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"
12 import TcBinds ( mkPragFun, tcPrags, badBootDeclErr )
13 import TcClassDcl ( tcMethodBind, mkMethodBind, badMethodErr,
14 tcClassDecl2, getGenericInstances )
16 import TcMType ( tcSkolSigType, checkValidTheta, checkValidInstHead, instTypeErr,
17 checkAmbiguity, SourceTyCtxt(..) )
18 import TcType ( mkClassPred, tyVarsOfType,
19 tcSplitSigmaTy, tcSplitDFunHead, mkTyVarTys,
20 SkolemInfo(InstSkol), tcSplitDFunTy, pprClassPred )
21 import Inst ( tcInstClassOp, newDicts, instToId, showLIE,
22 getOverlapFlag, tcExtendLocalInstEnv )
23 import InstEnv ( mkLocalInstance, instanceDFunId )
24 import TcDeriv ( tcDeriving )
25 import TcEnv ( InstInfo(..), InstBindings(..),
26 newDFunName, tcExtendIdEnv
28 import TcHsType ( kcHsSigType, tcHsKindedType )
29 import TcUnify ( checkSigTyVars )
30 import TcSimplify ( tcSimplifyCheck, tcSimplifySuperClasses )
31 import Type ( zipOpenTvSubst, substTheta, substTys )
32 import DataCon ( classDataCon )
33 import Class ( classBigSig )
34 import Var ( Id, idName, idType )
35 import MkId ( mkDictFunId, rUNTIME_ERROR_ID )
36 import FunDeps ( checkInstFDs )
37 import Name ( Name, getSrcLoc )
38 import UnicodeUtil ( stringToUtf8 )
39 import Maybe ( catMaybes )
40 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart )
41 import ListSetOps ( minusList )
44 import BasicTypes ( Activation( AlwaysActive ), InlineSpec(..) )
48 Typechecking instance declarations is done in two passes. The first
49 pass, made by @tcInstDecls1@, collects information to be used in the
52 This pre-processed info includes the as-yet-unprocessed bindings
53 inside the instance declaration. These are type-checked in the second
54 pass, when the class-instance envs and GVE contain all the info from
55 all the instance and value decls. Indeed that's the reason we need
56 two passes over the instance decls.
59 Here is the overall algorithm.
60 Assume that we have an instance declaration
62 instance c => k (t tvs) where b
66 $LIE_c$ is the LIE for the context of class $c$
68 $betas_bar$ is the free variables in the class method type, excluding the
71 $LIE_cop$ is the LIE constraining a particular class method
73 $tau_cop$ is the tau type of a class method
75 $LIE_i$ is the LIE for the context of instance $i$
77 $X$ is the instance constructor tycon
79 $gammas_bar$ is the set of type variables of the instance
81 $LIE_iop$ is the LIE for a particular class method instance
83 $tau_iop$ is the tau type for this instance of a class method
85 $alpha$ is the class variable
87 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
89 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
92 ToDo: Update the list above with names actually in the code.
96 First, make the LIEs for the class and instance contexts, which means
97 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
98 and make LIElistI and LIEI.
100 Then process each method in turn.
102 order the instance methods according to the ordering of the class methods
104 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
106 Create final dictionary function from bindings generated already
108 df = lambda inst_tyvars
115 in <op1,op2,...,opn,sd1,...,sdm>
117 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
118 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
122 %************************************************************************
124 \subsection{Extracting instance decls}
126 %************************************************************************
128 Gather up the instance declarations from their various sources
131 tcInstDecls1 -- Deal with both source-code and imported instance decls
132 :: [LTyClDecl Name] -- For deriving stuff
133 -> [LInstDecl Name] -- Source code instance decls
134 -> TcM (TcGblEnv, -- The full inst env
135 [InstInfo], -- Source-code instance decls to process;
136 -- contains all dfuns for this module
137 HsValBinds Name) -- Supporting bindings for derived instances
139 tcInstDecls1 tycl_decls inst_decls
141 -- Stop if addInstInfos etc discovers any errors
142 -- (they recover, so that we get more than one error each round)
144 -- (1) Do the ordinary instance declarations
145 mappM tcLocalInstDecl1 inst_decls `thenM` \ local_inst_infos ->
148 local_inst_info = catMaybes local_inst_infos
149 clas_decls = filter (isClassDecl.unLoc) tycl_decls
151 -- (2) Instances from generic class declarations
152 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
154 -- Next, construct the instance environment so far, consisting of
155 -- a) local instance decls
156 -- b) generic instances
157 addInsts local_inst_info $
158 addInsts generic_inst_info $
160 -- (3) Compute instances from "deriving" clauses;
161 -- This stuff computes a context for the derived instance decl, so it
162 -- needs to know about all the instances possible; hence inst_env4
163 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
164 addInsts deriv_inst_info $
166 getGblEnv `thenM` \ gbl_env ->
168 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
171 addInsts :: [InstInfo] -> TcM a -> TcM a
172 addInsts infos thing_inside
173 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
177 tcLocalInstDecl1 :: LInstDecl Name
178 -> TcM (Maybe InstInfo) -- Nothing if there was an error
179 -- A source-file instance declaration
180 -- Type-check all the stuff before the "where"
182 -- We check for respectable instance type, and context
183 -- but only do this for non-imported instance decls.
