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(..), GRHSsAndBinds(..),
16 HsExpr(..), InPat(..), HsLit(..), Sig(..),
17 collectMonoBinders, andMonoBindList
19 import RnHsSyn ( RenamedHsBinds, RenamedInstDecl, RenamedHsDecl )
20 import TcHsSyn ( TcMonoBinds, TcIdOcc(..),
21 maybeBoxedPrimType, tcIdType
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 ( GlobalValueEnv, tcExtendGlobalValEnv, tcAddImportedIdInfo, tcInstId )
32 import TcInstUtil ( InstInfo(..), classDataCon )
33 import TcMonoType ( tcHsType )
34 import TcSimplify ( tcSimplifyAndCheck )
35 import TcType ( TcTyVar, zonkTcTyVarBndr )
37 import Bag ( emptyBag, unitBag, unionBags, unionManyBags,
38 foldBag, bagToList, Bag
40 import CmdLineOpts ( opt_GlasgowExts, opt_AllowUndecidableInstances )
41 import Class ( classBigSig, Class )
42 import Var ( setIdInfo, idName, Id, TyVar )
43 import DataCon ( isNullaryDataCon, dataConArgTys, dataConId )
44 import Maybes ( maybeToBool, catMaybes, expectJust )
45 import MkId ( mkDictFunId )
46 import Name ( nameOccName, isLocallyDefined, Module,
49 import PrelVals ( eRROR_ID )
50 import PprType ( pprConstraint )
51 import SrcLoc ( SrcLoc )
52 import TyCon ( isSynTyCon, isDataTyCon, tyConDerivings )
53 import Type ( Type, isUnLiftedType, mkTyVarTys,
54 splitSigmaTy, isTyVarTy,
55 splitTyConApp_maybe, splitDictTy_maybe,
56 splitAlgTyConApp_maybe,
57 tyVarsOfTypes, substFlexiTheta
59 import VarEnv ( zipVarEnv )
60 import VarSet ( mkVarSet, varSetElems )
61 import TysPrim ( byteArrayPrimTyCon, mutableByteArrayPrimTyCon )
62 import TysWiredIn ( stringTy )
63 import Unique ( Unique, cCallableClassKey, cReturnableClassKey, Uniquable(..) )
67 Typechecking instance declarations is done in two passes. The first
68 pass, made by @tcInstDecls1@, collects information to be used in the
71 This pre-processed info includes the as-yet-unprocessed bindings
72 inside the instance declaration. These are type-checked in the second
73 pass, when the class-instance envs and GVE contain all the info from
74 all the instance and value decls. Indeed that's the reason we need
75 two passes over the instance decls.
78 Here is the overall algorithm.
79 Assume that we have an instance declaration
81 instance c => k (t tvs) where b
85 $LIE_c$ is the LIE for the context of class $c$
87 $betas_bar$ is the free variables in the class method type, excluding the
90 $LIE_cop$ is the LIE constraining a particular class method
92 $tau_cop$ is the tau type of a class method
94 $LIE_i$ is the LIE for the context of instance $i$
96 $X$ is the instance constructor tycon
98 $gammas_bar$ is the set of type variables of the instance
100 $LIE_iop$ is the LIE for a particular class method instance
102 $tau_iop$ is the tau type for this instance of a class method
104 $alpha$ is the class variable
106 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
108 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
111 ToDo: Update the list above with names actually in the code.
115 First, make the LIEs for the class and instance contexts, which means
116 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
117 and make LIElistI and LIEI.
