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 ( tcSpecSigs )
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, tcExtendLocalInstEnv )
22 import TcDeriv ( tcDeriving )
23 import TcEnv ( tcExtendGlobalValEnv, tcExtendTyVarEnv,
24 InstInfo(..), InstBindings(..),
25 newDFunName, tcExtendIdEnv
27 import TcHsType ( kcHsSigType, tcHsKindedType )
28 import TcUnify ( checkSigTyVars )
29 import TcSimplify ( tcSimplifyCheck, tcSimplifySuperClasses )
30 import Type ( zipOpenTvSubst, substTheta, substTys )
31 import DataCon ( classDataCon )
32 import Class ( classBigSig )
33 import Var ( Id, idName, idType )
34 import MkId ( mkDictFunId, rUNTIME_ERROR_ID )
35 import FunDeps ( checkInstFDs )
36 import Name ( Name, getSrcLoc )
37 import NameSet ( unitNameSet, emptyNameSet )
38 import UnicodeUtil ( stringToUtf8 )
39 import Maybe ( catMaybes )
40 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart )
41 import ListSetOps ( minusList )
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 [HsBindGroup 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 iDFunId 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 -- but only do this for non-imported instance decls.
183 -- Imported ones should have been checked already, and may indeed
184 -- contain something illegal in normal Haskell, notably
185 -- instance CCallable [Char]
186 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags))
187 = -- Prime error recovery, set source location
188 recoverM (returnM Nothing) $
190 addErrCtxt (instDeclCtxt1 poly_ty) $
192 -- Typecheck the instance type itself. We can't use
193 -- tcHsSigType, because it's not a valid user type.
194 kcHsSigType poly_ty `thenM` \ kinded_ty ->
195 tcHsKindedType kinded_ty `thenM` \ poly_ty' ->
197 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
199 checkValidTheta InstThetaCtxt theta `thenM_`
200 checkAmbiguity tyvars theta (tyVarsOfType tau) `thenM_`
201 checkValidInstHead tau `thenM` \ (clas,inst_tys) ->
202 checkTc (checkInstFDs theta clas inst_tys)
203 (instTypeErr (pprClassPred clas inst_tys) msg) `thenM_`
204 newDFunName clas inst_tys (srcSpanStart loc) `thenM` \ dfun_name ->
205 returnM (Just (InstInfo { iDFunId = mkDictFunId dfun_name tyvars theta clas inst_tys,
206 iBinds = VanillaInst binds uprags }))
208 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
212 %************************************************************************
214 \subsection{Type-checking instance declarations, pass 2}
216 %************************************************************************
219 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
220 -> TcM (TcLclEnv, LHsBinds Id)
221 -- (a) From each class declaration,
222 -- generate any default-method bindings
223 -- (b) From each instance decl
224 -- generate the dfun binding
226 tcInstDecls2 tycl_decls inst_decls
227 = do { -- (a) Default methods from class decls
228 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
229 filter (isClassDecl.unLoc) tycl_decls
230 ; tcExtendIdEnv (concat dm_ids_s) $ do
232 -- (b) instance declarations
233 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
236 ; tcl_env <- getLclEnv
237 ; returnM (tcl_env, unionManyBags dm_binds_s `unionBags`
238 unionManyBags inst_binds_s) }
241 ======= New documentation starts here (Sept 92) ==============
243 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
244 the dictionary function for this instance declaration. For example
246 instance Foo a => Foo [a] where
250 might generate something like
252 dfun.Foo.List dFoo_a = let op1 x = ...
258 HOWEVER, if the instance decl has no context, then it returns a
259 bigger @HsBinds@ with declarations for each method. For example
261 instance Foo [a] where
267 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
268 const.Foo.op1.List a x = ...
269 const.Foo.op2.List a y = ...
271 This group may be mutually recursive, because (for example) there may
272 be no method supplied for op2 in which case we'll get
274 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
276 that is, the default method applied to the dictionary at this type.
278 What we actually produce in either case is:
280 AbsBinds [a] [dfun_theta_dicts]
281 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
282 { d = (sd1,sd2, ..., op1, op2, ...)
287 The "maybe" says that we only ask AbsBinds to make global constant methods
288 if the dfun_theta is empty.
