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, 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 ErrUtils ( dumpIfSet_dyn )
69 import ListSetOps ( Assoc, emptyAssoc, plusAssoc_C, mapAssoc,
70 assocElts, extendAssoc_C, equivClassesByUniq, minusList
72 import Maybe ( catMaybes )
76 Typechecking instance declarations is done in two passes. The first
77 pass, made by @tcInstDecls1@, collects information to be used in the
80 This pre-processed info includes the as-yet-unprocessed bindings
81 inside the instance declaration. These are type-checked in the second
82 pass, when the class-instance envs and GVE contain all the info from
83 all the instance and value decls. Indeed that's the reason we need
84 two passes over the instance decls.
87 Here is the overall algorithm.
88 Assume that we have an instance declaration
90 instance c => k (t tvs) where b
94 $LIE_c$ is the LIE for the context of class $c$
96 $betas_bar$ is the free variables in the class method type, excluding the
99 $LIE_cop$ is the LIE constraining a particular class method
101 $tau_cop$ is the tau type of a class method
103 $LIE_i$ is the LIE for the context of instance $i$
105 $X$ is the instance constructor tycon
107 $gammas_bar$ is the set of type variables of the instance
109 $LIE_iop$ is the LIE for a particular class method instance
111 $tau_iop$ is the tau type for this instance of a class method
113 $alpha$ is the class variable
115 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
117 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
120 ToDo: Update the list above with names actually in the code.
124 First, make the LIEs for the class and instance contexts, which means
125 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
126 and make LIElistI and LIEI.
128 Then process each method in turn.
130 order the instance methods according to the ordering of the class methods
132 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
134 Create final dictionary function from bindings generated already
136 df = lambda inst_tyvars
143 in <op1,op2,...,opn,sd1,...,sdm>
145 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
146 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
150 %************************************************************************
152 \subsection{Extracting instance decls}
154 %************************************************************************
156 Gather up the instance declarations from their various sources
159 tcInstDecls1 -- Deal with source-code instance decls
160 :: PersistentRenamerState
161 -> InstEnv -- Imported instance envt
162 -> FixityEnv -- for deriving Show and Read
163 -> Module -- Module for deriving
164 -> [RenamedTyClDecl] -- For deriving stuff
165 -> [RenamedInstDecl] -- Source code instance decls
166 -> TcM (InstEnv, -- the full inst env
167 [InstInfo], -- instance decls to process; contains all dfuns
169 RenamedHsBinds) -- derived instances
171 tcInstDecls1 prs inst_env get_fixity this_mod
172 tycl_decls inst_decls
173 -- The incoming inst_env includes all the imported instances already
175 -- Stop if addInstInfos etc discovers any errors
176 -- (they recover, so that we get more than one error each round)
177 -- (1) Do the ordinary instance declarations
178 mapNF_Tc tcLocalInstDecl1 inst_decls `thenNF_Tc` \ local_inst_infos ->
181 local_inst_info = catMaybes local_inst_infos
182 clas_decls = filter isClassDecl tycl_decls
184 -- (2) Instances from generic class declarations
185 getGenericInstances clas_decls `thenTc` \ generic_inst_info ->
187 -- Next, construct the instance environment so far, consisting of
188 -- a) imported instance decls (from this module) inst_env1
189 -- b) local instance decls inst_env2
190 -- c) generic instances final_inst_env
191 addInstInfos inst_env local_inst_info `thenNF_Tc` \ inst_env1 ->
192 addInstInfos inst_env1 generic_inst_info `thenNF_Tc` \ inst_env2 ->
194 -- (3) Compute instances from "deriving" clauses;
195 -- note that we only do derivings for things in this module;
196 -- we ignore deriving decls from interfaces!
