2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcInstDecls]{Typechecking instance declarations}
7 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
9 #include "HsVersions.h"
11 import HsSyn ( HsDecl(..), InstDecl(..),
12 MonoBinds(..), HsExpr(..), HsLit(..), Sig(..),
15 import RnHsSyn ( RenamedHsBinds, RenamedInstDecl, RenamedHsDecl )
16 import TcHsSyn ( TcMonoBinds, mkHsConApp )
18 import TcBinds ( tcSpecSigs )
19 import TcClassDcl ( tcMethodBind, checkFromThisClass )
21 import RnMonad ( RnNameSupply, FixityEnv )
22 import Inst ( InstOrigin(..),
23 newDicts, newClassDicts,
24 LIE, emptyLIE, plusLIE, plusLIEs )
25 import TcDeriv ( tcDeriving )
26 import TcEnv ( ValueEnv, tcExtendGlobalValEnv, tcExtendTyVarEnvForMeths,
27 tcAddImportedIdInfo, tcInstId, newDFunName
29 import TcInstUtil ( InstInfo(..), classDataCon )
30 import TcMonoType ( tcHsSigType )
31 import TcSimplify ( tcSimplifyAndCheck )
32 import TcType ( zonkTcSigTyVars )
34 import Bag ( emptyBag, unitBag, unionBags, unionManyBags,
37 import CmdLineOpts ( opt_GlasgowExts, opt_AllowUndecidableInstances )
38 import Class ( classBigSig )
39 import Var ( idName, idType )
40 import Maybes ( maybeToBool, expectJust )
41 import MkId ( mkDictFunId )
42 import Module ( Module )
43 import Name ( isLocallyDefined )
44 import NameSet ( emptyNameSet )
45 import PrelInfo ( eRROR_ID )
46 import PprType ( pprConstraint )
47 import TyCon ( isSynTyCon, tyConDerivings )
48 import Type ( mkTyVarTys, splitSigmaTy, isTyVarTy,
49 splitTyConApp_maybe, splitDictTy_maybe,
50 splitAlgTyConApp_maybe,
51 classesToPreds, classesOfPreds,
52 unUsgTy, tyVarsOfTypes
54 import Subst ( mkTopTyVarSubst, substClasses )
55 import VarSet ( mkVarSet, varSetElems )
56 import TysWiredIn ( isFFIArgumentTy, isFFIResultTy )
57 import PrelNames ( cCallableClassKey, cReturnableClassKey, hasKey )
61 Typechecking instance declarations is done in two passes. The first
62 pass, made by @tcInstDecls1@, collects information to be used in the
65 This pre-processed info includes the as-yet-unprocessed bindings
66 inside the instance declaration. These are type-checked in the second
67 pass, when the class-instance envs and GVE contain all the info from
68 all the instance and value decls. Indeed that's the reason we need
69 two passes over the instance decls.
72 Here is the overall algorithm.
73 Assume that we have an instance declaration
75 instance c => k (t tvs) where b
79 $LIE_c$ is the LIE for the context of class $c$
81 $betas_bar$ is the free variables in the class method type, excluding the
84 $LIE_cop$ is the LIE constraining a particular class method
86 $tau_cop$ is the tau type of a class method
88 $LIE_i$ is the LIE for the context of instance $i$
90 $X$ is the instance constructor tycon
92 $gammas_bar$ is the set of type variables of the instance
94 $LIE_iop$ is the LIE for a particular class method instance
96 $tau_iop$ is the tau type for this instance of a class method
98 $alpha$ is the class variable
100 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
102 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
105 ToDo: Update the list above with names actually in the code.
109 First, make the LIEs for the class and instance contexts, which means
110 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
111 and make LIElistI and LIEI.
