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,
17 checkInstTermination, instTypeErr,
18 checkAmbiguity, SourceTyCtxt(..) )
19 import TcType ( mkClassPred, tyVarsOfType,
20 tcSplitSigmaTy, tcSplitDFunHead, mkTyVarTys,
21 SkolemInfo(InstSkol), tcSplitDFunTy, pprClassPred )
22 import Inst ( tcInstClassOp, newDicts, instToId, showLIE,
23 getOverlapFlag, tcExtendLocalInstEnv )
24 import InstEnv ( mkLocalInstance, instanceDFunId )
25 import TcDeriv ( tcDeriving )
26 import TcEnv ( 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 Maybe ( catMaybes )
40 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart )
41 import ListSetOps ( minusList )
44 import BasicTypes ( Activation( AlwaysActive ), InlineSpec(..) )
48 Typechecking instance declarations is done in two passes. The first
49 pass, made by @tcInstDecls1@, collects information to be used in the
52 This pre-processed info includes the as-yet-unprocessed bindings
53 inside the instance declaration. These are type-checked in the second
54 pass, when the class-instance envs and GVE contain all the info from
55 all the instance and value decls. Indeed that's the reason we need
56 two passes over the instance decls.
59 Here is the overall algorithm.
60 Assume that we have an instance declaration
62 instance c => k (t tvs) where b
66 $LIE_c$ is the LIE for the context of class $c$
68 $betas_bar$ is the free variables in the class method type, excluding the
71 $LIE_cop$ is the LIE constraining a particular class method
73 $tau_cop$ is the tau type of a class method
75 $LIE_i$ is the LIE for the context of instance $i$
77 $X$ is the instance constructor tycon
79 $gammas_bar$ is the set of type variables of the instance
81 $LIE_iop$ is the LIE for a particular class method instance
83 $tau_iop$ is the tau type for this instance of a class method
85 $alpha$ is the class variable
87 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
89 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
92 ToDo: Update the list above with names actually in the code.
96 First, make the LIEs for the class and instance contexts, which means
97 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
98 and make LIElistI and LIEI.
100 Then process each method in turn.
102 order the instance methods according to the ordering of the class methods
104 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
106 Create final dictionary function from bindings generated already
108 df = lambda inst_tyvars
115 in <op1,op2,...,opn,sd1,...,sdm>
117 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
118 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
122 %************************************************************************
124 \subsection{Extracting instance decls}
126 %************************************************************************
128 Gather up the instance declarations from their various sources
131 tcInstDecls1 -- Deal with both source-code and imported instance decls
132 :: [LTyClDecl Name] -- For deriving stuff
133 -> [LInstDecl Name] -- Source code instance decls
134 -> TcM (TcGblEnv, -- The full inst env
135 [InstInfo], -- Source-code instance decls to process;
136 -- contains all dfuns for this module
137 HsValBinds Name) -- Supporting bindings for derived instances
139 tcInstDecls1 tycl_decls inst_decls
141 -- Stop if addInstInfos etc discovers any errors
142 -- (they recover, so that we get more than one error each round)
144 -- (1) Do the ordinary instance declarations
145 mappM tcLocalInstDecl1 inst_decls `thenM` \ local_inst_infos ->
148 local_inst_info = catMaybes local_inst_infos
149 clas_decls = filter (isClassDecl.unLoc) tycl_decls
151 -- (2) Instances from generic class declarations
152 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
154 -- Next, construct the instance environment so far, consisting of
155 -- a) local instance decls
156 -- b) generic instances
157 addInsts local_inst_info $
158 addInsts generic_inst_info $
160 -- (3) Compute instances from "deriving" clauses;
161 -- This stuff computes a context for the derived instance decl, so it
162 -- needs to know about all the instances possible; hence inst_env4
163 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
164 addInsts deriv_inst_info $
166 getGblEnv `thenM` \ gbl_env ->
168 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
171 addInsts :: [InstInfo] -> TcM a -> TcM a
172 addInsts infos thing_inside
173 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
177 tcLocalInstDecl1 :: LInstDecl Name
178 -> TcM (Maybe InstInfo) -- Nothing if there was an error
179 -- A source-file instance declaration
180 -- Type-check all the stuff before the "where"
182 -- We check for respectable instance type, and context
183 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags))
184 = -- Prime error recovery, set source location
185 recoverM (returnM Nothing) $
187 addErrCtxt (instDeclCtxt1 poly_ty) $
189 -- Typecheck the instance type itself. We can't use
190 -- tcHsSigType, because it's not a valid user type.