184 -- Imported ones should have been checked already, and may indeed
185 -- contain something illegal in normal Haskell, notably
186 -- instance CCallable [Char]
187 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags))
188 = -- Prime error recovery, set source location
189 recoverM (returnM Nothing) $
191 addErrCtxt (instDeclCtxt1 poly_ty) $
193 -- Typecheck the instance type itself. We can't use
194 -- tcHsSigType, because it's not a valid user type.
195 kcHsSigType poly_ty `thenM` \ kinded_ty ->
196 tcHsKindedType kinded_ty `thenM` \ poly_ty' ->
198 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
200 checkValidTheta InstThetaCtxt theta `thenM_`
201 checkAmbiguity tyvars theta (tyVarsOfType tau) `thenM_`
202 checkValidInstHead tau `thenM` \ (clas,inst_tys) ->
203 checkTc (checkInstFDs theta clas inst_tys)
204 (instTypeErr (pprClassPred clas inst_tys) msg) `thenM_`
205 newDFunName clas inst_tys (srcSpanStart loc) `thenM` \ dfun_name ->
206 getOverlapFlag `thenM` \ overlap_flag ->
207 let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
208 ispec = mkLocalInstance dfun overlap_flag
211 tcIsHsBoot `thenM` \ is_boot ->
212 checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
213 badBootDeclErr `thenM_`
215 returnM (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags }))
217 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
221 %************************************************************************
223 \subsection{Type-checking instance declarations, pass 2}
225 %************************************************************************
228 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
229 -> TcM (LHsBinds Id, TcLclEnv)
230 -- (a) From each class declaration,
231 -- generate any default-method bindings
232 -- (b) From each instance decl
233 -- generate the dfun binding
235 tcInstDecls2 tycl_decls inst_decls
236 = do { -- (a) Default methods from class decls
237 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
238 filter (isClassDecl.unLoc) tycl_decls
239 ; tcExtendIdEnv (concat dm_ids_s) $ do
241 -- (b) instance declarations
242 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
245 ; let binds = unionManyBags dm_binds_s `unionBags`
246 unionManyBags inst_binds_s
247 ; tcl_env <- getLclEnv -- Default method Ids in here
248 ; returnM (binds, tcl_env) }
251 ======= New documentation starts here (Sept 92) ==============
253 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
254 the dictionary function for this instance declaration. For example
256 instance Foo a => Foo [a] where
260 might generate something like
262 dfun.Foo.List dFoo_a = let op1 x = ...
268 HOWEVER, if the instance decl has no context, then it returns a
269 bigger @HsBinds@ with declarations for each method. For example
271 instance Foo [a] where
277 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
278 const.Foo.op1.List a x = ...
279 const.Foo.op2.List a y = ...
281 This group may be mutually recursive, because (for example) there may
282 be no method supplied for op2 in which case we'll get
284 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
286 that is, the default method applied to the dictionary at this type.
288 What we actually produce in either case is:
290 AbsBinds [a] [dfun_theta_dicts]
291 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
292 { d = (sd1,sd2, ..., op1, op2, ...)
297 The "maybe" says that we only ask AbsBinds to make global constant methods
298 if the dfun_theta is empty.
301 For an instance declaration, say,
303 instance (C1 a, C2 b) => C (T a b) where
306 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
307 function whose type is
309 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
311 Notice that we pass it the superclass dictionaries at the instance type; this
312 is the ``Mark Jones optimisation''. The stuff before the "=>" here
313 is the @dfun_theta@ below.