119 Then process each method in turn.
121 order the instance methods according to the ordering of the class methods
123 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
125 Create final dictionary function from bindings generated already
127 df = lambda inst_tyvars
134 in <op1,op2,...,opn,sd1,...,sdm>
136 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
137 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
141 tcInstDecls1 :: GlobalValueEnv -- Contains IdInfo for dfun ids
143 -> Module -- module name for deriving
144 -> RnNameSupply -- for renaming derivings
145 -> TcM s (Bag InstInfo,
149 tcInstDecls1 unf_env decls mod_name rn_name_supply
150 = -- Do the ordinary instance declarations
151 mapNF_Tc (tcInstDecl1 unf_env mod_name)
152 [inst_decl | InstD inst_decl <- decls] `thenNF_Tc` \ inst_info_bags ->
154 decl_inst_info = unionManyBags inst_info_bags
156 -- Handle "derived" instances; note that we only do derivings
157 -- for things in this module; we ignore deriving decls from
159 tcDeriving mod_name rn_name_supply decl_inst_info
160 `thenTc` \ (deriv_inst_info, deriv_binds, ddump_deriv) ->
163 full_inst_info = deriv_inst_info `unionBags` decl_inst_info
165 returnTc (full_inst_info, deriv_binds, ddump_deriv)
168 tcInstDecl1 :: GlobalValueEnv -> Module -> RenamedInstDecl -> NF_TcM s (Bag InstInfo)
170 tcInstDecl1 unf_env mod_name (InstDecl poly_ty binds uprags (Just dfun_name) src_loc)
171 = -- Prime error recovery, set source location
172 recoverNF_Tc (returnNF_Tc emptyBag) $
173 tcAddSrcLoc src_loc $
175 -- Type-check all the stuff before the "where"
176 tcHsType poly_ty `thenTc` \ poly_ty' ->
178 (tyvars, theta, dict_ty) = splitSigmaTy poly_ty'
179 (clas, inst_tys) = case splitDictTy_maybe dict_ty of
180 Nothing -> pprPanic "tcInstDecl1" (ppr poly_ty)
184 -- Check for respectable instance type, and context
185 scrutiniseInstanceHead clas inst_tys `thenNF_Tc_`
186 mapNF_Tc scrutiniseInstanceConstraint theta `thenNF_Tc_`
188 -- Make the dfun id and constant-method ids
190 dfun_id = mkDictFunId dfun_name clas tyvars inst_tys theta
192 -- Add info from interface file
193 final_dfun_id = tcAddImportedIdInfo unf_env dfun_id
195 returnTc (unitBag (InstInfo clas tyvars inst_tys theta
197 binds src_loc uprags))
201 %************************************************************************
203 \subsection{Type-checking instance declarations, pass 2}
205 %************************************************************************
208 tcInstDecls2 :: Bag InstInfo
209 -> NF_TcM s (LIE s, TcMonoBinds s)
211 tcInstDecls2 inst_decls
212 = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
214 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
215 tc2 `thenNF_Tc` \ (lie2, binds2) ->
216 returnNF_Tc (lie1 `plusLIE` lie2,
217 binds1 `AndMonoBinds` binds2)
221 ======= New documentation starts here (Sept 92) ==============
223 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
224 the dictionary function for this instance declaration. For example
226 instance Foo a => Foo [a] where
230 might generate something like
232 dfun.Foo.List dFoo_a = let op1 x = ...
238 HOWEVER, if the instance decl has no context, then it returns a
239 bigger @HsBinds@ with declarations for each method. For example
241 instance Foo [a] where
247 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
248 const.Foo.op1.List a x = ...
249 const.Foo.op2.List a y = ...
251 This group may be mutually recursive, because (for example) there may
252 be no method supplied for op2 in which case we'll get
254 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
256 that is, the default method applied to the dictionary at this type.
258 What we actually produce in either case is:
260 AbsBinds [a] [dfun_theta_dicts]
261 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
262 { d = (sd1,sd2, ..., op1, op2, ...)
267 The "maybe" says that we only ask AbsBinds to make global constant methods
268 if the dfun_theta is empty.
271 For an instance declaration, say,
273 instance (C1 a, C2 b) => C (T a b) where
276 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
277 function whose type is
279 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
281 Notice that we pass it the superclass dictionaries at the instance type; this
282 is the ``Mark Jones optimisation''. The stuff before the "=>" here
283 is the @dfun_theta@ below.
285 First comes the easy case of a non-local instance decl.
288 tcInstDecl2 :: InstInfo -> NF_TcM s (LIE s, TcMonoBinds s)
290 tcInstDecl2 (InstInfo clas inst_tyvars inst_tys
294 | not (isLocallyDefined dfun_id)
295 = returnNF_Tc (emptyLIE, EmptyMonoBinds)
298 -- I deleted this "optimisation" because when importing these
299 -- instance decls the renamer would look for the dfun bindings and they weren't there.
