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 ats))
179 -- !!!TODO: Handle the `ats' parameter!!! -=chak
180 = -- Prime error recovery, set source location
181 recoverM (returnM Nothing) $
183 addErrCtxt (instDeclCtxt1 poly_ty) $
185 do { is_boot <- tcIsHsBoot
186 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
189 -- Typecheck the instance type itself. We can't use
190 -- tcHsSigType, because it's not a valid user type.
191 ; kinded_ty <- kcHsSigType poly_ty
192 ; poly_ty' <- tcHsKindedType kinded_ty
193 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
195 ; (clas, inst_tys) <- checkValidInstHead tau
196 ; checkValidInstance tyvars theta clas inst_tys
198 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
199 ; overlap_flag <- getOverlapFlag
200 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
201 ispec = mkLocalInstance dfun overlap_flag
203 ; return (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags })) }
207 %************************************************************************
209 \subsection{Type-checking instance declarations, pass 2}
211 %************************************************************************
214 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
215 -> TcM (LHsBinds Id, TcLclEnv)
216 -- (a) From each class declaration,
217 -- generate any default-method bindings
218 -- (b) From each instance decl
219 -- generate the dfun binding
221 tcInstDecls2 tycl_decls inst_decls
222 = do { -- (a) Default methods from class decls
223 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
224 filter (isClassDecl.unLoc) tycl_decls
225 ; tcExtendIdEnv (concat dm_ids_s) $ do
227 -- (b) instance declarations
228 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
231 ; let binds = unionManyBags dm_binds_s `unionBags`
232 unionManyBags inst_binds_s
233 ; tcl_env <- getLclEnv -- Default method Ids in here
234 ; returnM (binds, tcl_env) }
237 ======= New documentation starts here (Sept 92) ==============
239 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
240 the dictionary function for this instance declaration. For example
242 instance Foo a => Foo [a] where
246 might generate something like
248 dfun.Foo.List dFoo_a = let op1 x = ...
254 HOWEVER, if the instance decl has no context, then it returns a
255 bigger @HsBinds@ with declarations for each method. For example
257 instance Foo [a] where
263 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
264 const.Foo.op1.List a x = ...
265 const.Foo.op2.List a y = ...
267 This group may be mutually recursive, because (for example) there may
268 be no method supplied for op2 in which case we'll get
270 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
272 that is, the default method applied to the dictionary at this type.
274 What we actually produce in either case is:
276 AbsBinds [a] [dfun_theta_dicts]
277 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
278 { d = (sd1,sd2, ..., op1, op2, ...)
283 The "maybe" says that we only ask AbsBinds to make global constant methods
284 if the dfun_theta is empty.
287 For an instance declaration, say,
289 instance (C1 a, C2 b) => C (T a b) where
292 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
293 function whose type is
295 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
297 Notice that we pass it the superclass dictionaries at the instance type; this
298 is the ``Mark Jones optimisation''. The stuff before the "=>" here
299 is the @dfun_theta@ below.
301 First comes the easy case of a non-local instance decl.
305 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
306 -- Returns a binding for the dfun
308 ** Explain superclass stuff ***
310 -- Derived newtype instances
311 -- In the case of a newtype, things are rather easy
312 -- class Show a => Foo a b where ...
313 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
314 -- The newtype gives an FC axiom looking like
315 -- axiom CoT a :: Tree [a] = T a
317 -- So all need is to generate a binding looking like
318 -- dfunFooT :: forall a. (Show (T a), Foo Int (Tree [a]) => Foo Int (T a)
319 -- dfunFooT = /\a. \(ds:Show (T a) (df:Foo (Tree [a])).
