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.
54 Here is the overall algorithm.
55 Assume that we have an instance declaration
57 instance c => k (t tvs) where b
61 $LIE_c$ is the LIE for the context of class $c$
63 $betas_bar$ is the free variables in the class method type, excluding the
66 $LIE_cop$ is the LIE constraining a particular class method
68 $tau_cop$ is the tau type of a class method
70 $LIE_i$ is the LIE for the context of instance $i$
72 $X$ is the instance constructor tycon
74 $gammas_bar$ is the set of type variables of the instance
76 $LIE_iop$ is the LIE for a particular class method instance
78 $tau_iop$ is the tau type for this instance of a class method
80 $alpha$ is the class variable
82 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
84 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
87 ToDo: Update the list above with names actually in the code.
91 First, make the LIEs for the class and instance contexts, which means
92 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
93 and make LIElistI and LIEI.
95 Then process each method in turn.
97 order the instance methods according to the ordering of the class methods
99 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
101 Create final dictionary function from bindings generated already
103 df = lambda inst_tyvars
110 in <op1,op2,...,opn,sd1,...,sdm>
112 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
113 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
117 %************************************************************************
119 \subsection{Extracting instance decls}
121 %************************************************************************
123 Gather up the instance declarations from their various sources
126 tcInstDecls1 -- Deal with both source-code and imported instance decls
127 :: [LTyClDecl Name] -- For deriving stuff
128 -> [LInstDecl Name] -- Source code instance decls
129 -> TcM (TcGblEnv, -- The full inst env
130 [InstInfo], -- Source-code instance decls to process;
131 -- contains all dfuns for this module
132 HsValBinds Name) -- Supporting bindings for derived instances
134 tcInstDecls1 tycl_decls inst_decls
136 -- Stop if addInstInfos etc discovers any errors
137 -- (they recover, so that we get more than one error each round)
139 -- (1) Do the ordinary instance declarations
140 mappM tcLocalInstDecl1 inst_decls `thenM` \ local_inst_infos ->
143 local_inst_info = catMaybes local_inst_infos
144 clas_decls = filter (isClassDecl.unLoc) tycl_decls
146 -- (2) Instances from generic class declarations
147 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
149 -- Next, construct the instance environment so far, consisting of
150 -- a) local instance decls
151 -- b) generic instances
152 addInsts local_inst_info $
153 addInsts generic_inst_info $
155 -- (3) Compute instances from "deriving" clauses;
156 -- This stuff computes a context for the derived instance decl, so it
157 -- needs to know about all the instances possible; hence inst_env4
158 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
159 addInsts deriv_inst_info $
161 getGblEnv `thenM` \ gbl_env ->
163 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
166 addInsts :: [InstInfo] -> TcM a -> TcM a
167 addInsts infos thing_inside
168 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
172 tcLocalInstDecl1 :: LInstDecl Name
173 -> TcM (Maybe InstInfo) -- Nothing if there was an error
174 -- A source-file instance declaration
175 -- Type-check all the stuff before the "where"
177 -- We check for respectable instance type, and context
178 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
179 = -- Prime error recovery, set source location
181 -- !!!TODO: Handle the `ats' parameter!!! -=chak
182 recoverM (returnM Nothing) $
184 addErrCtxt (instDeclCtxt1 poly_ty) $
186 do { is_boot <- tcIsHsBoot
187 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
190 -- Typecheck the instance type itself. We can't use
191 -- tcHsSigType, because it's not a valid user type.
192 ; kinded_ty <- kcHsSigType poly_ty
193 ; poly_ty' <- tcHsKindedType kinded_ty
194 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
196 ; (clas, inst_tys) <- checkValidInstHead tau
197 ; checkValidInstance tyvars theta clas inst_tys
199 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
200 ; overlap_flag <- getOverlapFlag
201 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
202 ispec = mkLocalInstance dfun overlap_flag
204 ; return (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags })) }
208 %************************************************************************
210 \subsection{Type-checking instance declarations, pass 2}
212 %************************************************************************
215 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
216 -> TcM (LHsBinds Id, TcLclEnv)
217 -- (a) From each class declaration,
218 -- generate any default-method bindings
219 -- (b) From each instance decl
220 -- generate the dfun binding
222 tcInstDecls2 tycl_decls inst_decls
223 = do { -- (a) Default methods from class decls
224 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
225 filter (isClassDecl.unLoc) tycl_decls
226 ; tcExtendIdEnv (concat dm_ids_s) $ do
228 -- (b) instance declarations
229 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
232 ; let binds = unionManyBags dm_binds_s `unionBags`
233 unionManyBags inst_binds_s
234 ; tcl_env <- getLclEnv -- Default method Ids in here
235 ; returnM (binds, tcl_env) }
238 ======= New documentation starts here (Sept 92) ==============
240 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
241 the dictionary function for this instance declaration. For example
243 instance Foo a => Foo [a] where
247 might generate something like
249 dfun.Foo.List dFoo_a = let op1 x = ...