300 -- This would be fixable, but it seems simpler just to produce a tiny void binding instead,
301 -- even though it's never used.
303 -- This case deals with CCallable etc, which don't need any bindings
305 = returnNF_Tc (emptyLIE, EmptyBinds)
309 = -- Prime error recovery
310 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
313 -- Instantiate the instance decl with tc-style type variables
314 tcInstId dfun_id `thenNF_Tc` \ (inst_tyvars', dfun_theta', dict_ty') ->
316 (clas, inst_tys') = expectJust "tcInstDecl2" (splitDictTy_maybe dict_ty')
318 origin = InstanceDeclOrigin
321 sc_theta, sc_sel_ids,
322 op_sel_ids, defm_ids) = classBigSig clas
324 -- Instantiate the theta found in the original instance decl
325 inst_decl_theta' = substFlexiTheta (zipVarEnv inst_tyvars (mkTyVarTys inst_tyvars'))
328 -- Instantiate the super-class context with inst_tys
329 sc_theta' = substFlexiTheta (zipVarEnv class_tyvars inst_tys') sc_theta
331 -- Create dictionary Ids from the specified instance contexts.
332 newDicts origin sc_theta' `thenNF_Tc` \ (sc_dicts, sc_dict_ids) ->
333 newDicts origin dfun_theta' `thenNF_Tc` \ (dfun_arg_dicts, dfun_arg_dicts_ids) ->
334 newDicts origin inst_decl_theta' `thenNF_Tc` \ (inst_decl_dicts, _) ->
335 newDicts origin [(clas,inst_tys')] `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
337 -- Check that all the method bindings come from this class
339 check_from_this_class (bndr, loc)
340 | nameOccName bndr `elem` sel_names = returnNF_Tc ()
341 | otherwise = tcAddSrcLoc loc $
342 addErrTc (badMethodErr bndr clas)
343 sel_names = map getOccName op_sel_ids
344 bndrs = bagToList (collectMonoBinders monobinds)
346 mapNF_Tc check_from_this_class bndrs `thenNF_Tc_`
348 tcExtendGlobalValEnv (catMaybes defm_ids) (
350 -- Default-method Ids may be mentioned in synthesised RHSs
351 mapAndUnzip3Tc (tcMethodBind clas origin inst_tys' inst_tyvars' monobinds uprags True)
352 (op_sel_ids `zip` defm_ids)
353 ) `thenTc` \ (method_binds_s, insts_needed_s, meth_lies_w_ids) ->
355 -- Deal with SPECIALISE instance pragmas
357 dfun_prags = [Sig (idName dfun_id) ty loc | SpecInstSig ty loc <- uprags]
359 tcExtendGlobalValEnv [dfun_id] (
360 tcPragmaSigs dfun_prags
361 ) `thenTc` \ (prag_info_fn, prag_binds, prag_lie) ->
363 -- Check the overloading constraints of the methods and superclasses
365 -- tcMethodBind has checked that the class_tyvars havn't
366 -- been unified with each other or another type, but we must
368 mapNF_Tc zonkTcTyVarBndr inst_tyvars' `thenNF_Tc` \ zonked_inst_tyvars ->
370 inst_tyvars_set = mkVarSet zonked_inst_tyvars
372 (meth_lies, meth_ids) = unzip meth_lies_w_ids
374 -- These insts are in scope; quite a few, eh?
375 avail_insts = this_dict `plusLIE`
376 dfun_arg_dicts `plusLIE`
378 unionManyBags meth_lies
380 methods_lie = plusLIEs insts_needed_s
383 -- Ditto method bindings
384 tcAddErrCtxt methodCtxt (
386 (ptext SLIT("instance declaration context"))
387 inst_tyvars_set -- Local tyvars
390 ) `thenTc` \ (const_lie1, lie_binds1) ->
392 -- Check that we *could* construct the superclass dictionaries,
393 -- even though we are *actually* going to pass the superclass dicts in;
394 -- the check ensures that the caller will never have
395 --a problem building them.
