2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcInstDecls]{Typechecking instance declarations}
12 #include "HsVersions.h"
14 import HsSyn ( HsDecl(..), InstDecl(..),
15 HsBinds(..), MonoBinds(..),
16 HsExpr(..), InPat(..), HsLit(..), Sig(..),
19 import RnHsSyn ( RenamedHsBinds, RenamedInstDecl, RenamedHsDecl )
20 import TcHsSyn ( TcMonoBinds,
24 import TcBinds ( tcSpecSigs )
25 import TcClassDcl ( tcMethodBind, checkFromThisClass )
27 import RnMonad ( RnNameSupply, Fixities )
28 import Inst ( Inst, InstOrigin(..),
29 newDicts, LIE, emptyLIE, plusLIE, plusLIEs )
30 import TcDeriv ( tcDeriving )
31 import TcEnv ( ValueEnv, tcExtendGlobalValEnv, tcExtendTyVarEnvForMeths,
32 tcAddImportedIdInfo, tcInstId
34 import TcInstUtil ( InstInfo(..), classDataCon )
35 import TcMonoType ( tcHsTopType )
36 import TcSimplify ( tcSimplifyAndCheck )
37 import TcType ( TcTyVar, zonkTcTyVarBndr )
39 import Bag ( emptyBag, unitBag, unionBags, unionManyBags,
42 import CmdLineOpts ( opt_GlasgowExts, opt_AllowUndecidableInstances )
43 import Class ( classBigSig, Class )
44 import Var ( idName, idType, Id, TyVar )
45 import DataCon ( isNullaryDataCon, dataConArgTys, dataConId )
46 import Maybes ( maybeToBool, catMaybes, expectJust )
47 import MkId ( mkDictFunId )
48 import Module ( ModuleName )
49 import Name ( isLocallyDefined, NamedThing(..) )
50 import NameSet ( emptyNameSet )
51 import PrelInfo ( eRROR_ID )
52 import PprType ( pprConstraint )
53 import SrcLoc ( SrcLoc )
54 import TyCon ( isSynTyCon, isDataTyCon, tyConDerivings )
55 import Type ( Type, isUnLiftedType, mkTyVarTys,
56 splitSigmaTy, isTyVarTy,
57 splitTyConApp_maybe, splitDictTy_maybe, unUsgTy,
58 splitAlgTyConApp_maybe,
61 import Subst ( mkTopTyVarSubst, substTheta )
62 import VarSet ( mkVarSet, varSetElems )
63 import TysPrim ( byteArrayPrimTyCon, mutableByteArrayPrimTyCon )
64 import TysWiredIn ( stringTy )
65 import Unique ( Unique, cCallableClassKey, cReturnableClassKey, Uniquable(..) )
69 Typechecking instance declarations is done in two passes. The first
70 pass, made by @tcInstDecls1@, collects information to be used in the
73 This pre-processed info includes the as-yet-unprocessed bindings
74 inside the instance declaration. These are type-checked in the second
75 pass, when the class-instance envs and GVE contain all the info from
76 all the instance and value decls. Indeed that's the reason we need
77 two passes over the instance decls.
80 Here is the overall algorithm.
81 Assume that we have an instance declaration
83 instance c => k (t tvs) where b
87 $LIE_c$ is the LIE for the context of class $c$
89 $betas_bar$ is the free variables in the class method type, excluding the
92 $LIE_cop$ is the LIE constraining a particular class method
94 $tau_cop$ is the tau type of a class method
96 $LIE_i$ is the LIE for the context of instance $i$
98 $X$ is the instance constructor tycon
100 $gammas_bar$ is the set of type variables of the instance
102 $LIE_iop$ is the LIE for a particular class method instance
104 $tau_iop$ is the tau type for this instance of a class method
106 $alpha$ is the class variable
108 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
110 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
113 ToDo: Update the list above with names actually in the code.
117 First, make the LIEs for the class and instance contexts, which means
118 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
119 and make LIElistI and LIEI.
