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(..),
17 collectMonoBinders, andMonoBindList
19 import RnHsSyn ( RenamedHsBinds, RenamedInstDecl, RenamedHsDecl )
20 import TcHsSyn ( TcMonoBinds,
24 import TcBinds ( tcPragmaSigs )
25 import TcClassDcl ( tcMethodBind, badMethodErr )
27 import RnMonad ( RnNameSupply )
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,
40 foldBag, bagToList, Bag
42 import CmdLineOpts ( opt_GlasgowExts, opt_AllowUndecidableInstances )
43 import Class ( classBigSig, Class )
44 import Var ( setIdInfo, idName, idType, Id, TyVar )
45 import DataCon ( isNullaryDataCon, dataConArgTys, dataConId )
46 import Maybes ( maybeToBool, catMaybes, expectJust )
47 import MkId ( mkDictFunId )
48 import Name ( nameOccName, isLocallyDefined, Module,
51 import PrelVals ( 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,
58 splitAlgTyConApp_maybe,
59 tyVarsOfTypes, substTopTheta
61 import VarEnv ( zipVarEnv )
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 -> Module -- module name for deriving
146 -> RnNameSupply -- for renaming derivings
147 -> TcM s (Bag InstInfo,
150 tcInstDecls1 unf_env decls mod_name rn_name_supply
151 = -- Do the ordinary instance declarations
152 mapNF_Tc (tcInstDecl1 unf_env mod_name)
153 [inst_decl | InstD inst_decl <- decls] `thenNF_Tc` \ inst_info_bags ->
155 decl_inst_info = unionManyBags inst_info_bags
157 -- Handle "derived" instances; note that we only do derivings
158 -- for things in this module; we ignore deriving decls from
160 tcDeriving mod_name rn_name_supply decl_inst_info
161 `thenTc` \ (deriv_inst_info, deriv_binds) ->
164 full_inst_info = deriv_inst_info `unionBags` decl_inst_info
166 returnTc (full_inst_info, deriv_binds)
169 tcInstDecl1 :: ValueEnv -> Module -> RenamedInstDecl -> NF_TcM s (Bag InstInfo)
171 tcInstDecl1 unf_env mod_name (InstDecl poly_ty binds uprags (Just dfun_name) src_loc)
172 = -- Prime error recovery, set source location
173 recoverNF_Tc (returnNF_Tc emptyBag) $
174 tcAddSrcLoc src_loc $
176 -- Type-check all the stuff before the "where"
177 tcHsTopType poly_ty `thenTc` \ poly_ty' ->
179 (tyvars, theta, dict_ty) = splitSigmaTy poly_ty'
180 (clas, inst_tys) = case splitDictTy_maybe dict_ty of
181 Nothing -> pprPanic "tcInstDecl1" (ppr poly_ty)
185 -- Check for respectable instance type, and context
186 scrutiniseInstanceHead clas inst_tys `thenNF_Tc_`
187 mapNF_Tc scrutiniseInstanceConstraint theta `thenNF_Tc_`
189 -- Make the dfun id and constant-method ids
191 dfun_id = mkDictFunId dfun_name clas tyvars inst_tys theta
193 -- Add info from interface file
194 final_dfun_id = tcAddImportedIdInfo unf_env dfun_id
196 returnTc (unitBag (InstInfo clas tyvars inst_tys theta
198 binds src_loc uprags))
202 %************************************************************************
204 \subsection{Type-checking instance declarations, pass 2}
206 %************************************************************************
209 tcInstDecls2 :: Bag InstInfo
210 -> NF_TcM s (LIE, TcMonoBinds)
212 tcInstDecls2 inst_decls
213 = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
215 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
216 tc2 `thenNF_Tc` \ (lie2, binds2) ->
217 returnNF_Tc (lie1 `plusLIE` lie2,
218 binds1 `AndMonoBinds` binds2)
222 ======= New documentation starts here (Sept 92) ==============
224 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
225 the dictionary function for this instance declaration. For example
227 instance Foo a => Foo [a] where
231 might generate something like
233 dfun.Foo.List dFoo_a = let op1 x = ...
239 HOWEVER, if the instance decl has no context, then it returns a
240 bigger @HsBinds@ with declarations for each method. For example
242 instance Foo [a] where
248 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
249 const.Foo.op1.List a x = ...
250 const.Foo.op2.List a y = ...
252 This group may be mutually recursive, because (for example) there may
253 be no method supplied for op2 in which case we'll get
255 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
257 that is, the default method applied to the dictionary at this type.
259 What we actually produce in either case is:
261 AbsBinds [a] [dfun_theta_dicts]
262 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
263 { d = (sd1,sd2, ..., op1, op2, ...)
