, toHsType
20 import RnHsSyn ( RenamedHsBinds, RenamedInstDecl,
21 RenamedMonoBinds, RenamedTyClDecl, RenamedHsType,
22 extractHsTyVars, maybeGenericMatch
24 import TcHsSyn ( TcMonoBinds, mkHsConApp )
25 import TcBinds ( tcSpecSigs )
26 import TcClassDcl ( tcMethodBind, mkMethodBind, badMethodErr )
28 import TcMType ( tcInstType, checkValidTheta, checkValidInstHead, instTypeErr,
29 checkAmbiguity, UserTypeCtxt(..), SourceTyCtxt(..) )
30 import TcType ( mkClassPred, mkTyVarTy, tcSplitForAllTys, tyVarsOfType,
31 tcSplitSigmaTy, getClassPredTys, tcSplitPredTy_maybe, mkTyVarTys,
34 import Inst ( InstOrigin(..), tcInstClassOp, newDicts, instToId, showLIE )
35 import TcDeriv ( tcDeriving )
36 import TcEnv ( tcExtendGlobalValEnv,
37 tcLookupClass, tcExtendTyVarEnv2,
38 tcExtendInstEnv, tcExtendLocalInstEnv, tcLookupGlobalId,
39 InstInfo(..), InstBindings(..), pprInstInfo, simpleInstInfoTyCon,
40 simpleInstInfoTy, newDFunName
42 import PprType ( pprClassPred )
43 import TcMonoType ( tcHsTyVars, kcHsSigType, tcHsType, tcHsSigType )
44 import TcUnify ( checkSigTyVars )
45 import TcSimplify ( tcSimplifyCheck, tcSimplifyTop )
46 import HscTypes ( DFunId )
47 import Subst ( mkTyVarSubst, substTheta, substTy )
48 import DataCon ( classDataCon )
49 import Class ( Class, classBigSig )
50 import Var ( idName, idType )
52 import Id ( setIdLocalExported )
53 import MkId ( mkDictFunId, rUNTIME_ERROR_ID )
54 import FunDeps ( checkInstFDs )
55 import Generics ( validGenericInstanceType )
56 import Name ( getSrcLoc )
57 import NameSet ( unitNameSet, emptyNameSet, nameSetToList )
58 import TyCon ( TyCon )
59 import TysWiredIn ( genericTyCons )
60 import SrcLoc ( SrcLoc )
61 import Unique ( Uniquable(..) )
62 import Util ( lengthExceeds )
63 import BasicTypes ( NewOrData(..) )
64 import UnicodeUtil ( stringToUtf8 )
65 import ErrUtils ( dumpIfSet_dyn )
66 import ListSetOps ( Assoc, emptyAssoc, plusAssoc_C, mapAssoc,
67 assocElts, extendAssoc_C, equivClassesByUniq, minusList
69 import Maybe ( catMaybes )
70 import List ( partition )
75 Typechecking instance declarations is done in two passes. The first
76 pass, made by @tcInstDecls1@, collects information to be used in the
79 This pre-processed info includes the as-yet-unprocessed bindings
80 inside the instance declaration. These are type-checked in the second
81 pass, when the class-instance envs and GVE contain all the info from
82 all the instance and value decls. Indeed that's the reason we need
83 two passes over the instance decls.
86 Here is the overall algorithm.
87 Assume that we have an instance declaration
89 instance c => k (t tvs) where b
93 $LIE_c$ is the LIE for the context of class $c$
95 $betas_bar$ is the free variables in the class method type, excluding the
98 $LIE_cop$ is the LIE constraining a particular class method
100 $tau_cop$ is the tau type of a class method
102 $LIE_i$ is the LIE for the context of instance $i$
104 $X$ is the instance constructor tycon
106 $gammas_bar$ is the set of type variables of the instance
108 $LIE_iop$ is the LIE for a particular class method instance
110 $tau_iop$ is the tau type for this instance of a class method
112 $alpha$ is the class variable
114 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
116 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
119 ToDo: Update the list above with names actually in the code.
123 First, make the LIEs for the class and instance contexts, which means
124 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
125 and make LIElistI and LIEI.
