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, 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 ( tcExtendGlobalValEnv, tcExtendTyVarEnv,
26 InstInfo(..), InstBindings(..),
27 newDFunName, tcExtendIdEnv
29 import TcHsType ( kcHsSigType, tcHsKindedType )
30 import TcUnify ( checkSigTyVars )
31 import TcSimplify ( tcSimplifyCheck, tcSimplifySuperClasses )
32 import Type ( zipOpenTvSubst, substTheta, substTys )
33 import DataCon ( classDataCon )
34 import Class ( classBigSig )
35 import Var ( Id, idName, idType )
36 import MkId ( mkDictFunId, rUNTIME_ERROR_ID )
37 import FunDeps ( checkInstFDs )
38 import Name ( Name, getSrcLoc )
39 import NameSet ( unitNameSet, emptyNameSet )
40 import UnicodeUtil ( stringToUtf8 )
41 import Maybe ( catMaybes )
42 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart )
43 import ListSetOps ( minusList )
49 Typechecking instance declarations is done in two passes. The first
50 pass, made by @tcInstDecls1@, collects information to be used in the
53 This pre-processed info includes the as-yet-unprocessed bindings
54 inside the instance declaration. These are type-checked in the second
55 pass, when the class-instance envs and GVE contain all the info from
56 all the instance and value decls. Indeed that's the reason we need
57 two passes over the instance decls.
60 Here is the overall algorithm.
61 Assume that we have an instance declaration
63 instance c => k (t tvs) where b
67 $LIE_c$ is the LIE for the context of class $c$
69 $betas_bar$ is the free variables in the class method type, excluding the
72 $LIE_cop$ is the LIE constraining a particular class method
74 $tau_cop$ is the tau type of a class method
76 $LIE_i$ is the LIE for the context of instance $i$
78 $X$ is the instance constructor tycon
80 $gammas_bar$ is the set of type variables of the instance
82 $LIE_iop$ is the LIE for a particular class method instance
84 $tau_iop$ is the tau type for this instance of a class method
86 $alpha$ is the class variable
88 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
90 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
93 ToDo: Update the list above with names actually in the code.
97 First, make the LIEs for the class and instance contexts, which means
98 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
99 and make LIElistI and LIEI.
101 Then process each method in turn.
103 order the instance methods according to the ordering of the class methods
105 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
107 Create final dictionary function from bindings generated already
109 df = lambda inst_tyvars
116 in <op1,op2,...,opn,sd1,...,sdm>
118 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
119 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
123 %************************************************************************
125 \subsection{Extracting instance decls}
127 %************************************************************************
129 Gather up the instance declarations from their various sources
132 tcInstDecls1 -- Deal with both source-code and imported instance decls
133 :: [LTyClDecl Name] -- For deriving stuff
134 -> [LInstDecl Name] -- Source code instance decls
135 -> TcM (TcGblEnv, -- The full inst env
136 [InstInfo], -- Source-code instance decls to process;
137 -- contains all dfuns for this module
138 [HsBindGroup Name]) -- Supporting bindings for derived instances
140 tcInstDecls1 tycl_decls inst_decls
142 -- Stop if addInstInfos etc discovers any errors
143 -- (they recover, so that we get more than one error each round)
145 -- (1) Do the ordinary instance declarations
146 mappM tcLocalInstDecl1 inst_decls `thenM` \ local_inst_infos ->
149 local_inst_info = catMaybes local_inst_infos
150 clas_decls = filter (isClassDecl.unLoc) tycl_decls
152 -- (2) Instances from generic class declarations
153 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
155 -- Next, construct the instance environment so far, consisting of
156 -- a) local instance decls
157 -- b) generic instances
158 addInsts local_inst_info $
159 addInsts generic_inst_info $
161 -- (3) Compute instances from "deriving" clauses;
162 -- This stuff computes a context for the derived instance decl, so it
163 -- needs to know about all the instances possible; hence inst_env4
164 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
165 addInsts deriv_inst_info $
167 getGblEnv `thenM` \ gbl_env ->
169 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
172 addInsts :: [InstInfo] -> TcM a -> TcM a
173 addInsts infos thing_inside
174 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
178 tcLocalInstDecl1 :: LInstDecl Name
179 -> TcM (Maybe InstInfo) -- Nothing if there was an error
180 -- A source-file instance declaration
181 -- Type-check all the stuff before the "where"
183 -- We check for respectable instance type, and context
184 -- but only do this for non-imported instance decls.
