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 HsBinds(..), MonoBinds(..),
13 HsExpr(..), InPat(..), HsLit(..), Sig(..),
16 import RnHsSyn ( RenamedHsBinds, RenamedInstDecl, RenamedHsDecl )
17 import TcHsSyn ( TcMonoBinds, mkHsConApp )
19 import TcBinds ( tcSpecSigs )
20 import TcClassDcl ( tcMethodBind, checkFromThisClass )
22 import RnMonad ( RnNameSupply, FixityEnv )
23 import Inst ( Inst, InstOrigin(..),
24 newDicts, newClassDicts,
25 LIE, emptyLIE, plusLIE, plusLIEs )
26 import TcDeriv ( tcDeriving )
27 import TcEnv ( ValueEnv, tcExtendGlobalValEnv, tcExtendTyVarEnvForMeths,
28 tcAddImportedIdInfo, tcInstId
30 import TcInstUtil ( InstInfo(..), classDataCon )
31 import TcMonoType ( tcHsSigType )
32 import TcSimplify ( tcSimplifyAndCheck )
33 import TcType ( TcTyVar, zonkTcSigTyVars )
35 import Bag ( emptyBag, unitBag, unionBags, unionManyBags,
38 import CmdLineOpts ( opt_GlasgowExts, opt_AllowUndecidableInstances )
39 import Class ( classBigSig, Class )
40 import Var ( idName, idType, Id, TyVar )
41 import Maybes ( maybeToBool, catMaybes, expectJust )
42 import MkId ( mkDictFunId )
43 import Module ( ModuleName )
44 import Name ( isLocallyDefined, NamedThing(..) )
45 import NameSet ( emptyNameSet )
46 import PrelInfo ( eRROR_ID )
47 import PprType ( pprConstraint )
48 import TyCon ( isSynTyCon, tyConDerivings )
49 import Type ( Type, isUnLiftedType, mkTyVarTys,
50 splitSigmaTy, isTyVarTy,
51 splitTyConApp_maybe, splitDictTy_maybe,
52 getClassTys_maybe, splitAlgTyConApp_maybe,
53 classesToPreds, classesOfPreds,
54 unUsgTy, tyVarsOfTypes
56 import Subst ( mkTopTyVarSubst, substClasses )
57 import VarSet ( mkVarSet, varSetElems )
58 import TysWiredIn ( stringTy, isFFIArgumentTy, isFFIResultTy )
59 import Unique ( Unique, cCallableClassKey, cReturnableClassKey, hasKey, Uniquable(..) )
63 Typechecking instance declarations is done in two passes. The first
64 pass, made by @tcInstDecls1@, collects information to be used in the
67 This pre-processed info includes the as-yet-unprocessed bindings
68 inside the instance declaration. These are type-checked in the second
69 pass, when the class-instance envs and GVE contain all the info from
70 all the instance and value decls. Indeed that's the reason we need
71 two passes over the instance decls.
74 Here is the overall algorithm.
75 Assume that we have an instance declaration
77 instance c => k (t tvs) where b
81 $LIE_c$ is the LIE for the context of class $c$
83 $betas_bar$ is the free variables in the class method type, excluding the
86 $LIE_cop$ is the LIE constraining a particular class method
88 $tau_cop$ is the tau type of a class method
90 $LIE_i$ is the LIE for the context of instance $i$
92 $X$ is the instance constructor tycon
94 $gammas_bar$ is the set of type variables of the instance
96 $LIE_iop$ is the LIE for a particular class method instance
98 $tau_iop$ is the tau type for this instance of a class method
100 $alpha$ is the class variable
102 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
104 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
107 ToDo: Update the list above with names actually in the code.
111 First, make the LIEs for the class and instance contexts, which means
112 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
113 and make LIElistI and LIEI.