127 Then process each method in turn.
129 order the instance methods according to the ordering of the class methods
131 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
133 Create final dictionary function from bindings generated already
135 df = lambda inst_tyvars
142 in <op1,op2,...,opn,sd1,...,sdm>
144 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
145 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
149 %************************************************************************
151 \subsection{Extracting instance decls}
153 %************************************************************************
155 Gather up the instance declarations from their various sources
158 tcInstDecls1 -- Deal with both source-code and imported instance decls
159 :: [RenamedTyClDecl] -- For deriving stuff
160 -> [RenamedInstDecl] -- Source code instance decls
161 -> TcM (TcGblEnv, -- The full inst env
162 [InstInfo], -- Source-code instance decls to process;
163 -- contains all dfuns for this module
164 RenamedHsBinds, -- Supporting bindings for derived instances
165 FreeVars) -- And the free vars of the derived code
167 tcInstDecls1 tycl_decls inst_decls
169 -- Stop if addInstInfos etc discovers any errors
170 -- (they recover, so that we get more than one error each round)
172 (src_inst_decls, iface_inst_decls) = partition isSourceInstDecl inst_decls
175 -- (0) Deal with the imported instance decls
176 tcIfaceInstDecls iface_inst_decls `thenM` \ imp_dfuns ->
177 tcExtendInstEnv imp_dfuns $
179 -- (1) Do the ordinary instance declarations
180 mappM tcLocalInstDecl1 src_inst_decls `thenM` \ local_inst_infos ->
183 local_inst_info = catMaybes local_inst_infos
184 clas_decls = filter isClassDecl tycl_decls
186 -- (2) Instances from generic class declarations
187 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
189 -- Next, construct the instance environment so far, consisting of
190 -- a) imported instance decls (from this module)
191 -- b) local instance decls
192 -- c) generic instances
193 tcExtendLocalInstEnv local_inst_info $
194 tcExtendLocalInstEnv generic_inst_info $
196 -- (3) Compute instances from "deriving" clauses;
197 -- note that we only do derivings for things in this module;
198 -- we ignore deriving decls from interfaces!
199 -- This stuff computes a context for the derived instance decl, so it
200 -- needs to know about all the instances possible; hence inst_env4
201 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds, fvs) ->
202 tcExtendLocalInstEnv deriv_inst_info $
204 getGblEnv `thenM` \ gbl_env ->
206 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
211 tcLocalInstDecl1 :: RenamedInstDecl
212 -> TcM (Maybe InstInfo) -- Nothing if there was an error
213 -- A source-file instance declaration
214 -- Type-check all the stuff before the "where"
216 -- We check for respectable instance type, and context
217 -- but only do this for non-imported instance decls.
218 -- Imported ones should have been checked already, and may indeed
219 -- contain something illegal in normal Haskell, notably
220 -- instance CCallable [Char]
221 tcLocalInstDecl1 decl@(InstDecl poly_ty binds uprags Nothing src_loc)
222 = -- Prime error recovery, set source location
223 recoverM (returnM Nothing) $
225 addErrCtxt (instDeclCtxt poly_ty) $
227 -- Typecheck the instance type itself. We can't use
228 -- tcHsSigType, because it's not a valid user type.
