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, checkValidInstance, checkValidInstHead )
17 import TcType ( mkClassPred, tcSplitSigmaTy, tcSplitDFunHead, mkTyVarTys,
18 SkolemInfo(InstSkol), tcSplitDFunTy )
19 import Inst ( tcInstClassOp, newDicts, instToId, showLIE,
20 getOverlapFlag, tcExtendLocalInstEnv )
21 import InstEnv ( mkLocalInstance, instanceDFunId )
22 import TcDeriv ( tcDeriving )
23 import TcEnv ( InstInfo(..), InstBindings(..),
24 newDFunName, tcExtendIdEnv
26 import TcHsType ( kcHsSigType, tcHsKindedType )
27 import TcUnify ( checkSigTyVars )
28 import TcSimplify ( tcSimplifyCheck, tcSimplifySuperClasses )
29 import Type ( zipOpenTvSubst, substTheta, substTys )
30 import DataCon ( classDataCon )
31 import Class ( classBigSig )
32 import Var ( Id, idName, idType )
33 import MkId ( mkDictFunId )
34 import Name ( Name, getSrcLoc )
35 import Maybe ( catMaybes )
36 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart )
37 import ListSetOps ( minusList )
40 import BasicTypes ( Activation( AlwaysActive ), InlineSpec(..) )
44 Typechecking instance declarations is done in two passes. The first
45 pass, made by @tcInstDecls1@, collects information to be used in the
48 This pre-processed info includes the as-yet-unprocessed bindings
49 inside the instance declaration. These are type-checked in the second
50 pass, when the class-instance envs and GVE contain all the info from
51 all the instance and value decls. Indeed that's the reason we need
52 two passes over the instance decls.
55 Here is the overall algorithm.
56 Assume that we have an instance declaration
58 instance c => k (t tvs) where b
62 $LIE_c$ is the LIE for the context of class $c$
64 $betas_bar$ is the free variables in the class method type, excluding the
67 $LIE_cop$ is the LIE constraining a particular class method
69 $tau_cop$ is the tau type of a class method
71 $LIE_i$ is the LIE for the context of instance $i$
73 $X$ is the instance constructor tycon
75 $gammas_bar$ is the set of type variables of the instance
77 $LIE_iop$ is the LIE for a particular class method instance
79 $tau_iop$ is the tau type for this instance of a class method
81 $alpha$ is the class variable
83 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
85 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
88 ToDo: Update the list above with names actually in the code.
92 First, make the LIEs for the class and instance contexts, which means
93 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
94 and make LIElistI and LIEI.
96 Then process each method in turn.
98 order the instance methods according to the ordering of the class methods
100 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
102 Create final dictionary function from bindings generated already
104 df = lambda inst_tyvars
111 in <op1,op2,...,opn,sd1,...,sdm>
113 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
114 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
118 %************************************************************************
120 \subsection{Extracting instance decls}
122 %************************************************************************
124 Gather up the instance declarations from their various sources
127 tcInstDecls1 -- Deal with both source-code and imported instance decls
128 :: [LTyClDecl Name] -- For deriving stuff
129 -> [LInstDecl Name] -- Source code instance decls
130 -> TcM (TcGblEnv, -- The full inst env
131 [InstInfo], -- Source-code instance decls to process;
132 -- contains all dfuns for this module
133 HsValBinds Name) -- Supporting bindings for derived instances
135 tcInstDecls1 tycl_decls inst_decls
137 -- Stop if addInstInfos etc discovers any errors
138 -- (they recover, so that we get more than one error each round)
140 -- (1) Do the ordinary instance declarations
141 mappM tcLocalInstDecl1 inst_decls `thenM` \ local_inst_infos ->
144 local_inst_info = catMaybes local_inst_infos
145 clas_decls = filter (isClassDecl.unLoc) tycl_decls
147 -- (2) Instances from generic class declarations
148 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
150 -- Next, construct the instance environment so far, consisting of
151 -- a) local instance decls
152 -- b) generic instances
153 addInsts local_inst_info $
154 addInsts generic_inst_info $
156 -- (3) Compute instances from "deriving" clauses;
157 -- This stuff computes a context for the derived instance decl, so it
158 -- needs to know about all the instances possible; hence inst_env4
159 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
160 addInsts deriv_inst_info $
162 getGblEnv `thenM` \ gbl_env ->
164 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
167 addInsts :: [InstInfo] -> TcM a -> TcM a
168 addInsts infos thing_inside
169 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
173 tcLocalInstDecl1 :: LInstDecl Name
174 -> TcM (Maybe InstInfo) -- Nothing if there was an error
175 -- A source-file instance declaration
176 -- Type-check all the stuff before the "where"
178 -- We check for respectable instance type, and context
179 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags))
180 = -- Prime error recovery, set source location
181 recoverM (returnM Nothing) $
183 addErrCtxt (instDeclCtxt1 poly_ty) $
185 do { is_boot <- tcIsHsBoot
186 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
189 -- Typecheck the instance type itself. We can't use
190 -- tcHsSigType, because it's not a valid user type.
