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 -- 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 getOverlapFlag `thenM` \ overlap_flag ->
206 let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
207 ispec = mkLocalInstance dfun overlap_flag
210 tcIsHsBoot `thenM` \ is_boot ->
211 checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
212 badBootDeclErr `thenM_`
214 returnM (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags }))
216 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
220 %************************************************************************
222 \subsection{Type-checking instance declarations, pass 2}
224 %************************************************************************
227 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
228 -> TcM (LHsBinds Id, TcLclEnv)
229 -- (a) From each class declaration,
230 -- generate any default-method bindings
231 -- (b) From each instance decl
232 -- generate the dfun binding
234 tcInstDecls2 tycl_decls inst_decls
235 = do { -- (a) Default methods from class decls
236 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
237 filter (isClassDecl.unLoc) tycl_decls
238 ; tcExtendIdEnv (concat dm_ids_s) $ do
240 -- (b) instance declarations
241 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
244 ; let binds = unionManyBags dm_binds_s `unionBags`
245 unionManyBags inst_binds_s
246 ; tcl_env <- getLclEnv -- Default method Ids in here
247 ; returnM (binds, tcl_env) }
250 ======= New documentation starts here (Sept 92) ==============
252 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
253 the dictionary function for this instance declaration. For example
255 instance Foo a => Foo [a] where
259 might generate something like
261 dfun.Foo.List dFoo_a = let op1 x = ...
267 HOWEVER, if the instance decl has no context, then it returns a
268 bigger @HsBinds@ with declarations for each method. For example
270 instance Foo [a] where
276 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
277 const.Foo.op1.List a x = ...
278 const.Foo.op2.List a y = ...
280 This group may be mutually recursive, because (for example) there may
281 be no method supplied for op2 in which case we'll get
283 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
285 that is, the default method applied to the dictionary at this type.
287 What we actually produce in either case is:
289 AbsBinds [a] [dfun_theta_dicts]
290 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
291 { d = (sd1,sd2, ..., op1, op2, ...)
296 The "maybe" says that we only ask AbsBinds to make global constant methods
297 if the dfun_theta is empty.
300 For an instance declaration, say,
302 instance (C1 a, C2 b) => C (T a b) where
305 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
306 function whose type is
308 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
310 Notice that we pass it the superclass dictionaries at the instance type; this
311 is the ``Mark Jones optimisation''. The stuff before the "=>" here
312 is the @dfun_theta@ below.
314 First comes the easy case of a non-local instance decl.
318 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
320 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = binds })
322 dfun_id = instanceDFunId ispec
323 rigid_info = InstSkol dfun_id
324 inst_ty = idType dfun_id
326 -- Prime error recovery
327 recoverM (returnM emptyLHsBinds) $
328 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
329 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
331 -- Instantiate the instance decl with skolem constants
332 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
333 -- These inst_tyvars' scope over the 'where' part
334 -- Those tyvars are inside the dfun_id's type, which is a bit
335 -- bizarre, but OK so long as you realise it!
337 (clas, inst_tys') = tcSplitDFunHead inst_head'
338 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
340 -- Instantiate the super-class context with inst_tys
341 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
342 origin = SigOrigin rigid_info
344 -- Create dictionary Ids from the specified instance contexts.
345 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
346 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
347 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
348 -- Default-method Ids may be mentioned in synthesised RHSs,
349 -- but they'll already be in the environment.
351 -- Typecheck the methods
352 let -- These insts are in scope; quite a few, eh?
353 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
355 tcMethods origin clas inst_tyvars'
356 dfun_theta' inst_tys' avail_insts
357 op_items binds `thenM` \ (meth_ids, meth_binds) ->
359 -- Figure out bindings for the superclass context
360 -- Don't include this_dict in the 'givens', else
361 -- sc_dicts get bound by just selecting from this_dict!!
362 addErrCtxt superClassCtxt
363 (tcSimplifySuperClasses inst_tyvars'
365 sc_dicts) `thenM` \ sc_binds ->
367 -- It's possible that the superclass stuff might unified one
368 -- of the inst_tyavars' with something in the envt
369 checkSigTyVars inst_tyvars' `thenM_`
371 -- Deal with 'SPECIALISE instance' pragmas
373 specs = case binds of
374 VanillaInst _ prags -> filter isSpecInstLSig prags
377 tcPrags dfun_id specs `thenM` \ prags ->
379 -- Create the result bindings
381 dict_constr = classDataCon clas
382 scs_and_meths = map instToId sc_dicts ++ meth_ids
383 this_dict_id = instToId this_dict
384 inline_prag | null dfun_arg_dicts = []
385 | otherwise = [InlinePrag (Inline AlwaysActive True)]
386 -- Always inline the dfun; this is an experimental decision
387 -- because it makes a big performance difference sometimes.
388 -- Often it means we can do the method selection, and then
389 -- inline the method as well. Marcin's idea; see comments below.
391 -- BUT: don't inline it if it's a constant dictionary;
392 -- we'll get all the benefit without inlining, and we get
393 -- a **lot** of code duplication if we inline it
395 -- See Note [Inline dfuns] below
399 = -- Blatant special case for CCallable, CReturnable
400 -- If the dictionary is empty then we should never
401 -- select anything from it, so we make its RHS just
402 -- emit an error message. This in turn means that we don't
403 -- mention the constructor, which doesn't exist for CCallable, CReturnable
404 -- Hardly beautiful, but only three extra lines.
