2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcInstDecls: Typechecking instance declarations
9 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
24 import RnEnv ( lookupGlobalOccRn )
25 import RnSource ( addTcgDUs )
35 import CoreUnfold ( mkDFunUnfolding )
36 import PrelNames ( inlineIdName )
54 #include "HsVersions.h"
57 Typechecking instance declarations is done in two passes. The first
58 pass, made by @tcInstDecls1@, collects information to be used in the
61 This pre-processed info includes the as-yet-unprocessed bindings
62 inside the instance declaration. These are type-checked in the second
63 pass, when the class-instance envs and GVE contain all the info from
64 all the instance and value decls. Indeed that's the reason we need
65 two passes over the instance decls.
68 Note [How instance declarations are translated]
69 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
70 Here is how we translation instance declarations into Core
74 op1, op2 :: Ix b => a -> b -> b
78 {-# INLINE [2] op1 #-}
82 op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
86 -- Default methods get the 'self' dictionary as argument
87 -- so they can call other methods at the same type
88 -- Default methods get the same type as their method selector
89 $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
90 $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
91 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
92 -- Note [Tricky type variable scoping]
94 -- A top-level definition for each instance method
95 -- Here op1_i, op2_i are the "instance method Ids"
96 -- The INLINE pragma comes from the user pragma
97 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
98 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
99 op1_i = /\a. \(d:C a).
102 -- Note [Subtle interaction of recursion and overlap]
104 local_op1 :: forall b. Ix b => [a] -> b -> b
106 -- Source code; run the type checker on this
107 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
108 -- Note [Tricky type variable scoping]
112 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
114 -- The dictionary function itself
115 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
116 df_i :: forall a. C a -> C [a]
117 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
118 -- But see Note [Default methods in instances]
119 -- We can't apply the type checker to the default-method call
121 -- Use a RULE to short-circuit applications of the class ops
122 {-# RULE "op1@C[a]" forall a, d:C a.
123 op1 [a] (df_i d) = op1_i a d #-}
125 * We want to inline the dictionary function itself as vigorously as we
126 possibly can, so that we expose that dictionary constructor to
127 selectors as much as poss. We don't actually inline it; rather, we
128 use a Builtin RULE for the ClassOps (see MkId.mkDictSelId) to short
129 circuit such applications. But the RULE only applies if it can "see"
130 the dfun's DFunUnfolding.
132 * Note that df_i may be mutually recursive with both op1_i and op2_i.
133 It's crucial that df_i is not chosen as the loop breaker, even
134 though op1_i has a (user-specified) INLINE pragma.
136 * Instead the idea is to inline df_i into op1_i, which may then select
137 methods from the MkC record, and thereby break the recursion with
138 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
139 the same type, it won't mention df_i, so there won't be recursion in
142 * If op1_i is marked INLINE by the user there's a danger that we won't
143 inline df_i in it, and that in turn means that (since it'll be a
144 loop-breaker because df_i isn't), op1_i will ironically never be
145 inlined. But this is OK: the recursion breaking happens by way of
146 a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
147 unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
150 Note [Subtle interaction of recursion and overlap]
151 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
153 class C a where { op1,op2 :: a -> a }
154 instance C a => C [a] where
155 op1 x = op2 x ++ op2 x
157 intance C [Int] where
160 When type-checking the C [a] instance, we need a C [a] dictionary (for
161 the call of op2). If we look up in the instance environment, we find
162 an overlap. And in *general* the right thing is to complain (see Note
163 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
164 complain, because we just want to delegate to the op2 of this same
167 Why is this justified? Because we generate a (C [a]) constraint in
168 a context in which 'a' cannot be instantiated to anything that matches
169 other overlapping instances, or else we would not be excecuting this
170 version of op1 in the first place.
172 It might even be a bit disguised:
174 nullFail :: C [a] => [a] -> [a]
175 nullFail x = op2 x ++ op2 x
177 instance C a => C [a] where
180 Precisely this is used in package 'regex-base', module Context.hs.
181 See the overlapping instances for RegexContext, and the fact that they
182 call 'nullFail' just like the example above. The DoCon package also
183 does the same thing; it shows up in module Fraction.hs
185 Conclusion: when typechecking the methods in a C [a] instance, we want
186 to have C [a] available. That is why we have the strange local
187 definition for 'this' in the definition of op1_i in the example above.
