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 )
78 Typechecking instance declarations is done in two passes. The first
79 pass, made by @tcInstDecls1@, collects information to be used in the
82 This pre-processed info includes the as-yet-unprocessed bindings
83 inside the instance declaration. These are type-checked in the second
84 pass, when the class-instance envs and GVE contain all the info from
85 all the instance and value decls. Indeed that's the reason we need
86 two passes over the instance decls.
89 Here is the overall algorithm.
90 Assume that we have an instance declaration
92 instance c => k (t tvs) where b
96 $LIE_c$ is the LIE for the context of class $c$
98 $betas_bar$ is the free variables in the class method type, excluding the
101 $LIE_cop$ is the LIE constraining a particular class method
103 $tau_cop$ is the tau type of a class method
105 $LIE_i$ is the LIE for the context of instance $i$
107 $X$ is the instance constructor tycon
109 $gammas_bar$ is the set of type variables of the instance
111 $LIE_iop$ is the LIE for a particular class method instance
113 $tau_iop$ is the tau type for this instance of a class method
115 $alpha$ is the class variable
117 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
119 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
122 ToDo: Update the list above with names actually in the code.
126 First, make the LIEs for the class and instance contexts, which means
127 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
128 and make LIElistI and LIEI.
130 Then process each method in turn.
132 order the instance methods according to the ordering of the class methods
134 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
136 Create final dictionary function from bindings generated already
138 df = lambda inst_tyvars
145 in <op1,op2,...,opn,sd1,...,sdm>
147 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
148 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
152 %************************************************************************
154 \subsection{Extracting instance decls}
156 %************************************************************************
158 Gather up the instance declarations from their various sources
161 tcInstDecls1 -- Deal with source-code instance decls
162 :: PersistentRenamerState
163 -> InstEnv -- Imported instance envt
164 -> FixityEnv -- for deriving Show and Read
165 -> Module -- Module for deriving
166 -> [RenamedTyClDecl] -- For deriving stuff
167 -> [RenamedInstDecl] -- Source code instance decls
168 -> TcM (InstEnv, -- the full inst env
169 [InstInfo], -- instance decls to process; contains all dfuns
171 RenamedHsBinds) -- derived instances
173 tcInstDecls1 prs inst_env get_fixity this_mod
174 tycl_decls inst_decls
175 -- The incoming inst_env includes all the imported instances already
177 -- Stop if addInstInfos etc discovers any errors
178 -- (they recover, so that we get more than one error each round)
179 -- (1) Do the ordinary instance declarations
180 mapNF_Tc tcLocalInstDecl1 inst_decls `thenNF_Tc` \ 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 `thenTc` \ generic_inst_info ->
189 -- Next, construct the instance environment so far, consisting of
190 -- a) imported instance decls (from this module) inst_env1
191 -- b) local instance decls inst_env2
192 -- c) generic instances final_inst_env
193 addInstInfos inst_env local_inst_info `thenNF_Tc` \ inst_env1 ->
194 addInstInfos inst_env1 generic_inst_info `thenNF_Tc` \ inst_env2 ->
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 prs this_mod inst_env2
202 get_fixity tycl_decls `thenTc` \ (deriv_inst_info, deriv_binds) ->
203 addInstInfos inst_env2 deriv_inst_info `thenNF_Tc` \ final_inst_env ->
205 returnTc (final_inst_env,
206 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
209 initInstEnv :: PersistentCompilerState -> HomeSymbolTable -> NF_TcM InstEnv
210 -- Initialise the instance environment from the
211 -- persistent compiler state and the home symbol table
214 pkg_inst_env = pcs_insts pcs
215 hst_dfuns = foldModuleEnv ((++) . md_insts) [] hst
217 addInstDFuns pkg_inst_env hst_dfuns
219 addInstInfos :: InstEnv -> [InstInfo] -> NF_TcM InstEnv
220 addInstInfos inst_env infos = addInstDFuns inst_env (map iDFunId infos)
222 addInstDFuns :: InstEnv -> [DFunId] -> NF_TcM InstEnv
223 addInstDFuns inst_env dfuns
224 = getDOptsTc `thenNF_Tc` \ dflags ->
226 (inst_env', errs) = extendInstEnv dflags inst_env dfuns
228 addErrsTc errs `thenNF_Tc_`
229 traceTc (text "Adding instances:" <+> vcat (map pp dfuns)) `thenTc_`
232 pp dfun = ppr dfun <+> dcolon <+> ppr (idType dfun)
236 tcIfaceInstDecls1 :: [RenamedInstDecl] -> NF_TcM [DFunId]
237 tcIfaceInstDecls1 decls = mapNF_Tc tcIfaceInstDecl1 decls
239 tcIfaceInstDecl1 :: RenamedInstDecl -> NF_TcM DFunId
240 -- An interface-file instance declaration
241 -- Should be in scope by now, because we should
242 -- have sucked in its interface-file definition
243 -- So it will be replete with its unfolding etc
244 tcIfaceInstDecl1 decl@(InstDecl poly_ty binds uprags (Just dfun_name) src_loc)
245 = tcLookupId dfun_name
248 tcLocalInstDecl1 :: RenamedInstDecl
249 -> NF_TcM (Maybe InstInfo) -- Nothing if there was an error
250 -- A source-file instance declaration
251 -- Type-check all the stuff before the "where"
253 -- We check for respectable instance type, and context
254 -- but only do this for non-imported instance decls.