191 ; kinded_ty <- kcHsSigType poly_ty
192 ; poly_ty' <- tcHsKindedType kinded_ty
193 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
195 ; (clas, inst_tys) <- checkValidInstHead tau
196 ; checkValidInstance tyvars theta clas inst_tys
198 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
199 ; overlap_flag <- getOverlapFlag
200 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
201 ispec = mkLocalInstance dfun overlap_flag
203 ; return (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags })) }
207 %************************************************************************
209 \subsection{Type-checking instance declarations, pass 2}
211 %************************************************************************
214 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
215 -> TcM (LHsBinds Id, TcLclEnv)
216 -- (a) From each class declaration,
217 -- generate any default-method bindings
218 -- (b) From each instance decl
219 -- generate the dfun binding
221 tcInstDecls2 tycl_decls inst_decls
222 = do { -- (a) Default methods from class decls
223 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
224 filter (isClassDecl.unLoc) tycl_decls
225 ; tcExtendIdEnv (concat dm_ids_s) $ do
227 -- (b) instance declarations
228 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
231 ; let binds = unionManyBags dm_binds_s `unionBags`
232 unionManyBags inst_binds_s
233 ; tcl_env <- getLclEnv -- Default method Ids in here
234 ; returnM (binds, tcl_env) }
237 ======= New documentation starts here (Sept 92) ==============
239 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
240 the dictionary function for this instance declaration. For example
242 instance Foo a => Foo [a] where
246 might generate something like
248 dfun.Foo.List dFoo_a = let op1 x = ...
254 HOWEVER, if the instance decl has no context, then it returns a
255 bigger @HsBinds@ with declarations for each method. For example
257 instance Foo [a] where
263 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
264 const.Foo.op1.List a x = ...
265 const.Foo.op2.List a y = ...
267 This group may be mutually recursive, because (for example) there may
268 be no method supplied for op2 in which case we'll get
270 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
272 that is, the default method applied to the dictionary at this type.
274 What we actually produce in either case is:
276 AbsBinds [a] [dfun_theta_dicts]
277 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
278 { d = (sd1,sd2, ..., op1, op2, ...)
283 The "maybe" says that we only ask AbsBinds to make global constant methods
284 if the dfun_theta is empty.
287 For an instance declaration, say,
289 instance (C1 a, C2 b) => C (T a b) where
292 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
293 function whose type is
295 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
297 Notice that we pass it the superclass dictionaries at the instance type; this
298 is the ``Mark Jones optimisation''. The stuff before the "=>" here
299 is the @dfun_theta@ below.
301 First comes the easy case of a non-local instance decl.
305 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
307 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = binds })
309 dfun_id = instanceDFunId ispec
310 rigid_info = InstSkol dfun_id
311 inst_ty = idType dfun_id
313 -- Prime error recovery
314 recoverM (returnM emptyLHsBinds) $
315 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
316 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
318 -- Instantiate the instance decl with skolem constants
319 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
320 -- These inst_tyvars' scope over the 'where' part
321 -- Those tyvars are inside the dfun_id's type, which is a bit
322 -- bizarre, but OK so long as you realise it!