191 kcHsSigType poly_ty `thenM` \ kinded_ty ->
192 tcHsKindedType kinded_ty `thenM` \ poly_ty' ->
194 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
196 checkValidTheta InstThetaCtxt theta `thenM_`
197 checkAmbiguity tyvars theta (tyVarsOfType tau) `thenM_`
198 checkValidInstHead tau `thenM` \ (clas,inst_tys) ->
199 checkInstTermination theta inst_tys `thenM_`
200 checkTc (checkInstFDs theta clas inst_tys)
201 (instTypeErr (pprClassPred clas inst_tys) msg) `thenM_`
202 newDFunName clas inst_tys (srcSpanStart loc) `thenM` \ dfun_name ->
203 getOverlapFlag `thenM` \ overlap_flag ->
204 let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
205 ispec = mkLocalInstance dfun overlap_flag
208 tcIsHsBoot `thenM` \ is_boot ->
209 checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
210 badBootDeclErr `thenM_`
212 returnM (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags }))
214 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
218 %************************************************************************
220 \subsection{Type-checking instance declarations, pass 2}
222 %************************************************************************
225 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
226 -> TcM (LHsBinds Id, TcLclEnv)
227 -- (a) From each class declaration,
228 -- generate any default-method bindings
229 -- (b) From each instance decl
230 -- generate the dfun binding
232 tcInstDecls2 tycl_decls inst_decls
233 = do { -- (a) Default methods from class decls
234 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
235 filter (isClassDecl.unLoc) tycl_decls
236 ; tcExtendIdEnv (concat dm_ids_s) $ do
238 -- (b) instance declarations
239 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
242 ; let binds = unionManyBags dm_binds_s `unionBags`
243 unionManyBags inst_binds_s
244 ; tcl_env <- getLclEnv -- Default method Ids in here
245 ; returnM (binds, tcl_env) }
248 ======= New documentation starts here (Sept 92) ==============
250 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
251 the dictionary function for this instance declaration. For example
253 instance Foo a => Foo [a] where
257 might generate something like
259 dfun.Foo.List dFoo_a = let op1 x = ...
265 HOWEVER, if the instance decl has no context, then it returns a
266 bigger @HsBinds@ with declarations for each method. For example
268 instance Foo [a] where
274 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
275 const.Foo.op1.List a x = ...
276 const.Foo.op2.List a y = ...
278 This group may be mutually recursive, because (for example) there may
279 be no method supplied for op2 in which case we'll get
281 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
283 that is, the default method applied to the dictionary at this type.
285 What we actually produce in either case is:
287 AbsBinds [a] [dfun_theta_dicts]
288 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
289 { d = (sd1,sd2, ..., op1, op2, ...)
294 The "maybe" says that we only ask AbsBinds to make global constant methods
295 if the dfun_theta is empty.
298 For an instance declaration, say,
300 instance (C1 a, C2 b) => C (T a b) where
303 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
304 function whose type is
306 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
308 Notice that we pass it the superclass dictionaries at the instance type; this
309 is the ``Mark Jones optimisation''. The stuff before the "=>" here
310 is the @dfun_theta@ below.
