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,
21 import TcBinds ( tcSpecSigs )
22 import TcClassDcl ( tcMethodBind, checkFromThisClass )
24 import RnMonad ( RnNameSupply, Fixities )
25 import Inst ( Inst, InstOrigin(..),
26 newDicts, LIE, emptyLIE, plusLIE, plusLIEs )
27 import TcDeriv ( tcDeriving )
28 import TcEnv ( ValueEnv, tcExtendGlobalValEnv, tcExtendTyVarEnvForMeths,
29 tcAddImportedIdInfo, tcInstId
31 import TcInstUtil ( InstInfo(..), classDataCon )
32 import TcMonoType ( tcHsTopType )
33 import TcSimplify ( tcSimplifyAndCheck )
34 import TcType ( TcTyVar, zonkTcTyVarBndr )
36 import Bag ( emptyBag, unitBag, unionBags, unionManyBags,
39 import CmdLineOpts ( opt_GlasgowExts, opt_AllowUndecidableInstances )
40 import Class ( classBigSig, Class )
41 import Var ( idName, idType, Id, TyVar )
42 import DataCon ( isNullaryDataCon, dataConArgTys, dataConId )
43 import Maybes ( maybeToBool, catMaybes, expectJust )
44 import MkId ( mkDictFunId )
45 import Module ( ModuleName )
46 import Name ( isLocallyDefined, NamedThing(..) )
47 import NameSet ( emptyNameSet )
48 import PrelInfo ( eRROR_ID )
49 import PprType ( pprConstraint )
50 import SrcLoc ( SrcLoc )
51 import TyCon ( isSynTyCon, isDataTyCon, tyConDerivings )
52 import Type ( Type, isUnLiftedType, mkTyVarTys,
53 splitSigmaTy, isTyVarTy,
54 splitTyConApp_maybe, splitDictTy_maybe, unUsgTy,
55 splitAlgTyConApp_maybe,
58 import Subst ( mkTopTyVarSubst, substTheta )
59 import VarSet ( mkVarSet, varSetElems )
60 import TysPrim ( byteArrayPrimTyCon, mutableByteArrayPrimTyCon )
61 import TysWiredIn ( stringTy )
62 import Unique ( Unique, cCallableClassKey, cReturnableClassKey, Uniquable(..) )
66 Typechecking instance declarations is done in two passes. The first
67 pass, made by @tcInstDecls1@, collects information to be used in the
70 This pre-processed info includes the as-yet-unprocessed bindings
71 inside the instance declaration. These are type-checked in the second
72 pass, when the class-instance envs and GVE contain all the info from
73 all the instance and value decls. Indeed that's the reason we need
74 two passes over the instance decls.
77 Here is the overall algorithm.
78 Assume that we have an instance declaration
80 instance c => k (t tvs) where b
84 $LIE_c$ is the LIE for the context of class $c$
86 $betas_bar$ is the free variables in the class method type, excluding the
89 $LIE_cop$ is the LIE constraining a particular class method
91 $tau_cop$ is the tau type of a class method
93 $LIE_i$ is the LIE for the context of instance $i$
95 $X$ is the instance constructor tycon
97 $gammas_bar$ is the set of type variables of the instance
99 $LIE_iop$ is the LIE for a particular class method instance
101 $tau_iop$ is the tau type for this instance of a class method
103 $alpha$ is the class variable
105 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
107 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
110 ToDo: Update the list above with names actually in the code.
114 First, make the LIEs for the class and instance contexts, which means
115 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
116 and make LIElistI and LIEI.