324 (clas, inst_tys') = tcSplitDFunHead inst_head'
325 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
327 -- Instantiate the super-class context with inst_tys
328 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
329 origin = SigOrigin rigid_info
331 -- Create dictionary Ids from the specified instance contexts.
332 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
333 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
334 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
335 -- Default-method Ids may be mentioned in synthesised RHSs,
336 -- but they'll already be in the environment.
338 -- Typecheck the methods
339 let -- These insts are in scope; quite a few, eh?
340 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
342 tcMethods origin clas inst_tyvars'
343 dfun_theta' inst_tys' avail_insts
344 op_items binds `thenM` \ (meth_ids, meth_binds) ->
346 -- Figure out bindings for the superclass context
347 -- Don't include this_dict in the 'givens', else
348 -- sc_dicts get bound by just selecting from this_dict!!
349 addErrCtxt superClassCtxt
350 (tcSimplifySuperClasses inst_tyvars'
352 sc_dicts) `thenM` \ sc_binds ->
354 -- It's possible that the superclass stuff might unified one
355 -- of the inst_tyavars' with something in the envt
356 checkSigTyVars inst_tyvars' `thenM_`
358 -- Deal with 'SPECIALISE instance' pragmas
360 specs = case binds of
361 VanillaInst _ prags -> filter isSpecInstLSig prags
364 tcPrags dfun_id specs `thenM` \ prags ->
366 -- Create the result bindings
368 dict_constr = classDataCon clas
369 scs_and_meths = map instToId sc_dicts ++ meth_ids
370 this_dict_id = instToId this_dict
371 inline_prag | null dfun_arg_dicts = []
372 | otherwise = [InlinePrag (Inline AlwaysActive True)]
373 -- Always inline the dfun; this is an experimental decision
374 -- because it makes a big performance difference sometimes.
375 -- Often it means we can do the method selection, and then
376 -- inline the method as well. Marcin's idea; see comments below.
378 -- BUT: don't inline it if it's a constant dictionary;
379 -- we'll get all the benefit without inlining, and we get
380 -- a **lot** of code duplication if we inline it
382 -- See Note [Inline dfuns] below
385 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
386 -- We don't produce a binding for the dict_constr; instead we
387 -- rely on the simplifier to unfold this saturated application
388 -- We do this rather than generate an HsCon directly, because
389 -- it means that the special cases (e.g. dictionary with only one
390 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
391 -- than needing to be repeated here.
393 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
394 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
396 main_bind = noLoc $ AbsBinds
398 (map instToId dfun_arg_dicts)
399 [(inst_tyvars', dfun_id, this_dict_id,
400 inline_prag ++ prags)]
403 showLIE (text "instance") `thenM_`
404 returnM (unitBag main_bind)
407 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
408 avail_insts op_items (VanillaInst monobinds uprags)
409 = -- Check that all the method bindings come from this class
411 sel_names = [idName sel_id | (sel_id, _) <- op_items]
412 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
414 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
416 -- Make the method bindings
418 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
420 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
422 -- And type check them
423 -- It's really worth making meth_insts available to the tcMethodBind
424 -- Consider instance Monad (ST s) where
425 -- {-# INLINE (>>) #-}
426 -- (>>) = ...(>>=)...
427 -- If we don't include meth_insts, we end up with bindings like this:
428 -- rec { dict = MkD then bind ...
429 -- then = inline_me (... (GHC.Base.>>= dict) ...)
431 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
432 -- and (b) the inline_me prevents us inlining the >>= selector, which
433 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
434 -- is not inlined across modules. Rather ironic since this does not
435 -- happen without the INLINE pragma!
437 -- Solution: make meth_insts available, so that 'then' refers directly
438 -- to the local 'bind' rather than going via the dictionary.