185 -- Imported ones should have been checked already, and may indeed
186 -- contain something illegal in normal Haskell, notably
187 -- instance CCallable [Char]
188 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags))
189 = -- Prime error recovery, set source location
190 recoverM (returnM Nothing) $
192 addErrCtxt (instDeclCtxt1 poly_ty) $
194 -- Typecheck the instance type itself. We can't use
195 -- tcHsSigType, because it's not a valid user type.
196 kcHsSigType poly_ty `thenM` \ kinded_ty ->
197 tcHsKindedType kinded_ty `thenM` \ poly_ty' ->
199 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
201 checkValidTheta InstThetaCtxt theta `thenM_`
202 checkAmbiguity tyvars theta (tyVarsOfType tau) `thenM_`
203 checkValidInstHead tau `thenM` \ (clas,inst_tys) ->
204 checkTc (checkInstFDs theta clas inst_tys)
205 (instTypeErr (pprClassPred clas inst_tys) msg) `thenM_`
206 newDFunName clas inst_tys (srcSpanStart loc) `thenM` \ dfun_name ->
207 getOverlapFlag `thenM` \ overlap_flag ->
208 let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
209 ispec = mkLocalInstance dfun overlap_flag
212 tcIsHsBoot `thenM` \ is_boot ->
213 checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
214 badBootDeclErr `thenM_`
216 returnM (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags }))
218 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
222 %************************************************************************
224 \subsection{Type-checking instance declarations, pass 2}
226 %************************************************************************
229 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
230 -> TcM (TcLclEnv, LHsBinds Id)
231 -- (a) From each class declaration,
232 -- generate any default-method bindings
233 -- (b) From each instance decl
234 -- generate the dfun binding
236 tcInstDecls2 tycl_decls inst_decls
237 = do { -- (a) Default methods from class decls
238 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
239 filter (isClassDecl.unLoc) tycl_decls
240 ; tcExtendIdEnv (concat dm_ids_s) $ do
242 -- (b) instance declarations
243 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
246 ; tcl_env <- getLclEnv
247 ; returnM (tcl_env, unionManyBags dm_binds_s `unionBags`
248 unionManyBags inst_binds_s) }
251 ======= New documentation starts here (Sept 92) ==============
253 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
254 the dictionary function for this instance declaration. For example
256 instance Foo a => Foo [a] where
260 might generate something like
262 dfun.Foo.List dFoo_a = let op1 x = ...
268 HOWEVER, if the instance decl has no context, then it returns a
269 bigger @HsBinds@ with declarations for each method. For example
271 instance Foo [a] where
277 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
278 const.Foo.op1.List a x = ...
279 const.Foo.op2.List a y = ...
281 This group may be mutually recursive, because (for example) there may
282 be no method supplied for op2 in which case we'll get
284 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
286 that is, the default method applied to the dictionary at this type.
288 What we actually produce in either case is:
290 AbsBinds [a] [dfun_theta_dicts]
291 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
292 { d = (sd1,sd2, ..., op1, op2, ...)
297 The "maybe" says that we only ask AbsBinds to make global constant methods
298 if the dfun_theta is empty.
301 For an instance declaration, say,
303 instance (C1 a, C2 b) => C (T a b) where
306 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
307 function whose type is
309 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
311 Notice that we pass it the superclass dictionaries at the instance type; this
312 is the ``Mark Jones optimisation''. The stuff before the "=>" here
313 is the @dfun_theta@ below.
315 First comes the easy case of a non-local instance decl.
319 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
321 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = binds })
323 dfun_id = instanceDFunId ispec
324 rigid_info = InstSkol dfun_id
325 inst_ty = idType dfun_id
327 -- Prime error recovery
328 recoverM (returnM emptyLHsBinds) $
329 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
330 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
332 -- Instantiate the instance decl with skolem constants
333 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
334 -- These inst_tyvars' scope over the 'where' part
335 -- Those tyvars are inside the dfun_id's type, which is a bit
336 -- bizarre, but OK so long as you realise it!
