2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcInstDecls]{Typechecking instance declarations}
7 module TcInstDcls ( tcInstDecls1, tcIfaceInstDecls1, addInstDFuns,
8 tcInstDecls2, initInstEnv, 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,
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 UserTypeCtxt(..), SourceTyCtxt(..) )
30 import TcType ( mkClassPred, mkTyVarTy, tcSplitForAllTys,
31 tcSplitSigmaTy, getClassPredTys, tcSplitPredTy_maybe,
34 import Inst ( InstOrigin(..), newDicts, instToId,
35 LIE, mkLIE, emptyLIE, plusLIE, plusLIEs )
36 import TcDeriv ( tcDeriving )
37 import TcEnv ( tcExtendGlobalValEnv, tcExtendLocalValEnv2,
38 tcLookupId, tcLookupClass, tcExtendTyVarEnv2,
39 InstInfo(..), pprInstInfo, simpleInstInfoTyCon,
40 simpleInstInfoTy, newDFunName
42 import InstEnv ( InstEnv, extendInstEnv )
43 import PprType ( pprClassPred )
44 import TcMonoType ( tcSigPolyId, tcHsTyVars, kcHsSigType, tcHsType, tcHsSigType )
45 import TcUnify ( checkSigTyVars )
46 import TcSimplify ( tcSimplifyCheck, tcSimplifyTop )
47 import HscTypes ( HomeSymbolTable, DFunId, FixityEnv,
48 PersistentCompilerState(..), PersistentRenamerState,
51 import Subst ( mkTyVarSubst, substTheta )
52 import DataCon ( classDataCon )
53 import Class ( Class, classBigSig )
54 import Var ( idName, idType )
55 import Id ( setIdLocalExported )
56 import MkId ( mkDictFunId, unsafeCoerceId, rUNTIME_ERROR_ID )
57 import FunDeps ( checkInstFDs )
58 import Generics ( validGenericInstanceType )
59 import Module ( Module, foldModuleEnv )
60 import Name ( getSrcLoc )
61 import NameSet ( unitNameSet, emptyNameSet, nameSetToList )
62 import TyCon ( TyCon )
63 import TysWiredIn ( genericTyCons )
64 import SrcLoc ( SrcLoc )
65 import Unique ( Uniquable(..) )
66 import Util ( lengthExceeds, isSingleton )
67 import BasicTypes ( NewOrData(..) )
68 import UnicodeUtil ( stringToUtf8 )
69 import ErrUtils ( dumpIfSet_dyn )
70 import ListSetOps ( Assoc, emptyAssoc, plusAssoc_C, mapAssoc,
71 assocElts, extendAssoc_C, equivClassesByUniq, minusList
73 import Maybe ( catMaybes )
77 Typechecking instance declarations is done in two passes. The first
78 pass, made by @tcInstDecls1@, collects information to be used in the
81 This pre-processed info includes the as-yet-unprocessed bindings
82 inside the instance declaration. These are type-checked in the second
83 pass, when the class-instance envs and GVE contain all the info from
84 all the instance and value decls. Indeed that's the reason we need
85 two passes over the instance decls.
88 Here is the overall algorithm.
89 Assume that we have an instance declaration
91 instance c => k (t tvs) where b
95 $LIE_c$ is the LIE for the context of class $c$
97 $betas_bar$ is the free variables in the class method type, excluding the
100 $LIE_cop$ is the LIE constraining a particular class method
102 $tau_cop$ is the tau type of a class method
104 $LIE_i$ is the LIE for the context of instance $i$
106 $X$ is the instance constructor tycon
108 $gammas_bar$ is the set of type variables of the instance
110 $LIE_iop$ is the LIE for a particular class method instance
112 $tau_iop$ is the tau type for this instance of a class method
114 $alpha$ is the class variable
116 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
118 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
121 ToDo: Update the list above with names actually in the code.
125 First, make the LIEs for the class and instance contexts, which means
126 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
127 and make LIElistI and LIEI.