113 Then process each method in turn.
115 order the instance methods according to the ordering of the class methods
117 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
119 Create final dictionary function from bindings generated already
121 df = lambda inst_tyvars
128 in <op1,op2,...,opn,sd1,...,sdm>
130 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
131 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
135 tcInstDecls1 :: ValueEnv -- Contains IdInfo for dfun ids
137 -> Module -- Module for deriving
138 -> FixityEnv -- For derivings
139 -> RnNameSupply -- For renaming derivings
140 -> TcM s (Bag InstInfo,
143 tcInstDecls1 unf_env decls mod fixs rn_name_supply
144 = -- Do the ordinary instance declarations
145 mapNF_Tc (tcInstDecl1 mod unf_env)
146 [inst_decl | InstD inst_decl <- decls] `thenNF_Tc` \ inst_info_bags ->
148 decl_inst_info = unionManyBags inst_info_bags
150 -- Handle "derived" instances; note that we only do derivings
151 -- for things in this module; we ignore deriving decls from
153 tcDeriving mod fixs rn_name_supply decl_inst_info
154 `thenTc` \ (deriv_inst_info, deriv_binds) ->
157 full_inst_info = deriv_inst_info `unionBags` decl_inst_info
159 returnTc (full_inst_info, deriv_binds)
162 tcInstDecl1 :: Module -> ValueEnv -> RenamedInstDecl -> NF_TcM s (Bag InstInfo)
164 tcInstDecl1 mod unf_env (InstDecl poly_ty binds uprags maybe_dfun_name src_loc)
165 = -- Prime error recovery, set source location
166 recoverNF_Tc (returnNF_Tc emptyBag) $
167 tcAddSrcLoc src_loc $
169 -- Type-check all the stuff before the "where"
170 tcHsSigType poly_ty `thenTc` \ poly_ty' ->
172 (tyvars, theta, dict_ty) = splitSigmaTy poly_ty'
173 constr = classesOfPreds theta
174 (clas, inst_tys) = case splitDictTy_maybe dict_ty of
176 Nothing -> pprPanic "tcInstDecl1" (ppr poly_ty)
179 (case maybe_dfun_name of
180 Nothing -> -- A source-file instance declaration
182 -- Check for respectable instance type, and context
183 -- but only do this for non-imported instance decls.
184 -- Imported ones should have been checked already, and may indeed
185 -- contain something illegal in normal Haskell, notably
186 -- instance CCallable [Char]
187 scrutiniseInstanceHead clas inst_tys `thenNF_Tc_`
188 mapNF_Tc scrutiniseInstanceConstraint constr `thenNF_Tc_`
190 -- Make the dfun id and return it
191 newDFunName mod clas inst_tys src_loc `thenNF_Tc` \ dfun_name ->
192 returnNF_Tc (mkDictFunId dfun_name clas tyvars inst_tys constr)
194 Just dfun_name -> -- An interface-file instance declaration
195 -- Make the dfun id and add info from interface file
197 dfun_id = mkDictFunId dfun_name clas tyvars inst_tys constr
199 returnNF_Tc (tcAddImportedIdInfo unf_env dfun_id)
200 ) `thenNF_Tc` \ dfun_id ->
202 returnTc (unitBag (InstInfo clas tyvars inst_tys constr dfun_id binds src_loc uprags))
206 %************************************************************************
208 \subsection{Type-checking instance declarations, pass 2}
210 %************************************************************************
213 tcInstDecls2 :: Bag InstInfo
214 -> NF_TcM s (LIE, TcMonoBinds)
216 tcInstDecls2 inst_decls
217 = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
219 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
220 tc2 `thenNF_Tc` \ (lie2, binds2) ->
221 returnNF_Tc (lie1 `plusLIE` lie2,
222 binds1 `AndMonoBinds` binds2)
226 ======= New documentation starts here (Sept 92) ==============
228 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
229 the dictionary function for this instance declaration. For example
231 instance Foo a => Foo [a] where
235 might generate something like
237 dfun.Foo.List dFoo_a = let op1 x = ...
243 HOWEVER, if the instance decl has no context, then it returns a
244 bigger @HsBinds@ with declarations for each method. For example
246 instance Foo [a] where
252 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
253 const.Foo.op1.List a x = ...
254 const.Foo.op2.List a y = ...
256 This group may be mutually recursive, because (for example) there may
257 be no method supplied for op2 in which case we'll get
259 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
261 that is, the default method applied to the dictionary at this type.
263 What we actually produce in either case is:
265 AbsBinds [a] [dfun_theta_dicts]
266 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
267 { d = (sd1,sd2, ..., op1, op2, ...)
272 The "maybe" says that we only ask AbsBinds to make global constant methods
273 if the dfun_theta is empty.
276 For an instance declaration, say,
278 instance (C1 a, C2 b) => C (T a b) where
281 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
282 function whose type is
284 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
286 Notice that we pass it the superclass dictionaries at the instance type; this
287 is the ``Mark Jones optimisation''. The stuff before the "=>" here
288 is the @dfun_theta@ below.
290 First comes the easy case of a non-local instance decl.
293 tcInstDecl2 :: InstInfo -> NF_TcM s (LIE, TcMonoBinds)
295 tcInstDecl2 (InstInfo clas inst_tyvars inst_tys
299 | not (isLocallyDefined dfun_id)
300 = returnNF_Tc (emptyLIE, EmptyMonoBinds)
303 = -- Prime error recovery
304 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
307 -- Check that all the method bindings come from this class
308 checkFromThisClass clas monobinds `thenNF_Tc_`
310 -- Instantiate the instance decl with tc-style type variables
311 tcInstId dfun_id `thenNF_Tc` \ (inst_tyvars', dfun_theta', dict_ty') ->
313 (clas, inst_tys') = expectJust "tcInstDecl2" (splitDictTy_maybe dict_ty')
315 origin = InstanceDeclOrigin
317 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
319 dm_ids = [dm_id | (_, dm_id, _) <- op_items]
321 -- Instantiate the theta found in the original instance decl
322 inst_decl_theta' = substClasses (mkTopTyVarSubst inst_tyvars (mkTyVarTys inst_tyvars'))
325 -- Instantiate the super-class context with inst_tys
326 sc_theta' = substClasses (mkTopTyVarSubst class_tyvars inst_tys') sc_theta
328 -- Create dictionary Ids from the specified instance contexts.