255 -- Imported ones should have been checked already, and may indeed
256 -- contain something illegal in normal Haskell, notably
257 -- instance CCallable [Char]
258 tcLocalInstDecl1 decl@(InstDecl poly_ty binds uprags Nothing src_loc)
259 = -- Prime error recovery, set source location
260 recoverNF_Tc (returnNF_Tc Nothing) $
261 tcAddSrcLoc src_loc $
262 tcAddErrCtxt (instDeclCtxt poly_ty) $
264 -- Typecheck the instance type itself. We can't use
265 -- tcHsSigType, because it's not a valid user type.
266 kcHsSigType poly_ty `thenTc_`
267 tcHsType poly_ty `thenTc` \ poly_ty' ->
269 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
271 checkValidTheta InstThetaCtxt theta `thenTc_`
272 checkValidInstHead tau `thenTc` \ (clas,inst_tys) ->
273 checkTc (checkInstFDs theta clas inst_tys)
274 (instTypeErr (pprClassPred clas inst_tys) msg) `thenTc_`
275 newDFunName clas inst_tys src_loc `thenNF_Tc` \ dfun_name ->
276 returnTc (Just (InstInfo { iDFunId = mkDictFunId dfun_name clas tyvars inst_tys theta,
277 iBinds = binds, iPrags = uprags }))
279 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
283 %************************************************************************
285 \subsection{Extracting generic instance declaration from class declarations}
287 %************************************************************************
289 @getGenericInstances@ extracts the generic instance declarations from a class
290 declaration. For exmaple
295 op{ x+y } (Inl v) = ...
296 op{ x+y } (Inr v) = ...
297 op{ x*y } (v :*: w) = ...
300 gives rise to the instance declarations
302 instance C (x+y) where
306 instance C (x*y) where
314 getGenericInstances :: [RenamedTyClDecl] -> TcM [InstInfo]
315 getGenericInstances class_decls
316 = mapTc get_generics class_decls `thenTc` \ gen_inst_infos ->
318 gen_inst_info = concat gen_inst_infos
320 if null gen_inst_info then
323 getDOptsTc `thenNF_Tc` \ dflags ->
324 ioToTc (dumpIfSet_dyn dflags Opt_D_dump_deriv "Generic instances"
325 (vcat (map pprInstInfo gen_inst_info)))
327 returnTc gen_inst_info
329 get_generics decl@(ClassDecl {tcdMeths = Nothing})
330 = returnTc [] -- Imported class decls
332 get_generics decl@(ClassDecl {tcdName = class_name, tcdMeths = Just def_methods, tcdLoc = loc})
334 = returnTc [] -- The comon case: no generic default methods
336 | otherwise -- A source class decl with generic default methods
337 = recoverNF_Tc (returnNF_Tc []) $
339 tcLookupClass class_name `thenTc` \ clas ->
341 -- Make an InstInfo out of each group
342 mapTc (mkGenericInstance clas loc) groups `thenTc` \ inst_infos ->
344 -- Check that there is only one InstInfo for each type constructor
345 -- The main way this can fail is if you write
346 -- f {| a+b |} ... = ...