115 Then process each method in turn.
117 order the instance methods according to the ordering of the class methods
119 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
121 Create final dictionary function from bindings generated already
123 df = lambda inst_tyvars
130 in <op1,op2,...,opn,sd1,...,sdm>
132 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
133 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
137 tcInstDecls1 :: ValueEnv -- Contains IdInfo for dfun ids
139 -> ModuleName -- module name for deriving
141 -> RnNameSupply -- for renaming derivings
142 -> TcM s (Bag InstInfo,
145 tcInstDecls1 unf_env decls mod_name fixs rn_name_supply
146 = -- Do the ordinary instance declarations
147 mapNF_Tc (tcInstDecl1 unf_env)
148 [inst_decl | InstD inst_decl <- decls] `thenNF_Tc` \ inst_info_bags ->
150 decl_inst_info = unionManyBags inst_info_bags
152 -- Handle "derived" instances; note that we only do derivings
153 -- for things in this module; we ignore deriving decls from
155 tcDeriving mod_name fixs rn_name_supply decl_inst_info
156 `thenTc` \ (deriv_inst_info, deriv_binds) ->
159 full_inst_info = deriv_inst_info `unionBags` decl_inst_info
161 returnTc (full_inst_info, deriv_binds)
164 tcInstDecl1 :: ValueEnv -> RenamedInstDecl -> NF_TcM s (Bag InstInfo)
166 tcInstDecl1 unf_env (InstDecl poly_ty binds uprags dfun_name src_loc)
167 = -- Prime error recovery, set source location
168 recoverNF_Tc (returnNF_Tc emptyBag) $
169 tcAddSrcLoc src_loc $
171 -- Type-check all the stuff before the "where"
172 tcHsSigType poly_ty `thenTc` \ poly_ty' ->
174 (tyvars, theta, dict_ty) = splitSigmaTy poly_ty'
175 constr = classesOfPreds theta
176 (clas, inst_tys) = case splitDictTy_maybe dict_ty of
178 Nothing -> pprPanic "tcInstDecl1" (ppr poly_ty)
181 -- Check for respectable instance type, and context
182 -- but only do this for non-imported instance decls.
183 -- Imported ones should have been checked already, and may indeed
184 -- contain something illegal in normal Haskell, notably
185 -- instance CCallable [Char]
186 (if isLocallyDefined dfun_name then
187 scrutiniseInstanceHead clas inst_tys `thenNF_Tc_`
188 mapNF_Tc scrutiniseInstanceConstraint constr
195 dfun_id = mkDictFunId dfun_name clas tyvars inst_tys constr
197 -- Add info from interface file
198 final_dfun_id = tcAddImportedIdInfo unf_env dfun_id
200 returnTc (unitBag (InstInfo clas tyvars inst_tys constr
202 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 -- I deleted this "optimisation" because when importing these
304 -- instance decls the renamer would look for the dfun bindings and they weren't there.
305 -- This would be fixable, but it seems simpler just to produce a tiny void binding instead,
306 -- even though it's never used.
308 -- This case deals with CCallable etc, which don't need any bindings
310 = returnNF_Tc (emptyLIE, EmptyBinds)
314 = -- Prime error recovery
315 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
318 -- Instantiate the instance decl with tc-style type variables
319 tcInstId dfun_id `thenNF_Tc` \ (inst_tyvars', dfun_theta', dict_ty') ->
321 (clas, inst_tys') = expectJust "tcInstDecl2" (splitDictTy_maybe dict_ty')
323 origin = InstanceDeclOrigin
325 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
327 dm_ids = [dm_id | (_, dm_id, _) <- op_items]
329 -- Instantiate the theta found in the original instance decl
330 inst_decl_theta' = substClasses (mkTopTyVarSubst inst_tyvars (mkTyVarTys inst_tyvars'))
333 -- Instantiate the super-class context with inst_tys
334 sc_theta' = substClasses (mkTopTyVarSubst class_tyvars inst_tys') sc_theta
336 -- Create dictionary Ids from the specified instance contexts.
