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 -- but only do this for non-imported instance decls.
187 -- Imported ones should have been checked already, and may indeed
188 -- contain something illegal in normal Haskell, notably
189 -- instance CCallable [Char]
190 (if isLocallyDefined dfun_name then
191 scrutiniseInstanceHead clas inst_tys `thenNF_Tc_`
192 mapNF_Tc scrutiniseInstanceConstraint theta
197 -- Make the dfun id and constant-method ids
199 dfun_id = mkDictFunId dfun_name clas tyvars inst_tys theta
201 -- Add info from interface file
202 final_dfun_id = tcAddImportedIdInfo unf_env dfun_id
204 returnTc (unitBag (InstInfo clas tyvars inst_tys theta
206 binds src_loc uprags))
210 %************************************************************************
212 \subsection{Type-checking instance declarations, pass 2}
214 %************************************************************************
217 tcInstDecls2 :: Bag InstInfo
218 -> NF_TcM s (LIE, TcMonoBinds)
220 tcInstDecls2 inst_decls
221 = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
223 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
224 tc2 `thenNF_Tc` \ (lie2, binds2) ->
225 returnNF_Tc (lie1 `plusLIE` lie2,
226 binds1 `AndMonoBinds` binds2)
230 ======= New documentation starts here (Sept 92) ==============
232 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
233 the dictionary function for this instance declaration. For example
235 instance Foo a => Foo [a] where
239 might generate something like
241 dfun.Foo.List dFoo_a = let op1 x = ...
247 HOWEVER, if the instance decl has no context, then it returns a
248 bigger @HsBinds@ with declarations for each method. For example
250 instance Foo [a] where
256 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
257 const.Foo.op1.List a x = ...
258 const.Foo.op2.List a y = ...
260 This group may be mutually recursive, because (for example) there may
261 be no method supplied for op2 in which case we'll get
263 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
265 that is, the default method applied to the dictionary at this type.
267 What we actually produce in either case is:
269 AbsBinds [a] [dfun_theta_dicts]
270 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
271 { d = (sd1,sd2, ..., op1, op2, ...)
276 The "maybe" says that we only ask AbsBinds to make global constant methods
277 if the dfun_theta is empty.
280 For an instance declaration, say,
282 instance (C1 a, C2 b) => C (T a b) where
285 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
286 function whose type is
288 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
290 Notice that we pass it the superclass dictionaries at the instance type; this
291 is the ``Mark Jones optimisation''. The stuff before the "=>" here
292 is the @dfun_theta@ below.
294 First comes the easy case of a non-local instance decl.
297 tcInstDecl2 :: InstInfo -> NF_TcM s (LIE, TcMonoBinds)
299 tcInstDecl2 (InstInfo clas inst_tyvars inst_tys
303 | not (isLocallyDefined dfun_id)
304 = returnNF_Tc (emptyLIE, EmptyMonoBinds)
307 -- I deleted this "optimisation" because when importing these
308 -- instance decls the renamer would look for the dfun bindings and they weren't there.
309 -- This would be fixable, but it seems simpler just to produce a tiny void binding instead,
310 -- even though it's never used.
312 -- This case deals with CCallable etc, which don't need any bindings
314 = returnNF_Tc (emptyLIE, EmptyBinds)
318 = -- Prime error recovery
319 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
322 -- Instantiate the instance decl with tc-style type variables
323 tcInstId dfun_id `thenNF_Tc` \ (inst_tyvars', dfun_theta', dict_ty') ->
325 (clas, inst_tys') = expectJust "tcInstDecl2" (splitDictTy_maybe dict_ty')
327 origin = InstanceDeclOrigin
330 sc_theta, sc_sel_ids,
331 op_sel_ids, defm_ids) = classBigSig clas
333 -- Instantiate the theta found in the original instance decl
334 inst_decl_theta' = substTopTheta (zipVarEnv inst_tyvars (mkTyVarTys inst_tyvars'))
337 -- Instantiate the super-class context with inst_tys
338 sc_theta' = substTopTheta (zipVarEnv class_tyvars inst_tys') sc_theta
340 -- Create dictionary Ids from the specified instance contexts.