320 -- case df `cast` (Foo Int (CoT a)) of
321 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
323 tcInstDecl2 (InstInfo { iSpec = ispec,
324 iBinds = NewTypeDerived rep_tys })
325 = do { let dfun_id = instanceDFunId ispec
326 rigid_info = InstSkol dfun_id
327 origin = SigOrigin rigid_info
328 inst_ty = idType dfun_id
329 ; (tvs, theta, inst_head) <- tcSkolSigType rigid_info inst_ty
330 ; ASSERT( isSingleton theta ) -- Always the case for NewTypeDerived
331 rep_dict <- newDict origin (head theta)
333 ; let rep_dict_id = instToId rep_dict
335 co_fn = CoTyLams tvs <.> CoLams [rep_dict_id] <.> ExprCoFn cast
337 ; return (unitBag (VarBind dfun_id (HsCoerce co_fn (HsVar rep_dict_id))))
339 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
340 avail_insts op_items (NewTypeDerived rep_tys)
341 = getInstLoc origin `thenM` \ inst_loc ->
342 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
345 (ptext SLIT("newtype derived instance"))
346 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
348 -- I don't think we have to do the checkSigTyVars thing
350 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
353 do_one inst_loc (sel_id, _)
354 = -- The binding is like "op @ NewTy = op @ RepTy"
355 -- Make the *binder*, like in mkMethodBind
356 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
358 -- Make the *occurrence on the rhs*
359 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
361 meth_id = instToId meth_inst
363 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
365 -- Instantiate rep_tys with the relevant type variables
366 -- This looks a bit odd, because inst_tyvars' are the skolemised version
367 -- of the type variables in the instance declaration; but rep_tys doesn't
368 -- have the skolemised version, so we substitute them in here
369 rep_tys' = substTys subst rep_tys
370 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
374 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
376 dfun_id = instanceDFunId ispec
377 rigid_info = InstSkol dfun_id
378 inst_ty = idType dfun_id
380 -- Prime error recovery
381 recoverM (returnM emptyLHsBinds) $
382 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
383 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
385 -- Instantiate the instance decl with skolem constants
386 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
387 -- These inst_tyvars' scope over the 'where' part
388 -- Those tyvars are inside the dfun_id's type, which is a bit
389 -- bizarre, but OK so long as you realise it!
391 (clas, inst_tys') = tcSplitDFunHead inst_head'
392 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
394 -- Instantiate the super-class context with inst_tys
395 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
396 origin = SigOrigin rigid_info
398 -- Create dictionary Ids from the specified instance contexts.
399 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
400 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
401 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
402 -- Default-method Ids may be mentioned in synthesised RHSs,
403 -- but they'll already be in the environment.
405 -- Typecheck the methods
406 let -- These insts are in scope; quite a few, eh?
407 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
409 tcMethods origin clas inst_tyvars'
410 dfun_theta' inst_tys' avail_insts
411 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
413 -- Figure out bindings for the superclass context
414 -- Don't include this_dict in the 'givens', else
415 -- sc_dicts get bound by just selecting from this_dict!!
416 addErrCtxt superClassCtxt
417 (tcSimplifySuperClasses inst_tyvars'
419 sc_dicts) `thenM` \ sc_binds ->
421 -- It's possible that the superclass stuff might unified one
422 -- of the inst_tyavars' with something in the envt
423 checkSigTyVars inst_tyvars' `thenM_`
425 -- Deal with 'SPECIALISE instance' pragmas
426 tcPrags dfun_id (filter isSpecInstLSig prags) `thenM` \ prags ->
428 -- Create the result bindings
430 dict_constr = classDataCon clas
431 scs_and_meths = map instToId sc_dicts ++ meth_ids
432 this_dict_id = instToId this_dict
433 inline_prag | null dfun_arg_dicts = []
434 | otherwise = [InlinePrag (Inline AlwaysActive True)]
435 -- Always inline the dfun; this is an experimental decision
436 -- because it makes a big performance difference sometimes.
437 -- Often it means we can do the method selection, and then
438 -- inline the method as well. Marcin's idea; see comments below.
440 -- BUT: don't inline it if it's a constant dictionary;
441 -- we'll get all the benefit without inlining, and we get
442 -- a **lot** of code duplication if we inline it
444 -- See Note [Inline dfuns] below
447 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
448 -- We don't produce a binding for the dict_constr; instead we
449 -- rely on the simplifier to unfold this saturated application
450 -- We do this rather than generate an HsCon directly, because
451 -- it means that the special cases (e.g. dictionary with only one
452 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
453 -- than needing to be repeated here.