188 We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck
189 we supply 'this' as a given dictionary. Only needed, though, if there
190 are some type variables involved; otherwise there can be no overlap and
193 Note [Tricky type variable scoping]
194 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
197 op1, op2 :: Ix b => a -> b -> b
200 instance C a => C [a]
201 {-# INLINE [2] op1 #-}
204 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
205 in scope in <rhs>. In particular, we must make sure that 'b' is in
206 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
207 which brings appropriate tyvars into scope. This happens for both
208 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
209 complained if 'b' is mentioned in <rhs>.
213 %************************************************************************
215 \subsection{Extracting instance decls}
217 %************************************************************************
219 Gather up the instance declarations from their various sources
222 tcInstDecls1 -- Deal with both source-code and imported instance decls
223 :: [LTyClDecl Name] -- For deriving stuff
224 -> [LInstDecl Name] -- Source code instance decls
225 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
226 -> TcM (TcGblEnv, -- The full inst env
227 [InstInfo Name], -- Source-code instance decls to process;
228 -- contains all dfuns for this module
229 HsValBinds Name) -- Supporting bindings for derived instances
231 tcInstDecls1 tycl_decls inst_decls deriv_decls
233 do { -- Stop if addInstInfos etc discovers any errors
234 -- (they recover, so that we get more than one error each
237 -- (1) Do class and family instance declarations
238 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
239 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
240 ; idx_tycons <- mapAndRecoverM tcIdxTyInstDeclTL idxty_decls
243 at_tycons_s) = unzip local_info_tycons
244 ; at_idx_tycons = concat at_tycons_s ++ idx_tycons
245 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
246 ; implicit_things = concatMap implicitTyThings at_idx_tycons
247 ; aux_binds = mkAuxBinds at_idx_tycons
250 -- (2) Add the tycons of indexed types and their implicit
251 -- tythings to the global environment
252 ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {
254 -- (3) Instances from generic class declarations
255 ; generic_inst_info <- getGenericInstances clas_decls
257 -- Next, construct the instance environment so far, consisting
259 -- a) local instance decls
260 -- b) generic instances
261 -- c) local family instance decls
262 ; addInsts local_info $
263 addInsts generic_inst_info $
264 addFamInsts at_idx_tycons $ do {
266 -- (4) Compute instances from "deriving" clauses;
267 -- This stuff computes a context for the derived instance
268 -- decl, so it needs to know about all the instances possible
269 -- NB: class instance declarations can contain derivings as
270 -- part of associated data type declarations
271 failIfErrsM -- If the addInsts stuff gave any errors, don't
272 -- try the deriving stuff, becuase that may give
274 ; (deriv_inst_info, deriv_binds, deriv_dus)
275 <- tcDeriving tycl_decls inst_decls deriv_decls
276 ; gbl_env <- addInsts deriv_inst_info getGblEnv
277 ; return ( addTcgDUs gbl_env deriv_dus,
278 generic_inst_info ++ deriv_inst_info ++ local_info,
279 aux_binds `plusHsValBinds` deriv_binds)
282 -- Make sure that toplevel type instance are not for associated types.
283 -- !!!TODO: Need to perform this check for the TyThing of type functions,
285 tcIdxTyInstDeclTL ldecl@(L loc decl) =
286 do { tything <- tcFamInstDecl ldecl
288 when (isAssocFamily tything) $
289 addErr $ assocInClassErr (tcdName decl)
292 isAssocFamily (ATyCon tycon) =
293 case tyConFamInst_maybe tycon of
294 Nothing -> panic "isAssocFamily: no family?!?"
295 Just (fam, _) -> isTyConAssoc fam
296 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
298 assocInClassErr :: Name -> SDoc
299 assocInClassErr name =
300 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
301 ptext (sLit "must be inside a class instance")
303 addInsts :: [InstInfo Name] -> TcM a -> TcM a
304 addInsts infos thing_inside
305 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
307 addFamInsts :: [TyThing] -> TcM a -> TcM a
308 addFamInsts tycons thing_inside
309 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
311 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
312 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
317 tcLocalInstDecl1 :: LInstDecl Name
318 -> TcM (InstInfo Name, [TyThing])
319 -- A source-file instance declaration
320 -- Type-check all the stuff before the "where"
322 -- We check for respectable instance type, and context
323 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
325 addErrCtxt (instDeclCtxt1 poly_ty) $
327 do { is_boot <- tcIsHsBoot
328 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
331 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
333 -- Now, check the validity of the instance.