291 For an instance declaration, say,
293 instance (C1 a, C2 b) => C (T a b) where
296 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
297 function whose type is
299 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
301 Notice that we pass it the superclass dictionaries at the instance type; this
302 is the ``Mark Jones optimisation''. The stuff before the "=>" here
303 is the @dfun_theta@ below.
305 First comes the easy case of a non-local instance decl.
309 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
311 tcInstDecl2 (InstInfo { iDFunId = dfun_id, iBinds = binds })
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
319 rigid_info = InstSkol dfun_id
320 inst_ty = idType dfun_id
322 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
323 -- These inst_tyvars' scope over the 'where' part
324 -- Those tyvars are inside the dfun_id's type, which is a bit
325 -- bizarre, but OK so long as you realise it!
327 (clas, inst_tys') = tcSplitDFunHead inst_head'
328 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
330 -- Instantiate the super-class context with inst_tys
331 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
332 origin = SigOrigin rigid_info
334 -- Create dictionary Ids from the specified instance contexts.
335 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
336 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
337 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
338 -- Default-method Ids may be mentioned in synthesised RHSs,
339 -- but they'll already be in the environment.
341 -- Typecheck the methods
342 let -- These insts are in scope; quite a few, eh?
343 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
345 tcMethods origin clas inst_tyvars'
346 dfun_theta' inst_tys' avail_insts
347 op_items binds `thenM` \ (meth_ids, meth_binds) ->
349 -- Figure out bindings for the superclass context
350 -- Don't include this_dict in the 'givens', else
351 -- sc_dicts get bound by just selecting from this_dict!!
352 addErrCtxt superClassCtxt
353 (tcSimplifySuperClasses inst_tyvars'
355 sc_dicts) `thenM` \ sc_binds ->
357 -- It's possible that the superclass stuff might unified one
358 -- of the inst_tyavars' with something in the envt
359 checkSigTyVars inst_tyvars' `thenM_`
361 -- Deal with 'SPECIALISE instance' pragmas by making them
362 -- look like SPECIALISE pragmas for the dfun
364 uprags = case binds of
365 VanillaInst _ uprags -> uprags
367 spec_prags = [ L loc (SpecSig (L loc (idName dfun_id)) ty)
368 | L loc (SpecInstSig ty) <- uprags ]
370 tcExtendGlobalValEnv [dfun_id] (
371 tcExtendTyVarEnv inst_tyvars' $
372 tcSpecSigs spec_prags
373 ) `thenM` \ prag_binds ->
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 inlines | null dfun_arg_dicts = emptyNameSet
381 | otherwise = unitNameSet (idName dfun_id)
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
395 = -- Blatant special case for CCallable, CReturnable
396 -- If the dictionary is empty then we should never
397 -- select anything from it, so we make its RHS just
398 -- emit an error message. This in turn means that we don't
399 -- mention the constructor, which doesn't exist for CCallable, CReturnable
400 -- Hardly beautiful, but only three extra lines.
401 nlHsApp (noLoc $ TyApp (nlHsVar rUNTIME_ERROR_ID)
402 [idType this_dict_id])
403 (nlHsLit (HsStringPrim (mkFastString (stringToUtf8 msg))))
405 | otherwise -- The common case
406 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
407 -- We don't produce a binding for the dict_constr; instead we
408 -- rely on the simplifier to unfold this saturated application
409 -- We do this rather than generate an HsCon directly, because
410 -- it means that the special cases (e.g. dictionary with only one
411 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
412 -- than needing to be repeated here.
415 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
417 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
418 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
420 main_bind = noLoc $ AbsBinds
422 (map instToId dfun_arg_dicts)
423 [(inst_tyvars', dfun_id, this_dict_id)]
426 showLIE (text "instance") `thenM_`
427 returnM (unitBag main_bind `unionBags`
431 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
432 avail_insts op_items (VanillaInst monobinds uprags)
433 = -- Check that all the method bindings come from this class
435 sel_names = [idName sel_id | (sel_id, _) <- op_items]
436 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
438 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
440 -- Make the method bindings
442 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
444 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
446 -- And type check them
447 -- It's really worth making meth_insts available to the tcMethodBind
448 -- Consider instance Monad (ST s) where
449 -- {-# INLINE (>>) #-}
450 -- (>>) = ...(>>=)...