129 Then process each method in turn.
131 order the instance methods according to the ordering of the class methods
133 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
135 Create final dictionary function from bindings generated already
137 df = lambda inst_tyvars
144 in <op1,op2,...,opn,sd1,...,sdm>
146 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
147 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
151 %************************************************************************
153 \subsection{Extracting instance decls}
155 %************************************************************************
157 Gather up the instance declarations from their various sources
160 tcInstDecls1 -- Deal with source-code instance decls
161 :: PersistentRenamerState
162 -> InstEnv -- Imported instance envt
163 -> FixityEnv -- for deriving Show and Read
164 -> Module -- Module for deriving
165 -> [RenamedTyClDecl] -- For deriving stuff
166 -> [RenamedInstDecl] -- Source code instance decls
167 -> TcM (InstEnv, -- the full inst env
168 [InstInfo], -- instance decls to process; contains all dfuns
170 RenamedHsBinds) -- derived instances
172 tcInstDecls1 prs inst_env get_fixity this_mod
173 tycl_decls inst_decls
174 -- The incoming inst_env includes all the imported instances already
176 -- Stop if addInstInfos etc discovers any errors
177 -- (they recover, so that we get more than one error each round)
178 -- (1) Do the ordinary instance declarations
179 mapNF_Tc tcLocalInstDecl1 inst_decls `thenNF_Tc` \ local_inst_infos ->
182 local_inst_info = catMaybes local_inst_infos
183 clas_decls = filter isClassDecl tycl_decls
185 -- (2) Instances from generic class declarations
186 getGenericInstances clas_decls `thenTc` \ generic_inst_info ->
188 -- Next, construct the instance environment so far, consisting of
189 -- a) imported instance decls (from this module) inst_env1
190 -- b) local instance decls inst_env2
191 -- c) generic instances final_inst_env
192 addInstInfos inst_env local_inst_info `thenNF_Tc` \ inst_env1 ->
193 addInstInfos inst_env1 generic_inst_info `thenNF_Tc` \ inst_env2 ->
195 -- (3) Compute instances from "deriving" clauses;
196 -- note that we only do derivings for things in this module;
197 -- we ignore deriving decls from interfaces!
198 -- This stuff computes a context for the derived instance decl, so it
199 -- needs to know about all the instances possible; hence inst_env4
200 tcDeriving prs this_mod inst_env2
201 get_fixity tycl_decls `thenTc` \ (deriv_inst_info, deriv_binds) ->
202 addInstInfos inst_env2 deriv_inst_info `thenNF_Tc` \ final_inst_env ->
204 returnTc (final_inst_env,
205 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
208 initInstEnv :: PersistentCompilerState -> HomeSymbolTable -> NF_TcM InstEnv
209 -- Initialise the instance environment from the
210 -- persistent compiler state and the home symbol table
213 pkg_inst_env = pcs_insts pcs
214 hst_dfuns = foldModuleEnv ((++) . md_insts) [] hst
216 addInstDFuns pkg_inst_env hst_dfuns
218 addInstInfos :: InstEnv -> [InstInfo] -> NF_TcM InstEnv
219 addInstInfos inst_env infos = addInstDFuns inst_env (map iDFunId infos)
221 addInstDFuns :: InstEnv -> [DFunId] -> NF_TcM InstEnv
222 addInstDFuns inst_env dfuns
223 = getDOptsTc `thenNF_Tc` \ dflags ->
225 (inst_env', errs) = extendInstEnv dflags inst_env dfuns
227 addErrsTc errs `thenNF_Tc_`
228 traceTc (text "Adding instances:" <+> vcat (map pp dfuns)) `thenTc_`
231 pp dfun = ppr dfun <+> dcolon <+> ppr (idType dfun)
235 tcIfaceInstDecls1 :: [RenamedInstDecl] -> NF_TcM [DFunId]
236 tcIfaceInstDecls1 decls = mapNF_Tc tcIfaceInstDecl1 decls
238 tcIfaceInstDecl1 :: RenamedInstDecl -> NF_TcM DFunId
239 -- An interface-file instance declaration
240 -- Should be in scope by now, because we should
241 -- have sucked in its interface-file definition
242 -- So it will be replete with its unfolding etc
243 tcIfaceInstDecl1 decl@(InstDecl poly_ty binds uprags (Just dfun_name) src_loc)
244 = tcLookupId dfun_name
247 tcLocalInstDecl1 :: RenamedInstDecl
248 -> NF_TcM (Maybe InstInfo) -- Nothing if there was an error
249 -- A source-file instance declaration
250 -- Type-check all the stuff before the "where"
252 -- We check for respectable instance type, and context
253 -- but only do this for non-imported instance decls.