315 First comes the easy case of a non-local instance decl.
319 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
321 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = binds })
323 dfun_id = instanceDFunId ispec
324 rigid_info = InstSkol dfun_id
325 inst_ty = idType dfun_id
327 -- Prime error recovery
328 recoverM (returnM emptyLHsBinds) $
329 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
330 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
332 -- Instantiate the instance decl with skolem constants
333 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
334 -- These inst_tyvars' scope over the 'where' part
335 -- Those tyvars are inside the dfun_id's type, which is a bit
336 -- bizarre, but OK so long as you realise it!
338 (clas, inst_tys') = tcSplitDFunHead inst_head'
339 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
341 -- Instantiate the super-class context with inst_tys
342 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
343 origin = SigOrigin rigid_info
345 -- Create dictionary Ids from the specified instance contexts.
346 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
347 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
348 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
349 -- Default-method Ids may be mentioned in synthesised RHSs,
350 -- but they'll already be in the environment.
352 -- Typecheck the methods
353 let -- These insts are in scope; quite a few, eh?
354 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
356 tcMethods origin clas inst_tyvars'
357 dfun_theta' inst_tys' avail_insts
358 op_items binds `thenM` \ (meth_ids, meth_binds) ->
360 -- Figure out bindings for the superclass context
361 -- Don't include this_dict in the 'givens', else
362 -- sc_dicts get bound by just selecting from this_dict!!
363 addErrCtxt superClassCtxt
364 (tcSimplifySuperClasses inst_tyvars'
366 sc_dicts) `thenM` \ sc_binds ->
368 -- It's possible that the superclass stuff might unified one
369 -- of the inst_tyavars' with something in the envt
370 checkSigTyVars inst_tyvars' `thenM_`
372 -- Deal with 'SPECIALISE instance' pragmas
374 specs = case binds of
375 VanillaInst _ prags -> filter isSpecInstLSig prags
378 tcPrags dfun_id specs `thenM` \ prags ->
380 -- Create the result bindings
382 dict_constr = classDataCon clas
383 scs_and_meths = map instToId sc_dicts ++ meth_ids
384 this_dict_id = instToId this_dict
385 inline_prag | null dfun_arg_dicts = []
386 | otherwise = [InlinePrag (Inline AlwaysActive True)]
387 -- Always inline the dfun; this is an experimental decision
388 -- because it makes a big performance difference sometimes.
389 -- Often it means we can do the method selection, and then
390 -- inline the method as well. Marcin's idea; see comments below.
392 -- BUT: don't inline it if it's a constant dictionary;
393 -- we'll get all the benefit without inlining, and we get
394 -- a **lot** of code duplication if we inline it
396 -- See Note [Inline dfuns] below
400 = -- Blatant special case for CCallable, CReturnable
401 -- If the dictionary is empty then we should never
402 -- select anything from it, so we make its RHS just
403 -- emit an error message. This in turn means that we don't
404 -- mention the constructor, which doesn't exist for CCallable, CReturnable
405 -- Hardly beautiful, but only three extra lines.
406 nlHsApp (noLoc $ TyApp (nlHsVar rUNTIME_ERROR_ID)
407 [idType this_dict_id])
408 (nlHsLit (HsStringPrim (mkFastString (stringToUtf8 msg))))
410 | otherwise -- The common case
411 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
412 -- We don't produce a binding for the dict_constr; instead we
413 -- rely on the simplifier to unfold this saturated application
414 -- We do this rather than generate an HsCon directly, because
415 -- it means that the special cases (e.g. dictionary with only one
416 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
417 -- than needing to be repeated here.
420 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
422 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
423 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
425 main_bind = noLoc $ AbsBinds
427 (map instToId dfun_arg_dicts)
428 [(inst_tyvars', dfun_id, this_dict_id,
429 inline_prag ++ prags)]
432 showLIE (text "instance") `thenM_`
433 returnM (unitBag main_bind)
436 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
437 avail_insts op_items (VanillaInst monobinds uprags)
438 = -- Check that all the method bindings come from this class
440 sel_names = [idName sel_id | (sel_id, _) <- op_items]
441 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
443 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
445 -- Make the method bindings
447 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
449 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
451 -- And type check them
452 -- It's really worth making meth_insts available to the tcMethodBind
453 -- Consider instance Monad (ST s) where
454 -- {-# INLINE (>>) #-}
455 -- (>>) = ...(>>=)...