334 ; (clas, inst_tys) <- checkValidInstHead tau
335 ; checkValidInstance tyvars theta clas inst_tys
337 -- Next, process any associated types.
338 ; idx_tycons <- recoverM (return []) $
339 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
340 ; checkValidAndMissingATs clas (tyvars, inst_tys)
342 ; return idx_tycons }
344 -- Finally, construct the Core representation of the instance.
345 -- (This no longer includes the associated types.)
346 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
347 -- Dfun location is that of instance *header*
348 ; overlap_flag <- getOverlapFlag
349 ; let (eq_theta,dict_theta) = partition isEqPred theta
350 theta' = eq_theta ++ dict_theta
351 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
352 ispec = mkLocalInstance dfun overlap_flag
354 ; return (InstInfo { iSpec = ispec,
355 iBinds = VanillaInst binds uprags False },
359 -- We pass in the source form and the type checked form of the ATs. We
360 -- really need the source form only to be able to produce more informative
362 checkValidAndMissingATs :: Class
363 -> ([TyVar], [TcType]) -- instance types
364 -> [(LTyClDecl Name, -- source form of AT
365 TyThing)] -- Core form of AT
367 checkValidAndMissingATs clas inst_tys ats
368 = do { -- Issue a warning for each class AT that is not defined in this
370 ; let class_ats = map tyConName (classATs clas)
371 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
372 omitted = filterOut (`elemNameSet` defined_ats) class_ats
373 ; warn <- doptM Opt_WarnMissingMethods
374 ; mapM_ (warnTc warn . omittedATWarn) omitted
376 -- Ensure that all AT indexes that correspond to class parameters
377 -- coincide with the types in the instance head. All remaining
378 -- AT arguments must be variables. Also raise an error for any
379 -- type instances that are not associated with this class.
380 ; mapM_ (checkIndexes clas inst_tys) ats
383 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
384 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
385 = checkIndexes' clas inst_tys hsAT
387 snd . fromJust . tyConFamInst_maybe $ tycon)
388 checkIndexes _ _ _ = panic "checkIndexes"
390 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
391 = let atName = tcdName . unLoc $ hsAT
393 setSrcSpan (getLoc hsAT) $
394 addErrCtxt (atInstCtxt atName) $
395 case find ((atName ==) . tyConName) (classATs clas) of
396 Nothing -> addErrTc $ badATErr clas atName -- not in this class
398 case assocTyConArgPoss_maybe atycon of
399 Nothing -> panic "checkIndexes': AT has no args poss?!?"
402 -- The following is tricky! We need to deal with three
403 -- complications: (1) The AT possibly only uses a subset of
404 -- the class parameters as indexes and those it uses may be in
405 -- a different order; (2) the AT may have extra arguments,
406 -- which must be type variables; and (3) variables in AT and
407 -- instance head will be different `Name's even if their
408 -- source lexemes are identical.
410 -- e.g. class C a b c where
411 -- data D b a :: * -> * -- NB (1) b a, omits c
412 -- instance C [x] Bool Char where
413 -- data D Bool [x] v = MkD x [v] -- NB (2) v
414 -- -- NB (3) the x in 'instance C...' have differnt
415 -- -- Names to x's in 'data D...'
417 -- Re (1), `poss' contains a permutation vector to extract the
418 -- class parameters in the right order.
420 -- Re (2), we wrap the (permuted) class parameters in a Maybe
421 -- type and use Nothing for any extra AT arguments. (First
422 -- equation of `checkIndex' below.)