255 HOWEVER, if the instance decl has no context, then it returns a
256 bigger @HsBinds@ with declarations for each method. For example
258 instance Foo [a] where
264 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
265 const.Foo.op1.List a x = ...
266 const.Foo.op2.List a y = ...
268 This group may be mutually recursive, because (for example) there may
269 be no method supplied for op2 in which case we'll get
271 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
273 that is, the default method applied to the dictionary at this type.
275 What we actually produce in either case is:
277 AbsBinds [a] [dfun_theta_dicts]
278 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
279 { d = (sd1,sd2, ..., op1, op2, ...)
284 The "maybe" says that we only ask AbsBinds to make global constant methods
285 if the dfun_theta is empty.
288 For an instance declaration, say,
290 instance (C1 a, C2 b) => C (T a b) where
293 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
294 function whose type is
296 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
298 Notice that we pass it the superclass dictionaries at the instance type; this
299 is the ``Mark Jones optimisation''. The stuff before the "=>" here
300 is the @dfun_theta@ below.
302 First comes the easy case of a non-local instance decl.
306 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
308 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = binds })
310 dfun_id = instanceDFunId ispec
311 rigid_info = InstSkol dfun_id
312 inst_ty = idType dfun_id
314 -- Prime error recovery
315 recoverM (returnM emptyLHsBinds) $
316 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
317 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
319 -- Instantiate the instance decl with skolem constants
320 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
321 -- These inst_tyvars' scope over the 'where' part
322 -- Those tyvars are inside the dfun_id's type, which is a bit
323 -- bizarre, but OK so long as you realise it!
325 (clas, inst_tys') = tcSplitDFunHead inst_head'
326 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
328 -- Instantiate the super-class context with inst_tys
329 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
330 origin = SigOrigin rigid_info
332 -- Create dictionary Ids from the specified instance contexts.
333 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
334 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
335 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
336 -- Default-method Ids may be mentioned in synthesised RHSs,
337 -- but they'll already be in the environment.
339 -- Typecheck the methods
340 let -- These insts are in scope; quite a few, eh?
341 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
343 tcMethods origin clas inst_tyvars'
344 dfun_theta' inst_tys' avail_insts
345 op_items binds `thenM` \ (meth_ids, meth_binds) ->
347 -- Figure out bindings for the superclass context
348 -- Don't include this_dict in the 'givens', else
349 -- sc_dicts get bound by just selecting from this_dict!!
350 addErrCtxt superClassCtxt
351 (tcSimplifySuperClasses inst_tyvars'
353 sc_dicts) `thenM` \ sc_binds ->
355 -- It's possible that the superclass stuff might unified one
356 -- of the inst_tyavars' with something in the envt
357 checkSigTyVars inst_tyvars' `thenM_`
359 -- Deal with 'SPECIALISE instance' pragmas
361 specs = case binds of
362 VanillaInst _ prags -> filter isSpecInstLSig prags
365 tcPrags dfun_id specs `thenM` \ prags ->
367 -- Create the result bindings
369 dict_constr = classDataCon clas
370 scs_and_meths = map instToId sc_dicts ++ meth_ids
371 this_dict_id = instToId this_dict
372 inline_prag | null dfun_arg_dicts = []
373 | otherwise = [InlinePrag (Inline AlwaysActive True)]
374 -- Always inline the dfun; this is an experimental decision
375 -- because it makes a big performance difference sometimes.
376 -- Often it means we can do the method selection, and then
377 -- inline the method as well. Marcin's idea; see comments below.
379 -- BUT: don't inline it if it's a constant dictionary;
380 -- we'll get all the benefit without inlining, and we get
381 -- a **lot** of code duplication if we inline it
383 -- See Note [Inline dfuns] below
386 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
387 -- We don't produce a binding for the dict_constr; instead we
388 -- rely on the simplifier to unfold this saturated application
389 -- We do this rather than generate an HsCon directly, because
390 -- it means that the special cases (e.g. dictionary with only one
391 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
392 -- than needing to be repeated here.
394 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
395 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
397 main_bind = noLoc $ AbsBinds
399 (map instToId dfun_arg_dicts)
400 [(inst_tyvars', dfun_id, this_dict_id,
401 inline_prag ++ prags)]
404 showLIE (text "instance") `thenM_`
405 returnM (unitBag main_bind)
408 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
409 avail_insts op_items (VanillaInst monobinds uprags)
410 = -- Check that all the method bindings come from this class
412 sel_names = [idName sel_id | (sel_id, _) <- op_items]
413 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
415 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
417 -- Make the method bindings
419 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
421 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
423 -- And type check them
424 -- It's really worth making meth_insts available to the tcMethodBind
425 -- Consider instance Monad (ST s) where
426 -- {-# INLINE (>>) #-}
427 -- (>>) = ...(>>=)...