347 -- f {| x+y |} ... = ...
348 -- Then at this point we'll have an InstInfo for each
350 tc_inst_infos :: [(TyCon, InstInfo)]
351 tc_inst_infos = [(simpleInstInfoTyCon i, i) | i <- inst_infos]
353 bad_groups = [group | group <- equivClassesByUniq get_uniq tc_inst_infos,
354 group `lengthExceeds` 1]
355 get_uniq (tc,_) = getUnique tc
357 mapTc (addErrTc . dupGenericInsts) bad_groups `thenTc_`
359 -- Check that there is an InstInfo for each generic type constructor
361 missing = genericTyCons `minusList` [tc | (tc,_) <- tc_inst_infos]
363 checkTc (null missing) (missingGenericInstances missing) `thenTc_`
368 -- Group the declarations by type pattern
369 groups :: [(RenamedHsType, RenamedMonoBinds)]
370 groups = assocElts (getGenericBinds def_methods)
373 ---------------------------------
374 getGenericBinds :: RenamedMonoBinds -> Assoc RenamedHsType RenamedMonoBinds
375 -- Takes a group of method bindings, finds the generic ones, and returns
376 -- them in finite map indexed by the type parameter in the definition.
378 getGenericBinds EmptyMonoBinds = emptyAssoc
379 getGenericBinds (AndMonoBinds m1 m2)
380 = plusAssoc_C AndMonoBinds (getGenericBinds m1) (getGenericBinds m2)
382 getGenericBinds (FunMonoBind id infixop matches loc)
383 = mapAssoc wrap (foldl add emptyAssoc matches)
384 -- Using foldl not foldr is vital, else
385 -- we reverse the order of the bindings!
387 add env match = case maybeGenericMatch match of
389 Just (ty, match') -> extendAssoc_C (++) env (ty, [match'])
391 wrap ms = FunMonoBind id infixop ms loc
393 ---------------------------------
394 mkGenericInstance :: Class -> SrcLoc
395 -> (RenamedHsType, RenamedMonoBinds)
398 mkGenericInstance clas loc (hs_ty, binds)
399 -- Make a generic instance declaration
400 -- For example: instance (C a, C b) => C (a+b) where { binds }
402 = -- Extract the universally quantified type variables
404 sig_tvs = map UserTyVar (nameSetToList (extractHsTyVars hs_ty))
406 tcHsTyVars sig_tvs (kcHsSigType hs_ty) $ \ tyvars ->
408 -- Type-check the instance type, and check its form
409 tcHsSigType GenPatCtxt hs_ty `thenTc` \ inst_ty ->
410 checkTc (validGenericInstanceType inst_ty)
411 (badGenericInstanceType binds) `thenTc_`
413 -- Make the dictionary function.
414 newDFunName clas [inst_ty] loc `thenNF_Tc` \ dfun_name ->
416 inst_theta = [mkClassPred clas [mkTyVarTy tv] | tv <- tyvars]
417 dfun_id = mkDictFunId dfun_name clas tyvars [inst_ty] inst_theta
420 returnTc (InstInfo { iDFunId = dfun_id, iBinds = binds, iPrags = [] })
424 %************************************************************************
426 \subsection{Type-checking instance declarations, pass 2}
428 %************************************************************************
431 tcInstDecls2 :: [InstInfo]
432 -> NF_TcM (LIE, TcMonoBinds)
434 tcInstDecls2 inst_decls
435 -- = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
436 = foldr combine (returnNF_Tc (emptyLIE, EmptyMonoBinds))
437 (map tcInstDecl2 inst_decls)
439 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
440 tc2 `thenNF_Tc` \ (lie2, binds2) ->
441 returnNF_Tc (lie1 `plusLIE` lie2,
442 binds1 `AndMonoBinds` binds2)
445 ======= New documentation starts here (Sept 92) ==============
447 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
448 the dictionary function for this instance declaration. For example
450 instance Foo a => Foo [a] where
454 might generate something like
456 dfun.Foo.List dFoo_a = let op1 x = ...
462 HOWEVER, if the instance decl has no context, then it returns a
463 bigger @HsBinds@ with declarations for each method. For example
465 instance Foo [a] where
471 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
472 const.Foo.op1.List a x = ...