229 kcHsSigType poly_ty `thenM_`
230 tcHsType poly_ty `thenM` \ poly_ty' ->
232 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
234 checkValidTheta InstThetaCtxt theta `thenM_`
235 checkAmbiguity tyvars theta (tyVarsOfType tau) `thenM_`
236 checkValidInstHead tau `thenM` \ (clas,inst_tys) ->
237 checkTc (checkInstFDs theta clas inst_tys)
238 (instTypeErr (pprClassPred clas inst_tys) msg) `thenM_`
239 newDFunName clas inst_tys src_loc `thenM` \ dfun_name ->
240 returnM (Just (InstInfo { iDFunId = mkDictFunId dfun_name tyvars theta clas inst_tys,
241 iBinds = VanillaInst binds uprags }))
243 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
246 Imported instance declarations
249 tcIfaceInstDecls :: [RenamedInstDecl] -> TcM [DFunId]
250 -- Deal with the instance decls,
251 tcIfaceInstDecls decls = mappM tcIfaceInstDecl decls
253 tcIfaceInstDecl :: RenamedInstDecl -> TcM DFunId
254 -- An interface-file instance declaration
255 -- Should be in scope by now, because we should
256 -- have sucked in its interface-file definition
257 -- So it will be replete with its unfolding etc
258 tcIfaceInstDecl decl@(InstDecl poly_ty binds uprags (Just dfun_name) src_loc)
259 = tcLookupGlobalId dfun_name
263 %************************************************************************
265 \subsection{Extracting generic instance declaration from class declarations}
267 %************************************************************************
269 @getGenericInstances@ extracts the generic instance declarations from a class
270 declaration. For exmaple
275 op{ x+y } (Inl v) = ...
276 op{ x+y } (Inr v) = ...
277 op{ x*y } (v :*: w) = ...
280 gives rise to the instance declarations
282 instance C (x+y) where
286 instance C (x*y) where
294 getGenericInstances :: [RenamedTyClDecl] -> TcM [InstInfo]
295 getGenericInstances class_decls
296 = mappM get_generics class_decls `thenM` \ gen_inst_infos ->
298 gen_inst_info = concat gen_inst_infos
300 if null gen_inst_info then
303 getDOpts `thenM` \ dflags ->
304 ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Generic instances"
305 (vcat (map pprInstInfo gen_inst_info)))
307 returnM gen_inst_info
309 get_generics decl@(ClassDecl {tcdMeths = Nothing})
310 = returnM [] -- Imported class decls
312 get_generics decl@(ClassDecl {tcdName = class_name, tcdMeths = Just def_methods, tcdLoc = loc})
314 = returnM [] -- The comon case: no generic default methods
316 | otherwise -- A source class decl with generic default methods
317 = recoverM (returnM []) $
319 tcLookupClass class_name `thenM` \ clas ->
321 -- Make an InstInfo out of each group
322 mappM (mkGenericInstance clas loc) groups `thenM` \ inst_infos ->
324 -- Check that there is only one InstInfo for each type constructor
325 -- The main way this can fail is if you write
326 -- f {| a+b |} ... = ...
327 -- f {| x+y |} ... = ...
328 -- Then at this point we'll have an InstInfo for each
330 tc_inst_infos :: [(TyCon, InstInfo)]
331 tc_inst_infos = [(simpleInstInfoTyCon i, i) | i <- inst_infos]
333 bad_groups = [group | group <- equivClassesByUniq get_uniq tc_inst_infos,
334 group `lengthExceeds` 1]
335 get_uniq (tc,_) = getUnique tc
337 mappM (addErrTc . dupGenericInsts) bad_groups `thenM_`
339 -- Check that there is an InstInfo for each generic type constructor
341 missing = genericTyCons `minusList` [tc | (tc,_) <- tc_inst_infos]
343 checkTc (null missing) (missingGenericInstances missing) `thenM_`
348 -- Group the declarations by type pattern
349 groups :: [(RenamedHsType, RenamedMonoBinds)]
350 groups = assocElts (getGenericBinds def_methods)
353 ---------------------------------
354 getGenericBinds :: RenamedMonoBinds -> Assoc RenamedHsType RenamedMonoBinds
355 -- Takes a group of method bindings, finds the generic ones, and returns
356 -- them in finite map indexed by the type parameter in the definition.
358 getGenericBinds EmptyMonoBinds = emptyAssoc
359 getGenericBinds (AndMonoBinds m1 m2)
360 = plusAssoc_C AndMonoBinds (getGenericBinds m1) (getGenericBinds m2)
362 getGenericBinds (FunMonoBind id infixop matches loc)
363 = mapAssoc wrap (foldl add emptyAssoc matches)
364 -- Using foldl not foldr is vital, else
365 -- we reverse the order of the bindings!