456 -- If we don't include meth_insts, we end up with bindings like this:
457 -- rec { dict = MkD then bind ...
458 -- then = inline_me (... (GHC.Base.>>= dict) ...)
460 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
461 -- and (b) the inline_me prevents us inlining the >>= selector, which
462 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
463 -- is not inlined across modules. Rather ironic since this does not
464 -- happen without the INLINE pragma!
466 -- Solution: make meth_insts available, so that 'then' refers directly
467 -- to the local 'bind' rather than going via the dictionary.
469 -- BUT WATCH OUT! If the method type mentions the class variable, then
470 -- this optimisation is not right. Consider
474 -- instance C Int where
476 -- The occurrence of 'op' on the rhs gives rise to a constraint
478 -- The trouble is that the 'meth_inst' for op, which is 'available', also
479 -- looks like 'op at Int'. But they are not the same.
481 prag_fn = mkPragFun uprags
482 all_insts = avail_insts ++ catMaybes meth_insts
483 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts prag_fn
484 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
487 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
489 returnM (meth_ids, unionManyBags meth_binds_s)
492 -- Derived newtype instances
493 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
494 avail_insts op_items (NewTypeDerived rep_tys)
495 = getInstLoc origin `thenM` \ inst_loc ->
496 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
499 (ptext SLIT("newtype derived instance"))
500 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
502 -- I don't think we have to do the checkSigTyVars thing
504 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
507 do_one inst_loc (sel_id, _)
508 = -- The binding is like "op @ NewTy = op @ RepTy"
509 -- Make the *binder*, like in mkMethodBind
510 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
512 -- Make the *occurrence on the rhs*
513 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
515 meth_id = instToId meth_inst
517 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
519 -- Instantiate rep_tys with the relevant type variables
520 -- This looks a bit odd, because inst_tyvars' are the skolemised version
521 -- of the type variables in the instance declaration; but rep_tys doesn't
522 -- have the skolemised version, so we substitute them in here
523 rep_tys' = substTys subst rep_tys
524 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
528 ------------------------------
529 [Inline dfuns] Inlining dfuns unconditionally
530 ------------------------------
532 The code above unconditionally inlines dict funs. Here's why.
533 Consider this program:
535 test :: Int -> Int -> Bool
536 test x y = (x,y) == (y,x) || test y x
537 -- Recursive to avoid making it inline.
539 This needs the (Eq (Int,Int)) instance. If we inline that dfun
540 the code we end up with is good:
543 \r -> case ==# [ww ww1] of wild {
544 PrelBase.False -> Test.$wtest ww1 ww;
546 case ==# [ww1 ww] of wild1 {
547 PrelBase.False -> Test.$wtest ww1 ww;
548 PrelBase.True -> PrelBase.True [];
551 Test.test = \r [w w1]
554 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
557 If we don't inline the dfun, the code is not nearly as good:
559 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
560 PrelBase.:DEq tpl1 tpl2 -> tpl2;
565 let { y = PrelBase.I#! [ww1]; } in
566 let { x = PrelBase.I#! [ww]; } in
567 let { sat_slx = PrelTup.(,)! [y x]; } in
568 let { sat_sly = PrelTup.(,)! [x y];
570 case == sat_sly sat_slx of wild {
571 PrelBase.False -> Test.$wtest ww1 ww;
572 PrelBase.True -> PrelBase.True [];
579 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
582 Why doesn't GHC inline $fEq? Because it looks big:
584 PrelTup.zdfEqZ1T{-rcX-}
585 = \ @ a{-reT-} :: * @ b{-reS-} :: *
586 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
587 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
589 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
590 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
592 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
593 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
595 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
596 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
597 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
599 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
601 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
603 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
604 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
608 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
609 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
610 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
611 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
613 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
615 and it's not as bad as it seems, because it's further dramatically
616 simplified: only zeze2 is extracted and its body is simplified.
619 %************************************************************************
621 \subsection{Error messages}
623 %************************************************************************
626 instDeclCtxt1 hs_inst_ty
627 = inst_decl_ctxt (case unLoc hs_inst_ty of
628 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
629 HsPredTy pred -> ppr pred
630 other -> ppr hs_inst_ty) -- Don't expect this
631 instDeclCtxt2 dfun_ty
632 = inst_decl_ctxt (ppr (mkClassPred cls tys))
634 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
636 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
638 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")