424 -- Re (3), we replace any type variable in the AT parameters
425 -- that has the same source lexeme as some variable in the
426 -- instance types with the instance type variable sharing its
429 let relevantInstTys = map (instTys !!) poss
430 instArgs = map Just relevantInstTys ++
431 repeat Nothing -- extra arguments
432 renaming = substSameTyVar atTvs instTvs
434 zipWithM_ checkIndex (substTys renaming atTys) instArgs
436 checkIndex ty Nothing
437 | isTyVarTy ty = return ()
438 | otherwise = addErrTc $ mustBeVarArgErr ty
439 checkIndex ty (Just instTy)
440 | ty `tcEqType` instTy = return ()
441 | otherwise = addErrTc $ wrongATArgErr ty instTy
443 listToNameSet = addListToNameSet emptyNameSet
445 substSameTyVar [] _ = emptyTvSubst
446 substSameTyVar (tv:tvs) replacingTvs =
447 let replacement = case find (tv `sameLexeme`) replacingTvs of
448 Nothing -> mkTyVarTy tv
449 Just rtv -> mkTyVarTy rtv
451 tv1 `sameLexeme` tv2 =
452 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
454 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
458 %************************************************************************
460 Type-checking instance declarations, pass 2
462 %************************************************************************
465 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
466 -> TcM (LHsBinds Id, TcLclEnv)
467 -- (a) From each class declaration,
468 -- generate any default-method bindings
469 -- (b) From each instance decl
470 -- generate the dfun binding
472 tcInstDecls2 tycl_decls inst_decls
473 = do { -- (a) Default methods from class decls
474 let class_decls = filter (isClassDecl . unLoc) tycl_decls
475 ; (dm_ids_s, dm_binds_s) <- mapAndUnzipM tcClassDecl2 class_decls
477 ; tcExtendIdEnv (concat dm_ids_s) $ do
479 -- (b) instance declarations
480 { inst_binds_s <- mapM tcInstDecl2 inst_decls
483 ; let binds = unionManyBags dm_binds_s `unionBags`
484 unionManyBags inst_binds_s
485 ; tcl_env <- getLclEnv -- Default method Ids in here
486 ; return (binds, tcl_env) } }
488 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
489 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
490 = recoverM (return emptyLHsBinds) $
492 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
493 tc_inst_decl2 dfun_id ibinds
495 dfun_id = instanceDFunId ispec
496 loc = getSrcSpan dfun_id
501 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
502 -- Returns a binding for the dfun
504 ------------------------
505 -- Derived newtype instances; surprisingly tricky!
507 -- class Show a => Foo a b where ...
508 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
510 -- The newtype gives an FC axiom looking like
511 -- axiom CoN a :: N a ~ Tree [a]
512 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
514 -- So all need is to generate a binding looking like:
515 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
516 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
517 -- case df `cast` (Foo Int (sym (CoN a))) of
518 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
520 -- If there are no superclasses, matters are simpler, because we don't need the case
521 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
523 tc_inst_decl2 dfun_id (NewTypeDerived coi)
524 = do { let rigid_info = InstSkol
525 origin = SigOrigin rigid_info
526 inst_ty = idType dfun_id
527 inst_tvs = fst (tcSplitForAllTys inst_ty)
528 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
529 -- inst_head_ty is a PredType
531 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
532 (class_tyvars, sc_theta, _, _) = classBigSig cls
533 cls_tycon = classTyCon cls
534 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
535 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
539 IdCo -> (last_ty, idHsWrapper)
540 ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
542 co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
543 -- NB: the free variable of coi are bound by the
544 -- universally quantified variables of the dfun_id
545 -- This is weird, and maybe we should make NewTypeDerived
546 -- carry a type-variable list too; but it works fine
548 -----------------------
550 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
551 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
552 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
553 -- where rep_ty is the (eta-reduced) type rep of T
554 -- So we just replace T with CoT, and insert a 'sym'
555 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
557 mk_full_coercion co = mkTyConApp cls_tycon
558 (initial_cls_inst_tys ++ [mkSymCoercion co])
559 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
561 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
562 -- In our example, rep_pred is (Foo Int (Tree [a]))
564 ; sc_loc <- getInstLoc InstScOrigin
565 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
566 ; inst_loc <- getInstLoc origin
567 ; dfun_dicts <- newDictBndrs inst_loc theta
568 ; rep_dict <- newDictBndr inst_loc rep_pred
569 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
571 -- Figure out bindings for the superclass context from dfun_dicts
572 -- Don't include this_dict in the 'givens', else
573 -- sc_dicts get bound by just selecting from this_dict!!
