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))
179 = -- Prime error recovery, set source location
180 recoverM (returnM Nothing) $
182 addErrCtxt (instDeclCtxt1 poly_ty) $
184 do { is_boot <- tcIsHsBoot
185 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
188 -- Typecheck the instance type itself. We can't use
189 -- tcHsSigType, because it's not a valid user type.
190 ; kinded_ty <- kcHsSigType poly_ty
191 ; poly_ty' <- tcHsKindedType kinded_ty
192 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
194 ; (clas, inst_tys) <- checkValidInstHead tau
195 ; checkValidInstance tyvars theta clas inst_tys
197 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
198 ; overlap_flag <- getOverlapFlag
199 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
200 ispec = mkLocalInstance dfun overlap_flag
202 ; return (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags })) }
206 %************************************************************************
208 \subsection{Type-checking instance declarations, pass 2}
210 %************************************************************************
213 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
214 -> TcM (LHsBinds Id, TcLclEnv)
215 -- (a) From each class declaration,
216 -- generate any default-method bindings
217 -- (b) From each instance decl
218 -- generate the dfun binding
220 tcInstDecls2 tycl_decls inst_decls
221 = do { -- (a) Default methods from class decls
222 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
223 filter (isClassDecl.unLoc) tycl_decls
224 ; tcExtendIdEnv (concat dm_ids_s) $ do
226 -- (b) instance declarations
227 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
230 ; let binds = unionManyBags dm_binds_s `unionBags`
231 unionManyBags inst_binds_s
232 ; tcl_env <- getLclEnv -- Default method Ids in here
233 ; returnM (binds, tcl_env) }
236 ======= New documentation starts here (Sept 92) ==============
238 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
239 the dictionary function for this instance declaration. For example
241 instance Foo a => Foo [a] where
245 might generate something like
247 dfun.Foo.List dFoo_a = let op1 x = ...
253 HOWEVER, if the instance decl has no context, then it returns a
254 bigger @HsBinds@ with declarations for each method. For example
256 instance Foo [a] where
262 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
263 const.Foo.op1.List a x = ...
264 const.Foo.op2.List a y = ...
266 This group may be mutually recursive, because (for example) there may
267 be no method supplied for op2 in which case we'll get
269 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
271 that is, the default method applied to the dictionary at this type.
273 What we actually produce in either case is:
275 AbsBinds [a] [dfun_theta_dicts]
276 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
277 { d = (sd1,sd2, ..., op1, op2, ...)
282 The "maybe" says that we only ask AbsBinds to make global constant methods
283 if the dfun_theta is empty.
286 For an instance declaration, say,
288 instance (C1 a, C2 b) => C (T a b) where
291 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
292 function whose type is
294 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
296 Notice that we pass it the superclass dictionaries at the instance type; this
297 is the ``Mark Jones optimisation''. The stuff before the "=>" here
298 is the @dfun_theta@ below.
300 First comes the easy case of a non-local instance decl.
304 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
306 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = binds })
308 dfun_id = instanceDFunId ispec
309 rigid_info = InstSkol dfun_id
310 inst_ty = idType dfun_id
312 -- Prime error recovery
313 recoverM (returnM emptyLHsBinds) $
314 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
315 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
317 -- Instantiate the instance decl with skolem constants
318 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
319 -- These inst_tyvars' scope over the 'where' part
320 -- Those tyvars are inside the dfun_id's type, which is a bit
321 -- bizarre, but OK so long as you realise it!
323 (clas, inst_tys') = tcSplitDFunHead inst_head'
324 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
326 -- Instantiate the super-class context with inst_tys
327 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
328 origin = SigOrigin rigid_info
330 -- Create dictionary Ids from the specified instance contexts.
331 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
332 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
333 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
334 -- Default-method Ids may be mentioned in synthesised RHSs,
335 -- but they'll already be in the environment.
337 -- Typecheck the methods
338 let -- These insts are in scope; quite a few, eh?
339 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
341 tcMethods origin clas inst_tyvars'
342 dfun_theta' inst_tys' avail_insts
343 op_items binds `thenM` \ (meth_ids, meth_binds) ->
345 -- Figure out bindings for the superclass context
346 -- Don't include this_dict in the 'givens', else
347 -- sc_dicts get bound by just selecting from this_dict!!
348 addErrCtxt superClassCtxt
349 (tcSimplifySuperClasses inst_tyvars'
351 sc_dicts) `thenM` \ sc_binds ->
353 -- It's possible that the superclass stuff might unified one
354 -- of the inst_tyavars' with something in the envt
355 checkSigTyVars inst_tyvars' `thenM_`
357 -- Deal with 'SPECIALISE instance' pragmas
359 specs = case binds of
360 VanillaInst _ prags -> filter isSpecInstLSig prags
363 tcPrags dfun_id specs `thenM` \ prags ->
365 -- Create the result bindings
367 dict_constr = classDataCon clas
368 scs_and_meths = map instToId sc_dicts ++ meth_ids
369 this_dict_id = instToId this_dict
370 inline_prag | null dfun_arg_dicts = []
371 | otherwise = [InlinePrag (Inline AlwaysActive True)]
372 -- Always inline the dfun; this is an experimental decision
373 -- because it makes a big performance difference sometimes.
374 -- Often it means we can do the method selection, and then
375 -- inline the method as well. Marcin's idea; see comments below.