337 newClassDicts origin sc_theta' `thenNF_Tc` \ (sc_dicts, sc_dict_ids) ->
338 newDicts origin dfun_theta' `thenNF_Tc` \ (dfun_arg_dicts, dfun_arg_dicts_ids) ->
339 newClassDicts origin inst_decl_theta' `thenNF_Tc` \ (inst_decl_dicts, _) ->
340 newClassDicts origin [(clas,inst_tys')] `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
342 -- Check that all the method bindings come from this class
343 checkFromThisClass clas op_items monobinds `thenNF_Tc_`
345 tcExtendTyVarEnvForMeths inst_tyvars inst_tyvars' (
346 tcExtendGlobalValEnv dm_ids (
347 -- Default-method Ids may be mentioned in synthesised RHSs
349 mapAndUnzip3Tc (tcMethodBind clas origin inst_tyvars' inst_tys'
350 (classesToPreds inst_decl_theta')
351 monobinds uprags True)
353 )) `thenTc` \ (method_binds_s, insts_needed_s, meth_lies_w_ids) ->
355 -- Deal with SPECIALISE instance pragmas by making them
356 -- look like SPECIALISE pragmas for the dfun
358 dfun_prags = [SpecSig (idName dfun_id) ty loc | SpecInstSig ty loc <- uprags]
360 tcExtendGlobalValEnv [dfun_id] (
361 tcSpecSigs dfun_prags
362 ) `thenTc` \ (prag_binds, prag_lie) ->
364 -- Check the overloading constraints of the methods and superclasses
366 -- tcMethodBind has checked that the class_tyvars havn't
367 -- been unified with each other or another type, but we must
368 -- still zonk them before passing them to tcSimplifyAndCheck
369 zonkTcSigTyVars inst_tyvars' `thenNF_Tc` \ zonked_inst_tyvars ->
371 inst_tyvars_set = mkVarSet zonked_inst_tyvars
373 (meth_lies, meth_ids) = unzip meth_lies_w_ids
375 -- These insts are in scope; quite a few, eh?
376 avail_insts = this_dict `plusLIE`
377 dfun_arg_dicts `plusLIE`
379 unionManyBags meth_lies
381 methods_lie = plusLIEs insts_needed_s
384 -- Ditto method bindings
385 tcAddErrCtxt methodCtxt (
387 (ptext SLIT("instance declaration context"))
388 inst_tyvars_set -- Local tyvars
391 ) `thenTc` \ (const_lie1, lie_binds1) ->
393 -- Check that we *could* construct the superclass dictionaries,
394 -- even though we are *actually* going to pass the superclass dicts in;
395 -- the check ensures that the caller will never have
396 --a problem building them.
397 tcAddErrCtxt superClassCtxt (
399 (ptext SLIT("instance declaration context"))
400 inst_tyvars_set -- Local tyvars
401 inst_decl_dicts -- The instance dictionaries available
402 sc_dicts -- The superclass dicationaries reqd
404 -- Ignore the result; we're only doing
405 -- this to make sure it can be done.
407 -- Now do the simplification again, this time to get the
408 -- bindings; this time we use an enhanced "avails"
409 -- Ignore errors because they come from the *previous* tcSimplify
412 (ptext SLIT("instance declaration context"))
414 dfun_arg_dicts -- NB! Don't include this_dict here, else the sc_dicts
415 -- get bound by just selecting from this_dict!!
417 ) `thenTc` \ (const_lie2, lie_binds2) ->
420 -- Create the result bindings
422 dict_constr = classDataCon clas
423 scs_and_meths = sc_dict_ids ++ meth_ids
427 = -- Blatant special case for CCallable, CReturnable
428 -- If the dictionary is empty then we should never
429 -- select anything from it, so we make its RHS just
430 -- emit an error message. This in turn means that we don't
431 -- mention the constructor, which doesn't exist for CCallable, CReturnable
432 -- Hardly beautiful, but only three extra lines.
433 HsApp (TyApp (HsVar eRROR_ID) [(unUsgTy . idType) this_dict_id])
434 (HsLitOut (HsString msg) stringTy)
436 | otherwise -- The common case
437 = mkHsConApp dict_constr inst_tys' (map HsVar (sc_dict_ids ++ meth_ids))
438 -- We don't produce a binding for the dict_constr; instead we
439 -- rely on the simplifier to unfold this saturated application
440 -- We do this rather than generate an HsCon directly, because
441 -- it means that the special cases (e.g. dictionary with only one
442 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
443 -- than needing to be repeated here.