197 -- This stuff computes a context for the derived instance decl, so it
198 -- needs to know about all the instances possible; hence inst_env4
199 tcDeriving prs this_mod inst_env2
200 get_fixity tycl_decls `thenTc` \ (deriv_inst_info, deriv_binds) ->
201 addInstInfos inst_env2 deriv_inst_info `thenNF_Tc` \ final_inst_env ->
203 returnTc (final_inst_env,
204 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
207 initInstEnv :: PersistentCompilerState -> HomeSymbolTable -> NF_TcM InstEnv
208 -- Initialise the instance environment from the
209 -- persistent compiler state and the home symbol table
212 pkg_inst_env = pcs_insts pcs
213 hst_dfuns = foldModuleEnv ((++) . md_insts) [] hst
215 addInstDFuns pkg_inst_env hst_dfuns
217 addInstInfos :: InstEnv -> [InstInfo] -> NF_TcM InstEnv
218 addInstInfos inst_env infos = addInstDFuns inst_env (map iDFunId infos)
220 addInstDFuns :: InstEnv -> [DFunId] -> NF_TcM InstEnv
221 addInstDFuns inst_env dfuns
222 = getDOptsTc `thenNF_Tc` \ dflags ->
224 (inst_env', errs) = extendInstEnv dflags inst_env dfuns
226 addErrsTc errs `thenNF_Tc_`
227 traceTc (text "Adding instances:" <+> vcat (map pp dfuns)) `thenTc_`
230 pp dfun = ppr dfun <+> dcolon <+> ppr (idType dfun)
234 tcIfaceInstDecls1 :: [RenamedInstDecl] -> NF_TcM [DFunId]
235 tcIfaceInstDecls1 decls = mapNF_Tc tcIfaceInstDecl1 decls
237 tcIfaceInstDecl1 :: RenamedInstDecl -> NF_TcM DFunId
238 -- An interface-file instance declaration
239 -- Should be in scope by now, because we should
240 -- have sucked in its interface-file definition
241 -- So it will be replete with its unfolding etc
242 tcIfaceInstDecl1 decl@(InstDecl poly_ty binds uprags (Just dfun_name) src_loc)
243 = tcLookupId dfun_name
246 tcLocalInstDecl1 :: RenamedInstDecl
247 -> NF_TcM (Maybe InstInfo) -- Nothing if there was an error
248 -- A source-file instance declaration
249 -- Type-check all the stuff before the "where"
251 -- We check for respectable instance type, and context
252 -- but only do this for non-imported instance decls.
253 -- Imported ones should have been checked already, and may indeed
254 -- contain something illegal in normal Haskell, notably
255 -- instance CCallable [Char]
256 tcLocalInstDecl1 decl@(InstDecl poly_ty binds uprags Nothing src_loc)
257 = -- Prime error recovery, set source location
258 recoverNF_Tc (returnNF_Tc Nothing) $
259 tcAddSrcLoc src_loc $
260 tcAddErrCtxt (instDeclCtxt poly_ty) $
262 -- Typecheck the instance type itself. We can't use
263 -- tcHsSigType, because it's not a valid user type.
264 kcHsSigType poly_ty `thenTc_`
265 tcHsType poly_ty `thenTc` \ poly_ty' ->
267 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
269 checkValidTheta InstThetaCtxt theta `thenTc_`
270 checkValidInstHead tau `thenTc` \ (clas,inst_tys) ->
271 checkTc (checkInstFDs theta clas inst_tys)
272 (instTypeErr (pprClassPred clas inst_tys) msg) `thenTc_`
273 newDFunName clas inst_tys src_loc `thenNF_Tc` \ dfun_name ->
274 returnTc (Just (InstInfo { iDFunId = mkDictFunId dfun_name clas tyvars inst_tys theta,
275 iBinds = binds, iPrags = uprags }))
277 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
281 %************************************************************************
283 \subsection{Extracting generic instance declaration from class declarations}
285 %************************************************************************
287 @getGenericInstances@ extracts the generic instance declarations from a class
288 declaration. For exmaple
293 op{ x+y } (Inl v) = ...
294 op{ x+y } (Inr v) = ...
295 op{ x*y } (v :*: w) = ...
298 gives rise to the instance declarations
300 instance C (x+y) where
304 instance C (x*y) where
312 getGenericInstances :: [RenamedTyClDecl] -> TcM [InstInfo]
313 getGenericInstances class_decls
314 = mapTc get_generics class_decls `thenTc` \ gen_inst_infos ->
316 gen_inst_info = concat gen_inst_infos
318 if null gen_inst_info then
321 getDOptsTc `thenNF_Tc` \ dflags ->
322 ioToTc (dumpIfSet_dyn dflags Opt_D_dump_deriv "Generic instances"
323 (vcat (map pprInstInfo gen_inst_info)))
325 returnTc gen_inst_info
327 get_generics decl@(ClassDecl {tcdMeths = Nothing})
328 = returnTc [] -- Imported class decls
330 get_generics decl@(ClassDecl {tcdName = class_name, tcdMeths = Just def_methods, tcdLoc = loc})
332 = returnTc [] -- The comon case: no generic default methods
334 | otherwise -- A source class decl with generic default methods
335 = recoverNF_Tc (returnNF_Tc []) $
337 tcLookupClass class_name `thenTc` \ clas ->
339 -- Make an InstInfo out of each group
340 mapTc (mkGenericInstance clas loc) groups `thenTc` \ inst_infos ->
342 -- Check that there is only one InstInfo for each type constructor
343 -- The main way this can fail is if you write
344 -- f {| a+b |} ... = ...