455 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
456 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
458 main_bind = noLoc $ AbsBinds
460 (map instToId dfun_arg_dicts)
461 [(inst_tyvars', dfun_id, this_dict_id,
462 inline_prag ++ prags)]
465 showLIE (text "instance") `thenM_`
466 returnM (unitBag main_bind)
469 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
470 avail_insts op_items monobinds uprags
471 = -- Check that all the method bindings come from this class
473 sel_names = [idName sel_id | (sel_id, _) <- op_items]
474 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
476 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
478 -- Make the method bindings
480 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
482 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
484 -- And type check them
485 -- It's really worth making meth_insts available to the tcMethodBind
486 -- Consider instance Monad (ST s) where
487 -- {-# INLINE (>>) #-}
488 -- (>>) = ...(>>=)...
489 -- If we don't include meth_insts, we end up with bindings like this:
490 -- rec { dict = MkD then bind ...
491 -- then = inline_me (... (GHC.Base.>>= dict) ...)
493 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
494 -- and (b) the inline_me prevents us inlining the >>= selector, which
495 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
496 -- is not inlined across modules. Rather ironic since this does not
497 -- happen without the INLINE pragma!
499 -- Solution: make meth_insts available, so that 'then' refers directly
500 -- to the local 'bind' rather than going via the dictionary.
502 -- BUT WATCH OUT! If the method type mentions the class variable, then
503 -- this optimisation is not right. Consider
507 -- instance C Int where
509 -- The occurrence of 'op' on the rhs gives rise to a constraint
511 -- The trouble is that the 'meth_inst' for op, which is 'available', also
512 -- looks like 'op at Int'. But they are not the same.
514 prag_fn = mkPragFun uprags
515 all_insts = avail_insts ++ catMaybes meth_insts
516 sig_fn n = Just [] -- No scoped type variables, but every method has
517 -- a type signature, in effect, so that we check
518 -- the method has the right type
519 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
520 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
523 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
525 returnM (meth_ids, unionManyBags meth_binds_s)
529 ------------------------------
530 [Inline dfuns] Inlining dfuns unconditionally
531 ------------------------------
533 The code above unconditionally inlines dict funs. Here's why.
534 Consider this program:
536 test :: Int -> Int -> Bool
537 test x y = (x,y) == (y,x) || test y x
538 -- Recursive to avoid making it inline.
540 This needs the (Eq (Int,Int)) instance. If we inline that dfun
541 the code we end up with is good:
544 \r -> case ==# [ww ww1] of wild {
545 PrelBase.False -> Test.$wtest ww1 ww;
547 case ==# [ww1 ww] of wild1 {
548 PrelBase.False -> Test.$wtest ww1 ww;
549 PrelBase.True -> PrelBase.True [];
552 Test.test = \r [w w1]
555 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
558 If we don't inline the dfun, the code is not nearly as good:
560 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
561 PrelBase.:DEq tpl1 tpl2 -> tpl2;
566 let { y = PrelBase.I#! [ww1]; } in
567 let { x = PrelBase.I#! [ww]; } in
568 let { sat_slx = PrelTup.(,)! [y x]; } in
569 let { sat_sly = PrelTup.(,)! [x y];
571 case == sat_sly sat_slx of wild {
572 PrelBase.False -> Test.$wtest ww1 ww;
573 PrelBase.True -> PrelBase.True [];
580 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
583 Why doesn't GHC inline $fEq? Because it looks big:
585 PrelTup.zdfEqZ1T{-rcX-}
586 = \ @ a{-reT-} :: * @ b{-reS-} :: *
587 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
588 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
590 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
591 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
593 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
594 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
596 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
597 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
598 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
600 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
602 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
604 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
605 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
609 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
610 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
611 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
612 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
614 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
616 and it's not as bad as it seems, because it's further dramatically
617 simplified: only zeze2 is extracted and its body is simplified.
620 %************************************************************************
622 \subsection{Error messages}
624 %************************************************************************
627 instDeclCtxt1 hs_inst_ty
628 = inst_decl_ctxt (case unLoc hs_inst_ty of
629 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
630 HsPredTy pred -> ppr pred
631 other -> ppr hs_inst_ty) -- Don't expect this
632 instDeclCtxt2 dfun_ty
633 = inst_decl_ctxt (ppr (mkClassPred cls tys))
635 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
637 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
639 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")