451 -- If we don't include meth_insts, we end up with bindings like this:
452 -- rec { dict = MkD then bind ...
453 -- then = inline_me (... (GHC.Base.>>= dict) ...)
455 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
456 -- and (b) the inline_me prevents us inlining the >>= selector, which
457 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
458 -- is not inlined across modules. Rather ironic since this does not
459 -- happen without the INLINE pragma!
461 -- Solution: make meth_insts available, so that 'then' refers directly
462 -- to the local 'bind' rather than going via the dictionary.
464 -- BUT WATCH OUT! If the method type mentions the class variable, then
465 -- this optimisation is not right. Consider
469 -- instance C Int where
471 -- The occurrence of 'op' on the rhs gives rise to a constraint
473 -- The trouble is that the 'meth_inst' for op, which is 'available', also
474 -- looks like 'op at Int'. But they are not the same.
476 all_insts = avail_insts ++ catMaybes meth_insts
477 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts uprags
478 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
481 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
483 returnM (meth_ids, unionManyBags meth_binds_s)
486 -- Derived newtype instances
487 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
488 avail_insts op_items (NewTypeDerived rep_tys)
489 = getInstLoc origin `thenM` \ inst_loc ->
490 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
493 (ptext SLIT("newtype derived instance"))
494 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
496 -- I don't think we have to do the checkSigTyVars thing
498 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
501 do_one inst_loc (sel_id, _)
502 = -- The binding is like "op @ NewTy = op @ RepTy"
503 -- Make the *binder*, like in mkMethodBind
504 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
506 -- Make the *occurrence on the rhs*
507 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
509 meth_id = instToId meth_inst
511 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
513 -- Instantiate rep_tys with the relevant type variables
514 -- This looks a bit odd, because inst_tyvars' are the skolemised version
515 -- of the type variables in the instance declaration; but rep_tys doesn't
516 -- have the skolemised version, so we substitute them in here
517 rep_tys' = substTys subst rep_tys
518 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
522 ------------------------------
523 [Inline dfuns] Inlining dfuns unconditionally
524 ------------------------------
526 The code above unconditionally inlines dict funs. Here's why.
527 Consider this program:
529 test :: Int -> Int -> Bool
530 test x y = (x,y) == (y,x) || test y x
531 -- Recursive to avoid making it inline.
533 This needs the (Eq (Int,Int)) instance. If we inline that dfun
534 the code we end up with is good:
537 \r -> case ==# [ww ww1] of wild {
538 PrelBase.False -> Test.$wtest ww1 ww;
540 case ==# [ww1 ww] of wild1 {
541 PrelBase.False -> Test.$wtest ww1 ww;
542 PrelBase.True -> PrelBase.True [];
545 Test.test = \r [w w1]
548 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
551 If we don't inline the dfun, the code is not nearly as good:
553 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
554 PrelBase.:DEq tpl1 tpl2 -> tpl2;
559 let { y = PrelBase.I#! [ww1]; } in
560 let { x = PrelBase.I#! [ww]; } in
561 let { sat_slx = PrelTup.(,)! [y x]; } in
562 let { sat_sly = PrelTup.(,)! [x y];
564 case == sat_sly sat_slx of wild {
565 PrelBase.False -> Test.$wtest ww1 ww;
566 PrelBase.True -> PrelBase.True [];
573 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
576 Why doesn't GHC inline $fEq? Because it looks big:
578 PrelTup.zdfEqZ1T{-rcX-}
579 = \ @ a{-reT-} :: * @ b{-reS-} :: *
580 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
581 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
583 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
584 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
586 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
587 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
589 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
590 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
591 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
593 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
595 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
597 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
598 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
602 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
603 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
604 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
605 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
607 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
609 and it's not as bad as it seems, because it's further dramatically
610 simplified: only zeze2 is extracted and its body is simplified.
613 %************************************************************************
615 \subsection{Error messages}
617 %************************************************************************
620 instDeclCtxt1 hs_inst_ty
621 = inst_decl_ctxt (case unLoc hs_inst_ty of
622 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
623 HsPredTy pred -> ppr pred
624 other -> ppr hs_inst_ty) -- Don't expect this
625 instDeclCtxt2 dfun_ty
626 = inst_decl_ctxt (ppr (mkClassPred cls tys))
628 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
630 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
632 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")