446 msg = _PK_ ("Compiler error: bad dictionary " ++ showSDoc (ppr clas))
448 dict_bind = VarMonoBind this_dict_id dict_rhs
449 method_binds = andMonoBindList method_binds_s
455 [(inst_tyvars', dfun_id, this_dict_id)]
456 emptyNameSet -- No inlines (yet)
457 (lie_binds1 `AndMonoBinds`
458 lie_binds2 `AndMonoBinds`
459 method_binds `AndMonoBinds`
462 returnTc (const_lie1 `plusLIE` const_lie2 `plusLIE` prag_lie,
463 main_bind `AndMonoBinds` prag_binds)
467 %************************************************************************
469 \subsection{Checking for a decent instance type}
471 %************************************************************************
473 @scrutiniseInstanceHead@ checks the type {\em and} its syntactic constraints:
474 it must normally look like: @instance Foo (Tycon a b c ...) ...@
476 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
477 flag is on, or (2)~the instance is imported (they must have been
478 compiled elsewhere). In these cases, we let them go through anyway.
480 We can also have instances for functions: @instance Foo (a -> b) ...@.
483 scrutiniseInstanceConstraint (clas, tys)
485 || opt_AllowUndecidableInstances = returnNF_Tc ()
486 | otherwise = addErrTc (instConstraintErr clas tys)
488 scrutiniseInstanceHead clas inst_taus
490 -- A user declaration of a CCallable/CReturnable instance
491 -- must be for a "boxed primitive" type.
492 (clas `hasKey` cCallableClassKey && not (ccallable_type first_inst_tau)) ||
493 (clas `hasKey` cReturnableClassKey && not (creturnable_type first_inst_tau))
494 = addErrTc (nonBoxedPrimCCallErr clas first_inst_tau)
497 -- It is obviously illegal to have an explicit instance
498 -- for something that we are also planning to `derive'
499 | maybeToBool alg_tycon_app_maybe && clas `elem` (tyConDerivings alg_tycon)
500 = addErrTc (derivingWhenInstanceExistsErr clas first_inst_tau)
501 -- Kind check will have ensured inst_taus is of length 1
503 -- Allow anything for AllowUndecidableInstances
504 | opt_AllowUndecidableInstances
507 -- If GlasgowExts then check at least one isn't a type variable
509 = if all isTyVarTy inst_taus then
510 addErrTc (instTypeErr clas inst_taus (text "There must be at least one non-type-variable in the instance head"))
514 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
515 | not (length inst_taus == 1 &&
516 maybeToBool maybe_tycon_app && -- Yes, there's a type constuctor
517 not (isSynTyCon tycon) && -- ...but not a synonym
518 all isTyVarTy arg_tys && -- Applied to type variables
519 length (varSetElems (tyVarsOfTypes arg_tys)) == length arg_tys
520 -- This last condition checks that all the type variables are distinct
522 = addErrTc (instTypeErr clas inst_taus
523 (text "the instance type must be of form (T a b c)" $$
524 text "where T is not a synonym, and a,b,c are distinct type variables")
531 (first_inst_tau : _) = inst_taus
533 -- Stuff for algebraic or -> type
534 maybe_tycon_app = splitTyConApp_maybe first_inst_tau
535 Just (tycon, arg_tys) = maybe_tycon_app
537 -- Stuff for an *algebraic* data type
538 alg_tycon_app_maybe = splitAlgTyConApp_maybe first_inst_tau
539 -- The "Alg" part looks through synonyms
540 Just (alg_tycon, _, _) = alg_tycon_app_maybe
542 ccallable_type ty = isFFIArgumentTy False {- Not safe call -} ty
543 creturnable_type ty = isFFIResultTy ty
547 instConstraintErr clas tys
548 = hang (ptext SLIT("Illegal constraint") <+>
549 quotes (pprConstraint clas tys) <+>
550 ptext SLIT("in instance context"))
551 4 (ptext SLIT("(Instance contexts must constrain only type variables)"))
553 instTypeErr clas tys msg
554 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes (pprConstraint clas tys),
558 derivingWhenInstanceExistsErr clas tycon
559 = hang (hsep [ptext SLIT("Deriving class"),
561 ptext SLIT("type"), quotes (ppr tycon)])
562 4 (ptext SLIT("when an explicit instance exists"))
564 nonBoxedPrimCCallErr clas inst_ty
565 = hang (ptext SLIT("Unacceptable instance type for ccall-ish class"))
566 4 (hsep [ ptext SLIT("class"), ppr clas, ptext SLIT("type"),
569 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
570 superClassCtxt = ptext SLIT("When checking the superclasses of an instance declaration")