121 Then process each method in turn.
123 order the instance methods according to the ordering of the class methods
125 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
127 Create final dictionary function from bindings generated already
129 df = lambda inst_tyvars
136 in <op1,op2,...,opn,sd1,...,sdm>
138 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
139 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
143 tcInstDecls1 :: ValueEnv -- Contains IdInfo for dfun ids
145 -> ModuleName -- module name for deriving
147 -> RnNameSupply -- for renaming derivings
148 -> TcM s (Bag InstInfo,
151 tcInstDecls1 unf_env decls mod_name fixs rn_name_supply
152 = -- Do the ordinary instance declarations
153 mapNF_Tc (tcInstDecl1 unf_env)
154 [inst_decl | InstD inst_decl <- decls] `thenNF_Tc` \ inst_info_bags ->
156 decl_inst_info = unionManyBags inst_info_bags
158 -- Handle "derived" instances; note that we only do derivings
159 -- for things in this module; we ignore deriving decls from
161 tcDeriving mod_name fixs rn_name_supply decl_inst_info
162 `thenTc` \ (deriv_inst_info, deriv_binds) ->
165 full_inst_info = deriv_inst_info `unionBags` decl_inst_info
167 returnTc (full_inst_info, deriv_binds)
170 tcInstDecl1 :: ValueEnv -> RenamedInstDecl -> NF_TcM s (Bag InstInfo)
172 tcInstDecl1 unf_env (InstDecl poly_ty binds uprags dfun_name src_loc)
173 = -- Prime error recovery, set source location
174 recoverNF_Tc (returnNF_Tc emptyBag) $
175 tcAddSrcLoc src_loc $
177 -- Type-check all the stuff before the "where"
178 tcHsTopType poly_ty `thenTc` \ poly_ty' ->
180 (tyvars, theta, dict_ty) = splitSigmaTy poly_ty'
181 (clas, inst_tys) = case splitDictTy_maybe dict_ty of
182 Nothing -> pprPanic "tcInstDecl1" (ppr poly_ty)
186 -- Check for respectable instance type, and context
187 -- but only do this for non-imported instance decls.
188 -- Imported ones should have been checked already, and may indeed
189 -- contain something illegal in normal Haskell, notably
190 -- instance CCallable [Char]
191 (if isLocallyDefined dfun_name then
192 scrutiniseInstanceHead clas inst_tys `thenNF_Tc_`
193 mapNF_Tc scrutiniseInstanceConstraint theta
200 dfun_id = mkDictFunId dfun_name clas tyvars inst_tys theta
202 -- Add info from interface file
203 final_dfun_id = tcAddImportedIdInfo unf_env dfun_id
205 returnTc (unitBag (InstInfo clas tyvars inst_tys theta
207 binds src_loc uprags))
211 %************************************************************************
213 \subsection{Type-checking instance declarations, pass 2}
215 %************************************************************************
218 tcInstDecls2 :: Bag InstInfo
219 -> NF_TcM s (LIE, TcMonoBinds)
221 tcInstDecls2 inst_decls
222 = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
224 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
225 tc2 `thenNF_Tc` \ (lie2, binds2) ->
226 returnNF_Tc (lie1 `plusLIE` lie2,
227 binds1 `AndMonoBinds` binds2)
231 ======= New documentation starts here (Sept 92) ==============
233 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
234 the dictionary function for this instance declaration. For example
236 instance Foo a => Foo [a] where
240 might generate something like
242 dfun.Foo.List dFoo_a = let op1 x = ...
248 HOWEVER, if the instance decl has no context, then it returns a
249 bigger @HsBinds@ with declarations for each method. For example
251 instance Foo [a] where
257 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
258 const.Foo.op1.List a x = ...
259 const.Foo.op2.List a y = ...
261 This group may be mutually recursive, because (for example) there may
262 be no method supplied for op2 in which case we'll get
264 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
266 that is, the default method applied to the dictionary at this type.
268 What we actually produce in either case is:
270 AbsBinds [a] [dfun_theta_dicts]
271 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
272 { d = (sd1,sd2, ..., op1, op2, ...)
277 The "maybe" says that we only ask AbsBinds to make global constant methods
278 if the dfun_theta is empty.
281 For an instance declaration, say,
283 instance (C1 a, C2 b) => C (T a b) where
286 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
287 function whose type is
289 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
291 Notice that we pass it the superclass dictionaries at the instance type; this
292 is the ``Mark Jones optimisation''. The stuff before the "=>" here
293 is the @dfun_theta@ below.