338 (clas, inst_tys') = tcSplitDFunHead inst_head'
339 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
341 -- Instantiate the super-class context with inst_tys
342 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
343 origin = SigOrigin rigid_info
345 -- Create dictionary Ids from the specified instance contexts.
346 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
347 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
348 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
349 -- Default-method Ids may be mentioned in synthesised RHSs,
350 -- but they'll already be in the environment.
352 -- Typecheck the methods
353 let -- These insts are in scope; quite a few, eh?
354 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
356 tcMethods origin clas inst_tyvars'
357 dfun_theta' inst_tys' avail_insts
358 op_items binds `thenM` \ (meth_ids, meth_binds) ->
360 -- Figure out bindings for the superclass context
361 -- Don't include this_dict in the 'givens', else
362 -- sc_dicts get bound by just selecting from this_dict!!
363 addErrCtxt superClassCtxt
364 (tcSimplifySuperClasses inst_tyvars'
366 sc_dicts) `thenM` \ sc_binds ->
368 -- It's possible that the superclass stuff might unified one
369 -- of the inst_tyavars' with something in the envt
370 checkSigTyVars inst_tyvars' `thenM_`
372 -- Deal with 'SPECIALISE instance' pragmas by making them
373 -- look like SPECIALISE pragmas for the dfun
375 uprags = case binds of
376 VanillaInst _ uprags -> uprags
378 spec_prags = [ L loc (SpecSig (L loc (idName dfun_id)) ty)
379 | L loc (SpecInstSig ty) <- uprags ]
381 tcExtendGlobalValEnv [dfun_id] (
382 tcExtendTyVarEnv inst_tyvars' $
383 tcSpecSigs spec_prags
384 ) `thenM` \ prag_binds ->
386 -- Create the result bindings
388 dict_constr = classDataCon clas
389 scs_and_meths = map instToId sc_dicts ++ meth_ids
390 this_dict_id = instToId this_dict
391 inlines | null dfun_arg_dicts = emptyNameSet
392 | otherwise = unitNameSet (idName dfun_id)
393 -- Always inline the dfun; this is an experimental decision
394 -- because it makes a big performance difference sometimes.
395 -- Often it means we can do the method selection, and then
396 -- inline the method as well. Marcin's idea; see comments below.
398 -- BUT: don't inline it if it's a constant dictionary;
399 -- we'll get all the benefit without inlining, and we get
400 -- a **lot** of code duplication if we inline it
402 -- See Note [Inline dfuns] below
406 = -- Blatant special case for CCallable, CReturnable
407 -- If the dictionary is empty then we should never
408 -- select anything from it, so we make its RHS just
409 -- emit an error message. This in turn means that we don't
410 -- mention the constructor, which doesn't exist for CCallable, CReturnable
411 -- Hardly beautiful, but only three extra lines.
412 nlHsApp (noLoc $ TyApp (nlHsVar rUNTIME_ERROR_ID)
413 [idType this_dict_id])
414 (nlHsLit (HsStringPrim (mkFastString (stringToUtf8 msg))))
416 | otherwise -- The common case
417 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
418 -- We don't produce a binding for the dict_constr; instead we
419 -- rely on the simplifier to unfold this saturated application
420 -- We do this rather than generate an HsCon directly, because
421 -- it means that the special cases (e.g. dictionary with only one
422 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
423 -- than needing to be repeated here.
426 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
428 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
429 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
431 main_bind = noLoc $ AbsBinds
433 (map instToId dfun_arg_dicts)
434 [(inst_tyvars', dfun_id, this_dict_id)]
437 showLIE (text "instance") `thenM_`
438 returnM (unitBag main_bind `unionBags`
442 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
443 avail_insts op_items (VanillaInst monobinds uprags)
444 = -- Check that all the method bindings come from this class
446 sel_names = [idName sel_id | (sel_id, _) <- op_items]
447 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
449 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
451 -- Make the method bindings
453 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
455 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
457 -- And type check them
458 -- It's really worth making meth_insts available to the tcMethodBind
459 -- Consider instance Monad (ST s) where
460 -- {-# INLINE (>>) #-}
461 -- (>>) = ...(>>=)...
462 -- If we don't include meth_insts, we end up with bindings like this:
463 -- rec { dict = MkD then bind ...