396 tcAddErrCtxt superClassCtxt (
398 (ptext SLIT("instance declaration context"))
399 inst_tyvars_set -- Local tyvars
400 inst_decl_dicts -- The instance dictionaries available
401 sc_dicts -- The superclass dicationaries reqd
403 -- Ignore the result; we're only doing
404 -- this to make sure it can be done.
406 -- Now do the simplification again, this time to get the
407 -- bindings; this time we use an enhanced "avails"
408 -- Ignore errors because they come from the *previous* tcSimplify
411 (ptext SLIT("instance declaration context"))
413 dfun_arg_dicts -- NB! Don't include this_dict here, else the sc_dicts
414 -- get bound by just selecting from this_dict!!
416 ) `thenTc` \ (const_lie2, lie_binds2) ->
419 -- Create the result bindings
421 dict_constr = classDataCon clas
422 scs_and_meths = sc_dict_ids ++ meth_ids
426 = -- Blatant special case for CCallable, CReturnable [and Eval -- sof 5/98]
427 -- If the dictionary is empty then we should never
428 -- select anything from it, so we make its RHS just
429 -- emit an error message. This in turn means that we don't
430 -- mention the constructor, which doesn't exist for CCallable, CReturnable
431 -- Hardly beautiful, but only three extra lines.
432 HsApp (TyApp (HsVar (RealId eRROR_ID)) [tcIdType this_dict_id])
433 (HsLitOut (HsString msg) stringTy)
435 | otherwise -- The common case
436 = foldl HsApp (TyApp (HsVar (RealId (dataConId dict_constr))) inst_tys')
437 (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.mkDataConId 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
451 final_dfun_id = setIdInfo dfun_id (prag_info_fn (idName dfun_id))
457 [(inst_tyvars', RealId final_dfun_id, this_dict_id)]
458 (lie_binds1 `AndMonoBinds`
459 lie_binds2 `AndMonoBinds`
460 method_binds `AndMonoBinds`
463 returnTc (const_lie1 `plusLIE` const_lie2 `plusLIE` prag_lie,
464 main_bind `AndMonoBinds` prag_binds)
468 %************************************************************************
470 \subsection{Checking for a decent instance type}
472 %************************************************************************
474 @scrutiniseInstanceHead@ checks the type {\em and} its syntactic constraints:
475 it must normally look like: @instance Foo (Tycon a b c ...) ...@
477 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
478 flag is on, or (2)~the instance is imported (they must have been
479 compiled elsewhere). In these cases, we let them go through anyway.
481 We can also have instances for functions: @instance Foo (a -> b) ...@.
484 scrutiniseInstanceConstraint (clas, tys)
486 || opt_AllowUndecidableInstances = returnNF_Tc ()
487 | otherwise = addErrTc (instConstraintErr clas tys)
489 scrutiniseInstanceHead clas inst_taus
490 | -- CCALL CHECK (a).... urgh!
491 -- To verify that a user declaration of a CCallable/CReturnable
492 -- instance is OK, we must be able to see the constructor(s)
493 -- of the instance type (see next guard.)
495 -- We flag this separately to give a more precise error msg.
497 (getUnique clas == cCallableClassKey || getUnique clas == cReturnableClassKey)
498 && is_alg_tycon_app && not constructors_visible
499 = addErrTc (invisibleDataConPrimCCallErr clas first_inst_tau)
502 -- A user declaration of a CCallable/CReturnable instance
503 -- must be for a "boxed primitive" type.