254 -- Imported ones should have been checked already, and may indeed
255 -- contain something illegal in normal Haskell, notably
256 -- instance CCallable [Char]
257 tcLocalInstDecl1 decl@(InstDecl poly_ty binds uprags Nothing src_loc)
258 = -- Prime error recovery, set source location
259 recoverNF_Tc (returnNF_Tc Nothing) $
260 tcAddSrcLoc src_loc $
261 tcAddErrCtxt (instDeclCtxt poly_ty) $
263 -- Typecheck the instance type itself. We can't use
264 -- tcHsSigType, because it's not a valid user type.
265 kcHsSigType poly_ty `thenTc_`
266 tcHsType poly_ty `thenTc` \ poly_ty' ->
268 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
270 checkValidTheta InstThetaCtxt theta `thenTc_`
271 checkValidInstHead tau `thenTc` \ (clas,inst_tys) ->
272 checkTc (checkInstFDs theta clas inst_tys)
273 (instTypeErr (pprClassPred clas inst_tys) msg) `thenTc_`
274 newDFunName clas inst_tys src_loc `thenNF_Tc` \ dfun_name ->
275 returnTc (Just (InstInfo { iDFunId = mkDictFunId dfun_name clas tyvars inst_tys theta,
276 iBinds = binds, iPrags = uprags }))
278 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
282 %************************************************************************
284 \subsection{Extracting generic instance declaration from class declarations}
286 %************************************************************************
288 @getGenericInstances@ extracts the generic instance declarations from a class
289 declaration. For exmaple
294 op{ x+y } (Inl v) = ...
295 op{ x+y } (Inr v) = ...
296 op{ x*y } (v :*: w) = ...
299 gives rise to the instance declarations
301 instance C (x+y) where
305 instance C (x*y) where
313 getGenericInstances :: [RenamedTyClDecl] -> TcM [InstInfo]
314 getGenericInstances class_decls
315 = mapTc get_generics class_decls `thenTc` \ gen_inst_infos ->
317 gen_inst_info = concat gen_inst_infos
319 if null gen_inst_info then
322 getDOptsTc `thenNF_Tc` \ dflags ->
323 ioToTc (dumpIfSet_dyn dflags Opt_D_dump_deriv "Generic instances"
324 (vcat (map pprInstInfo gen_inst_info)))
326 returnTc gen_inst_info
328 get_generics decl@(ClassDecl {tcdMeths = Nothing})
329 = returnTc [] -- Imported class decls
331 get_generics decl@(ClassDecl {tcdName = class_name, tcdMeths = Just def_methods, tcdLoc = loc})
333 = returnTc [] -- The comon case: no generic default methods
335 | otherwise -- A source class decl with generic default methods
336 = recoverNF_Tc (returnNF_Tc []) $
338 tcLookupClass class_name `thenTc` \ clas ->
340 -- Make an InstInfo out of each group
341 mapTc (mkGenericInstance clas loc) groups `thenTc` \ inst_infos ->
343 -- Check that there is only one InstInfo for each type constructor
344 -- The main way this can fail is if you write
345 -- f {| a+b |} ... = ...
346 -- f {| x+y |} ... = ...