329 newClassDicts origin sc_theta' `thenNF_Tc` \ (sc_dicts, sc_dict_ids) ->
330 newDicts origin dfun_theta' `thenNF_Tc` \ (dfun_arg_dicts, dfun_arg_dicts_ids) ->
331 newClassDicts origin inst_decl_theta' `thenNF_Tc` \ (inst_decl_dicts, _) ->
332 newClassDicts origin [(clas,inst_tys')] `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
334 tcExtendTyVarEnvForMeths inst_tyvars inst_tyvars' (
335 tcExtendGlobalValEnv dm_ids (
336 -- Default-method Ids may be mentioned in synthesised RHSs
338 mapAndUnzip3Tc (tcMethodBind clas origin inst_tyvars' inst_tys'
339 (classesToPreds inst_decl_theta')
340 monobinds uprags True)
342 )) `thenTc` \ (method_binds_s, insts_needed_s, meth_lies_w_ids) ->
344 -- Deal with SPECIALISE instance pragmas by making them
345 -- look like SPECIALISE pragmas for the dfun
347 dfun_prags = [SpecSig (idName dfun_id) ty loc | SpecInstSig ty loc <- uprags]
349 tcExtendGlobalValEnv [dfun_id] (
350 tcSpecSigs dfun_prags
351 ) `thenTc` \ (prag_binds, prag_lie) ->
353 -- Check the overloading constraints of the methods and superclasses
355 -- tcMethodBind has checked that the class_tyvars havn't
356 -- been unified with each other or another type, but we must
357 -- still zonk them before passing them to tcSimplifyAndCheck
358 zonkTcSigTyVars inst_tyvars' `thenNF_Tc` \ zonked_inst_tyvars ->
360 inst_tyvars_set = mkVarSet zonked_inst_tyvars
362 (meth_lies, meth_ids) = unzip meth_lies_w_ids
364 -- These insts are in scope; quite a few, eh?
365 avail_insts = this_dict `plusLIE`
366 dfun_arg_dicts `plusLIE`
368 unionManyBags meth_lies
370 methods_lie = plusLIEs insts_needed_s
373 -- Ditto method bindings
374 tcAddErrCtxt methodCtxt (
376 (ptext SLIT("instance declaration context"))
377 inst_tyvars_set -- Local tyvars
380 ) `thenTc` \ (const_lie1, lie_binds1) ->
382 -- Check that we *could* construct the superclass dictionaries,
383 -- even though we are *actually* going to pass the superclass dicts in;
384 -- the check ensures that the caller will never have
385 --a problem building them.
386 tcAddErrCtxt superClassCtxt (
388 (ptext SLIT("instance declaration context"))
389 inst_tyvars_set -- Local tyvars
390 inst_decl_dicts -- The instance dictionaries available
391 sc_dicts -- The superclass dicationaries reqd
393 -- Ignore the result; we're only doing
394 -- this to make sure it can be done.
396 -- Now do the simplification again, this time to get the
397 -- bindings; this time we use an enhanced "avails"
398 -- Ignore errors because they come from the *previous* tcSimplify
401 (ptext SLIT("instance declaration context"))
403 dfun_arg_dicts -- NB! Don't include this_dict here, else the sc_dicts
404 -- get bound by just selecting from this_dict!!
406 ) `thenTc` \ (const_lie2, lie_binds2) ->
409 -- Create the result bindings
411 dict_constr = classDataCon clas
412 scs_and_meths = sc_dict_ids ++ meth_ids
416 = -- Blatant special case for CCallable, CReturnable
417 -- If the dictionary is empty then we should never
418 -- select anything from it, so we make its RHS just
419 -- emit an error message. This in turn means that we don't
420 -- mention the constructor, which doesn't exist for CCallable, CReturnable
421 -- Hardly beautiful, but only three extra lines.
422 HsApp (TyApp (HsVar eRROR_ID) [(unUsgTy . idType) this_dict_id])
423 (HsLit (HsString msg))
425 | otherwise -- The common case
426 = mkHsConApp dict_constr inst_tys' (map HsVar (sc_dict_ids ++ meth_ids))
427 -- We don't produce a binding for the dict_constr; instead we
428 -- rely on the simplifier to unfold this saturated application
429 -- We do this rather than generate an HsCon directly, because
430 -- it means that the special cases (e.g. dictionary with only one
431 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
432 -- than needing to be repeated here.