574 ; sc_binds <- addErrCtxt superClassCtxt $
575 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
578 -- It's possible that the superclass stuff might unified something
579 -- in the envt with one of the clas_tyvars
580 ; checkSigTyVars inst_tvs'
582 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
584 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
585 ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
587 ; return (unitBag $ noLoc $
588 AbsBinds inst_tvs' (map instToVar dfun_dicts)
589 [(inst_tvs', dfun_id, instToId this_dict, [])]
590 (dict_bind `consBag` sc_binds)) }
592 -----------------------
593 -- (make_body C tys scs coreced_rep_dict)
595 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
596 -- But if there are no superclasses, it returns just coerced_rep_dict
597 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
599 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
600 | null sc_dicts -- Case (a)
601 = return coerced_rep_dict
602 | otherwise -- Case (b)
603 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
604 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
605 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
606 pat_dicts = dummy_sc_dict_ids,
607 pat_binds = emptyLHsBinds,
608 pat_args = PrefixCon (map nlVarPat op_ids),
610 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
611 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
612 map HsVar (sc_dict_ids ++ op_ids)
614 -- Warning: this HsCase scrutinises a value with a PredTy, which is
615 -- never otherwise seen in Haskell source code. It'd be
616 -- nicer to generate Core directly!
617 ; return (HsCase (noLoc coerced_rep_dict) $
618 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
620 sc_dict_ids = map instToId sc_dicts
621 pat_ty = mkTyConApp cls_tycon cls_inst_tys
622 cls_data_con = head (tyConDataCons cls_tycon)
623 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
624 op_tys = dropList sc_dict_ids cls_arg_tys
626 ------------------------
627 -- Ordinary instances
629 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
630 = do { let rigid_info = InstSkol
631 inst_ty = idType dfun_id
632 loc = getSrcSpan dfun_id
634 -- Instantiate the instance decl with skolem constants
635 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
636 -- These inst_tyvars' scope over the 'where' part
637 -- Those tyvars are inside the dfun_id's type, which is a bit
638 -- bizarre, but OK so long as you realise it!
640 (clas, inst_tys') = tcSplitDFunHead inst_head'
641 (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
643 -- Instantiate the super-class context with inst_tys
644 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
645 origin = SigOrigin rigid_info
647 -- Create dictionary Ids from the specified instance contexts.
648 ; inst_loc <- getInstLoc origin
649 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
650 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
651 -- Default-method Ids may be mentioned in synthesised RHSs,
652 -- but they'll already be in the environment.
655 -- Cook up a binding for "this = df d1 .. dn",
656 -- to use in each method binding
657 -- Need to clone the dict in case it is floated out, and
658 -- then clashes with its friends
659 ; cloned_this <- cloneDict this_dict
660 ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
661 L loc $ wrapId app_wrapper dfun_id
662 app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
663 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
665 | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
666 | otherwise = (cloned_this, unitBag cloned_this_bind)
668 -- Deal with 'SPECIALISE instance' pragmas
669 -- See Note [SPECIALISE instance pragmas]
670 ; let spec_inst_sigs = filter isSpecInstLSig uprags
671 -- The filter removes the pragmas for methods
672 ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
674 -- Typecheck the methods
675 ; let prag_fn = mkPragFun uprags
676 tc_meth = tcInstanceMethod loc standalone_deriv
680 prag_fn spec_inst_prags monobinds
682 ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
683 mapAndUnzipM tc_meth op_items
685 -- Figure out bindings for the superclass context
686 ; sc_loc <- getInstLoc InstScOrigin
687 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
688 ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
689 ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
691 -- It's possible that the superclass stuff might unified
692 -- something in the envt with one of the inst_tyvars'
693 ; checkSigTyVars inst_tyvars'
695 -- Create the result bindings
696 ; let dict_constr = classDataCon clas
697 this_dict_id = instToId this_dict
698 dict_bind = mkVarBind this_dict_id dict_rhs
699 dict_rhs = foldl mk_app inst_constr (sc_ids ++ meth_ids)
700 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
701 (dataConWrapId dict_constr)
702 -- We don't produce a binding for the dict_constr; instead we
703 -- rely on the simplifier to unfold this saturated application
704 -- We do this rather than generate an HsCon directly, because
705 -- it means that the special cases (e.g. dictionary with only one
706 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
707 -- than needing to be repeated here.