377 -- BUT: don't inline it if it's a constant dictionary;
378 -- we'll get all the benefit without inlining, and we get
379 -- a **lot** of code duplication if we inline it
381 -- See Note [Inline dfuns] below
384 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
385 -- We don't produce a binding for the dict_constr; instead we
386 -- rely on the simplifier to unfold this saturated application
387 -- We do this rather than generate an HsCon directly, because
388 -- it means that the special cases (e.g. dictionary with only one
389 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
390 -- than needing to be repeated here.
392 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
393 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
395 main_bind = noLoc $ AbsBinds
397 (map instToId dfun_arg_dicts)
398 [(inst_tyvars', dfun_id, this_dict_id,
399 inline_prag ++ prags)]
402 showLIE (text "instance") `thenM_`
403 returnM (unitBag main_bind)
406 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
407 avail_insts op_items (VanillaInst monobinds uprags)
408 = -- Check that all the method bindings come from this class
410 sel_names = [idName sel_id | (sel_id, _) <- op_items]
411 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
413 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
415 -- Make the method bindings
417 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
419 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
421 -- And type check them
422 -- It's really worth making meth_insts available to the tcMethodBind
423 -- Consider instance Monad (ST s) where
424 -- {-# INLINE (>>) #-}
425 -- (>>) = ...(>>=)...
426 -- If we don't include meth_insts, we end up with bindings like this:
427 -- rec { dict = MkD then bind ...
428 -- then = inline_me (... (GHC.Base.>>= dict) ...)
430 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
431 -- and (b) the inline_me prevents us inlining the >>= selector, which
432 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
433 -- is not inlined across modules. Rather ironic since this does not
434 -- happen without the INLINE pragma!
436 -- Solution: make meth_insts available, so that 'then' refers directly
437 -- to the local 'bind' rather than going via the dictionary.
439 -- BUT WATCH OUT! If the method type mentions the class variable, then
440 -- this optimisation is not right. Consider
444 -- instance C Int where
446 -- The occurrence of 'op' on the rhs gives rise to a constraint
448 -- The trouble is that the 'meth_inst' for op, which is 'available', also
449 -- looks like 'op at Int'. But they are not the same.
451 prag_fn = mkPragFun uprags
452 all_insts = avail_insts ++ catMaybes meth_insts
453 sig_fn n = Just [] -- No scoped type variables, but every method has
454 -- a type signature, in effect, so that we check
455 -- the method has the right type
456 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
457 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
460 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
462 returnM (meth_ids, unionManyBags meth_binds_s)
465 -- Derived newtype instances
466 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
467 avail_insts op_items (NewTypeDerived rep_tys)
468 = getInstLoc origin `thenM` \ inst_loc ->
469 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
472 (ptext SLIT("newtype derived instance"))
473 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
475 -- I don't think we have to do the checkSigTyVars thing
477 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
480 do_one inst_loc (sel_id, _)
481 = -- The binding is like "op @ NewTy = op @ RepTy"
482 -- Make the *binder*, like in mkMethodBind
483 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
485 -- Make the *occurrence on the rhs*
486 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
488 meth_id = instToId meth_inst
490 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
492 -- Instantiate rep_tys with the relevant type variables
493 -- This looks a bit odd, because inst_tyvars' are the skolemised version
494 -- of the type variables in the instance declaration; but rep_tys doesn't
495 -- have the skolemised version, so we substitute them in here
496 rep_tys' = substTys subst rep_tys
497 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
501 ------------------------------
502 [Inline dfuns] Inlining dfuns unconditionally
503 ------------------------------
505 The code above unconditionally inlines dict funs. Here's why.
506 Consider this program:
508 test :: Int -> Int -> Bool
509 test x y = (x,y) == (y,x) || test y x
510 -- Recursive to avoid making it inline.
512 This needs the (Eq (Int,Int)) instance. If we inline that dfun
513 the code we end up with is good:
516 \r -> case ==# [ww ww1] of wild {
517 PrelBase.False -> Test.$wtest ww1 ww;
519 case ==# [ww1 ww] of wild1 {
520 PrelBase.False -> Test.$wtest ww1 ww;
521 PrelBase.True -> PrelBase.True [];
524 Test.test = \r [w w1]
527 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
530 If we don't inline the dfun, the code is not nearly as good:
532 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
533 PrelBase.:DEq tpl1 tpl2 -> tpl2;
538 let { y = PrelBase.I#! [ww1]; } in
539 let { x = PrelBase.I#! [ww]; } in
540 let { sat_slx = PrelTup.(,)! [y x]; } in
541 let { sat_sly = PrelTup.(,)! [x y];
543 case == sat_sly sat_slx of wild {
544 PrelBase.False -> Test.$wtest ww1 ww;
545 PrelBase.True -> PrelBase.True [];
552 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
555 Why doesn't GHC inline $fEq? Because it looks big:
557 PrelTup.zdfEqZ1T{-rcX-}
558 = \ @ a{-reT-} :: * @ b{-reS-} :: *
559 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
560 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
562 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
563 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
565 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
566 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
568 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
569 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
570 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
572 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
574 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
576 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
577 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
581 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
582 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
583 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
584 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
586 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
588 and it's not as bad as it seems, because it's further dramatically
589 simplified: only zeze2 is extracted and its body is simplified.
592 %************************************************************************
594 \subsection{Error messages}
596 %************************************************************************
599 instDeclCtxt1 hs_inst_ty
600 = inst_decl_ctxt (case unLoc hs_inst_ty of
601 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
602 HsPredTy pred -> ppr pred
603 other -> ppr hs_inst_ty) -- Don't expect this
604 instDeclCtxt2 dfun_ty
605 = inst_decl_ctxt (ppr (mkClassPred cls tys))
607 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
609 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
611 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")