312 First comes the easy case of a non-local instance decl.
316 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
318 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = binds })
320 dfun_id = instanceDFunId ispec
321 rigid_info = InstSkol dfun_id
322 inst_ty = idType dfun_id
324 -- Prime error recovery
325 recoverM (returnM emptyLHsBinds) $
326 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
327 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
329 -- Instantiate the instance decl with skolem constants
330 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
331 -- These inst_tyvars' scope over the 'where' part
332 -- Those tyvars are inside the dfun_id's type, which is a bit
333 -- bizarre, but OK so long as you realise it!
335 (clas, inst_tys') = tcSplitDFunHead inst_head'
336 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
338 -- Instantiate the super-class context with inst_tys
339 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
340 origin = SigOrigin rigid_info
342 -- Create dictionary Ids from the specified instance contexts.
343 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
344 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
345 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
346 -- Default-method Ids may be mentioned in synthesised RHSs,
347 -- but they'll already be in the environment.
349 -- Typecheck the methods
350 let -- These insts are in scope; quite a few, eh?
351 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
353 tcMethods origin clas inst_tyvars'
354 dfun_theta' inst_tys' avail_insts
355 op_items binds `thenM` \ (meth_ids, meth_binds) ->
357 -- Figure out bindings for the superclass context
358 -- Don't include this_dict in the 'givens', else
359 -- sc_dicts get bound by just selecting from this_dict!!
360 addErrCtxt superClassCtxt
361 (tcSimplifySuperClasses inst_tyvars'
363 sc_dicts) `thenM` \ sc_binds ->
365 -- It's possible that the superclass stuff might unified one
366 -- of the inst_tyavars' with something in the envt
367 checkSigTyVars inst_tyvars' `thenM_`
369 -- Deal with 'SPECIALISE instance' pragmas
371 specs = case binds of
372 VanillaInst _ prags -> filter isSpecInstLSig prags
375 tcPrags dfun_id specs `thenM` \ prags ->
377 -- Create the result bindings
379 dict_constr = classDataCon clas
380 scs_and_meths = map instToId sc_dicts ++ meth_ids
381 this_dict_id = instToId this_dict
382 inline_prag | null dfun_arg_dicts = []
383 | otherwise = [InlinePrag (Inline AlwaysActive True)]
384 -- Always inline the dfun; this is an experimental decision
385 -- because it makes a big performance difference sometimes.
386 -- Often it means we can do the method selection, and then
387 -- inline the method as well. Marcin's idea; see comments below.
389 -- BUT: don't inline it if it's a constant dictionary;
390 -- we'll get all the benefit without inlining, and we get
391 -- a **lot** of code duplication if we inline it
393 -- See Note [Inline dfuns] below
396 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
397 -- We don't produce a binding for the dict_constr; instead we
398 -- rely on the simplifier to unfold this saturated application
399 -- We do this rather than generate an HsCon directly, because
400 -- it means that the special cases (e.g. dictionary with only one
401 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
402 -- than needing to be repeated here.
405 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
407 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
408 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
410 main_bind = noLoc $ AbsBinds
412 (map instToId dfun_arg_dicts)
413 [(inst_tyvars', dfun_id, this_dict_id,
414 inline_prag ++ prags)]
417 showLIE (text "instance") `thenM_`
418 returnM (unitBag main_bind)
421 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
422 avail_insts op_items (VanillaInst monobinds uprags)
423 = -- Check that all the method bindings come from this class
425 sel_names = [idName sel_id | (sel_id, _) <- op_items]
426 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
428 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
430 -- Make the method bindings
432 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
434 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
436 -- And type check them
437 -- It's really worth making meth_insts available to the tcMethodBind
438 -- Consider instance Monad (ST s) where
439 -- {-# INLINE (>>) #-}
440 -- (>>) = ...(>>=)...
441 -- If we don't include meth_insts, we end up with bindings like this:
442 -- rec { dict = MkD then bind ...
443 -- then = inline_me (... (GHC.Base.>>= dict) ...)
445 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
446 -- and (b) the inline_me prevents us inlining the >>= selector, which
447 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
448 -- is not inlined across modules. Rather ironic since this does not
449 -- happen without the INLINE pragma!