268 The "maybe" says that we only ask AbsBinds to make global constant methods
269 if the dfun_theta is empty.
272 For an instance declaration, say,
274 instance (C1 a, C2 b) => C (T a b) where
277 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
278 function whose type is
280 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
282 Notice that we pass it the superclass dictionaries at the instance type; this
283 is the ``Mark Jones optimisation''. The stuff before the "=>" here
284 is the @dfun_theta@ below.
286 First comes the easy case of a non-local instance decl.
289 tcInstDecl2 :: InstInfo -> NF_TcM s (LIE, TcMonoBinds)
291 tcInstDecl2 (InstInfo clas inst_tyvars inst_tys
295 | not (isLocallyDefined dfun_id)
296 = returnNF_Tc (emptyLIE, EmptyMonoBinds)
299 -- I deleted this "optimisation" because when importing these
300 -- instance decls the renamer would look for the dfun bindings and they weren't there.
301 -- This would be fixable, but it seems simpler just to produce a tiny void binding instead,
302 -- even though it's never used.
304 -- This case deals with CCallable etc, which don't need any bindings
306 = returnNF_Tc (emptyLIE, EmptyBinds)
310 = -- Prime error recovery
311 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
314 -- Instantiate the instance decl with tc-style type variables
315 tcInstId dfun_id `thenNF_Tc` \ (inst_tyvars', dfun_theta', dict_ty') ->
317 (clas, inst_tys') = expectJust "tcInstDecl2" (splitDictTy_maybe dict_ty')
319 origin = InstanceDeclOrigin
322 sc_theta, sc_sel_ids,
323 op_sel_ids, defm_ids) = classBigSig clas
325 -- Instantiate the theta found in the original instance decl
326 inst_decl_theta' = substTopTheta (zipVarEnv inst_tyvars (mkTyVarTys inst_tyvars'))
329 -- Instantiate the super-class context with inst_tys
330 sc_theta' = substTopTheta (zipVarEnv class_tyvars inst_tys') sc_theta
332 -- Create dictionary Ids from the specified instance contexts.
333 newDicts origin sc_theta' `thenNF_Tc` \ (sc_dicts, sc_dict_ids) ->
334 newDicts origin dfun_theta' `thenNF_Tc` \ (dfun_arg_dicts, dfun_arg_dicts_ids) ->
335 newDicts origin inst_decl_theta' `thenNF_Tc` \ (inst_decl_dicts, _) ->
336 newDicts origin [(clas,inst_tys')] `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
338 -- Check that all the method bindings come from this class
340 check_from_this_class (bndr, loc)
341 | nameOccName bndr `elem` sel_names = returnNF_Tc ()
342 | otherwise = tcAddSrcLoc loc $
343 addErrTc (badMethodErr bndr clas)
344 sel_names = map getOccName op_sel_ids
345 bndrs = bagToList (collectMonoBinders monobinds)
347 mapNF_Tc check_from_this_class bndrs `thenNF_Tc_`
349 tcExtendTyVarEnvForMeths inst_tyvars inst_tyvars' (
350 tcExtendGlobalValEnv (catMaybes defm_ids) (
351 -- Default-method Ids may be mentioned in synthesised RHSs
353 mapAndUnzip3Tc (tcMethodBind clas origin inst_tyvars' inst_tys' inst_decl_theta'
354 monobinds uprags True)
355 (op_sel_ids `zip` defm_ids)
356 )) `thenTc` \ (method_binds_s, insts_needed_s, meth_lies_w_ids) ->
358 -- Deal with SPECIALISE instance pragmas
360 dfun_prags = [Sig (idName dfun_id) ty loc | SpecInstSig ty loc <- uprags]
362 tcExtendGlobalValEnv [dfun_id] (
363 tcPragmaSigs dfun_prags
364 ) `thenTc` \ (prag_info_fn, prag_binds, prag_lie) ->
366 -- Check the overloading constraints of the methods and superclasses
368 -- tcMethodBind has checked that the class_tyvars havn't
369 -- been unified with each other or another type, but we must
371 mapNF_Tc zonkTcTyVarBndr inst_tyvars' `thenNF_Tc` \ zonked_inst_tyvars ->
373 inst_tyvars_set = mkVarSet zonked_inst_tyvars
375 (meth_lies, meth_ids) = unzip meth_lies_w_ids
377 -- These insts are in scope; quite a few, eh?