367 add env match = case maybeGenericMatch match of
369 Just (ty, match') -> extendAssoc_C (++) env (ty, [match'])
371 wrap ms = FunMonoBind id infixop ms loc
373 ---------------------------------
374 mkGenericInstance :: Class -> SrcLoc
375 -> (RenamedHsType, RenamedMonoBinds)
378 mkGenericInstance clas loc (hs_ty, binds)
379 -- Make a generic instance declaration
380 -- For example: instance (C a, C b) => C (a+b) where { binds }
382 = -- Extract the universally quantified type variables
384 sig_tvs = map UserTyVar (nameSetToList (extractHsTyVars hs_ty))
386 tcHsTyVars sig_tvs (kcHsSigType hs_ty) $ \ tyvars ->
388 -- Type-check the instance type, and check its form
389 tcHsSigType GenPatCtxt hs_ty `thenM` \ inst_ty ->
390 checkTc (validGenericInstanceType inst_ty)
391 (badGenericInstanceType binds) `thenM_`
393 -- Make the dictionary function.
394 newDFunName clas [inst_ty] loc `thenM` \ dfun_name ->
396 inst_theta = [mkClassPred clas [mkTyVarTy tv] | tv <- tyvars]
397 dfun_id = mkDictFunId dfun_name tyvars inst_theta clas [inst_ty]
400 returnM (InstInfo { iDFunId = dfun_id, iBinds = VanillaInst binds [] })
404 %************************************************************************
406 \subsection{Type-checking instance declarations, pass 2}
408 %************************************************************************
411 tcInstDecls2 :: [InstInfo] -> TcM TcMonoBinds
412 tcInstDecls2 inst_decls
413 = mappM tcInstDecl2 inst_decls `thenM` \ binds_s ->
414 returnM (andMonoBindList binds_s)
417 ======= New documentation starts here (Sept 92) ==============
419 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
420 the dictionary function for this instance declaration. For example
422 instance Foo a => Foo [a] where
426 might generate something like
428 dfun.Foo.List dFoo_a = let op1 x = ...
434 HOWEVER, if the instance decl has no context, then it returns a
435 bigger @HsBinds@ with declarations for each method. For example
437 instance Foo [a] where
443 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
444 const.Foo.op1.List a x = ...
445 const.Foo.op2.List a y = ...
447 This group may be mutually recursive, because (for example) there may
448 be no method supplied for op2 in which case we'll get
450 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
452 that is, the default method applied to the dictionary at this type.
454 What we actually produce in either case is:
456 AbsBinds [a] [dfun_theta_dicts]
457 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
458 { d = (sd1,sd2, ..., op1, op2, ...)
463 The "maybe" says that we only ask AbsBinds to make global constant methods
464 if the dfun_theta is empty.
467 For an instance declaration, say,
469 instance (C1 a, C2 b) => C (T a b) where
472 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
473 function whose type is
475 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
477 Notice that we pass it the superclass dictionaries at the instance type; this
478 is the ``Mark Jones optimisation''. The stuff before the "=>" here
479 is the @dfun_theta@ below.
481 First comes the easy case of a non-local instance decl.
485 tcInstDecl2 :: InstInfo -> TcM TcMonoBinds
487 tcInstDecl2 (InstInfo { iDFunId = dfun_id, iBinds = binds })
488 = -- Prime error recovery
489 recoverM (returnM EmptyMonoBinds) $
490 addSrcLoc (getSrcLoc dfun_id) $
491 addErrCtxt (instDeclCtxt (toHsType (idType dfun_id))) $
493 inst_ty = idType dfun_id
494 (inst_tyvars, _) = tcSplitForAllTys inst_ty
495 -- The tyvars of the instance decl scope over the 'where' part
496 -- Those tyvars are inside the dfun_id's type, which is a bit
497 -- bizarre, but OK so long as you realise it!