341 newDicts origin sc_theta' `thenNF_Tc` \ (sc_dicts, sc_dict_ids) ->
342 newDicts origin dfun_theta' `thenNF_Tc` \ (dfun_arg_dicts, dfun_arg_dicts_ids) ->
343 newDicts origin inst_decl_theta' `thenNF_Tc` \ (inst_decl_dicts, _) ->
344 newDicts origin [(clas,inst_tys')] `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
346 -- Check that all the method bindings come from this class
348 check_from_this_class (bndr, loc)
349 | nameOccName bndr `elem` sel_names = returnNF_Tc ()
350 | otherwise = tcAddSrcLoc loc $
351 addErrTc (badMethodErr bndr clas)
352 sel_names = map getOccName op_sel_ids
353 bndrs = bagToList (collectMonoBinders monobinds)
355 mapNF_Tc check_from_this_class bndrs `thenNF_Tc_`
357 tcExtendTyVarEnvForMeths inst_tyvars inst_tyvars' (
358 tcExtendGlobalValEnv (catMaybes defm_ids) (
359 -- Default-method Ids may be mentioned in synthesised RHSs
361 mapAndUnzip3Tc (tcMethodBind clas origin inst_tyvars' inst_tys' inst_decl_theta'
362 monobinds uprags True)
363 (op_sel_ids `zip` defm_ids)
364 )) `thenTc` \ (method_binds_s, insts_needed_s, meth_lies_w_ids) ->
366 -- Deal with SPECIALISE instance pragmas
368 dfun_prags = [Sig (idName dfun_id) ty loc | SpecInstSig ty loc <- uprags]
370 tcExtendGlobalValEnv [dfun_id] (
371 tcPragmaSigs dfun_prags
372 ) `thenTc` \ (prag_info_fn, prag_binds, prag_lie) ->
374 -- Check the overloading constraints of the methods and superclasses
376 -- tcMethodBind has checked that the class_tyvars havn't
377 -- been unified with each other or another type, but we must
379 mapNF_Tc zonkTcTyVarBndr inst_tyvars' `thenNF_Tc` \ zonked_inst_tyvars ->
381 inst_tyvars_set = mkVarSet zonked_inst_tyvars
383 (meth_lies, meth_ids) = unzip meth_lies_w_ids
385 -- These insts are in scope; quite a few, eh?
386 avail_insts = this_dict `plusLIE`
387 dfun_arg_dicts `plusLIE`
389 unionManyBags meth_lies
391 methods_lie = plusLIEs insts_needed_s
394 -- Ditto method bindings
395 tcAddErrCtxt methodCtxt (
397 (ptext SLIT("instance declaration context"))
398 inst_tyvars_set -- Local tyvars
401 ) `thenTc` \ (const_lie1, lie_binds1) ->
403 -- Check that we *could* construct the superclass dictionaries,
404 -- even though we are *actually* going to pass the superclass dicts in;
405 -- the check ensures that the caller will never have
406 --a problem building them.
407 tcAddErrCtxt superClassCtxt (
409 (ptext SLIT("instance declaration context"))
410 inst_tyvars_set -- Local tyvars
411 inst_decl_dicts -- The instance dictionaries available
412 sc_dicts -- The superclass dicationaries reqd
414 -- Ignore the result; we're only doing
415 -- this to make sure it can be done.
417 -- Now do the simplification again, this time to get the
418 -- bindings; this time we use an enhanced "avails"
419 -- Ignore errors because they come from the *previous* tcSimplify
422 (ptext SLIT("instance declaration context"))
424 dfun_arg_dicts -- NB! Don't include this_dict here, else the sc_dicts
425 -- get bound by just selecting from this_dict!!