345 -- f {| x+y |} ... = ...
346 -- Then at this point we'll have an InstInfo for each
348 tc_inst_infos :: [(TyCon, InstInfo)]
349 tc_inst_infos = [(simpleInstInfoTyCon i, i) | i <- inst_infos]
351 bad_groups = [group | group <- equivClassesByUniq get_uniq tc_inst_infos,
352 group `lengthExceeds` 1]
353 get_uniq (tc,_) = getUnique tc
355 mapTc (addErrTc . dupGenericInsts) bad_groups `thenTc_`
357 -- Check that there is an InstInfo for each generic type constructor
359 missing = genericTyCons `minusList` [tc | (tc,_) <- tc_inst_infos]
361 checkTc (null missing) (missingGenericInstances missing) `thenTc_`
366 -- Group the declarations by type pattern
367 groups :: [(RenamedHsType, RenamedMonoBinds)]
368 groups = assocElts (getGenericBinds def_methods)
371 ---------------------------------
372 getGenericBinds :: RenamedMonoBinds -> Assoc RenamedHsType RenamedMonoBinds
373 -- Takes a group of method bindings, finds the generic ones, and returns
374 -- them in finite map indexed by the type parameter in the definition.
376 getGenericBinds EmptyMonoBinds = emptyAssoc
377 getGenericBinds (AndMonoBinds m1 m2)
378 = plusAssoc_C AndMonoBinds (getGenericBinds m1) (getGenericBinds m2)
380 getGenericBinds (FunMonoBind id infixop matches loc)
381 = mapAssoc wrap (foldl add emptyAssoc matches)
382 -- Using foldl not foldr is vital, else
383 -- we reverse the order of the bindings!
385 add env match = case maybeGenericMatch match of
387 Just (ty, match') -> extendAssoc_C (++) env (ty, [match'])
389 wrap ms = FunMonoBind id infixop ms loc
391 ---------------------------------
392 mkGenericInstance :: Class -> SrcLoc
393 -> (RenamedHsType, RenamedMonoBinds)
396 mkGenericInstance clas loc (hs_ty, binds)
397 -- Make a generic instance declaration
398 -- For example: instance (C a, C b) => C (a+b) where { binds }
400 = -- Extract the universally quantified type variables
402 sig_tvs = map UserTyVar (nameSetToList (extractHsTyVars hs_ty))
404 tcHsTyVars sig_tvs (kcHsSigType hs_ty) $ \ tyvars ->
406 -- Type-check the instance type, and check its form
407 tcHsSigType GenPatCtxt hs_ty `thenTc` \ inst_ty ->
408 checkTc (validGenericInstanceType inst_ty)
409 (badGenericInstanceType binds) `thenTc_`
411 -- Make the dictionary function.
412 newDFunName clas [inst_ty] loc `thenNF_Tc` \ dfun_name ->
414 inst_theta = [mkClassPred clas [mkTyVarTy tv] | tv <- tyvars]
415 dfun_id = mkDictFunId dfun_name clas tyvars [inst_ty] inst_theta
418 returnTc (InstInfo { iDFunId = dfun_id, iBinds = binds, iPrags = [] })
422 %************************************************************************
424 \subsection{Type-checking instance declarations, pass 2}
426 %************************************************************************
429 tcInstDecls2 :: [InstInfo]
430 -> NF_TcM (LIE, TcMonoBinds)
432 tcInstDecls2 inst_decls
433 -- = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
434 = foldr combine (returnNF_Tc (emptyLIE, EmptyMonoBinds))
435 (map tcInstDecl2 inst_decls)
437 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
438 tc2 `thenNF_Tc` \ (lie2, binds2) ->
439 returnNF_Tc (lie1 `plusLIE` lie2,
440 binds1 `AndMonoBinds` binds2)
443 ======= New documentation starts here (Sept 92) ==============
445 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
446 the dictionary function for this instance declaration. For example
448 instance Foo a => Foo [a] where
452 might generate something like
454 dfun.Foo.List dFoo_a = let op1 x = ...