118 Then process each method in turn.
120 order the instance methods according to the ordering of the class methods
122 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
124 Create final dictionary function from bindings generated already
126 df = lambda inst_tyvars
133 in <op1,op2,...,opn,sd1,...,sdm>
135 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
136 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
140 tcInstDecls1 :: ValueEnv -- Contains IdInfo for dfun ids
142 -> ModuleName -- module name for deriving
144 -> RnNameSupply -- for renaming derivings
145 -> TcM s (Bag InstInfo,
148 tcInstDecls1 unf_env decls mod_name fixs rn_name_supply
149 = -- Do the ordinary instance declarations
150 mapNF_Tc (tcInstDecl1 unf_env)
151 [inst_decl | InstD inst_decl <- decls] `thenNF_Tc` \ inst_info_bags ->
153 decl_inst_info = unionManyBags inst_info_bags
155 -- Handle "derived" instances; note that we only do derivings
156 -- for things in this module; we ignore deriving decls from
158 tcDeriving mod_name fixs rn_name_supply decl_inst_info
159 `thenTc` \ (deriv_inst_info, deriv_binds) ->
162 full_inst_info = deriv_inst_info `unionBags` decl_inst_info
164 returnTc (full_inst_info, deriv_binds)
167 tcInstDecl1 :: ValueEnv -> RenamedInstDecl -> NF_TcM s (Bag InstInfo)
169 tcInstDecl1 unf_env (InstDecl poly_ty binds uprags dfun_name src_loc)
170 = -- Prime error recovery, set source location
171 recoverNF_Tc (returnNF_Tc emptyBag) $
172 tcAddSrcLoc src_loc $
174 -- Type-check all the stuff before the "where"
175 tcHsTopType poly_ty `thenTc` \ poly_ty' ->
177 (tyvars, theta, dict_ty) = splitSigmaTy poly_ty'
178 (clas, inst_tys) = case splitDictTy_maybe dict_ty of
179 Nothing -> pprPanic "tcInstDecl1" (ppr poly_ty)
183 -- Check for respectable instance type, and context
184 -- but only do this for non-imported instance decls.
185 -- Imported ones should have been checked already, and may indeed
186 -- contain something illegal in normal Haskell, notably
187 -- instance CCallable [Char]
188 (if isLocallyDefined dfun_name then
189 scrutiniseInstanceHead clas inst_tys `thenNF_Tc_`
190 mapNF_Tc scrutiniseInstanceConstraint theta
197 dfun_id = mkDictFunId dfun_name clas tyvars inst_tys theta
199 -- Add info from interface file
200 final_dfun_id = tcAddImportedIdInfo unf_env dfun_id
202 returnTc (unitBag (InstInfo clas tyvars inst_tys theta
204 binds src_loc uprags))
208 %************************************************************************
210 \subsection{Type-checking instance declarations, pass 2}
212 %************************************************************************
215 tcInstDecls2 :: Bag InstInfo
216 -> NF_TcM s (LIE, TcMonoBinds)
218 tcInstDecls2 inst_decls
219 = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
221 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
222 tc2 `thenNF_Tc` \ (lie2, binds2) ->
223 returnNF_Tc (lie1 `plusLIE` lie2,
224 binds1 `AndMonoBinds` binds2)
228 ======= New documentation starts here (Sept 92) ==============
230 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
231 the dictionary function for this instance declaration. For example
233 instance Foo a => Foo [a] where
237 might generate something like
239 dfun.Foo.List dFoo_a = let op1 x = ...
245 HOWEVER, if the instance decl has no context, then it returns a
246 bigger @HsBinds@ with declarations for each method. For example
248 instance Foo [a] where
254 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
255 const.Foo.op1.List a x = ...
256 const.Foo.op2.List a y = ...
258 This group may be mutually recursive, because (for example) there may
259 be no method supplied for op2 in which case we'll get
261 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
263 that is, the default method applied to the dictionary at this type.
265 What we actually produce in either case is:
267 AbsBinds [a] [dfun_theta_dicts]
268 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
269 { d = (sd1,sd2, ..., op1, op2, ...)
274 The "maybe" says that we only ask AbsBinds to make global constant methods
275 if the dfun_theta is empty.
278 For an instance declaration, say,
280 instance (C1 a, C2 b) => C (T a b) where
283 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
284 function whose type is
286 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
288 Notice that we pass it the superclass dictionaries at the instance type; this
289 is the ``Mark Jones optimisation''. The stuff before the "=>" here
290 is the @dfun_theta@ below.
292 First comes the easy case of a non-local instance decl.
295 tcInstDecl2 :: InstInfo -> NF_TcM s (LIE, TcMonoBinds)
297 tcInstDecl2 (InstInfo clas inst_tyvars inst_tys
301 | not (isLocallyDefined dfun_id)
302 = returnNF_Tc (emptyLIE, EmptyMonoBinds)
305 -- I deleted this "optimisation" because when importing these
306 -- instance decls the renamer would look for the dfun bindings and they weren't there.