500 -- Instantiate the instance decl with tc-style type variables
501 tcInstType InstTv inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
503 Just pred = tcSplitPredTy_maybe inst_head'
504 (clas, inst_tys') = getClassPredTys pred
505 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
507 -- Instantiate the super-class context with inst_tys
508 sc_theta' = substTheta (mkTyVarSubst class_tyvars inst_tys') sc_theta
509 origin = InstanceDeclOrigin
511 -- Create dictionary Ids from the specified instance contexts.
512 newDicts origin sc_theta' `thenM` \ sc_dicts ->
513 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
514 newDicts origin [pred] `thenM` \ [this_dict] ->
515 -- Default-method Ids may be mentioned in synthesised RHSs,
516 -- but they'll already be in the environment.
519 -- Typecheck the methods
520 let -- These insts are in scope; quite a few, eh?
521 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
523 tcMethods clas inst_tyvars inst_tyvars'
524 dfun_theta' inst_tys' avail_insts
525 op_items binds `thenM` \ (meth_ids, meth_binds) ->
527 -- Figure out bindings for the superclass context
528 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
529 `thenM` \ (zonked_inst_tyvars, sc_binds_inner, sc_binds_outer) ->
531 -- Deal with 'SPECIALISE instance' pragmas by making them
532 -- look like SPECIALISE pragmas for the dfun
534 uprags = case binds of
535 VanillaInst _ uprags -> uprags
537 spec_prags = [ SpecSig (idName dfun_id) ty loc
538 | SpecInstSig ty loc <- uprags ]
539 xtve = inst_tyvars `zip` inst_tyvars'
541 tcExtendGlobalValEnv [dfun_id] (
542 tcExtendTyVarEnv2 xtve $
543 tcSpecSigs spec_prags
544 ) `thenM` \ prag_binds ->
546 -- Create the result bindings
548 dict_constr = classDataCon clas
549 scs_and_meths = map instToId sc_dicts ++ meth_ids
550 this_dict_id = instToId this_dict
551 inlines | null dfun_arg_dicts = emptyNameSet
552 | otherwise = unitNameSet (idName dfun_id)
553 -- Always inline the dfun; this is an experimental decision
554 -- because it makes a big performance difference sometimes.
555 -- Often it means we can do the method selection, and then
556 -- inline the method as well. Marcin's idea; see comments below.
558 -- BUT: don't inline it if it's a constant dictionary;
559 -- we'll get all the benefit without inlining, and we get
560 -- a **lot** of code duplication if we inline it
562 -- See Note [Inline dfuns] below
566 = -- Blatant special case for CCallable, CReturnable
567 -- If the dictionary is empty then we should never
568 -- select anything from it, so we make its RHS just
569 -- emit an error message. This in turn means that we don't
570 -- mention the constructor, which doesn't exist for CCallable, CReturnable
571 -- Hardly beautiful, but only three extra lines.
572 HsApp (TyApp (HsVar rUNTIME_ERROR_ID) [idType this_dict_id])
573 (HsLit (HsStringPrim (mkFastString (stringToUtf8 msg))))
575 | otherwise -- The common case
576 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
577 -- We don't produce a binding for the dict_constr; instead we
578 -- rely on the simplifier to unfold this saturated application
579 -- We do this rather than generate an HsCon directly, because
580 -- it means that the special cases (e.g. dictionary with only one
581 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
582 -- than needing to be repeated here.
585 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
587 dict_bind = VarMonoBind this_dict_id dict_rhs
588 all_binds = sc_binds_inner `AndMonoBinds` meth_binds `AndMonoBinds` dict_bind
592 (map instToId dfun_arg_dicts)
593 [(inst_tyvars', dfun_id, this_dict_id)]
596 showLIE "instance" `thenM_`
597 returnM (main_bind `AndMonoBinds` prag_binds `AndMonoBinds` sc_binds_outer)
600 tcMethods clas inst_tyvars inst_tyvars' dfun_theta' inst_tys'
601 avail_insts op_items (VanillaInst monobinds uprags)
602 = -- Check that all the method bindings come from this class
604 sel_names = [idName sel_id | (sel_id, _) <- op_items]
605 bad_bndrs = collectMonoBinders monobinds `minusList` sel_names
607 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
609 -- Make the method bindings
611 mk_method_bind = mkMethodBind InstanceDeclOrigin clas inst_tys' monobinds
613 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
615 -- And type check them
616 -- It's really worth making meth_insts available to the tcMethodBind
617 -- Consider instance Monad (ST s) where
618 -- {-# INLINE (>>) #-}
619 -- (>>) = ...(>>=)...