427 ) `thenTc` \ (const_lie2, lie_binds2) ->
430 -- Create the result bindings
432 dict_constr = classDataCon clas
433 scs_and_meths = sc_dict_ids ++ meth_ids
437 = -- Blatant special case for CCallable, CReturnable
438 -- If the dictionary is empty then we should never
439 -- select anything from it, so we make its RHS just
440 -- emit an error message. This in turn means that we don't
441 -- mention the constructor, which doesn't exist for CCallable, CReturnable
442 -- Hardly beautiful, but only three extra lines.
443 HsApp (TyApp (HsVar eRROR_ID) [idType this_dict_id])
444 (HsLitOut (HsString msg) stringTy)
446 | otherwise -- The common case
447 = foldl HsApp (TyApp (HsVar (dataConId dict_constr)) inst_tys')
448 (map HsVar (sc_dict_ids ++ meth_ids))
449 -- We don't produce a binding for the dict_constr; instead we
450 -- rely on the simplifier to unfold this saturated application
451 -- We do this rather than generate an HsCon directly, because
452 -- it means that the special cases (e.g. dictionary with only one
453 -- member) are dealt with by the common MkId.mkDataConId code rather
454 -- than needing to be repeated here.
457 msg = _PK_ ("Compiler error: bad dictionary " ++ showSDoc (ppr clas))
459 dict_bind = VarMonoBind this_dict_id dict_rhs
460 method_binds = andMonoBindList method_binds_s
462 final_dfun_id = setIdInfo dfun_id (prag_info_fn (idName dfun_id))
468 [(inst_tyvars', final_dfun_id, this_dict_id)]
469 (lie_binds1 `AndMonoBinds`
470 lie_binds2 `AndMonoBinds`
471 method_binds `AndMonoBinds`
474 returnTc (const_lie1 `plusLIE` const_lie2 `plusLIE` prag_lie,
475 main_bind `AndMonoBinds` prag_binds)
479 %************************************************************************
481 \subsection{Checking for a decent instance type}
483 %************************************************************************
485 @scrutiniseInstanceHead@ checks the type {\em and} its syntactic constraints:
486 it must normally look like: @instance Foo (Tycon a b c ...) ...@
488 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
489 flag is on, or (2)~the instance is imported (they must have been
490 compiled elsewhere). In these cases, we let them go through anyway.
492 We can also have instances for functions: @instance Foo (a -> b) ...@.
495 scrutiniseInstanceConstraint (clas, tys)
497 || opt_AllowUndecidableInstances = returnNF_Tc ()
498 | otherwise = addErrTc (instConstraintErr clas tys)
500 scrutiniseInstanceHead clas inst_taus
501 | -- CCALL CHECK (a).... urgh!
502 -- To verify that a user declaration of a CCallable/CReturnable
503 -- instance is OK, we must be able to see the constructor(s)
504 -- of the instance type (see next guard.)
506 -- We flag this separately to give a more precise error msg.
508 (getUnique clas == cCallableClassKey || getUnique clas == cReturnableClassKey)
509 && is_alg_tycon_app && not constructors_visible
510 = addErrTc (invisibleDataConPrimCCallErr clas first_inst_tau)
513 -- A user declaration of a CCallable/CReturnable instance
514 -- must be for a "boxed primitive" type.