347 -- Then at this point we'll have an InstInfo for each
349 tc_inst_infos :: [(TyCon, InstInfo)]
350 tc_inst_infos = [(simpleInstInfoTyCon i, i) | i <- inst_infos]
352 bad_groups = [group | group <- equivClassesByUniq get_uniq tc_inst_infos,
353 group `lengthExceeds` 1]
354 get_uniq (tc,_) = getUnique tc
356 mapTc (addErrTc . dupGenericInsts) bad_groups `thenTc_`
358 -- Check that there is an InstInfo for each generic type constructor
360 missing = genericTyCons `minusList` [tc | (tc,_) <- tc_inst_infos]
362 checkTc (null missing) (missingGenericInstances missing) `thenTc_`
367 -- Group the declarations by type pattern
368 groups :: [(RenamedHsType, RenamedMonoBinds)]
369 groups = assocElts (getGenericBinds def_methods)
372 ---------------------------------
373 getGenericBinds :: RenamedMonoBinds -> Assoc RenamedHsType RenamedMonoBinds
374 -- Takes a group of method bindings, finds the generic ones, and returns
375 -- them in finite map indexed by the type parameter in the definition.
377 getGenericBinds EmptyMonoBinds = emptyAssoc
378 getGenericBinds (AndMonoBinds m1 m2)
379 = plusAssoc_C AndMonoBinds (getGenericBinds m1) (getGenericBinds m2)
381 getGenericBinds (FunMonoBind id infixop matches loc)
382 = mapAssoc wrap (foldl add emptyAssoc matches)
383 -- Using foldl not foldr is vital, else
384 -- we reverse the order of the bindings!
386 add env match = case maybeGenericMatch match of
388 Just (ty, match') -> extendAssoc_C (++) env (ty, [match'])
390 wrap ms = FunMonoBind id infixop ms loc
392 ---------------------------------
393 mkGenericInstance :: Class -> SrcLoc
394 -> (RenamedHsType, RenamedMonoBinds)
397 mkGenericInstance clas loc (hs_ty, binds)
398 -- Make a generic instance declaration
399 -- For example: instance (C a, C b) => C (a+b) where { binds }
401 = -- Extract the universally quantified type variables
403 sig_tvs = map UserTyVar (nameSetToList (extractHsTyVars hs_ty))
405 tcHsTyVars sig_tvs (kcHsSigType hs_ty) $ \ tyvars ->
407 -- Type-check the instance type, and check its form
408 tcHsSigType GenPatCtxt hs_ty `thenTc` \ inst_ty ->
409 checkTc (validGenericInstanceType inst_ty)
410 (badGenericInstanceType binds) `thenTc_`
412 -- Make the dictionary function.
413 newDFunName clas [inst_ty] loc `thenNF_Tc` \ dfun_name ->
415 inst_theta = [mkClassPred clas [mkTyVarTy tv] | tv <- tyvars]
416 dfun_id = mkDictFunId dfun_name clas tyvars [inst_ty] inst_theta
419 returnTc (InstInfo { iDFunId = dfun_id, iBinds = binds, iPrags = [] })
423 %************************************************************************
425 \subsection{Type-checking instance declarations, pass 2}
427 %************************************************************************
430 tcInstDecls2 :: [InstInfo]
431 -> NF_TcM (LIE, TcMonoBinds)
433 tcInstDecls2 inst_decls
434 -- = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
435 = foldr combine (returnNF_Tc (emptyLIE, EmptyMonoBinds))
436 (map tcInstDecl2 inst_decls)
438 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
439 tc2 `thenNF_Tc` \ (lie2, binds2) ->
440 returnNF_Tc (lie1 `plusLIE` lie2,
441 binds1 `AndMonoBinds` binds2)
444 ======= New documentation starts here (Sept 92) ==============
446 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
447 the dictionary function for this instance declaration. For example
449 instance Foo a => Foo [a] where
453 might generate something like
455 dfun.Foo.List dFoo_a = let op1 x = ...
461 HOWEVER, if the instance decl has no context, then it returns a
462 bigger @HsBinds@ with declarations for each method. For example
464 instance Foo [a] where
470 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
471 const.Foo.op1.List a x = ...
472 const.Foo.op2.List a y = ...
474 This group may be mutually recursive, because (for example) there may
475 be no method supplied for op2 in which case we'll get
477 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
479 that is, the default method applied to the dictionary at this type.
481 What we actually produce in either case is:
483 AbsBinds [a] [dfun_theta_dicts]
484 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
485 { d = (sd1,sd2, ..., op1, op2, ...)
490 The "maybe" says that we only ask AbsBinds to make global constant methods
491 if the dfun_theta is empty.