620 -- If we don't include meth_insts, we end up with bindings like this:
621 -- rec { dict = MkD then bind ...
622 -- then = inline_me (... (GHC.Base.>>= dict) ...)
624 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
625 -- and (b) the inline_me prevents us inlining the >>= selector, which
626 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
627 -- is not inlined across modules. Rather ironic since this does not
628 -- happen without the INLINE pragma!
630 -- Solution: make meth_insts available, so that 'then' refers directly
631 -- to the local 'bind' rather than going via the dictionary.
633 -- BUT WATCH OUT! If the method type mentions the class variable, then
634 -- this optimisation is not right. Consider
638 -- instance C Int where
640 -- The occurrence of 'op' on the rhs gives rise to a constraint
642 -- The trouble is that the 'meth_inst' for op, which is 'available', also
643 -- looks like 'op at Int'. But they are not the same.
645 all_insts = avail_insts ++ catMaybes meth_insts
646 xtve = inst_tyvars `zip` inst_tyvars'
647 tc_method_bind = tcMethodBind xtve inst_tyvars' dfun_theta' all_insts uprags
649 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
651 returnM ([meth_id | (_,meth_id,_) <- meth_infos],
652 andMonoBindList meth_binds_s)
655 -- Derived newtype instances
656 tcMethods clas inst_tyvars inst_tyvars' dfun_theta' inst_tys'
657 avail_insts op_items (NewTypeDerived rep_tys)
658 = getInstLoc InstanceDeclOrigin `thenM` \ inst_loc ->
659 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
662 (ptext SLIT("newtype derived instance"))
663 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
665 -- I don't think we have to do the checkSigTyVars thing
667 returnM (meth_ids, lie_binds `AndMonoBinds` andMonoBindList meth_binds)
670 do_one inst_loc (sel_id, _)
671 = -- The binding is like "op @ NewTy = op @ RepTy"
672 -- Make the *binder*, like in mkMethodBind
673 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
675 -- Make the *occurrence on the rhs*
676 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
678 meth_id = instToId meth_inst
680 return (meth_id, VarMonoBind meth_id (HsVar (instToId rhs_inst)), rhs_inst)
682 -- Instantiate rep_tys with the relevant type variables
683 rep_tys' = map (substTy subst) rep_tys
684 subst = mkTyVarSubst inst_tyvars (mkTyVarTys inst_tyvars')
687 Note: [Superclass loops]
688 ~~~~~~~~~~~~~~~~~~~~~~~~~
689 We have to be very, very careful when generating superclasses, lest we
690 accidentally build a loop. Here's an example:
694 class S a => C a where { opc :: a -> a }
695 class S b => D b where { opd :: b -> b }
703 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
704 Simplifying, we may well get:
705 $dfCInt = :C ds1 (opd dd)
708 Notice that we spot that we can extract ds1 from dd.
710 Alas! Alack! We can do the same for (instance D Int):
712 $dfDInt = :D ds2 (opc dc)
716 And now we've defined the superclass in terms of itself.
719 Solution: treat the superclass context separately, and simplify it
720 all the way down to nothing on its own. Don't toss any 'free' parts
721 out to be simplified together with other bits of context.