460 HOWEVER, if the instance decl has no context, then it returns a
461 bigger @HsBinds@ with declarations for each method. For example
463 instance Foo [a] where
469 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
470 const.Foo.op1.List a x = ...
471 const.Foo.op2.List a y = ...
473 This group may be mutually recursive, because (for example) there may
474 be no method supplied for op2 in which case we'll get
476 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
478 that is, the default method applied to the dictionary at this type.
480 What we actually produce in either case is:
482 AbsBinds [a] [dfun_theta_dicts]
483 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
484 { d = (sd1,sd2, ..., op1, op2, ...)
489 The "maybe" says that we only ask AbsBinds to make global constant methods
490 if the dfun_theta is empty.
493 For an instance declaration, say,
495 instance (C1 a, C2 b) => C (T a b) where
498 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
499 function whose type is
501 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
503 Notice that we pass it the superclass dictionaries at the instance type; this
504 is the ``Mark Jones optimisation''. The stuff before the "=>" here
505 is the @dfun_theta@ below.
507 First comes the easy case of a non-local instance decl.
511 tcInstDecl2 :: InstInfo -> TcM (LIE, TcMonoBinds)
513 tcInstDecl2 (NewTypeDerived { iDFunId = dfun_id })
514 = tcInstType InstTv (idType dfun_id) `thenNF_Tc` \ (inst_tyvars', dfun_theta', inst_head') ->
515 newDicts InstanceDeclOrigin dfun_theta' `thenNF_Tc` \ rep_dicts ->
517 rep_dict_id = ASSERT( isSingleton rep_dicts )
518 instToId (head rep_dicts) -- Derived newtypes have just one dict arg
520 body = TyLam inst_tyvars' $
521 DictLam [rep_dict_id] $
522 (HsVar unsafeCoerceId `TyApp` [idType rep_dict_id, inst_head'])
525 -- You might wonder why we have the 'coerce'. It's because the
526 -- type equality mechanism isn't clever enough; see comments with Type.eqType.
527 -- So Lint complains if we don't have this.
529 returnTc (emptyLIE, VarMonoBind dfun_id body)
531 tcInstDecl2 (InstInfo { iDFunId = dfun_id, iBinds = monobinds, iPrags = uprags })
532 = -- Prime error recovery
533 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
534 tcAddSrcLoc (getSrcLoc dfun_id) $
535 tcAddErrCtxt (instDeclCtxt (toHsType (idType dfun_id))) $
537 inst_ty = idType dfun_id
538 (inst_tyvars, _) = tcSplitForAllTys inst_ty
539 -- The tyvars of the instance decl scope over the 'where' part
540 -- Those tyvars are inside the dfun_id's type, which is a bit
541 -- bizarre, but OK so long as you realise it!
544 -- Instantiate the instance decl with tc-style type variables
545 tcInstType InstTv inst_ty `thenNF_Tc` \ (inst_tyvars', dfun_theta', inst_head') ->
547 Just pred = tcSplitPredTy_maybe inst_head'
548 (clas, inst_tys') = getClassPredTys pred
549 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
551 -- Instantiate the super-class context with inst_tys
552 sc_theta' = substTheta (mkTyVarSubst class_tyvars inst_tys') sc_theta
553 origin = InstanceDeclOrigin
555 -- Create dictionary Ids from the specified instance contexts.
556 newDicts origin sc_theta' `thenNF_Tc` \ sc_dicts ->
557 newDicts origin dfun_theta' `thenNF_Tc` \ dfun_arg_dicts ->
558 newDicts origin [pred] `thenNF_Tc` \ [this_dict] ->
559 -- Default-method Ids may be mentioned in synthesised RHSs,
560 -- but they'll already be in the environment.
562 -- Check that all the method bindings come from this class
563 mkMethodBinds clas inst_tys' op_items monobinds `thenTc` \ (meth_insts, meth_infos) ->
565 let -- These insts are in scope; quite a few, eh?
