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 Maybe ( catMaybes )
39 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart )
40 import ListSetOps ( minusList )
43 import BasicTypes ( Activation( AlwaysActive ), InlineSpec(..) )
47 Typechecking instance declarations is done in two passes. The first
48 pass, made by @tcInstDecls1@, collects information to be used in the
51 This pre-processed info includes the as-yet-unprocessed bindings
52 inside the instance declaration. These are type-checked in the second
53 pass, when the class-instance envs and GVE contain all the info from
54 all the instance and value decls. Indeed that's the reason we need
55 two passes over the instance decls.
58 Here is the overall algorithm.
59 Assume that we have an instance declaration
61 instance c => k (t tvs) where b
65 $LIE_c$ is the LIE for the context of class $c$
67 $betas_bar$ is the free variables in the class method type, excluding the
70 $LIE_cop$ is the LIE constraining a particular class method
72 $tau_cop$ is the tau type of a class method
74 $LIE_i$ is the LIE for the context of instance $i$
76 $X$ is the instance constructor tycon
78 $gammas_bar$ is the set of type variables of the instance
80 $LIE_iop$ is the LIE for a particular class method instance
82 $tau_iop$ is the tau type for this instance of a class method
84 $alpha$ is the class variable
86 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
88 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
91 ToDo: Update the list above with names actually in the code.
95 First, make the LIEs for the class and instance contexts, which means
96 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
97 and make LIElistI and LIEI.
99 Then process each method in turn.
101 order the instance methods according to the ordering of the class methods
103 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
105 Create final dictionary function from bindings generated already
107 df = lambda inst_tyvars
114 in <op1,op2,...,opn,sd1,...,sdm>
116 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
117 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
121 %************************************************************************
123 \subsection{Extracting instance decls}
125 %************************************************************************
127 Gather up the instance declarations from their various sources
130 tcInstDecls1 -- Deal with both source-code and imported instance decls
131 :: [LTyClDecl Name] -- For deriving stuff
132 -> [LInstDecl Name] -- Source code instance decls
133 -> TcM (TcGblEnv, -- The full inst env
134 [InstInfo], -- Source-code instance decls to process;
135 -- contains all dfuns for this module
136 HsValBinds Name) -- Supporting bindings for derived instances
138 tcInstDecls1 tycl_decls inst_decls
140 -- Stop if addInstInfos etc discovers any errors
141 -- (they recover, so that we get more than one error each round)
143 -- (1) Do the ordinary instance declarations
144 mappM tcLocalInstDecl1 inst_decls `thenM` \ local_inst_infos ->
147 local_inst_info = catMaybes local_inst_infos
148 clas_decls = filter (isClassDecl.unLoc) tycl_decls
150 -- (2) Instances from generic class declarations
151 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
153 -- Next, construct the instance environment so far, consisting of
154 -- a) local instance decls
155 -- b) generic instances
156 addInsts local_inst_info $
157 addInsts generic_inst_info $
159 -- (3) Compute instances from "deriving" clauses;
160 -- This stuff computes a context for the derived instance decl, so it
161 -- needs to know about all the instances possible; hence inst_env4
162 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
163 addInsts deriv_inst_info $
165 getGblEnv `thenM` \ gbl_env ->
167 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
170 addInsts :: [InstInfo] -> TcM a -> TcM a
171 addInsts infos thing_inside
172 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
176 tcLocalInstDecl1 :: LInstDecl Name
177 -> TcM (Maybe InstInfo) -- Nothing if there was an error
178 -- A source-file instance declaration
179 -- Type-check all the stuff before the "where"
181 -- We check for respectable instance type, and context
182 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags))
183 = -- Prime error recovery, set source location
184 recoverM (returnM Nothing) $
186 addErrCtxt (instDeclCtxt1 poly_ty) $
188 -- Typecheck the instance type itself. We can't use
189 -- tcHsSigType, because it's not a valid user type.
190 kcHsSigType poly_ty `thenM` \ kinded_ty ->
191 tcHsKindedType kinded_ty `thenM` \ poly_ty' ->
193 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
195 checkValidTheta InstThetaCtxt theta `thenM_`
196 checkAmbiguity tyvars theta (tyVarsOfType tau) `thenM_`
197 checkValidInstHead tau `thenM` \ (clas,inst_tys) ->
198 checkTc (checkInstFDs theta clas inst_tys)
199 (instTypeErr (pprClassPred clas inst_tys) msg) `thenM_`
200 newDFunName clas inst_tys (srcSpanStart loc) `thenM` \ dfun_name ->
201 getOverlapFlag `thenM` \ overlap_flag ->
202 let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
203 ispec = mkLocalInstance dfun overlap_flag
206 tcIsHsBoot `thenM` \ is_boot ->
207 checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
208 badBootDeclErr `thenM_`
210 returnM (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags }))
212 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
216 %************************************************************************
218 \subsection{Type-checking instance declarations, pass 2}
220 %************************************************************************
223 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
224 -> TcM (LHsBinds Id, TcLclEnv)
225 -- (a) From each class declaration,
226 -- generate any default-method bindings
227 -- (b) From each instance decl
228 -- generate the dfun binding
230 tcInstDecls2 tycl_decls inst_decls
231 = do { -- (a) Default methods from class decls
232 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
233 filter (isClassDecl.unLoc) tycl_decls
234 ; tcExtendIdEnv (concat dm_ids_s) $ do
236 -- (b) instance declarations
237 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
240 ; let binds = unionManyBags dm_binds_s `unionBags`
241 unionManyBags inst_binds_s
242 ; tcl_env <- getLclEnv -- Default method Ids in here
243 ; returnM (binds, tcl_env) }
246 ======= New documentation starts here (Sept 92) ==============
248 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
249 the dictionary function for this instance declaration. For example
251 instance Foo a => Foo [a] where
255 might generate something like
257 dfun.Foo.List dFoo_a = let op1 x = ...