295 First comes the easy case of a non-local instance decl.
298 tcInstDecl2 :: InstInfo -> NF_TcM s (LIE, TcMonoBinds)
300 tcInstDecl2 (InstInfo clas inst_tyvars inst_tys
304 | not (isLocallyDefined dfun_id)
305 = returnNF_Tc (emptyLIE, EmptyMonoBinds)
308 -- I deleted this "optimisation" because when importing these
309 -- instance decls the renamer would look for the dfun bindings and they weren't there.
310 -- This would be fixable, but it seems simpler just to produce a tiny void binding instead,
311 -- even though it's never used.
313 -- This case deals with CCallable etc, which don't need any bindings
315 = returnNF_Tc (emptyLIE, EmptyBinds)
319 = -- Prime error recovery
320 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
323 -- Instantiate the instance decl with tc-style type variables
324 tcInstId dfun_id `thenNF_Tc` \ (inst_tyvars', dfun_theta', dict_ty') ->
326 (clas, inst_tys') = expectJust "tcInstDecl2" (splitDictTy_maybe dict_ty')
328 origin = InstanceDeclOrigin
331 sc_theta, sc_sel_ids,
332 op_sel_ids, defm_ids) = classBigSig clas
334 -- Instantiate the theta found in the original instance decl
335 inst_decl_theta' = substTheta (mkTopTyVarSubst inst_tyvars (mkTyVarTys inst_tyvars'))
338 -- Instantiate the super-class context with inst_tys
339 sc_theta' = substTheta (mkTopTyVarSubst class_tyvars inst_tys') sc_theta
341 -- Create dictionary Ids from the specified instance contexts.
342 newDicts origin sc_theta' `thenNF_Tc` \ (sc_dicts, sc_dict_ids) ->
343 newDicts origin dfun_theta' `thenNF_Tc` \ (dfun_arg_dicts, dfun_arg_dicts_ids) ->
344 newDicts origin inst_decl_theta' `thenNF_Tc` \ (inst_decl_dicts, _) ->
345 newDicts origin [(clas,inst_tys')] `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
347 -- Check that all the method bindings come from this class
348 checkFromThisClass clas op_sel_ids monobinds `thenNF_Tc_`
350 tcExtendTyVarEnvForMeths inst_tyvars inst_tyvars' (
351 tcExtendGlobalValEnv (catMaybes defm_ids) (
352 -- Default-method Ids may be mentioned in synthesised RHSs
354 mapAndUnzip3Tc (tcMethodBind clas origin inst_tyvars' inst_tys' inst_decl_theta'
355 monobinds uprags True)
356 (op_sel_ids `zip` defm_ids)
357 )) `thenTc` \ (method_binds_s, insts_needed_s, meth_lies_w_ids) ->
359 -- Deal with SPECIALISE instance pragmas by making them
360 -- look like SPECIALISE pragmas for the dfun
362 dfun_prags = [SpecSig (idName dfun_id) ty loc | SpecInstSig ty loc <- uprags]
364 tcExtendGlobalValEnv [dfun_id] (
365 tcSpecSigs dfun_prags
366 ) `thenTc` \ (prag_binds, prag_lie) ->
368 -- Check the overloading constraints of the methods and superclasses
370 -- tcMethodBind has checked that the class_tyvars havn't
371 -- been unified with each other or another type, but we must
373 mapNF_Tc zonkTcTyVarBndr inst_tyvars' `thenNF_Tc` \ zonked_inst_tyvars ->
375 inst_tyvars_set = mkVarSet zonked_inst_tyvars
377 (meth_lies, meth_ids) = unzip meth_lies_w_ids
379 -- These insts are in scope; quite a few, eh?
380 avail_insts = this_dict `plusLIE`
381 dfun_arg_dicts `plusLIE`
383 unionManyBags meth_lies
385 methods_lie = plusLIEs insts_needed_s
388 -- Ditto method bindings
389 tcAddErrCtxt methodCtxt (
391 (ptext SLIT("instance declaration context"))
392 inst_tyvars_set -- Local tyvars
395 ) `thenTc` \ (const_lie1, lie_binds1) ->
397 -- Check that we *could* construct the superclass dictionaries,
398 -- even though we are *actually* going to pass the superclass dicts in;
399 -- the check ensures that the caller will never have
400 --a problem building them.
401 tcAddErrCtxt superClassCtxt (
403 (ptext SLIT("instance declaration context"))
404 inst_tyvars_set -- Local tyvars
405 inst_decl_dicts -- The instance dictionaries available
406 sc_dicts -- The superclass dicationaries reqd
408 -- Ignore the result; we're only doing
409 -- this to make sure it can be done.