515 (getUnique clas == cCallableClassKey && not (ccallable_type first_inst_tau)) ||
516 (getUnique clas == cReturnableClassKey && not (creturnable_type first_inst_tau))
517 = addErrTc (nonBoxedPrimCCallErr clas first_inst_tau)
520 -- It is obviously illegal to have an explicit instance
521 -- for something that we are also planning to `derive'
522 | maybeToBool alg_tycon_app_maybe && clas `elem` (tyConDerivings alg_tycon)
523 = addErrTc (derivingWhenInstanceExistsErr clas first_inst_tau)
524 -- Kind check will have ensured inst_taus is of length 1
526 -- Allow anything for AllowUndecidableInstances
527 | opt_AllowUndecidableInstances
530 -- If GlasgowExts then check at least one isn't a type variable
532 = if all isTyVarTy inst_taus then
533 addErrTc (instTypeErr clas inst_taus (text "There must be at least one non-type-variable in the instance head"))
537 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
538 | not (length inst_taus == 1 &&
539 maybeToBool maybe_tycon_app && -- Yes, there's a type constuctor
540 not (isSynTyCon tycon) && -- ...but not a synonym
541 all isTyVarTy arg_tys && -- Applied to type variables
542 length (varSetElems (tyVarsOfTypes arg_tys)) == length arg_tys
543 -- This last condition checks that all the type variables are distinct
545 = addErrTc (instTypeErr clas inst_taus
546 (text "the instance type must be of form (T a b c)" $$
547 text "where T is not a synonym, and a,b,c are distinct type variables")
554 (first_inst_tau : _) = inst_taus
556 -- Stuff for algebraic or -> type
557 maybe_tycon_app = splitTyConApp_maybe first_inst_tau
558 Just (tycon, arg_tys) = maybe_tycon_app
560 -- Stuff for an *algebraic* data type
561 alg_tycon_app_maybe = splitAlgTyConApp_maybe first_inst_tau
562 -- The "Alg" part looks through synonyms
563 is_alg_tycon_app = maybeToBool alg_tycon_app_maybe
564 Just (alg_tycon, _, data_cons) = alg_tycon_app_maybe
566 constructors_visible = not (null data_cons)
569 -- These conditions come directly from what the DsCCall is capable of.
570 -- Totally grotesque. Green card should solve this.
572 ccallable_type ty = isUnLiftedType ty || -- Allow CCallable Int# etc
573 maybeToBool (maybeBoxedPrimType ty) || -- Ditto Int etc
577 byte_arr_thing = case splitAlgTyConApp_maybe ty of
578 Just (tycon, ty_args, [data_con]) | isDataTyCon tycon ->
579 length data_con_arg_tys == 2 &&
580 maybeToBool maybe_arg2_tycon &&
581 (arg2_tycon == byteArrayPrimTyCon ||
582 arg2_tycon == mutableByteArrayPrimTyCon)
584 data_con_arg_tys = dataConArgTys data_con ty_args
585 (data_con_arg_ty1 : data_con_arg_ty2 : _) = data_con_arg_tys
586 maybe_arg2_tycon = splitTyConApp_maybe data_con_arg_ty2
587 Just (arg2_tycon,_) = maybe_arg2_tycon
591 creturnable_type ty = maybeToBool (maybeBoxedPrimType ty) ||
592 -- Or, a data type with a single nullary constructor
593 case (splitAlgTyConApp_maybe ty) of
594 Just (tycon, tys_applied, [data_con])
595 -> isNullaryDataCon data_con
600 instConstraintErr clas tys
601 = hang (ptext SLIT("Illegal constaint") <+>
602 quotes (pprConstraint clas tys) <+>
603 ptext SLIT("in instance context"))
604 4 (ptext SLIT("(Instance contexts must constrain only type variables)"))
606 instTypeErr clas tys msg
607 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes (pprConstraint clas tys),
611 derivingWhenInstanceExistsErr clas tycon
612 = hang (hsep [ptext SLIT("Deriving class"),
614 ptext SLIT("type"), quotes (ppr tycon)])
615 4 (ptext SLIT("when an explicit instance exists"))
617 nonBoxedPrimCCallErr clas inst_ty
618 = hang (ptext SLIT("Unacceptable instance type for ccall-ish class"))
619 4 (hsep [ ptext SLIT("class"), ppr clas, ptext SLIT("type"),
623 Declaring CCallable & CReturnable instances in a module different
624 from where the type was defined. Caused by importing data type
625 abstractly (either programmatically or by the renamer being over-eager
628 invisibleDataConPrimCCallErr clas inst_ty
629 = hang (hsep [ptext SLIT("Constructors for"), quotes (ppr inst_ty),
630 ptext SLIT("not visible when checking"),
631 quotes (ppr clas), ptext SLIT("instance")])
632 4 (hsep [text "(Try either importing", ppr inst_ty,
633 text "non-abstractly or compile using -fno-prune-tydecls ..)"])
635 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
636 superClassCtxt = ptext SLIT("When checking the superclasses of an instance declaration")