378 avail_insts = this_dict `plusLIE`
379 dfun_arg_dicts `plusLIE`
381 unionManyBags meth_lies
383 methods_lie = plusLIEs insts_needed_s
386 -- Ditto method bindings
387 tcAddErrCtxt methodCtxt (
389 (ptext SLIT("instance declaration context"))
390 inst_tyvars_set -- Local tyvars
393 ) `thenTc` \ (const_lie1, lie_binds1) ->
395 -- Check that we *could* construct the superclass dictionaries,
396 -- even though we are *actually* going to pass the superclass dicts in;
397 -- the check ensures that the caller will never have
398 --a problem building them.
399 tcAddErrCtxt superClassCtxt (
401 (ptext SLIT("instance declaration context"))
402 inst_tyvars_set -- Local tyvars
403 inst_decl_dicts -- The instance dictionaries available
404 sc_dicts -- The superclass dicationaries reqd
406 -- Ignore the result; we're only doing
407 -- this to make sure it can be done.
409 -- Now do the simplification again, this time to get the
410 -- bindings; this time we use an enhanced "avails"
411 -- Ignore errors because they come from the *previous* tcSimplify
414 (ptext SLIT("instance declaration context"))
416 dfun_arg_dicts -- NB! Don't include this_dict here, else the sc_dicts
417 -- get bound by just selecting from this_dict!!
419 ) `thenTc` \ (const_lie2, lie_binds2) ->
422 -- Create the result bindings
424 dict_constr = classDataCon clas
425 scs_and_meths = sc_dict_ids ++ meth_ids
429 = -- Blatant special case for CCallable, CReturnable [and Eval -- sof 5/98]
430 -- If the dictionary is empty then we should never
431 -- select anything from it, so we make its RHS just
432 -- emit an error message. This in turn means that we don't
433 -- mention the constructor, which doesn't exist for CCallable, CReturnable
434 -- Hardly beautiful, but only three extra lines.
435 HsApp (TyApp (HsVar eRROR_ID) [idType this_dict_id])
436 (HsLitOut (HsString msg) stringTy)
438 | otherwise -- The common case
439 = foldl HsApp (TyApp (HsVar (dataConId dict_constr)) inst_tys')
440 (map HsVar (sc_dict_ids ++ meth_ids))
441 -- We don't produce a binding for the dict_constr; instead we
442 -- rely on the simplifier to unfold this saturated application
443 -- We do this rather than generate an HsCon directly, because
444 -- it means that the special cases (e.g. dictionary with only one
445 -- member) are dealt with by the common MkId.mkDataConId code rather
446 -- than needing to be repeated here.
449 msg = _PK_ ("Compiler error: bad dictionary " ++ showSDoc (ppr clas))
451 dict_bind = VarMonoBind this_dict_id dict_rhs
452 method_binds = andMonoBindList method_binds_s
454 final_dfun_id = setIdInfo dfun_id (prag_info_fn (idName dfun_id))
460 [(inst_tyvars', final_dfun_id, this_dict_id)]
461 (lie_binds1 `AndMonoBinds`
462 lie_binds2 `AndMonoBinds`
463 method_binds `AndMonoBinds`
466 returnTc (const_lie1 `plusLIE` const_lie2 `plusLIE` prag_lie,
467 main_bind `AndMonoBinds` prag_binds)
471 %************************************************************************
473 \subsection{Checking for a decent instance type}
475 %************************************************************************
477 @scrutiniseInstanceHead@ checks the type {\em and} its syntactic constraints:
478 it must normally look like: @instance Foo (Tycon a b c ...) ...@
480 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
481 flag is on, or (2)~the instance is imported (they must have been
482 compiled elsewhere). In these cases, we let them go through anyway.
484 We can also have instances for functions: @instance Foo (a -> b) ...@.
487 scrutiniseInstanceConstraint (clas, tys)
489 || opt_AllowUndecidableInstances = returnNF_Tc ()
490 | otherwise = addErrTc (instConstraintErr clas tys)
492 scrutiniseInstanceHead clas inst_taus
493 | -- CCALL CHECK (a).... urgh!
494 -- To verify that a user declaration of a CCallable/CReturnable
495 -- instance is OK, we must be able to see the constructor(s)
496 -- of the instance type (see next guard.)
498 -- We flag this separately to give a more precise error msg.
500 (getUnique clas == cCallableClassKey || getUnique clas == cReturnableClassKey)
501 && is_alg_tycon_app && not constructors_visible
502 = addErrTc (invisibleDataConPrimCCallErr clas first_inst_tau)
505 -- A user declaration of a CCallable/CReturnable instance
506 -- must be for a "boxed primitive" type.