411 -- Now do the simplification again, this time to get the
412 -- bindings; this time we use an enhanced "avails"
413 -- Ignore errors because they come from the *previous* tcSimplify
416 (ptext SLIT("instance declaration context"))
418 dfun_arg_dicts -- NB! Don't include this_dict here, else the sc_dicts
419 -- get bound by just selecting from this_dict!!
421 ) `thenTc` \ (const_lie2, lie_binds2) ->
424 -- Create the result bindings
426 dict_constr = classDataCon clas
427 scs_and_meths = sc_dict_ids ++ meth_ids
431 = -- Blatant special case for CCallable, CReturnable
432 -- If the dictionary is empty then we should never
433 -- select anything from it, so we make its RHS just
434 -- emit an error message. This in turn means that we don't
435 -- mention the constructor, which doesn't exist for CCallable, CReturnable
436 -- Hardly beautiful, but only three extra lines.
437 HsApp (TyApp (HsVar eRROR_ID) [(unUsgTy . idType) this_dict_id])
438 (HsLitOut (HsString msg) stringTy)
440 | otherwise -- The common case
441 = foldl HsApp (TyApp (HsVar (dataConId dict_constr)) inst_tys')
442 (map HsVar (sc_dict_ids ++ meth_ids))
443 -- We don't produce a binding for the dict_constr; instead we
444 -- rely on the simplifier to unfold this saturated application
445 -- We do this rather than generate an HsCon directly, because
446 -- it means that the special cases (e.g. dictionary with only one
447 -- member) are dealt with by the common MkId.mkDataConId code rather
448 -- than needing to be repeated here.
451 msg = _PK_ ("Compiler error: bad dictionary " ++ showSDoc (ppr clas))
453 dict_bind = VarMonoBind this_dict_id dict_rhs
454 method_binds = andMonoBindList method_binds_s
460 [(inst_tyvars', dfun_id, this_dict_id)]
461 emptyNameSet -- No inlines (yet)
462 (lie_binds1 `AndMonoBinds`
463 lie_binds2 `AndMonoBinds`
464 method_binds `AndMonoBinds`
467 returnTc (const_lie1 `plusLIE` const_lie2 `plusLIE` prag_lie,
468 main_bind `AndMonoBinds` prag_binds)
472 %************************************************************************
474 \subsection{Checking for a decent instance type}
476 %************************************************************************
478 @scrutiniseInstanceHead@ checks the type {\em and} its syntactic constraints:
479 it must normally look like: @instance Foo (Tycon a b c ...) ...@
481 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
482 flag is on, or (2)~the instance is imported (they must have been
483 compiled elsewhere). In these cases, we let them go through anyway.
485 We can also have instances for functions: @instance Foo (a -> b) ...@.
488 scrutiniseInstanceConstraint (clas, tys)
490 || opt_AllowUndecidableInstances = returnNF_Tc ()
491 | otherwise = addErrTc (instConstraintErr clas tys)
493 scrutiniseInstanceHead clas inst_taus
494 | -- CCALL CHECK (a).... urgh!
495 -- To verify that a user declaration of a CCallable/CReturnable
496 -- instance is OK, we must be able to see the constructor(s)
497 -- of the instance type (see next guard.)
499 -- We flag this separately to give a more precise error msg.
501 (getUnique clas == cCallableClassKey || getUnique clas == cReturnableClassKey)
502 && is_alg_tycon_app && not constructors_visible
503 = addErrTc (invisibleDataConPrimCCallErr clas first_inst_tau)
506 -- A user declaration of a CCallable/CReturnable instance
507 -- must be for a "boxed primitive" type.