473 const.Foo.op2.List a y = ...
475 This group may be mutually recursive, because (for example) there may
476 be no method supplied for op2 in which case we'll get
478 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
480 that is, the default method applied to the dictionary at this type.
482 What we actually produce in either case is:
484 AbsBinds [a] [dfun_theta_dicts]
485 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
486 { d = (sd1,sd2, ..., op1, op2, ...)
491 The "maybe" says that we only ask AbsBinds to make global constant methods
492 if the dfun_theta is empty.
495 For an instance declaration, say,
497 instance (C1 a, C2 b) => C (T a b) where
500 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
501 function whose type is
503 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
505 Notice that we pass it the superclass dictionaries at the instance type; this
506 is the ``Mark Jones optimisation''. The stuff before the "=>" here
507 is the @dfun_theta@ below.
509 First comes the easy case of a non-local instance decl.
513 tcInstDecl2 :: InstInfo -> TcM (LIE, TcMonoBinds)
515 tcInstDecl2 (NewTypeDerived { iDFunId = dfun_id })
516 = tcInstType InstTv (idType dfun_id) `thenNF_Tc` \ (inst_tyvars', dfun_theta', inst_head') ->
517 newDicts InstanceDeclOrigin dfun_theta' `thenNF_Tc` \ rep_dicts ->
519 rep_dict_id = ASSERT( isSingleton rep_dicts )
520 instToId (head rep_dicts) -- Derived newtypes have just one dict arg
522 body = TyLam inst_tyvars' $
523 DictLam [rep_dict_id] $
524 (HsVar unsafeCoerceId `TyApp` [idType rep_dict_id, inst_head'])
527 -- You might wonder why we have the 'coerce'. It's because the
528 -- type equality mechanism isn't clever enough; see comments with Type.eqType.
529 -- So Lint complains if we don't have this.
531 returnTc (emptyLIE, VarMonoBind dfun_id body)
533 tcInstDecl2 (InstInfo { iDFunId = dfun_id, iBinds = monobinds, iPrags = uprags })
534 = -- Prime error recovery
535 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
536 tcAddSrcLoc (getSrcLoc dfun_id) $
537 tcAddErrCtxt (instDeclCtxt (toHsType (idType dfun_id))) $
539 inst_ty = idType dfun_id
540 (inst_tyvars, _) = tcSplitForAllTys inst_ty
541 -- The tyvars of the instance decl scope over the 'where' part
542 -- Those tyvars are inside the dfun_id's type, which is a bit
543 -- bizarre, but OK so long as you realise it!
546 -- Instantiate the instance decl with tc-style type variables
547 tcInstType InstTv inst_ty `thenNF_Tc` \ (inst_tyvars', dfun_theta', inst_head') ->
549 Just pred = tcSplitPredTy_maybe inst_head'
550 (clas, inst_tys') = getClassPredTys pred
551 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
553 -- Instantiate the super-class context with inst_tys
554 sc_theta' = substTheta (mkTyVarSubst class_tyvars inst_tys') sc_theta
555 origin = InstanceDeclOrigin
557 -- Create dictionary Ids from the specified instance contexts.
558 newDicts origin sc_theta' `thenNF_Tc` \ sc_dicts ->
559 newDicts origin dfun_theta' `thenNF_Tc` \ dfun_arg_dicts ->
560 newDicts origin [pred] `thenNF_Tc` \ [this_dict] ->
561 -- Default-method Ids may be mentioned in synthesised RHSs,
562 -- but they'll already be in the environment.
564 -- Check that all the method bindings come from this class
565 mkMethodBinds clas inst_tys' op_items monobinds `thenTc` \ (meth_insts, meth_infos) ->
567 let -- These insts are in scope; quite a few, eh?