464 -- then = inline_me (... (GHC.Base.>>= dict) ...)
466 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
467 -- and (b) the inline_me prevents us inlining the >>= selector, which
468 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
469 -- is not inlined across modules. Rather ironic since this does not
470 -- happen without the INLINE pragma!
472 -- Solution: make meth_insts available, so that 'then' refers directly
473 -- to the local 'bind' rather than going via the dictionary.
475 -- BUT WATCH OUT! If the method type mentions the class variable, then
476 -- this optimisation is not right. Consider
480 -- instance C Int where
482 -- The occurrence of 'op' on the rhs gives rise to a constraint
484 -- The trouble is that the 'meth_inst' for op, which is 'available', also
485 -- looks like 'op at Int'. But they are not the same.
487 all_insts = avail_insts ++ catMaybes meth_insts
488 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts uprags
489 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
492 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
494 returnM (meth_ids, unionManyBags meth_binds_s)
497 -- Derived newtype instances
498 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
499 avail_insts op_items (NewTypeDerived rep_tys)
500 = getInstLoc origin `thenM` \ inst_loc ->
501 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
504 (ptext SLIT("newtype derived instance"))
505 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
507 -- I don't think we have to do the checkSigTyVars thing
509 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
512 do_one inst_loc (sel_id, _)
513 = -- The binding is like "op @ NewTy = op @ RepTy"
514 -- Make the *binder*, like in mkMethodBind
515 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
517 -- Make the *occurrence on the rhs*
518 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
520 meth_id = instToId meth_inst
522 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
524 -- Instantiate rep_tys with the relevant type variables
525 -- This looks a bit odd, because inst_tyvars' are the skolemised version
526 -- of the type variables in the instance declaration; but rep_tys doesn't
527 -- have the skolemised version, so we substitute them in here
528 rep_tys' = substTys subst rep_tys
529 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
533 ------------------------------
534 [Inline dfuns] Inlining dfuns unconditionally
535 ------------------------------
537 The code above unconditionally inlines dict funs. Here's why.
538 Consider this program:
540 test :: Int -> Int -> Bool
541 test x y = (x,y) == (y,x) || test y x
542 -- Recursive to avoid making it inline.
544 This needs the (Eq (Int,Int)) instance. If we inline that dfun
545 the code we end up with is good:
548 \r -> case ==# [ww ww1] of wild {
549 PrelBase.False -> Test.$wtest ww1 ww;
551 case ==# [ww1 ww] of wild1 {
552 PrelBase.False -> Test.$wtest ww1 ww;
553 PrelBase.True -> PrelBase.True [];
556 Test.test = \r [w w1]
559 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
562 If we don't inline the dfun, the code is not nearly as good:
564 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
565 PrelBase.:DEq tpl1 tpl2 -> tpl2;
570 let { y = PrelBase.I#! [ww1]; } in
571 let { x = PrelBase.I#! [ww]; } in
572 let { sat_slx = PrelTup.(,)! [y x]; } in
573 let { sat_sly = PrelTup.(,)! [x y];
575 case == sat_sly sat_slx of wild {
576 PrelBase.False -> Test.$wtest ww1 ww;
577 PrelBase.True -> PrelBase.True [];
584 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
587 Why doesn't GHC inline $fEq? Because it looks big:
589 PrelTup.zdfEqZ1T{-rcX-}
590 = \ @ a{-reT-} :: * @ b{-reS-} :: *
591 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
592 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
594 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
595 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
597 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
598 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
600 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
601 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
602 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
604 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
606 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
608 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
609 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
613 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
614 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
615 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
616 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
618 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
620 and it's not as bad as it seems, because it's further dramatically
621 simplified: only zeze2 is extracted and its body is simplified.
624 %************************************************************************
626 \subsection{Error messages}
628 %************************************************************************
631 instDeclCtxt1 hs_inst_ty
632 = inst_decl_ctxt (case unLoc hs_inst_ty of
633 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
634 HsPredTy pred -> ppr pred
635 other -> ppr hs_inst_ty) -- Don't expect this
636 instDeclCtxt2 dfun_ty
637 = inst_decl_ctxt (ppr (mkClassPred cls tys))
639 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
641 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
643 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")