709 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
710 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
711 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
713 dfun_id_w_fun = dfun_id
714 `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
715 `setInlinePragma` dfunInlinePragma
717 main_bind = noLoc $ AbsBinds
720 [(inst_tyvars', dfun_id_w_fun, this_dict_id, spec_inst_prags)]
723 ; showLIE (text "instance")
724 ; return (unitBag main_bind `unionBags`
725 listToBag meth_binds `unionBags`
726 listToBag sc_binds) }
729 ------------------------------
730 tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
731 -> (Inst, LHsBinds Id)
732 -> (Id, Inst) -> TcM (Id, LHsBind Id)
733 -- Build a top level decl like
734 -- sc_op = /\a \d. let this = ... in
737 -- The "this" part is just-in-case (discarded if not used)
738 -- See Note [Recursive superclasses]
739 tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
741 = addErrCtxt superClassCtxt $
742 do { sc_binds <- tcSimplifySuperClasses inst_loc
743 this_dict dicts [sc_dict]
744 -- Don't include this_dict in the 'givens', else
745 -- sc_dicts get bound by just selecting from this_dict!!
748 ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
749 (mkPredTy (dictPred sc_dict))
750 sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
752 sc_op_id = mkLocalId sc_op_name sc_op_ty
753 sc_id = instToVar sc_dict
754 sc_op_bind = AbsBinds tyvars
755 (map instToVar dicts)
756 [(tyvars, sc_op_id, sc_id, [])]
757 (this_bind `unionBags` sc_binds)
759 ; return (sc_op_id, noLoc sc_op_bind) }
762 Note [Recursive superclasses]
763 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
764 See Trac #1470 for why we would *like* to add "this_dict" to the
765 available instances here. But we can't do so because then the superclases
766 get satisfied by selection from this_dict, and that leads to an immediate
767 loop. What we need is to add this_dict to Avails without adding its
768 superclasses, and we currently have no way to do that.
770 Note [SPECIALISE instance pragmas]
771 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
774 instance (Ix a, Ix b) => Ix (a,b) where
775 {-# SPECIALISE instance Ix (Int,Int) #-}
778 We do *not* want to make a specialised version of the dictionary
779 function. Rather, we want specialised versions of each method.
780 Thus we should generate something like this:
782 $dfIx :: (Ix a, Ix x) => Ix (a,b)
783 {- DFUN [$crange, ...] -}
784 $dfIx da db = Ix ($crange da db) (...other methods...)
786 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
787 {- DFUN [$crangePair, ...] -}
788 $dfIxPair = Ix ($crangePair da db) (...other methods...)
790 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
791 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
792 $crange da db = <blah>
794 {-# RULE range ($dfIx da db) = $crange da db #-}
798 * The RULE is unaffected by the specialisation. We don't want to
799 specialise $dfIx, because then it would need a specialised RULE
800 which is a pain. The single RULE works fine at all specialisations.
801 See Note [How instance declarations are translated] above
803 * Instead, we want to specialise the *method*, $crange
805 In practice, rather than faking up a SPECIALISE pragama for each
806 method (which is painful, since we'd have to figure out its
807 specialised type), we call tcSpecPrag *as if* were going to specialise
808 $dfIx -- you can see that in the call to tcSpecInst. That generates a
809 SpecPrag which, as it turns out, can be used unchanged for each method.
810 The "it turns out" bit is delicate, but it works fine!
813 tcSpecInst :: Id -> Sig Name -> TcM SpecPrag
814 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
815 = addErrCtxt (spec_ctxt prag) $
816 do { let name = idName dfun_id
817 ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
818 ; let spec_ty = mkSigmaTy tyvars theta tau
819 ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
820 ; return (SpecPrag co_fn defaultInlinePragma) }
822 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
824 tcSpecInst _ _ = panic "tcSpecInst"
827 %************************************************************************
829 Type-checking an instance method
831 %************************************************************************
834 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
835 - Remembering to use fresh Name (the instance method Name) as the binder
836 - Bring the instance method Ids into scope, for the benefit of tcInstSig
837 - Use sig_fn mapping instance method Name -> instance tyvars
839 - Use tcValBinds to do the checking
842 tcInstanceMethod :: SrcSpan -> Bool -> Class -> [TcTyVar] -> [Inst]
844 -> (Inst, LHsBinds Id) -- "This" and its binding
845 -> TcPragFun -- Local prags
846 -> [LSpecPrag] -- Arising from 'SPECLALISE instance'
849 -> TcM (Id, LHsBind Id)
850 -- The returned inst_meth_ids all have types starting
851 -- forall tvs. theta => ...