568 avail_insts = [this_dict] ++ dfun_arg_dicts ++
569 sc_dicts ++ meth_insts
571 xtve = inst_tyvars `zip` inst_tyvars'
572 tc_meth = tcMethodBind xtve inst_tyvars' dfun_theta' avail_insts
574 mapAndUnzipTc tc_meth meth_infos `thenTc` \ (meth_binds_s, meth_lie_s) ->
576 -- Figure out bindings for the superclass context
577 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
578 `thenTc` \ (zonked_inst_tyvars, sc_binds_inner, sc_binds_outer) ->
580 -- Deal with SPECIALISE instance pragmas by making them
581 -- look like SPECIALISE pragmas for the dfun
583 spec_prags = [ SpecSig (idName dfun_id) ty loc
584 | SpecInstSig ty loc <- uprags]
587 tcExtendGlobalValEnv [dfun_id] (
588 tcExtendTyVarEnv2 xtve $
589 tcExtendLocalValEnv2 [(idName sel_id, tcSigPolyId sig)
590 | (sel_id, sig, _) <- meth_infos] $
591 -- Map sel_id to the local method name we are using
592 tcSpecSigs spec_prags
593 ) `thenTc` \ (prag_binds, prag_lie) ->
595 -- Create the result bindings
597 local_dfun_id = setIdLocalExported dfun_id
598 -- Reason for setIdLocalExported: see notes with MkId.mkDictFunId
600 dict_constr = classDataCon clas
601 scs_and_meths = map instToId (sc_dicts ++ meth_insts)
602 this_dict_id = instToId this_dict
603 inlines | null dfun_arg_dicts = emptyNameSet
604 | otherwise = unitNameSet (idName dfun_id)
605 -- Always inline the dfun; this is an experimental decision
606 -- because it makes a big performance difference sometimes.
607 -- Often it means we can do the method selection, and then
608 -- inline the method as well. Marcin's idea; see comments below.
610 -- BUT: don't inline it if it's a constant dictionary;
611 -- we'll get all the benefit without inlining, and we get
612 -- a **lot** of code duplication if we inline it
616 = -- Blatant special case for CCallable, CReturnable
617 -- If the dictionary is empty then we should never
618 -- select anything from it, so we make its RHS just
619 -- emit an error message. This in turn means that we don't
620 -- mention the constructor, which doesn't exist for CCallable, CReturnable
621 -- Hardly beautiful, but only three extra lines.
622 HsApp (TyApp (HsVar rUNTIME_ERROR_ID) [idType this_dict_id])
623 (HsLit (HsStringPrim (mkFastString (stringToUtf8 msg))))
625 | otherwise -- The common case
626 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
627 -- We don't produce a binding for the dict_constr; instead we
628 -- rely on the simplifier to unfold this saturated application
629 -- We do this rather than generate an HsCon directly, because
630 -- it means that the special cases (e.g. dictionary with only one
631 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
632 -- than needing to be repeated here.
635 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
637 dict_bind = VarMonoBind this_dict_id dict_rhs
638 meth_binds = andMonoBindList meth_binds_s
639 all_binds = sc_binds_inner `AndMonoBinds` meth_binds `AndMonoBinds` dict_bind
643 (map instToId dfun_arg_dicts)
644 [(inst_tyvars', local_dfun_id, this_dict_id)]
647 returnTc (plusLIEs meth_lie_s `plusLIE` prag_lie,
648 main_bind `AndMonoBinds` prag_binds `AndMonoBinds` sc_binds_outer)
653 We have to be very, very careful when generating superclasses, lest we
654 accidentally build a loop. Here's an example:
658 class S a => C a where { opc :: a -> a }
659 class S b => D b where { opd :: b -> b }
667 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
668 Simplifying, we may well get:
669 $dfCInt = :C ds1 (opd dd)
672 Notice that we spot that we can extract ds1 from dd.
674 Alas! Alack! We can do the same for (instance D Int):
676 $dfDInt = :D ds2 (opc dc)
680 And now we've defined the superclass in terms of itself.
683 Solution: treat the superclass context separately, and simplify it
684 all the way down to nothing on its own. Don't toss any 'free' parts
685 out to be simplified together with other bits of context.