504 (getUnique clas == cCallableClassKey && not (ccallable_type first_inst_tau)) ||
505 (getUnique clas == cReturnableClassKey && not (creturnable_type first_inst_tau))
506 = addErrTc (nonBoxedPrimCCallErr clas first_inst_tau)
509 -- It is obviously illegal to have an explicit instance
510 -- for something that we are also planning to `derive'
511 | maybeToBool alg_tycon_app_maybe && clas `elem` (tyConDerivings alg_tycon)
512 = addErrTc (derivingWhenInstanceExistsErr clas first_inst_tau)
513 -- Kind check will have ensured inst_taus is of length 1
515 -- Allow anything for AllowUndecidableInstances
516 | opt_AllowUndecidableInstances
519 -- If GlasgowExts then check at least one isn't a type variable
521 = if all isTyVarTy inst_taus then
522 addErrTc (instTypeErr clas inst_taus (text "There must be at least one non-type-variable in the instance head"))
526 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
527 | not (length inst_taus == 1 &&
528 maybeToBool maybe_tycon_app && -- Yes, there's a type constuctor
529 not (isSynTyCon tycon) && -- ...but not a synonym
530 all isTyVarTy arg_tys && -- Applied to type variables
531 length (varSetElems (tyVarsOfTypes arg_tys)) == length arg_tys
532 -- This last condition checks that all the type variables are distinct
534 = addErrTc (instTypeErr clas inst_taus
535 (text "the instance type must be of form (T a b c)" $$
536 text "where T is not a synonym, and a,b,c are distinct type variables")
543 (first_inst_tau : _) = inst_taus
545 -- Stuff for algebraic or -> type
546 maybe_tycon_app = splitTyConApp_maybe first_inst_tau
547 Just (tycon, arg_tys) = maybe_tycon_app
549 -- Stuff for an *algebraic* data type
550 alg_tycon_app_maybe = splitAlgTyConApp_maybe first_inst_tau
551 -- The "Alg" part looks through synonyms
552 is_alg_tycon_app = maybeToBool alg_tycon_app_maybe
553 Just (alg_tycon, _, data_cons) = alg_tycon_app_maybe
555 constructors_visible = not (null data_cons)
558 -- These conditions come directly from what the DsCCall is capable of.
559 -- Totally grotesque. Green card should solve this.
561 ccallable_type ty = isUnLiftedType ty || -- Allow CCallable Int# etc
562 maybeToBool (maybeBoxedPrimType ty) || -- Ditto Int etc
566 byte_arr_thing = case splitAlgTyConApp_maybe ty of
567 Just (tycon, ty_args, [data_con]) | isDataTyCon tycon ->
568 length data_con_arg_tys == 2 &&
569 maybeToBool maybe_arg2_tycon &&
570 (arg2_tycon == byteArrayPrimTyCon ||
571 arg2_tycon == mutableByteArrayPrimTyCon)
573 data_con_arg_tys = dataConArgTys data_con ty_args
574 (data_con_arg_ty1 : data_con_arg_ty2 : _) = data_con_arg_tys
575 maybe_arg2_tycon = splitTyConApp_maybe data_con_arg_ty2
576 Just (arg2_tycon,_) = maybe_arg2_tycon
580 creturnable_type ty = maybeToBool (maybeBoxedPrimType ty) ||
581 -- Or, a data type with a single nullary constructor
582 case (splitAlgTyConApp_maybe ty) of
583 Just (tycon, tys_applied, [data_con])
584 -> isNullaryDataCon data_con
589 instConstraintErr clas tys
590 = hang (ptext SLIT("Illegal constaint") <+>
591 quotes (pprConstraint clas tys) <+>
592 ptext SLIT("in instance context"))
593 4 (ptext SLIT("(Instance contexts must constrain only type variables)"))
595 instTypeErr clas tys msg
596 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes (pprConstraint clas tys),
600 derivingWhenInstanceExistsErr clas tycon
601 = hang (hsep [ptext SLIT("Deriving class"),
603 ptext SLIT("type"), quotes (ppr tycon)])
604 4 (ptext SLIT("when an explicit instance exists"))
606 nonBoxedPrimCCallErr clas inst_ty
607 = hang (ptext SLIT("Unacceptable instance type for ccall-ish class"))
608 4 (hsep [ ptext SLIT("class"), ppr clas, ptext SLIT("type"),
612 Declaring CCallable & CReturnable instances in a module different
613 from where the type was defined. Caused by importing data type
614 abstractly (either programmatically or by the renamer being over-eager
617 invisibleDataConPrimCCallErr clas inst_ty
618 = hang (hsep [ptext SLIT("Constructors for"), quotes (ppr inst_ty),
619 ptext SLIT("not visible when checking"),
620 quotes (ppr clas), ptext SLIT("instance")])
621 4 (hsep [text "(Try either importing", ppr inst_ty,
622 text "non-abstractly or compile using -fno-prune-tydecls ..)"])
624 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
625 superClassCtxt = ptext SLIT("When checking the superclasses of an instance declaration")