566 avail_insts = [this_dict] ++ dfun_arg_dicts ++
567 sc_dicts ++ meth_insts
569 xtve = inst_tyvars `zip` inst_tyvars'
570 tc_meth = tcMethodBind xtve inst_tyvars' dfun_theta' avail_insts
572 mapAndUnzipTc tc_meth meth_infos `thenTc` \ (meth_binds_s, meth_lie_s) ->
574 -- Figure out bindings for the superclass context
575 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
576 `thenTc` \ (zonked_inst_tyvars, sc_binds_inner, sc_binds_outer) ->
578 -- Deal with SPECIALISE instance pragmas by making them
579 -- look like SPECIALISE pragmas for the dfun
581 spec_prags = [ SpecSig (idName dfun_id) ty loc
582 | SpecInstSig ty loc <- uprags]
585 tcExtendGlobalValEnv [dfun_id] (
586 tcExtendTyVarEnv2 xtve $
587 tcExtendLocalValEnv2 [(idName sel_id, tcSigPolyId sig)
588 | (sel_id, sig, _) <- meth_infos] $
589 -- Map sel_id to the local method name we are using
590 tcSpecSigs spec_prags
591 ) `thenTc` \ (prag_binds, prag_lie) ->
593 -- Create the result bindings
595 local_dfun_id = setIdLocalExported dfun_id
596 -- Reason for setIdLocalExported: see notes with MkId.mkDictFunId
598 dict_constr = classDataCon clas
599 scs_and_meths = map instToId (sc_dicts ++ meth_insts)
600 this_dict_id = instToId this_dict
601 inlines | null dfun_arg_dicts = emptyNameSet
602 | otherwise = unitNameSet (idName dfun_id)
603 -- Always inline the dfun; this is an experimental decision
604 -- because it makes a big performance difference sometimes.
605 -- Often it means we can do the method selection, and then
606 -- inline the method as well. Marcin's idea; see comments below.
608 -- BUT: don't inline it if it's a constant dictionary;
609 -- we'll get all the benefit without inlining, and we get
610 -- a **lot** of code duplication if we inline it
614 = -- Blatant special case for CCallable, CReturnable
615 -- If the dictionary is empty then we should never
616 -- select anything from it, so we make its RHS just
617 -- emit an error message. This in turn means that we don't
618 -- mention the constructor, which doesn't exist for CCallable, CReturnable
619 -- Hardly beautiful, but only three extra lines.
620 HsApp (TyApp (HsVar eRROR_ID) [idType this_dict_id])
621 (HsLit (HsString msg))
623 | otherwise -- The common case
624 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
625 -- We don't produce a binding for the dict_constr; instead we
626 -- rely on the simplifier to unfold this saturated application
627 -- We do this rather than generate an HsCon directly, because
628 -- it means that the special cases (e.g. dictionary with only one
629 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
630 -- than needing to be repeated here.
633 msg = _PK_ ("Compiler error: bad dictionary " ++ showSDoc (ppr clas))
635 dict_bind = VarMonoBind this_dict_id dict_rhs
636 meth_binds = andMonoBindList meth_binds_s
637 all_binds = sc_binds_inner `AndMonoBinds` meth_binds `AndMonoBinds` dict_bind
641 (map instToId dfun_arg_dicts)
642 [(inst_tyvars', local_dfun_id, this_dict_id)]
645 returnTc (plusLIEs meth_lie_s `plusLIE` prag_lie,
646 main_bind `AndMonoBinds` prag_binds `AndMonoBinds` sc_binds_outer)
649 We have to be very, very careful when generating superclasses, lest we
650 accidentally build a loop. Here's an example:
654 class S a => C a where { opc :: a -> a }
655 class S b => D b where { opd :: b -> b }
663 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
664 Simplifying, we may well get:
665 $dfCInt = :C ds1 (opd dd)
668 Notice that we spot that we can extract ds1 from dd.
670 Alas! Alack! We can do the same for (instance D Int):
672 $dfDInt = :D ds2 (opc dc)
676 And now we've defined the superclass in terms of itself.
679 Solution: treat the superclass context separately, and simplify it
680 all the way down to nothing on its own. Don't toss any 'free' parts
681 out to be simplified together with other bits of context.