853 tcInstanceMethod loc standalone_deriv clas tyvars dfun_dicts inst_tys
854 (this_dict, this_dict_bind)
855 prag_fn spec_inst_prags binds_in (sel_id, dm_info)
856 = do { uniq <- newUnique
857 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
858 ; local_meth_name <- newLocalName sel_name
859 -- Base the local_meth_name on the selector name, becuase
860 -- type errors from tcInstanceMethodBody come from here
862 ; let local_meth_ty = instantiateMethod clas sel_id inst_tys
863 meth_ty = mkSigmaTy tyvars (map dictPred dfun_dicts) local_meth_ty
864 meth_id = mkLocalId meth_name meth_ty
865 local_meth_id = mkLocalId local_meth_name local_meth_ty
869 = add_meth_ctxt rn_bind $
870 do { (meth_id1, spec_prags) <- tcPrags NonRecursive False True
871 meth_id (prag_fn sel_name)
872 ; tcInstanceMethodBody (instLoc this_dict)
874 ([this_dict], this_dict_bind)
875 meth_id1 local_meth_id
877 (spec_inst_prags ++ spec_prags)
881 tc_default :: DefMeth -> TcM (Id, LHsBind Id)
882 -- The user didn't supply a method binding, so we have to make
883 -- up a default binding, in a way depending on the default-method info
885 tc_default NoDefMeth -- No default method at all
886 = do { warnMissingMethod sel_id
887 ; return (meth_id, mkVarBind meth_id $
888 mkLHsWrap lam_wrapper error_rhs) }
890 tc_default GenDefMeth -- Derivable type classes stuff
891 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
892 ; tc_body meth_bind }
894 tc_default DefMeth -- An polymorphic default method
895 = do { -- Build the typechecked version directly,
896 -- without calling typecheck_method;
897 -- see Note [Default methods in instances]
898 -- Generate /\as.\ds. let this = df as ds
899 -- in $dm inst_tys this
900 -- The 'let' is necessary only because HsSyn doesn't allow
901 -- you to apply a function to a dictionary *expression*.
902 dm_name <- lookupGlobalOccRn (mkDefMethRdrName sel_name)
903 -- Might not be imported, but will be an OrigName
904 ; dm_id <- tcLookupId dm_name
905 ; inline_id <- tcLookupId inlineIdName
906 ; let dm_inline_prag = idInlinePragma dm_id
907 dm_app = HsWrap (WpApp (instToId this_dict) <.> mkWpTyApps inst_tys) $
909 rhs | isInlinePragma dm_inline_prag -- See Note [INLINE and default methods]
910 = HsApp (L loc (HsWrap (WpTyApp local_meth_ty) (HsVar inline_id)))
914 meth_bind = L loc $ VarBind { var_id = local_meth_id
915 , var_rhs = L loc rhs
916 , var_inline = False }
917 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
918 -- Copy the inline pragma (if any) from the default
919 -- method to this version. Note [INLINE and default methods]
921 bind = AbsBinds { abs_tvs = tyvars, abs_dicts = dfun_lam_vars
922 , abs_exports = [( tyvars, meth_id1
923 , local_meth_id, spec_inst_prags)]
924 , abs_binds = this_dict_bind `unionBags` unitBag meth_bind }
925 -- Default methods in an instance declaration can't have their own
926 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
927 -- currently they are rejected with
928 -- "INLINE pragma lacks an accompanying binding"
930 ; return (meth_id1, L loc bind) }
932 ; case findMethodBind sel_name local_meth_name binds_in of
933 Just user_bind -> tc_body user_bind -- User-supplied method binding
934 Nothing -> tc_default dm_info -- None supplied
937 sel_name = idName sel_id
939 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
940 -- But there are no scoped type variables from local_method_id
941 -- Only the ones from the instance decl itself, which are already
942 -- in scope. Example:
943 -- class C a where { op :: forall b. Eq b => ... }
944 -- instance C [c] where { op = <rhs> }
945 -- In <rhs>, 'c' is scope but 'b' is not!