263 HOWEVER, if the instance decl has no context, then it returns a
264 bigger @HsBinds@ with declarations for each method. For example
266 instance Foo [a] where
272 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
273 const.Foo.op1.List a x = ...
274 const.Foo.op2.List a y = ...
276 This group may be mutually recursive, because (for example) there may
277 be no method supplied for op2 in which case we'll get
279 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
281 that is, the default method applied to the dictionary at this type.
283 What we actually produce in either case is:
285 AbsBinds [a] [dfun_theta_dicts]
286 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
287 { d = (sd1,sd2, ..., op1, op2, ...)
292 The "maybe" says that we only ask AbsBinds to make global constant methods
293 if the dfun_theta is empty.
296 For an instance declaration, say,
298 instance (C1 a, C2 b) => C (T a b) where
301 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
302 function whose type is
304 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
306 Notice that we pass it the superclass dictionaries at the instance type; this
307 is the ``Mark Jones optimisation''. The stuff before the "=>" here
308 is the @dfun_theta@ below.
310 First comes the easy case of a non-local instance decl.
314 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
316 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = binds })
318 dfun_id = instanceDFunId ispec
319 rigid_info = InstSkol dfun_id
320 inst_ty = idType dfun_id
322 -- Prime error recovery
323 recoverM (returnM emptyLHsBinds) $
324 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
325 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
327 -- Instantiate the instance decl with skolem constants
328 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
329 -- These inst_tyvars' scope over the 'where' part
330 -- Those tyvars are inside the dfun_id's type, which is a bit
331 -- bizarre, but OK so long as you realise it!
333 (clas, inst_tys') = tcSplitDFunHead inst_head'
334 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
336 -- Instantiate the super-class context with inst_tys
337 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
338 origin = SigOrigin rigid_info
340 -- Create dictionary Ids from the specified instance contexts.
341 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
342 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
343 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
344 -- Default-method Ids may be mentioned in synthesised RHSs,
345 -- but they'll already be in the environment.
347 -- Typecheck the methods
348 let -- These insts are in scope; quite a few, eh?
349 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
351 tcMethods origin clas inst_tyvars'
352 dfun_theta' inst_tys' avail_insts
353 op_items binds `thenM` \ (meth_ids, meth_binds) ->
355 -- Figure out bindings for the superclass context
356 -- Don't include this_dict in the 'givens', else
357 -- sc_dicts get bound by just selecting from this_dict!!
358 addErrCtxt superClassCtxt
359 (tcSimplifySuperClasses inst_tyvars'
361 sc_dicts) `thenM` \ sc_binds ->
363 -- It's possible that the superclass stuff might unified one
364 -- of the inst_tyavars' with something in the envt
365 checkSigTyVars inst_tyvars' `thenM_`
367 -- Deal with 'SPECIALISE instance' pragmas
369 specs = case binds of
370 VanillaInst _ prags -> filter isSpecInstLSig prags
373 tcPrags dfun_id specs `thenM` \ prags ->
375 -- Create the result bindings
377 dict_constr = classDataCon clas
378 scs_and_meths = map instToId sc_dicts ++ meth_ids
379 this_dict_id = instToId this_dict
380 inline_prag | null dfun_arg_dicts = []
381 | otherwise = [InlinePrag (Inline AlwaysActive True)]
382 -- Always inline the dfun; this is an experimental decision
383 -- because it makes a big performance difference sometimes.
384 -- Often it means we can do the method selection, and then
385 -- inline the method as well. Marcin's idea; see comments below.
387 -- BUT: don't inline it if it's a constant dictionary;
388 -- we'll get all the benefit without inlining, and we get
389 -- a **lot** of code duplication if we inline it
391 -- See Note [Inline dfuns] below
394 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
395 -- We don't produce a binding for the dict_constr; instead we
396 -- rely on the simplifier to unfold this saturated application
397 -- We do this rather than generate an HsCon directly, because
398 -- it means that the special cases (e.g. dictionary with only one
399 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
400 -- than needing to be repeated here.