507 (getUnique clas == cCallableClassKey && not (ccallable_type first_inst_tau)) ||
508 (getUnique clas == cReturnableClassKey && not (creturnable_type first_inst_tau))
509 = addErrTc (nonBoxedPrimCCallErr clas first_inst_tau)
512 -- It is obviously illegal to have an explicit instance
513 -- for something that we are also planning to `derive'
514 | maybeToBool alg_tycon_app_maybe && clas `elem` (tyConDerivings alg_tycon)
515 = addErrTc (derivingWhenInstanceExistsErr clas first_inst_tau)
516 -- Kind check will have ensured inst_taus is of length 1
518 -- Allow anything for AllowUndecidableInstances
519 | opt_AllowUndecidableInstances
522 -- If GlasgowExts then check at least one isn't a type variable
524 = if all isTyVarTy inst_taus then
525 addErrTc (instTypeErr clas inst_taus (text "There must be at least one non-type-variable in the instance head"))
529 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
530 | not (length inst_taus == 1 &&
531 maybeToBool maybe_tycon_app && -- Yes, there's a type constuctor
532 not (isSynTyCon tycon) && -- ...but not a synonym
533 all isTyVarTy arg_tys && -- Applied to type variables
534 length (varSetElems (tyVarsOfTypes arg_tys)) == length arg_tys
535 -- This last condition checks that all the type variables are distinct
537 = addErrTc (instTypeErr clas inst_taus
538 (text "the instance type must be of form (T a b c)" $$
539 text "where T is not a synonym, and a,b,c are distinct type variables")
546 (first_inst_tau : _) = inst_taus
548 -- Stuff for algebraic or -> type
549 maybe_tycon_app = splitTyConApp_maybe first_inst_tau
550 Just (tycon, arg_tys) = maybe_tycon_app
552 -- Stuff for an *algebraic* data type
553 alg_tycon_app_maybe = splitAlgTyConApp_maybe first_inst_tau
554 -- The "Alg" part looks through synonyms
555 is_alg_tycon_app = maybeToBool alg_tycon_app_maybe
556 Just (alg_tycon, _, data_cons) = alg_tycon_app_maybe
558 constructors_visible = not (null data_cons)
561 -- These conditions come directly from what the DsCCall is capable of.
562 -- Totally grotesque. Green card should solve this.
564 ccallable_type ty = isUnLiftedType ty || -- Allow CCallable Int# etc
565 maybeToBool (maybeBoxedPrimType ty) || -- Ditto Int etc
569 byte_arr_thing = case splitAlgTyConApp_maybe ty of
570 Just (tycon, ty_args, [data_con]) | isDataTyCon tycon ->
571 length data_con_arg_tys == 2 &&
572 maybeToBool maybe_arg2_tycon &&
573 (arg2_tycon == byteArrayPrimTyCon ||
574 arg2_tycon == mutableByteArrayPrimTyCon)
576 data_con_arg_tys = dataConArgTys data_con ty_args
577 (data_con_arg_ty1 : data_con_arg_ty2 : _) = data_con_arg_tys
578 maybe_arg2_tycon = splitTyConApp_maybe data_con_arg_ty2
579 Just (arg2_tycon,_) = maybe_arg2_tycon
583 creturnable_type ty = maybeToBool (maybeBoxedPrimType ty) ||
584 -- Or, a data type with a single nullary constructor
585 case (splitAlgTyConApp_maybe ty) of
586 Just (tycon, tys_applied, [data_con])
587 -> isNullaryDataCon data_con
592 instConstraintErr clas tys
593 = hang (ptext SLIT("Illegal constaint") <+>
594 quotes (pprConstraint clas tys) <+>
595 ptext SLIT("in instance context"))
596 4 (ptext SLIT("(Instance contexts must constrain only type variables)"))
598 instTypeErr clas tys msg
599 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes (pprConstraint clas tys),
603 derivingWhenInstanceExistsErr clas tycon
604 = hang (hsep [ptext SLIT("Deriving class"),
606 ptext SLIT("type"), quotes (ppr tycon)])
607 4 (ptext SLIT("when an explicit instance exists"))
609 nonBoxedPrimCCallErr clas inst_ty
610 = hang (ptext SLIT("Unacceptable instance type for ccall-ish class"))
611 4 (hsep [ ptext SLIT("class"), ppr clas, ptext SLIT("type"),
615 Declaring CCallable & CReturnable instances in a module different
616 from where the type was defined. Caused by importing data type
617 abstractly (either programmatically or by the renamer being over-eager
620 invisibleDataConPrimCCallErr clas inst_ty
621 = hang (hsep [ptext SLIT("Constructors for"), quotes (ppr inst_ty),
622 ptext SLIT("not visible when checking"),
623 quotes (ppr clas), ptext SLIT("instance")])
624 4 (hsep [text "(Try either importing", ppr inst_ty,
625 text "non-abstractly or compile using -fno-prune-tydecls ..)"])
627 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
628 superClassCtxt = ptext SLIT("When checking the superclasses of an instance declaration")