307 -- This would be fixable, but it seems simpler just to produce a tiny void binding instead,
308 -- even though it's never used.
310 -- This case deals with CCallable etc, which don't need any bindings
312 = returnNF_Tc (emptyLIE, EmptyBinds)
316 = -- Prime error recovery
317 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
320 -- Instantiate the instance decl with tc-style type variables
321 tcInstId dfun_id `thenNF_Tc` \ (inst_tyvars', dfun_theta', dict_ty') ->
323 (clas, inst_tys') = expectJust "tcInstDecl2" (splitDictTy_maybe dict_ty')
325 origin = InstanceDeclOrigin
328 sc_theta, sc_sel_ids,
329 op_sel_ids, defm_ids) = classBigSig clas
331 -- Instantiate the theta found in the original instance decl
332 inst_decl_theta' = substTheta (mkTopTyVarSubst inst_tyvars (mkTyVarTys inst_tyvars'))
335 -- Instantiate the super-class context with inst_tys
336 sc_theta' = substTheta (mkTopTyVarSubst class_tyvars inst_tys') sc_theta
338 -- Create dictionary Ids from the specified instance contexts.
339 newDicts origin sc_theta' `thenNF_Tc` \ (sc_dicts, sc_dict_ids) ->
340 newDicts origin dfun_theta' `thenNF_Tc` \ (dfun_arg_dicts, dfun_arg_dicts_ids) ->
341 newDicts origin inst_decl_theta' `thenNF_Tc` \ (inst_decl_dicts, _) ->
342 newDicts origin [(clas,inst_tys')] `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
344 -- Check that all the method bindings come from this class
345 checkFromThisClass clas op_sel_ids monobinds `thenNF_Tc_`
347 tcExtendTyVarEnvForMeths inst_tyvars inst_tyvars' (
348 tcExtendGlobalValEnv (catMaybes defm_ids) (
349 -- Default-method Ids may be mentioned in synthesised RHSs
351 mapAndUnzip3Tc (tcMethodBind clas origin inst_tyvars' inst_tys' inst_decl_theta'
352 monobinds uprags True)
353 (op_sel_ids `zip` defm_ids)
354 )) `thenTc` \ (method_binds_s, insts_needed_s, meth_lies_w_ids) ->
356 -- Deal with SPECIALISE instance pragmas by making them
357 -- look like SPECIALISE pragmas for the dfun
359 dfun_prags = [SpecSig (idName dfun_id) ty loc | SpecInstSig ty loc <- uprags]
361 tcExtendGlobalValEnv [dfun_id] (
362 tcSpecSigs dfun_prags
363 ) `thenTc` \ (prag_binds, prag_lie) ->
365 -- Check the overloading constraints of the methods and superclasses
367 -- tcMethodBind has checked that the class_tyvars havn't
368 -- been unified with each other or another type, but we must
370 mapNF_Tc zonkTcTyVarBndr inst_tyvars' `thenNF_Tc` \ zonked_inst_tyvars ->
372 inst_tyvars_set = mkVarSet zonked_inst_tyvars
374 (meth_lies, meth_ids) = unzip meth_lies_w_ids
376 -- These insts are in scope; quite a few, eh?
377 avail_insts = this_dict `plusLIE`
378 dfun_arg_dicts `plusLIE`
380 unionManyBags meth_lies
382 methods_lie = plusLIEs insts_needed_s
385 -- Ditto method bindings
386 tcAddErrCtxt methodCtxt (
388 (ptext SLIT("instance declaration context"))
389 inst_tyvars_set -- Local tyvars
392 ) `thenTc` \ (const_lie1, lie_binds1) ->
394 -- Check that we *could* construct the superclass dictionaries,
395 -- even though we are *actually* going to pass the superclass dicts in;
396 -- the check ensures that the caller will never have
397 --a problem building them.
398 tcAddErrCtxt superClassCtxt (
400 (ptext SLIT("instance declaration context"))
401 inst_tyvars_set -- Local tyvars
402 inst_decl_dicts -- The instance dictionaries available
403 sc_dicts -- The superclass dicationaries reqd
405 -- Ignore the result; we're only doing
406 -- this to make sure it can be done.