440 -- BUT WATCH OUT! If the method type mentions the class variable, then
441 -- this optimisation is not right. Consider
445 -- instance C Int where
447 -- The occurrence of 'op' on the rhs gives rise to a constraint
449 -- The trouble is that the 'meth_inst' for op, which is 'available', also
450 -- looks like 'op at Int'. But they are not the same.
452 prag_fn = mkPragFun uprags
453 all_insts = avail_insts ++ catMaybes meth_insts
454 sig_fn n = Just [] -- No scoped type variables, but every method has
455 -- a type signature, in effect, so that we check
456 -- the method has the right type
457 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
458 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
461 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
463 returnM (meth_ids, unionManyBags meth_binds_s)
466 -- Derived newtype instances
467 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
468 avail_insts op_items (NewTypeDerived rep_tys)
469 = getInstLoc origin `thenM` \ inst_loc ->
470 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
473 (ptext SLIT("newtype derived instance"))
474 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
476 -- I don't think we have to do the checkSigTyVars thing
478 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
481 do_one inst_loc (sel_id, _)
482 = -- The binding is like "op @ NewTy = op @ RepTy"
483 -- Make the *binder*, like in mkMethodBind
484 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
486 -- Make the *occurrence on the rhs*
487 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
489 meth_id = instToId meth_inst
491 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
493 -- Instantiate rep_tys with the relevant type variables
494 -- This looks a bit odd, because inst_tyvars' are the skolemised version
495 -- of the type variables in the instance declaration; but rep_tys doesn't
496 -- have the skolemised version, so we substitute them in here
497 rep_tys' = substTys subst rep_tys
498 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
502 ------------------------------
503 [Inline dfuns] Inlining dfuns unconditionally
504 ------------------------------
506 The code above unconditionally inlines dict funs. Here's why.
507 Consider this program:
509 test :: Int -> Int -> Bool
510 test x y = (x,y) == (y,x) || test y x
511 -- Recursive to avoid making it inline.
513 This needs the (Eq (Int,Int)) instance. If we inline that dfun
514 the code we end up with is good:
517 \r -> case ==# [ww ww1] of wild {
518 PrelBase.False -> Test.$wtest ww1 ww;
520 case ==# [ww1 ww] of wild1 {
521 PrelBase.False -> Test.$wtest ww1 ww;
522 PrelBase.True -> PrelBase.True [];
525 Test.test = \r [w w1]
528 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
531 If we don't inline the dfun, the code is not nearly as good:
533 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
534 PrelBase.:DEq tpl1 tpl2 -> tpl2;
539 let { y = PrelBase.I#! [ww1]; } in
540 let { x = PrelBase.I#! [ww]; } in
541 let { sat_slx = PrelTup.(,)! [y x]; } in
542 let { sat_sly = PrelTup.(,)! [x y];
544 case == sat_sly sat_slx of wild {
545 PrelBase.False -> Test.$wtest ww1 ww;
546 PrelBase.True -> PrelBase.True [];
553 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
556 Why doesn't GHC inline $fEq? Because it looks big:
558 PrelTup.zdfEqZ1T{-rcX-}
559 = \ @ a{-reT-} :: * @ b{-reS-} :: *
560 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
561 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
563 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
564 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
566 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
567 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
569 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
570 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
571 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
573 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
575 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
577 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
578 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
582 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
583 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
584 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
585 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
587 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
589 and it's not as bad as it seems, because it's further dramatically
590 simplified: only zeze2 is extracted and its body is simplified.
593 %************************************************************************
595 \subsection{Error messages}
597 %************************************************************************
600 instDeclCtxt1 hs_inst_ty
601 = inst_decl_ctxt (case unLoc hs_inst_ty of
602 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
603 HsPredTy pred -> ppr pred
604 other -> ppr hs_inst_ty) -- Don't expect this
605 instDeclCtxt2 dfun_ty
606 = inst_decl_ctxt (ppr (mkClassPred cls tys))
608 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
610 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
612 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")