405 nlHsApp (noLoc $ TyApp (nlHsVar rUNTIME_ERROR_ID)
406 [idType this_dict_id])
407 (nlHsLit (HsStringPrim (mkFastString msg)))
409 | otherwise -- The common case
410 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
411 -- We don't produce a binding for the dict_constr; instead we
412 -- rely on the simplifier to unfold this saturated application
413 -- We do this rather than generate an HsCon directly, because
414 -- it means that the special cases (e.g. dictionary with only one
415 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
416 -- than needing to be repeated here.
419 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
421 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
422 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
424 main_bind = noLoc $ AbsBinds
426 (map instToId dfun_arg_dicts)
427 [(inst_tyvars', dfun_id, this_dict_id,
428 inline_prag ++ prags)]
431 showLIE (text "instance") `thenM_`
432 returnM (unitBag main_bind)
435 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
436 avail_insts op_items (VanillaInst monobinds uprags)
437 = -- Check that all the method bindings come from this class
439 sel_names = [idName sel_id | (sel_id, _) <- op_items]
440 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
442 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
444 -- Make the method bindings
446 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
448 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
450 -- And type check them
451 -- It's really worth making meth_insts available to the tcMethodBind
452 -- Consider instance Monad (ST s) where
453 -- {-# INLINE (>>) #-}
454 -- (>>) = ...(>>=)...
455 -- If we don't include meth_insts, we end up with bindings like this:
456 -- rec { dict = MkD then bind ...
457 -- then = inline_me (... (GHC.Base.>>= dict) ...)
459 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
460 -- and (b) the inline_me prevents us inlining the >>= selector, which
461 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
462 -- is not inlined across modules. Rather ironic since this does not
463 -- happen without the INLINE pragma!
465 -- Solution: make meth_insts available, so that 'then' refers directly
466 -- to the local 'bind' rather than going via the dictionary.
468 -- BUT WATCH OUT! If the method type mentions the class variable, then
469 -- this optimisation is not right. Consider
473 -- instance C Int where
475 -- The occurrence of 'op' on the rhs gives rise to a constraint
477 -- The trouble is that the 'meth_inst' for op, which is 'available', also
478 -- looks like 'op at Int'. But they are not the same.
480 prag_fn = mkPragFun uprags
481 all_insts = avail_insts ++ catMaybes meth_insts
482 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts prag_fn
483 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
486 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
488 returnM (meth_ids, unionManyBags meth_binds_s)
491 -- Derived newtype instances
492 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
493 avail_insts op_items (NewTypeDerived rep_tys)
494 = getInstLoc origin `thenM` \ inst_loc ->
495 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
498 (ptext SLIT("newtype derived instance"))
499 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
501 -- I don't think we have to do the checkSigTyVars thing
503 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
506 do_one inst_loc (sel_id, _)
507 = -- The binding is like "op @ NewTy = op @ RepTy"
508 -- Make the *binder*, like in mkMethodBind
509 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
511 -- Make the *occurrence on the rhs*
512 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
514 meth_id = instToId meth_inst
516 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
518 -- Instantiate rep_tys with the relevant type variables
519 -- This looks a bit odd, because inst_tyvars' are the skolemised version
520 -- of the type variables in the instance declaration; but rep_tys doesn't
521 -- have the skolemised version, so we substitute them in here
522 rep_tys' = substTys subst rep_tys
523 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
527 ------------------------------
528 [Inline dfuns] Inlining dfuns unconditionally
529 ------------------------------
531 The code above unconditionally inlines dict funs. Here's why.
532 Consider this program:
534 test :: Int -> Int -> Bool
535 test x y = (x,y) == (y,x) || test y x
536 -- Recursive to avoid making it inline.
538 This needs the (Eq (Int,Int)) instance. If we inline that dfun
539 the code we end up with is good:
542 \r -> case ==# [ww ww1] of wild {
543 PrelBase.False -> Test.$wtest ww1 ww;
545 case ==# [ww1 ww] of wild1 {
546 PrelBase.False -> Test.$wtest ww1 ww;
547 PrelBase.True -> PrelBase.True [];
550 Test.test = \r [w w1]
553 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
556 If we don't inline the dfun, the code is not nearly as good:
558 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
559 PrelBase.:DEq tpl1 tpl2 -> tpl2;
564 let { y = PrelBase.I#! [ww1]; } in
565 let { x = PrelBase.I#! [ww]; } in
566 let { sat_slx = PrelTup.(,)! [y x]; } in
567 let { sat_sly = PrelTup.(,)! [x y];
569 case == sat_sly sat_slx of wild {
570 PrelBase.False -> Test.$wtest ww1 ww;
571 PrelBase.True -> PrelBase.True [];
578 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
581 Why doesn't GHC inline $fEq? Because it looks big:
583 PrelTup.zdfEqZ1T{-rcX-}
584 = \ @ a{-reT-} :: * @ b{-reS-} :: *
585 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
586 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
588 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
589 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
591 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
592 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
594 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
595 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
596 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
598 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
600 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
602 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
603 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
607 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
608 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
609 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
610 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
612 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
614 and it's not as bad as it seems, because it's further dramatically
615 simplified: only zeze2 is extracted and its body is simplified.
618 %************************************************************************
620 \subsection{Error messages}
622 %************************************************************************
625 instDeclCtxt1 hs_inst_ty
626 = inst_decl_ctxt (case unLoc hs_inst_ty of
627 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
628 HsPredTy pred -> ppr pred
629 other -> ppr hs_inst_ty) -- Don't expect this
630 instDeclCtxt2 dfun_ty
631 = inst_decl_ctxt (ppr (mkClassPred cls tys))
633 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
635 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
637 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")