682 Hence the tcSimplifyTop below.
684 At a more basic level, don't include this_dict in the context wrt
685 which we simplify sc_dicts, else sc_dicts get bound by just selecting
689 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
690 = tcAddErrCtxt superClassCtxt $
691 tcSimplifyCheck doc inst_tyvars'
693 (mkLIE sc_dicts) `thenTc` \ (sc_lie, sc_binds1) ->
695 -- It's possible that the superclass stuff might have done unification
696 checkSigTyVars inst_tyvars' `thenTc` \ zonked_inst_tyvars ->
698 -- We must simplify this all the way down
699 -- lest we build superclass loops
700 tcSimplifyTop sc_lie `thenTc` \ sc_binds2 ->
702 returnTc (zonked_inst_tyvars, sc_binds1, sc_binds2)
705 doc = ptext SLIT("instance declaration superclass context")
709 mkMethodBinds clas inst_tys' op_items monobinds
710 = -- Check that all the method bindings come from this class
711 mapTc (addErrTc . badMethodErr clas) bad_bndrs `thenNF_Tc_`
713 -- Make the method bindings
714 mapAndUnzipTc mk_method_bind op_items
717 mk_method_bind op_item = mkMethodBind InstanceDeclOrigin clas
718 inst_tys' monobinds op_item
720 -- Find any definitions in monobinds that aren't from the class
721 sel_names = [idName sel_id | (sel_id, _) <- op_items]
722 bad_bndrs = collectMonoBinders monobinds `minusList` sel_names
726 ------------------------------
727 Inlining dfuns unconditionally
728 ------------------------------
730 The code above unconditionally inlines dict funs. Here's why.
731 Consider this program:
733 test :: Int -> Int -> Bool
734 test x y = (x,y) == (y,x) || test y x
735 -- Recursive to avoid making it inline.
737 This needs the (Eq (Int,Int)) instance. If we inline that dfun
738 the code we end up with is good:
741 \r -> case ==# [ww ww1] of wild {
742 PrelBase.False -> Test.$wtest ww1 ww;
744 case ==# [ww1 ww] of wild1 {
745 PrelBase.False -> Test.$wtest ww1 ww;
746 PrelBase.True -> PrelBase.True [];
749 Test.test = \r [w w1]
752 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
755 If we don't inline the dfun, the code is not nearly as good:
757 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
758 PrelBase.:DEq tpl1 tpl2 -> tpl2;
763 let { y = PrelBase.I#! [ww1]; } in
764 let { x = PrelBase.I#! [ww]; } in
765 let { sat_slx = PrelTup.(,)! [y x]; } in
766 let { sat_sly = PrelTup.(,)! [x y];
768 case == sat_sly sat_slx of wild {
769 PrelBase.False -> Test.$wtest ww1 ww;
770 PrelBase.True -> PrelBase.True [];
777 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
780 Why doesn't GHC inline $fEq? Because it looks big:
782 PrelTup.zdfEqZ1T{-rcX-}
783 = \ @ a{-reT-} :: * @ b{-reS-} :: *
784 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
785 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
787 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
788 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
790 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
791 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
793 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
794 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
795 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
797 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
799 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
801 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
802 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
806 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
807 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
808 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
809 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
811 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
813 and it's not as bad as it seems, because it's further dramatically
814 simplified: only zeze2 is extracted and its body is simplified.
817 %************************************************************************
819 \subsection{Error messages}
821 %************************************************************************
824 tcAddDeclCtxt decl thing_inside
825 = tcAddSrcLoc (tcdLoc decl) $
830 ClassDecl {} -> "class"
831 TySynonym {} -> "type synonym"
832 TyData {tcdND = NewType} -> "newtype"
833 TyData {tcdND = DataType} -> "data type"
835 ctxt = hsep [ptext SLIT("In the"), text thing,
836 ptext SLIT("declaration for"), quotes (ppr (tcdName decl))]
838 instDeclCtxt inst_ty = ptext SLIT("In the instance declaration for") <+> quotes doc
840 doc = case inst_ty of
841 HsForAllTy _ _ (HsPredTy pred) -> ppr pred
842 HsPredTy pred -> ppr pred
843 other -> ppr inst_ty -- Don't expect this
847 badGenericInstanceType binds
848 = vcat [ptext SLIT("Illegal type pattern in the generic bindings"),
851 missingGenericInstances missing
852 = ptext SLIT("Missing type patterns for") <+> pprQuotedList missing
854 dupGenericInsts tc_inst_infos
855 = vcat [ptext SLIT("More than one type pattern for a single generic type constructor:"),
856 nest 4 (vcat (map ppr_inst_ty tc_inst_infos)),
857 ptext SLIT("All the type patterns for a generic type constructor must be identical")
860 ppr_inst_ty (tc,inst) = ppr (simpleInstInfoTy inst)
862 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
863 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")