451 -- Solution: make meth_insts available, so that 'then' refers directly
452 -- to the local 'bind' rather than going via the dictionary.
454 -- BUT WATCH OUT! If the method type mentions the class variable, then
455 -- this optimisation is not right. Consider
459 -- instance C Int where
461 -- The occurrence of 'op' on the rhs gives rise to a constraint
463 -- The trouble is that the 'meth_inst' for op, which is 'available', also
464 -- looks like 'op at Int'. But they are not the same.
466 prag_fn = mkPragFun uprags
467 all_insts = avail_insts ++ catMaybes meth_insts
468 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts prag_fn
469 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
472 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
474 returnM (meth_ids, unionManyBags meth_binds_s)
477 -- Derived newtype instances
478 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
479 avail_insts op_items (NewTypeDerived rep_tys)
480 = getInstLoc origin `thenM` \ inst_loc ->
481 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
484 (ptext SLIT("newtype derived instance"))
485 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
487 -- I don't think we have to do the checkSigTyVars thing
489 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
492 do_one inst_loc (sel_id, _)
493 = -- The binding is like "op @ NewTy = op @ RepTy"
494 -- Make the *binder*, like in mkMethodBind
495 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
497 -- Make the *occurrence on the rhs*
498 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
500 meth_id = instToId meth_inst
502 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
504 -- Instantiate rep_tys with the relevant type variables
505 -- This looks a bit odd, because inst_tyvars' are the skolemised version
506 -- of the type variables in the instance declaration; but rep_tys doesn't
507 -- have the skolemised version, so we substitute them in here
508 rep_tys' = substTys subst rep_tys
509 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
513 ------------------------------
514 [Inline dfuns] Inlining dfuns unconditionally
515 ------------------------------
517 The code above unconditionally inlines dict funs. Here's why.
518 Consider this program:
520 test :: Int -> Int -> Bool
521 test x y = (x,y) == (y,x) || test y x
522 -- Recursive to avoid making it inline.
524 This needs the (Eq (Int,Int)) instance. If we inline that dfun
525 the code we end up with is good:
528 \r -> case ==# [ww ww1] of wild {
529 PrelBase.False -> Test.$wtest ww1 ww;
531 case ==# [ww1 ww] of wild1 {
532 PrelBase.False -> Test.$wtest ww1 ww;
533 PrelBase.True -> PrelBase.True [];
536 Test.test = \r [w w1]
539 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
542 If we don't inline the dfun, the code is not nearly as good:
544 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
545 PrelBase.:DEq tpl1 tpl2 -> tpl2;
550 let { y = PrelBase.I#! [ww1]; } in
551 let { x = PrelBase.I#! [ww]; } in
552 let { sat_slx = PrelTup.(,)! [y x]; } in
553 let { sat_sly = PrelTup.(,)! [x y];
555 case == sat_sly sat_slx of wild {
556 PrelBase.False -> Test.$wtest ww1 ww;
557 PrelBase.True -> PrelBase.True [];
564 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
567 Why doesn't GHC inline $fEq? Because it looks big:
569 PrelTup.zdfEqZ1T{-rcX-}
570 = \ @ a{-reT-} :: * @ b{-reS-} :: *
571 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
572 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
574 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
575 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
577 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
578 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
580 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
581 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
582 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
584 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
586 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
588 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
589 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
593 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
594 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
595 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
596 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
598 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
600 and it's not as bad as it seems, because it's further dramatically
601 simplified: only zeze2 is extracted and its body is simplified.
604 %************************************************************************
606 \subsection{Error messages}
608 %************************************************************************
611 instDeclCtxt1 hs_inst_ty
612 = inst_decl_ctxt (case unLoc hs_inst_ty of
613 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
614 HsPredTy pred -> ppr pred
615 other -> ppr hs_inst_ty) -- Don't expect this
616 instDeclCtxt2 dfun_ty
617 = inst_decl_ctxt (ppr (mkClassPred cls tys))
619 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
621 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
623 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")