508 (getUnique clas == cCallableClassKey && not (ccallable_type first_inst_tau)) ||
509 (getUnique clas == cReturnableClassKey && not (creturnable_type first_inst_tau))
510 = addErrTc (nonBoxedPrimCCallErr clas first_inst_tau)
513 -- It is obviously illegal to have an explicit instance
514 -- for something that we are also planning to `derive'
515 | maybeToBool alg_tycon_app_maybe && clas `elem` (tyConDerivings alg_tycon)
516 = addErrTc (derivingWhenInstanceExistsErr clas first_inst_tau)
517 -- Kind check will have ensured inst_taus is of length 1
519 -- Allow anything for AllowUndecidableInstances
520 | opt_AllowUndecidableInstances
523 -- If GlasgowExts then check at least one isn't a type variable
525 = if all isTyVarTy inst_taus then
526 addErrTc (instTypeErr clas inst_taus (text "There must be at least one non-type-variable in the instance head"))
530 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
531 | not (length inst_taus == 1 &&
532 maybeToBool maybe_tycon_app && -- Yes, there's a type constuctor
533 not (isSynTyCon tycon) && -- ...but not a synonym
534 all isTyVarTy arg_tys && -- Applied to type variables
535 length (varSetElems (tyVarsOfTypes arg_tys)) == length arg_tys
536 -- This last condition checks that all the type variables are distinct
538 = addErrTc (instTypeErr clas inst_taus
539 (text "the instance type must be of form (T a b c)" $$
540 text "where T is not a synonym, and a,b,c are distinct type variables")
547 (first_inst_tau : _) = inst_taus
549 -- Stuff for algebraic or -> type
550 maybe_tycon_app = splitTyConApp_maybe first_inst_tau
551 Just (tycon, arg_tys) = maybe_tycon_app
553 -- Stuff for an *algebraic* data type
554 alg_tycon_app_maybe = splitAlgTyConApp_maybe first_inst_tau
555 -- The "Alg" part looks through synonyms
556 is_alg_tycon_app = maybeToBool alg_tycon_app_maybe
557 Just (alg_tycon, _, data_cons) = alg_tycon_app_maybe
559 constructors_visible = not (null data_cons)
562 -- These conditions come directly from what the DsCCall is capable of.
563 -- Totally grotesque. Green card should solve this.
565 ccallable_type ty = isUnLiftedType ty || -- Allow CCallable Int# etc
566 maybeToBool (maybeBoxedPrimType ty) || -- Ditto Int etc
570 byte_arr_thing = case splitAlgTyConApp_maybe ty of
571 Just (tycon, ty_args, [data_con]) | isDataTyCon tycon ->
572 length data_con_arg_tys == 2 &&
573 maybeToBool maybe_arg2_tycon &&
574 (arg2_tycon == byteArrayPrimTyCon ||
575 arg2_tycon == mutableByteArrayPrimTyCon)
577 data_con_arg_tys = dataConArgTys data_con ty_args
578 (data_con_arg_ty1 : data_con_arg_ty2 : _) = data_con_arg_tys
579 maybe_arg2_tycon = splitTyConApp_maybe data_con_arg_ty2
580 Just (arg2_tycon,_) = maybe_arg2_tycon
584 creturnable_type ty = maybeToBool (maybeBoxedPrimType ty) ||
585 -- Or, a data type with a single nullary constructor
586 case (splitAlgTyConApp_maybe ty) of
587 Just (tycon, tys_applied, [data_con])
588 -> isNullaryDataCon data_con
593 instConstraintErr clas tys
594 = hang (ptext SLIT("Illegal constaint") <+>
595 quotes (pprConstraint clas tys) <+>
596 ptext SLIT("in instance context"))
597 4 (ptext SLIT("(Instance contexts must constrain only type variables)"))
599 instTypeErr clas tys msg
600 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes (pprConstraint clas tys),
604 derivingWhenInstanceExistsErr clas tycon
605 = hang (hsep [ptext SLIT("Deriving class"),
607 ptext SLIT("type"), quotes (ppr tycon)])
608 4 (ptext SLIT("when an explicit instance exists"))
610 nonBoxedPrimCCallErr clas inst_ty
611 = hang (ptext SLIT("Unacceptable instance type for ccall-ish class"))
612 4 (hsep [ ptext SLIT("class"), ppr clas, ptext SLIT("type"),
616 Declaring CCallable & CReturnable instances in a module different
617 from where the type was defined. Caused by importing data type
618 abstractly (either programmatically or by the renamer being over-eager
621 invisibleDataConPrimCCallErr clas inst_ty
622 = hang (hsep [ptext SLIT("Constructors for"), quotes (ppr inst_ty),
623 ptext SLIT("not visible when checking"),
624 quotes (ppr clas), ptext SLIT("instance")])
625 4 (hsep [text "(Try either importing", ppr inst_ty,
626 text "non-abstractly or compile using -fno-prune-tydecls ..)"])
628 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
629 superClassCtxt = ptext SLIT("When checking the superclasses of an instance declaration")