408 -- Now do the simplification again, this time to get the
409 -- bindings; this time we use an enhanced "avails"
410 -- Ignore errors because they come from the *previous* tcSimplify
413 (ptext SLIT("instance declaration context"))
415 dfun_arg_dicts -- NB! Don't include this_dict here, else the sc_dicts
416 -- get bound by just selecting from this_dict!!
418 ) `thenTc` \ (const_lie2, lie_binds2) ->
421 -- Create the result bindings
423 dict_constr = classDataCon clas
424 scs_and_meths = sc_dict_ids ++ meth_ids
428 = -- Blatant special case for CCallable, CReturnable
429 -- If the dictionary is empty then we should never
430 -- select anything from it, so we make its RHS just
431 -- emit an error message. This in turn means that we don't
432 -- mention the constructor, which doesn't exist for CCallable, CReturnable
433 -- Hardly beautiful, but only three extra lines.
434 HsApp (TyApp (HsVar eRROR_ID) [(unUsgTy . idType) this_dict_id])
435 (HsLitOut (HsString msg) stringTy)
437 | otherwise -- The common case
438 = foldl HsApp (TyApp (HsVar (dataConId dict_constr)) inst_tys')
439 (map HsVar (sc_dict_ids ++ meth_ids))
440 -- We don't produce a binding for the dict_constr; instead we
441 -- rely on the simplifier to unfold this saturated application
442 -- We do this rather than generate an HsCon directly, because
443 -- it means that the special cases (e.g. dictionary with only one
444 -- member) are dealt with by the common MkId.mkDataConId code rather
445 -- than needing to be repeated here.
448 msg = _PK_ ("Compiler error: bad dictionary " ++ showSDoc (ppr clas))
450 dict_bind = VarMonoBind this_dict_id dict_rhs
451 method_binds = andMonoBindList method_binds_s
457 [(inst_tyvars', dfun_id, this_dict_id)]
458 emptyNameSet -- No inlines (yet)
459 (lie_binds1 `AndMonoBinds`
460 lie_binds2 `AndMonoBinds`
461 method_binds `AndMonoBinds`
464 returnTc (const_lie1 `plusLIE` const_lie2 `plusLIE` prag_lie,
465 main_bind `AndMonoBinds` prag_binds)
469 %************************************************************************
471 \subsection{Checking for a decent instance type}
473 %************************************************************************
475 @scrutiniseInstanceHead@ checks the type {\em and} its syntactic constraints:
476 it must normally look like: @instance Foo (Tycon a b c ...) ...@
478 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
479 flag is on, or (2)~the instance is imported (they must have been
480 compiled elsewhere). In these cases, we let them go through anyway.
482 We can also have instances for functions: @instance Foo (a -> b) ...@.
485 scrutiniseInstanceConstraint (clas, tys)
487 || opt_AllowUndecidableInstances = returnNF_Tc ()
488 | otherwise = addErrTc (instConstraintErr clas tys)
490 scrutiniseInstanceHead clas inst_taus
491 | -- CCALL CHECK (a).... urgh!
492 -- To verify that a user declaration of a CCallable/CReturnable
493 -- instance is OK, we must be able to see the constructor(s)
494 -- of the instance type (see next guard.)
496 -- We flag this separately to give a more precise error msg.
498 (getUnique clas == cCallableClassKey || getUnique clas == cReturnableClassKey)
499 && is_alg_tycon_app && not constructors_visible
500 = addErrTc (invisibleDataConPrimCCallErr clas first_inst_tau)
503 -- A user declaration of a CCallable/CReturnable instance
504 -- must be for a "boxed primitive" type.