494 For an instance declaration, say,
496 instance (C1 a, C2 b) => C (T a b) where
499 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
500 function whose type is
502 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
504 Notice that we pass it the superclass dictionaries at the instance type; this
505 is the ``Mark Jones optimisation''. The stuff before the "=>" here
506 is the @dfun_theta@ below.
508 First comes the easy case of a non-local instance decl.
512 tcInstDecl2 :: InstInfo -> TcM (LIE, TcMonoBinds)
514 tcInstDecl2 (NewTypeDerived { iDFunId = dfun_id })
515 = tcInstType InstTv (idType dfun_id) `thenNF_Tc` \ (inst_tyvars', dfun_theta', inst_head') ->
516 newDicts InstanceDeclOrigin dfun_theta' `thenNF_Tc` \ rep_dicts ->
518 rep_dict_id = ASSERT( isSingleton rep_dicts )
519 instToId (head rep_dicts) -- Derived newtypes have just one dict arg
521 body = TyLam inst_tyvars' $
522 DictLam [rep_dict_id] $
523 (HsVar unsafeCoerceId `TyApp` [idType rep_dict_id, inst_head'])
526 -- You might wonder why we have the 'coerce'. It's because the
527 -- type equality mechanism isn't clever enough; see comments with Type.eqType.
528 -- So Lint complains if we don't have this.
530 returnTc (emptyLIE, VarMonoBind dfun_id body)
532 tcInstDecl2 (InstInfo { iDFunId = dfun_id, iBinds = monobinds, iPrags = uprags })
533 = -- Prime error recovery
534 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
535 tcAddSrcLoc (getSrcLoc dfun_id) $
536 tcAddErrCtxt (instDeclCtxt (toHsType (idType dfun_id))) $
538 inst_ty = idType dfun_id
539 (inst_tyvars, _) = tcSplitForAllTys inst_ty
540 -- The tyvars of the instance decl scope over the 'where' part
541 -- Those tyvars are inside the dfun_id's type, which is a bit
542 -- bizarre, but OK so long as you realise it!
545 -- Instantiate the instance decl with tc-style type variables
546 tcInstType InstTv inst_ty `thenNF_Tc` \ (inst_tyvars', dfun_theta', inst_head') ->
548 Just pred = tcSplitPredTy_maybe inst_head'
549 (clas, inst_tys') = getClassPredTys pred
550 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
552 -- Instantiate the super-class context with inst_tys
553 sc_theta' = substTheta (mkTyVarSubst class_tyvars inst_tys') sc_theta
554 origin = InstanceDeclOrigin
556 -- Create dictionary Ids from the specified instance contexts.
557 newDicts origin sc_theta' `thenNF_Tc` \ sc_dicts ->
558 newDicts origin dfun_theta' `thenNF_Tc` \ dfun_arg_dicts ->
559 newDicts origin [pred] `thenNF_Tc` \ [this_dict] ->
560 -- Default-method Ids may be mentioned in synthesised RHSs,
561 -- but they'll already be in the environment.
563 -- Check that all the method bindings come from this class
564 mkMethodBinds clas inst_tys' op_items monobinds `thenTc` \ (meth_insts, meth_infos) ->
566 let -- These insts are in scope; quite a few, eh?