947 error_rhs = L loc $ HsApp error_fun error_msg
948 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
949 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
950 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
951 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
953 dfun_lam_vars = map instToVar dfun_dicts
954 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_lam_vars
956 -- For instance decls that come from standalone deriving clauses
957 -- we want to print out the full source code if there's an error
958 -- because otherwise the user won't see the code at all
959 add_meth_ctxt rn_bind thing
960 | standalone_deriv = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing
963 wrapId :: HsWrapper -> id -> HsExpr id
964 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
966 derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc
967 derivBindCtxt clas tys bind
968 = vcat [ ptext (sLit "When typechecking a standalone-derived method for")
969 <+> quotes (pprClassPred clas tys) <> colon
970 , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
972 warnMissingMethod :: Id -> TcM ()
973 warnMissingMethod sel_id
974 = do { warn <- doptM Opt_WarnMissingMethods
975 ; warnTc (warn -- Warn only if -fwarn-missing-methods
976 && not (startsWithUnderscore (getOccName sel_id)))
977 -- Don't warn about _foo methods
978 (ptext (sLit "No explicit method nor default method for")
979 <+> quotes (ppr sel_id)) }
982 Note [Export helper functions]
983 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
984 We arrange to export the "helper functions" of an instance declaration,
985 so that they are not subject to preInlineUnconditionally, even if their
986 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
987 the dict fun as Ids, not as CoreExprs, so we can't substitute a
988 non-variable for them.
990 We could change this by making DFunUnfoldings have CoreExprs, but it
991 seems a bit simpler this way.
993 Note [Default methods in instances]
994 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1001 instance Baz Int Int
1003 From the class decl we get
1005 $dmfoo :: forall v x. Baz v x => x -> x
1008 Notice that the type is ambiguous. That's fine, though. The instance decl generates
1010 $dBazIntInt = MkBaz fooIntInt
1011 fooIntInt = $dmfoo Int Int $dBazIntInt
1013 BUT this does mean we must generate the dictionary translation of
1014 fooIntInt directly, rather than generating source-code and
1015 type-checking it. That was the bug in Trac #1061. In any case it's
1016 less work to generate the translated version!
1018 Note [INLINE and default methods]
1019 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1020 We *copy* any INLINE pragma from the default method to the instance.
1023 op1, op2 :: Bool -> a -> a
1026 op1 b x = op2 (not b) x
1028 instance Foo Int where
1033 {-# INLINE $dmop1 #-}
1034 $dmop1 d b x = op2 d (not b) x
1036 $fFooInt = MkD $cop1 $cop2
1038 {-# INLINE $cop1 #-}
1039 $cop1 = inline $dmop1 $fFooInt
1044 a) We copy $dmop1's inline pragma to $cop1. Otherwise
1045 we'll just inline the former in the latter and stop, which
1046 isn't what the user expected
1048 b) We use the magic 'inline' Id to ensure that $dmop1 really is
1049 inlined in $cop1, even though the latter itself has an INLINE pragma
1050 That is important to allow the mutual recursion between $fooInt and
1053 This is all regrettably delicate.
1056 %************************************************************************
1058 \subsection{Error messages}
1060 %************************************************************************
1063 instDeclCtxt1 :: LHsType Name -> SDoc
1064 instDeclCtxt1 hs_inst_ty
1065 = inst_decl_ctxt (case unLoc hs_inst_ty of
1066 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1067 HsPredTy pred -> ppr pred
1068 _ -> ppr hs_inst_ty) -- Don't expect this
1069 instDeclCtxt2 :: Type -> SDoc
1070 instDeclCtxt2 dfun_ty
1071 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1073 (_,cls,tys) = tcSplitDFunTy dfun_ty
1075 inst_decl_ctxt :: SDoc -> SDoc
1076 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1078 superClassCtxt :: SDoc
1079 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
1081 atInstCtxt :: Name -> SDoc
1082 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1085 mustBeVarArgErr :: Type -> SDoc
1086 mustBeVarArgErr ty =
1087 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1088 ptext (sLit "must be variables")
1089 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1092 wrongATArgErr :: Type -> Type -> SDoc
1093 wrongATArgErr ty instTy =
1094 sep [ ptext (sLit "Type indexes must match class instance head")
1095 , ptext (sLit "Found") <+> quotes (ppr ty)
1096 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)