403 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
405 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
406 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
408 main_bind = noLoc $ AbsBinds
410 (map instToId dfun_arg_dicts)
411 [(inst_tyvars', dfun_id, this_dict_id,
412 inline_prag ++ prags)]
415 showLIE (text "instance") `thenM_`
416 returnM (unitBag main_bind)
419 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
420 avail_insts op_items (VanillaInst monobinds uprags)
421 = -- Check that all the method bindings come from this class
423 sel_names = [idName sel_id | (sel_id, _) <- op_items]
424 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
426 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
428 -- Make the method bindings
430 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
432 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
434 -- And type check them
435 -- It's really worth making meth_insts available to the tcMethodBind
436 -- Consider instance Monad (ST s) where
437 -- {-# INLINE (>>) #-}
438 -- (>>) = ...(>>=)...
439 -- If we don't include meth_insts, we end up with bindings like this:
440 -- rec { dict = MkD then bind ...
441 -- then = inline_me (... (GHC.Base.>>= dict) ...)
443 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
444 -- and (b) the inline_me prevents us inlining the >>= selector, which
445 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
446 -- is not inlined across modules. Rather ironic since this does not
447 -- happen without the INLINE pragma!
449 -- Solution: make meth_insts available, so that 'then' refers directly
450 -- to the local 'bind' rather than going via the dictionary.
452 -- BUT WATCH OUT! If the method type mentions the class variable, then
453 -- this optimisation is not right. Consider
457 -- instance C Int where
459 -- The occurrence of 'op' on the rhs gives rise to a constraint
461 -- The trouble is that the 'meth_inst' for op, which is 'available', also
462 -- looks like 'op at Int'. But they are not the same.
464 prag_fn = mkPragFun uprags
465 all_insts = avail_insts ++ catMaybes meth_insts
466 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts prag_fn
467 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
470 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
472 returnM (meth_ids, unionManyBags meth_binds_s)
475 -- Derived newtype instances
476 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
477 avail_insts op_items (NewTypeDerived rep_tys)
478 = getInstLoc origin `thenM` \ inst_loc ->
479 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
482 (ptext SLIT("newtype derived instance"))
483 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
485 -- I don't think we have to do the checkSigTyVars thing
487 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
490 do_one inst_loc (sel_id, _)
491 = -- The binding is like "op @ NewTy = op @ RepTy"
492 -- Make the *binder*, like in mkMethodBind
493 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
495 -- Make the *occurrence on the rhs*
496 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
498 meth_id = instToId meth_inst
500 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
502 -- Instantiate rep_tys with the relevant type variables
503 -- This looks a bit odd, because inst_tyvars' are the skolemised version
504 -- of the type variables in the instance declaration; but rep_tys doesn't
505 -- have the skolemised version, so we substitute them in here
506 rep_tys' = substTys subst rep_tys
507 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
511 ------------------------------
512 [Inline dfuns] Inlining dfuns unconditionally
513 ------------------------------
515 The code above unconditionally inlines dict funs. Here's why.
516 Consider this program:
518 test :: Int -> Int -> Bool
519 test x y = (x,y) == (y,x) || test y x
520 -- Recursive to avoid making it inline.
522 This needs the (Eq (Int,Int)) instance. If we inline that dfun
523 the code we end up with is good:
526 \r -> case ==# [ww ww1] of wild {
527 PrelBase.False -> Test.$wtest ww1 ww;
529 case ==# [ww1 ww] of wild1 {
530 PrelBase.False -> Test.$wtest ww1 ww;
531 PrelBase.True -> PrelBase.True [];
534 Test.test = \r [w w1]
537 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
540 If we don't inline the dfun, the code is not nearly as good:
542 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
543 PrelBase.:DEq tpl1 tpl2 -> tpl2;
548 let { y = PrelBase.I#! [ww1]; } in
549 let { x = PrelBase.I#! [ww]; } in
550 let { sat_slx = PrelTup.(,)! [y x]; } in
551 let { sat_sly = PrelTup.(,)! [x y];
553 case == sat_sly sat_slx of wild {
554 PrelBase.False -> Test.$wtest ww1 ww;
555 PrelBase.True -> PrelBase.True [];
562 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
565 Why doesn't GHC inline $fEq? Because it looks big:
567 PrelTup.zdfEqZ1T{-rcX-}
568 = \ @ a{-reT-} :: * @ b{-reS-} :: *
569 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
570 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
572 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
573 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
575 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
576 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
578 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
579 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
580 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
582 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
584 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
586 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
587 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
591 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
592 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
593 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
594 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
596 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
598 and it's not as bad as it seems, because it's further dramatically
599 simplified: only zeze2 is extracted and its body is simplified.
602 %************************************************************************
604 \subsection{Error messages}
606 %************************************************************************
609 instDeclCtxt1 hs_inst_ty
610 = inst_decl_ctxt (case unLoc hs_inst_ty of
611 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
612 HsPredTy pred -> ppr pred
613 other -> ppr hs_inst_ty) -- Don't expect this
614 instDeclCtxt2 dfun_ty
615 = inst_decl_ctxt (ppr (mkClassPred cls tys))
617 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
619 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
621 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")