722 Hence the tcSimplifyTop below.
724 At a more basic level, don't include this_dict in the context wrt
725 which we simplify sc_dicts, else sc_dicts get bound by just selecting
729 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
730 = addErrCtxt superClassCtxt $
731 getLIE (tcSimplifyCheck doc inst_tyvars'
733 sc_dicts) `thenM` \ (sc_binds1, sc_lie) ->
735 -- It's possible that the superclass stuff might have done unification
736 checkSigTyVars inst_tyvars' `thenM` \ zonked_inst_tyvars ->
738 -- We must simplify this all the way down
739 -- lest we build superclass loops
740 -- See Note [Superclass loops] above
741 tcSimplifyTop sc_lie `thenM` \ sc_binds2 ->
743 returnM (zonked_inst_tyvars, sc_binds1, sc_binds2)
746 doc = ptext SLIT("instance declaration superclass context")
750 ------------------------------
751 [Inline dfuns] Inlining dfuns unconditionally
752 ------------------------------
754 The code above unconditionally inlines dict funs. Here's why.
755 Consider this program:
757 test :: Int -> Int -> Bool
758 test x y = (x,y) == (y,x) || test y x
759 -- Recursive to avoid making it inline.
761 This needs the (Eq (Int,Int)) instance. If we inline that dfun
762 the code we end up with is good:
765 \r -> case ==# [ww ww1] of wild {
766 PrelBase.False -> Test.$wtest ww1 ww;
768 case ==# [ww1 ww] of wild1 {
769 PrelBase.False -> Test.$wtest ww1 ww;
770 PrelBase.True -> PrelBase.True [];
773 Test.test = \r [w w1]
776 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
779 If we don't inline the dfun, the code is not nearly as good:
781 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
782 PrelBase.:DEq tpl1 tpl2 -> tpl2;
787 let { y = PrelBase.I#! [ww1]; } in
788 let { x = PrelBase.I#! [ww]; } in
789 let { sat_slx = PrelTup.(,)! [y x]; } in
790 let { sat_sly = PrelTup.(,)! [x y];
792 case == sat_sly sat_slx of wild {
793 PrelBase.False -> Test.$wtest ww1 ww;
794 PrelBase.True -> PrelBase.True [];
801 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
804 Why doesn't GHC inline $fEq? Because it looks big:
806 PrelTup.zdfEqZ1T{-rcX-}
807 = \ @ a{-reT-} :: * @ b{-reS-} :: *
808 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
809 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
811 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
812 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
814 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
815 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
817 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
818 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
819 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
821 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
823 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
825 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
826 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
830 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
831 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
832 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
833 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
835 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
837 and it's not as bad as it seems, because it's further dramatically
838 simplified: only zeze2 is extracted and its body is simplified.
841 %************************************************************************
843 \subsection{Error messages}
845 %************************************************************************
848 tcAddDeclCtxt decl thing_inside
849 = addSrcLoc (tcdLoc decl) $
854 ClassDecl {} -> "class"
855 TySynonym {} -> "type synonym"
856 TyData {tcdND = NewType} -> "newtype"
857 TyData {tcdND = DataType} -> "data type"
859 ctxt = hsep [ptext SLIT("In the"), text thing,
860 ptext SLIT("declaration for"), quotes (ppr (tcdName decl))]
862 instDeclCtxt inst_ty = ptext SLIT("In the instance declaration for") <+> quotes doc
864 doc = case inst_ty of
865 HsForAllTy _ _ (HsPredTy pred) -> ppr pred
866 HsPredTy pred -> ppr pred
867 other -> ppr inst_ty -- Don't expect this
871 badGenericInstanceType binds
872 = vcat [ptext SLIT("Illegal type pattern in the generic bindings"),
875 missingGenericInstances missing
876 = ptext SLIT("Missing type patterns for") <+> pprQuotedList missing
878 dupGenericInsts tc_inst_infos
879 = vcat [ptext SLIT("More than one type pattern for a single generic type constructor:"),
880 nest 4 (vcat (map ppr_inst_ty tc_inst_infos)),
881 ptext SLIT("All the type patterns for a generic type constructor must be identical")
884 ppr_inst_ty (tc,inst) = ppr (simpleInstInfoTy inst)
886 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
887 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")