567 avail_insts = [this_dict] ++ dfun_arg_dicts ++
568 sc_dicts ++ meth_insts
570 xtve = inst_tyvars `zip` inst_tyvars'
571 tc_meth = tcMethodBind xtve inst_tyvars' dfun_theta' avail_insts
573 mapAndUnzipTc tc_meth meth_infos `thenTc` \ (meth_binds_s, meth_lie_s) ->
575 -- Figure out bindings for the superclass context
576 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
577 `thenTc` \ (zonked_inst_tyvars, sc_binds_inner, sc_binds_outer) ->
579 -- Deal with SPECIALISE instance pragmas by making them
580 -- look like SPECIALISE pragmas for the dfun
582 spec_prags = [ SpecSig (idName dfun_id) ty loc
583 | SpecInstSig ty loc <- uprags]
586 tcExtendGlobalValEnv [dfun_id] (
587 tcExtendTyVarEnv2 xtve $
588 tcExtendLocalValEnv2 [(idName sel_id, tcSigPolyId sig)
589 | (sel_id, sig, _) <- meth_infos] $
590 -- Map sel_id to the local method name we are using
591 tcSpecSigs spec_prags
592 ) `thenTc` \ (prag_binds, prag_lie) ->
594 -- Create the result bindings
596 local_dfun_id = setIdLocalExported dfun_id
597 -- Reason for setIdLocalExported: see notes with MkId.mkDictFunId
599 dict_constr = classDataCon clas
600 scs_and_meths = map instToId (sc_dicts ++ meth_insts)
601 this_dict_id = instToId this_dict
602 inlines | null dfun_arg_dicts = emptyNameSet
603 | otherwise = unitNameSet (idName dfun_id)
604 -- Always inline the dfun; this is an experimental decision
605 -- because it makes a big performance difference sometimes.
606 -- Often it means we can do the method selection, and then
607 -- inline the method as well. Marcin's idea; see comments below.
609 -- BUT: don't inline it if it's a constant dictionary;
610 -- we'll get all the benefit without inlining, and we get
611 -- a **lot** of code duplication if we inline it
615 = -- Blatant special case for CCallable, CReturnable
616 -- If the dictionary is empty then we should never
617 -- select anything from it, so we make its RHS just
618 -- emit an error message. This in turn means that we don't
619 -- mention the constructor, which doesn't exist for CCallable, CReturnable
620 -- Hardly beautiful, but only three extra lines.
621 HsApp (TyApp (HsVar rUNTIME_ERROR_ID) [idType this_dict_id])
622 (HsLit (HsStringPrim (_PK_ (stringToUtf8 msg))))
624 | otherwise -- The common case
625 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
626 -- We don't produce a binding for the dict_constr; instead we
627 -- rely on the simplifier to unfold this saturated application
628 -- We do this rather than generate an HsCon directly, because
629 -- it means that the special cases (e.g. dictionary with only one
630 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
631 -- than needing to be repeated here.
634 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
636 dict_bind = VarMonoBind this_dict_id dict_rhs
637 meth_binds = andMonoBindList meth_binds_s
638 all_binds = sc_binds_inner `AndMonoBinds` meth_binds `AndMonoBinds` dict_bind
642 (map instToId dfun_arg_dicts)
643 [(inst_tyvars', local_dfun_id, this_dict_id)]
646 returnTc (plusLIEs meth_lie_s `plusLIE` prag_lie,
647 main_bind `AndMonoBinds` prag_binds `AndMonoBinds` sc_binds_outer)
650 We have to be very, very careful when generating superclasses, lest we
651 accidentally build a loop. Here's an example:
655 class S a => C a where { opc :: a -> a }
656 class S b => D b where { opd :: b -> b }
664 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
665 Simplifying, we may well get:
666 $dfCInt = :C ds1 (opd dd)
669 Notice that we spot that we can extract ds1 from dd.
671 Alas! Alack! We can do the same for (instance D Int):
673 $dfDInt = :D ds2 (opc dc)
677 And now we've defined the superclass in terms of itself.
680 Solution: treat the superclass context separately, and simplify it
681 all the way down to nothing on its own. Don't toss any 'free' parts
682 out to be simplified together with other bits of context.