686 Hence the tcSimplifyTop below.
688 At a more basic level, don't include this_dict in the context wrt
689 which we simplify sc_dicts, else sc_dicts get bound by just selecting
693 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
694 = tcAddErrCtxt superClassCtxt $
695 tcSimplifyCheck doc inst_tyvars'
697 (mkLIE sc_dicts) `thenTc` \ (sc_lie, sc_binds1) ->
699 -- It's possible that the superclass stuff might have done unification
700 checkSigTyVars inst_tyvars' `thenTc` \ zonked_inst_tyvars ->
702 -- We must simplify this all the way down
703 -- lest we build superclass loops
704 -- See notes about superclass loops above
705 tcSimplifyTop sc_lie `thenTc` \ sc_binds2 ->
707 returnTc (zonked_inst_tyvars, sc_binds1, sc_binds2)
710 doc = ptext SLIT("instance declaration superclass context")
714 mkMethodBinds clas inst_tys' op_items monobinds
715 = -- Check that all the method bindings come from this class
716 mapTc (addErrTc . badMethodErr clas) bad_bndrs `thenNF_Tc_`
718 -- Make the method bindings
719 mapAndUnzipTc mk_method_bind op_items
722 mk_method_bind op_item = mkMethodBind InstanceDeclOrigin clas
723 inst_tys' monobinds op_item
725 -- Find any definitions in monobinds that aren't from the class
726 sel_names = [idName sel_id | (sel_id, _) <- op_items]
727 bad_bndrs = collectMonoBinders monobinds `minusList` sel_names
731 ------------------------------
732 Inlining dfuns unconditionally
733 ------------------------------
735 The code above unconditionally inlines dict funs. Here's why.
736 Consider this program:
738 test :: Int -> Int -> Bool
739 test x y = (x,y) == (y,x) || test y x
740 -- Recursive to avoid making it inline.
742 This needs the (Eq (Int,Int)) instance. If we inline that dfun
743 the code we end up with is good:
746 \r -> case ==# [ww ww1] of wild {
747 PrelBase.False -> Test.$wtest ww1 ww;
749 case ==# [ww1 ww] of wild1 {
750 PrelBase.False -> Test.$wtest ww1 ww;
751 PrelBase.True -> PrelBase.True [];
754 Test.test = \r [w w1]
757 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
760 If we don't inline the dfun, the code is not nearly as good:
762 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
763 PrelBase.:DEq tpl1 tpl2 -> tpl2;
768 let { y = PrelBase.I#! [ww1]; } in
769 let { x = PrelBase.I#! [ww]; } in
770 let { sat_slx = PrelTup.(,)! [y x]; } in
771 let { sat_sly = PrelTup.(,)! [x y];
773 case == sat_sly sat_slx of wild {
774 PrelBase.False -> Test.$wtest ww1 ww;
775 PrelBase.True -> PrelBase.True [];
782 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
785 Why doesn't GHC inline $fEq? Because it looks big:
787 PrelTup.zdfEqZ1T{-rcX-}
788 = \ @ a{-reT-} :: * @ b{-reS-} :: *
789 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
790 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
792 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
793 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
795 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
796 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
798 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
799 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
800 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
802 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
804 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
806 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
807 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
811 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
812 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
813 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
814 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
816 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
818 and it's not as bad as it seems, because it's further dramatically
819 simplified: only zeze2 is extracted and its body is simplified.
822 %************************************************************************
824 \subsection{Error messages}
826 %************************************************************************
829 tcAddDeclCtxt decl thing_inside
830 = tcAddSrcLoc (tcdLoc decl) $
835 ClassDecl {} -> "class"
836 TySynonym {} -> "type synonym"
837 TyData {tcdND = NewType} -> "newtype"
838 TyData {tcdND = DataType} -> "data type"
840 ctxt = hsep [ptext SLIT("In the"), text thing,
841 ptext SLIT("declaration for"), quotes (ppr (tcdName decl))]
843 instDeclCtxt inst_ty = ptext SLIT("In the instance declaration for") <+> quotes doc
845 doc = case inst_ty of
846 HsForAllTy _ _ (HsPredTy pred) -> ppr pred
847 HsPredTy pred -> ppr pred
848 other -> ppr inst_ty -- Don't expect this
852 badGenericInstanceType binds
853 = vcat [ptext SLIT("Illegal type pattern in the generic bindings"),
856 missingGenericInstances missing
857 = ptext SLIT("Missing type patterns for") <+> pprQuotedList missing
859 dupGenericInsts tc_inst_infos
860 = vcat [ptext SLIT("More than one type pattern for a single generic type constructor:"),
861 nest 4 (vcat (map ppr_inst_ty tc_inst_infos)),
862 ptext SLIT("All the type patterns for a generic type constructor must be identical")
865 ppr_inst_ty (tc,inst) = ppr (simpleInstInfoTy inst)
867 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
868 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")