428 -- If we don't include meth_insts, we end up with bindings like this:
429 -- rec { dict = MkD then bind ...
430 -- then = inline_me (... (GHC.Base.>>= dict) ...)
432 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
433 -- and (b) the inline_me prevents us inlining the >>= selector, which
434 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
435 -- is not inlined across modules. Rather ironic since this does not
436 -- happen without the INLINE pragma!
438 -- Solution: make meth_insts available, so that 'then' refers directly
439 -- to the local 'bind' rather than going via the dictionary.
441 -- BUT WATCH OUT! If the method type mentions the class variable, then
442 -- this optimisation is not right. Consider
446 -- instance C Int where
448 -- The occurrence of 'op' on the rhs gives rise to a constraint
450 -- The trouble is that the 'meth_inst' for op, which is 'available', also
451 -- looks like 'op at Int'. But they are not the same.
453 prag_fn = mkPragFun uprags
454 all_insts = avail_insts ++ catMaybes meth_insts
455 sig_fn n = Just [] -- No scoped type variables, but every method has
456 -- a type signature, in effect, so that we check
457 -- the method has the right type
458 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
459 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
462 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
464 returnM (meth_ids, unionManyBags meth_binds_s)
467 -- Derived newtype instances
468 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
469 avail_insts op_items (NewTypeDerived rep_tys)
470 = getInstLoc origin `thenM` \ inst_loc ->
471 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
474 (ptext SLIT("newtype derived instance"))
475 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
477 -- I don't think we have to do the checkSigTyVars thing
479 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
482 do_one inst_loc (sel_id, _)
483 = -- The binding is like "op @ NewTy = op @ RepTy"
484 -- Make the *binder*, like in mkMethodBind
485 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
487 -- Make the *occurrence on the rhs*
488 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
490 meth_id = instToId meth_inst
492 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
494 -- Instantiate rep_tys with the relevant type variables
495 -- This looks a bit odd, because inst_tyvars' are the skolemised version
496 -- of the type variables in the instance declaration; but rep_tys doesn't
497 -- have the skolemised version, so we substitute them in here
498 rep_tys' = substTys subst rep_tys
499 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
503 ------------------------------
504 [Inline dfuns] Inlining dfuns unconditionally
505 ------------------------------
507 The code above unconditionally inlines dict funs. Here's why.
508 Consider this program:
510 test :: Int -> Int -> Bool
511 test x y = (x,y) == (y,x) || test y x
512 -- Recursive to avoid making it inline.
514 This needs the (Eq (Int,Int)) instance. If we inline that dfun
515 the code we end up with is good:
518 \r -> case ==# [ww ww1] of wild {
519 PrelBase.False -> Test.$wtest ww1 ww;
521 case ==# [ww1 ww] of wild1 {
522 PrelBase.False -> Test.$wtest ww1 ww;
523 PrelBase.True -> PrelBase.True [];
526 Test.test = \r [w w1]
529 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
532 If we don't inline the dfun, the code is not nearly as good:
534 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
535 PrelBase.:DEq tpl1 tpl2 -> tpl2;
540 let { y = PrelBase.I#! [ww1]; } in
541 let { x = PrelBase.I#! [ww]; } in
542 let { sat_slx = PrelTup.(,)! [y x]; } in
543 let { sat_sly = PrelTup.(,)! [x y];
545 case == sat_sly sat_slx of wild {
546 PrelBase.False -> Test.$wtest ww1 ww;
547 PrelBase.True -> PrelBase.True [];
554 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
557 Why doesn't GHC inline $fEq? Because it looks big:
559 PrelTup.zdfEqZ1T{-rcX-}
560 = \ @ a{-reT-} :: * @ b{-reS-} :: *
561 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
562 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
564 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
565 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
567 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
568 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
570 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
571 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
572 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
574 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
576 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
578 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
579 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
583 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
584 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
585 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
586 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
588 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
590 and it's not as bad as it seems, because it's further dramatically
591 simplified: only zeze2 is extracted and its body is simplified.
594 %************************************************************************
596 \subsection{Error messages}
598 %************************************************************************
601 instDeclCtxt1 hs_inst_ty
602 = inst_decl_ctxt (case unLoc hs_inst_ty of
603 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
604 HsPredTy pred -> ppr pred
605 other -> ppr hs_inst_ty) -- Don't expect this
606 instDeclCtxt2 dfun_ty
607 = inst_decl_ctxt (ppr (mkClassPred cls tys))
609 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
611 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
613 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")