683 Hence the tcSimplifyTop below.
685 At a more basic level, don't include this_dict in the context wrt
686 which we simplify sc_dicts, else sc_dicts get bound by just selecting
690 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
691 = tcAddErrCtxt superClassCtxt $
692 tcSimplifyCheck doc inst_tyvars'
694 (mkLIE sc_dicts) `thenTc` \ (sc_lie, sc_binds1) ->
696 -- It's possible that the superclass stuff might have done unification
697 checkSigTyVars inst_tyvars' `thenTc` \ zonked_inst_tyvars ->
699 -- We must simplify this all the way down
700 -- lest we build superclass loops
701 tcSimplifyTop sc_lie `thenTc` \ sc_binds2 ->
703 returnTc (zonked_inst_tyvars, sc_binds1, sc_binds2)
706 doc = ptext SLIT("instance declaration superclass context")
710 mkMethodBinds clas inst_tys' op_items monobinds
711 = -- Check that all the method bindings come from this class
712 mapTc (addErrTc . badMethodErr clas) bad_bndrs `thenNF_Tc_`
714 -- Make the method bindings
715 mapAndUnzipTc mk_method_bind op_items
718 mk_method_bind op_item = mkMethodBind InstanceDeclOrigin clas
719 inst_tys' monobinds op_item
721 -- Find any definitions in monobinds that aren't from the class
722 sel_names = [idName sel_id | (sel_id, _) <- op_items]
723 bad_bndrs = collectMonoBinders monobinds `minusList` sel_names
727 ------------------------------
728 Inlining dfuns unconditionally
729 ------------------------------
731 The code above unconditionally inlines dict funs. Here's why.
732 Consider this program:
734 test :: Int -> Int -> Bool
735 test x y = (x,y) == (y,x) || test y x
736 -- Recursive to avoid making it inline.
738 This needs the (Eq (Int,Int)) instance. If we inline that dfun
739 the code we end up with is good:
742 \r -> case ==# [ww ww1] of wild {
743 PrelBase.False -> Test.$wtest ww1 ww;
745 case ==# [ww1 ww] of wild1 {
746 PrelBase.False -> Test.$wtest ww1 ww;
747 PrelBase.True -> PrelBase.True [];
750 Test.test = \r [w w1]
753 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
756 If we don't inline the dfun, the code is not nearly as good:
758 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
759 PrelBase.:DEq tpl1 tpl2 -> tpl2;
764 let { y = PrelBase.I#! [ww1]; } in
765 let { x = PrelBase.I#! [ww]; } in
766 let { sat_slx = PrelTup.(,)! [y x]; } in
767 let { sat_sly = PrelTup.(,)! [x y];
769 case == sat_sly sat_slx of wild {
770 PrelBase.False -> Test.$wtest ww1 ww;
771 PrelBase.True -> PrelBase.True [];
778 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
781 Why doesn't GHC inline $fEq? Because it looks big:
783 PrelTup.zdfEqZ1T{-rcX-}
784 = \ @ a{-reT-} :: * @ b{-reS-} :: *
785 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
786 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
788 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
789 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
791 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
792 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
794 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
795 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
796 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
798 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
800 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
802 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
803 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
807 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
808 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
809 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
810 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
812 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
814 and it's not as bad as it seems, because it's further dramatically
815 simplified: only zeze2 is extracted and its body is simplified.
818 %************************************************************************
820 \subsection{Error messages}
822 %************************************************************************
825 tcAddDeclCtxt decl thing_inside
826 = tcAddSrcLoc (tcdLoc decl) $
831 ClassDecl {} -> "class"
832 TySynonym {} -> "type synonym"
833 TyData {tcdND = NewType} -> "newtype"
834 TyData {tcdND = DataType} -> "data type"
836 ctxt = hsep [ptext SLIT("In the"), text thing,
837 ptext SLIT("declaration for"), quotes (ppr (tcdName decl))]
839 instDeclCtxt inst_ty = ptext SLIT("In the instance declaration for") <+> quotes doc
841 doc = case inst_ty of
842 HsForAllTy _ _ (HsPredTy pred) -> ppr pred
843 HsPredTy pred -> ppr pred
844 other -> ppr inst_ty -- Don't expect this
848 badGenericInstanceType binds
849 = vcat [ptext SLIT("Illegal type pattern in the generic bindings"),
852 missingGenericInstances missing
853 = ptext SLIT("Missing type patterns for") <+> pprQuotedList missing
855 dupGenericInsts tc_inst_infos
856 = vcat [ptext SLIT("More than one type pattern for a single generic type constructor:"),
857 nest 4 (vcat (map ppr_inst_ty tc_inst_infos)),
858 ptext SLIT("All the type patterns for a generic type constructor must be identical")
861 ppr_inst_ty (tc,inst) = ppr (simpleInstInfoTy inst)
863 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
864 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")