435 msg = _PK_ ("Compiler error: bad dictionary " ++ showSDoc (ppr clas))
437 dict_bind = VarMonoBind this_dict_id dict_rhs
438 method_binds = andMonoBindList method_binds_s
444 [(inst_tyvars', dfun_id, this_dict_id)]
445 emptyNameSet -- No inlines (yet)
446 (lie_binds1 `AndMonoBinds`
447 lie_binds2 `AndMonoBinds`
448 method_binds `AndMonoBinds`
451 returnTc (const_lie1 `plusLIE` const_lie2 `plusLIE` prag_lie,
452 main_bind `AndMonoBinds` prag_binds)
456 %************************************************************************
458 \subsection{Checking for a decent instance type}
460 %************************************************************************
462 @scrutiniseInstanceHead@ checks the type {\em and} its syntactic constraints:
463 it must normally look like: @instance Foo (Tycon a b c ...) ...@
465 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
466 flag is on, or (2)~the instance is imported (they must have been
467 compiled elsewhere). In these cases, we let them go through anyway.
469 We can also have instances for functions: @instance Foo (a -> b) ...@.
472 scrutiniseInstanceConstraint (clas, tys)
474 || opt_AllowUndecidableInstances = returnNF_Tc ()
475 | otherwise = addErrTc (instConstraintErr clas tys)
477 scrutiniseInstanceHead clas inst_taus
479 -- A user declaration of a CCallable/CReturnable instance
480 -- must be for a "boxed primitive" type.
481 (clas `hasKey` cCallableClassKey && not (ccallable_type first_inst_tau)) ||
482 (clas `hasKey` cReturnableClassKey && not (creturnable_type first_inst_tau))
483 = addErrTc (nonBoxedPrimCCallErr clas first_inst_tau)
486 -- It is obviously illegal to have an explicit instance
487 -- for something that we are also planning to `derive'
488 | maybeToBool alg_tycon_app_maybe && clas `elem` (tyConDerivings alg_tycon)
489 = addErrTc (derivingWhenInstanceExistsErr clas first_inst_tau)
490 -- Kind check will have ensured inst_taus is of length 1
492 -- Allow anything for AllowUndecidableInstances
493 | opt_AllowUndecidableInstances
496 -- If GlasgowExts then check at least one isn't a type variable
498 = if all isTyVarTy inst_taus then
499 addErrTc (instTypeErr clas inst_taus (text "There must be at least one non-type-variable in the instance head"))
503 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
504 | not (length inst_taus == 1 &&
505 maybeToBool maybe_tycon_app && -- Yes, there's a type constuctor
506 not (isSynTyCon tycon) && -- ...but not a synonym
507 all isTyVarTy arg_tys && -- Applied to type variables
508 length (varSetElems (tyVarsOfTypes arg_tys)) == length arg_tys
509 -- This last condition checks that all the type variables are distinct
511 = addErrTc (instTypeErr clas inst_taus
512 (text "the instance type must be of form (T a b c)" $$
513 text "where T is not a synonym, and a,b,c are distinct type variables")
520 (first_inst_tau : _) = inst_taus
522 -- Stuff for algebraic or -> type
523 maybe_tycon_app = splitTyConApp_maybe first_inst_tau
524 Just (tycon, arg_tys) = maybe_tycon_app
526 -- Stuff for an *algebraic* data type
527 alg_tycon_app_maybe = splitAlgTyConApp_maybe first_inst_tau
528 -- The "Alg" part looks through synonyms
529 Just (alg_tycon, _, _) = alg_tycon_app_maybe
531 ccallable_type ty = isFFIArgumentTy False {- Not safe call -} ty
532 creturnable_type ty = isFFIResultTy ty
536 instConstraintErr clas tys
537 = hang (ptext SLIT("Illegal constraint") <+>
538 quotes (pprConstraint clas tys) <+>
539 ptext SLIT("in instance context"))
540 4 (ptext SLIT("(Instance contexts must constrain only type variables)"))
542 instTypeErr clas tys msg
543 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes (pprConstraint clas tys),
547 derivingWhenInstanceExistsErr clas tycon
548 = hang (hsep [ptext SLIT("Deriving class"),
550 ptext SLIT("type"), quotes (ppr tycon)])
551 4 (ptext SLIT("when an explicit instance exists"))
553 nonBoxedPrimCCallErr clas inst_ty
554 = hang (ptext SLIT("Unacceptable instance type for ccall-ish class"))
555 4 (hsep [ ptext SLIT("class"), ppr clas, ptext SLIT("type"),
558 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
559 superClassCtxt = ptext SLIT("When checking the superclasses of an instance declaration")