505 (getUnique clas == cCallableClassKey && not (ccallable_type first_inst_tau)) ||
506 (getUnique clas == cReturnableClassKey && not (creturnable_type first_inst_tau))
507 = addErrTc (nonBoxedPrimCCallErr clas first_inst_tau)
510 -- It is obviously illegal to have an explicit instance
511 -- for something that we are also planning to `derive'
512 | maybeToBool alg_tycon_app_maybe && clas `elem` (tyConDerivings alg_tycon)
513 = addErrTc (derivingWhenInstanceExistsErr clas first_inst_tau)
514 -- Kind check will have ensured inst_taus is of length 1
516 -- Allow anything for AllowUndecidableInstances
517 | opt_AllowUndecidableInstances
520 -- If GlasgowExts then check at least one isn't a type variable
522 = if all isTyVarTy inst_taus then
523 addErrTc (instTypeErr clas inst_taus (text "There must be at least one non-type-variable in the instance head"))
527 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
528 | not (length inst_taus == 1 &&
529 maybeToBool maybe_tycon_app && -- Yes, there's a type constuctor
530 not (isSynTyCon tycon) && -- ...but not a synonym
531 all isTyVarTy arg_tys && -- Applied to type variables
532 length (varSetElems (tyVarsOfTypes arg_tys)) == length arg_tys
533 -- This last condition checks that all the type variables are distinct
535 = addErrTc (instTypeErr clas inst_taus
536 (text "the instance type must be of form (T a b c)" $$
537 text "where T is not a synonym, and a,b,c are distinct type variables")
544 (first_inst_tau : _) = inst_taus
546 -- Stuff for algebraic or -> type
547 maybe_tycon_app = splitTyConApp_maybe first_inst_tau
548 Just (tycon, arg_tys) = maybe_tycon_app
550 -- Stuff for an *algebraic* data type
551 alg_tycon_app_maybe = splitAlgTyConApp_maybe first_inst_tau
552 -- The "Alg" part looks through synonyms
553 is_alg_tycon_app = maybeToBool alg_tycon_app_maybe
554 Just (alg_tycon, _, data_cons) = alg_tycon_app_maybe
556 constructors_visible = not (null data_cons)
559 -- These conditions come directly from what the DsCCall is capable of.
560 -- Totally grotesque. Green card should solve this.
562 ccallable_type ty = isUnLiftedType ty || -- Allow CCallable Int# etc
563 maybeToBool (maybeBoxedPrimType ty) || -- Ditto Int etc
567 byte_arr_thing = case splitAlgTyConApp_maybe ty of
568 Just (tycon, ty_args, [data_con]) | isDataTyCon tycon ->
569 length data_con_arg_tys == 2 &&
570 maybeToBool maybe_arg2_tycon &&
571 (arg2_tycon == byteArrayPrimTyCon ||
572 arg2_tycon == mutableByteArrayPrimTyCon)
574 data_con_arg_tys = dataConArgTys data_con ty_args
575 (data_con_arg_ty1 : data_con_arg_ty2 : _) = data_con_arg_tys
576 maybe_arg2_tycon = splitTyConApp_maybe data_con_arg_ty2
577 Just (arg2_tycon,_) = maybe_arg2_tycon
581 creturnable_type ty = maybeToBool (maybeBoxedPrimType ty) ||
582 -- Or, a data type with a single nullary constructor
583 case (splitAlgTyConApp_maybe ty) of
584 Just (tycon, tys_applied, [data_con])
585 -> isNullaryDataCon data_con
590 instConstraintErr clas tys
591 = hang (ptext SLIT("Illegal constaint") <+>
592 quotes (pprConstraint clas tys) <+>
593 ptext SLIT("in instance context"))
594 4 (ptext SLIT("(Instance contexts must constrain only type variables)"))
596 instTypeErr clas tys msg
597 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes (pprConstraint clas tys),
601 derivingWhenInstanceExistsErr clas tycon
602 = hang (hsep [ptext SLIT("Deriving class"),
604 ptext SLIT("type"), quotes (ppr tycon)])
605 4 (ptext SLIT("when an explicit instance exists"))
607 nonBoxedPrimCCallErr clas inst_ty
608 = hang (ptext SLIT("Unacceptable instance type for ccall-ish class"))
609 4 (hsep [ ptext SLIT("class"), ppr clas, ptext SLIT("type"),
613 Declaring CCallable & CReturnable instances in a module different
614 from where the type was defined. Caused by importing data type
615 abstractly (either programmatically or by the renamer being over-eager
618 invisibleDataConPrimCCallErr clas inst_ty
619 = hang (hsep [ptext SLIT("Constructors for"), quotes (ppr inst_ty),
620 ptext SLIT("not visible when checking"),
621 quotes (ppr clas), ptext SLIT("instance")])
622 4 (hsep [text "(Try either importing", ppr inst_ty,
623 text "non-abstractly or compile using -fno-prune-tydecls ..)"])
625 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
626 superClassCtxt = ptext SLIT("When checking the superclasses of an instance declaration")