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. $dmop2 [a] (df_i a d)
114 -- The dictionary function itself
115 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
116 df_i :: forall a. C a -> C [a]
117 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
118 -- But see Note [Default methods in instances]
119 -- We can't apply the type checker to the default-method call
121 -- Use a RULE to short-circuit applications of the class ops
122 {-# RULE "op1@C[a]" forall a, d:C a.
123 op1 [a] (df_i d) = op1_i a d #-}
125 Note [Instances and loop breakers]
126 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
127 * Note that df_i may be mutually recursive with both op1_i and op2_i.
128 It's crucial that df_i is not chosen as the loop breaker, even
129 though op1_i has a (user-specified) INLINE pragma.
131 * Instead the idea is to inline df_i into op1_i, which may then select
132 methods from the MkC record, and thereby break the recursion with
133 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
134 the same type, it won't mention df_i, so there won't be recursion in
137 * If op1_i is marked INLINE by the user there's a danger that we won't
138 inline df_i in it, and that in turn means that (since it'll be a
139 loop-breaker because df_i isn't), op1_i will ironically never be
140 inlined. But this is OK: the recursion breaking happens by way of
141 a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
142 unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
144 Note [ClassOp/DFun selection]
145 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
146 One thing we see a lot is stuff like
148 where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
149 'op2' and 'df' to get
150 case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
151 MkD _ op2 _ _ _ -> op2
152 And that will reduce to ($cop2 d1 d2) which is what we wanted.
154 But it's tricky to make this work in practice, because it requires us to
155 inline both 'op2' and 'df'. But neither is keen to inline without having
156 seen the other's result; and it's very easy to get code bloat (from the
157 big intermediate) if you inline a bit too much.
159 Instead we use a cunning trick.
160 * We arrange that 'df' and 'op2' NEVER inline.
162 * We arrange that 'df' is ALWAYS defined in the sylised form
163 df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
165 * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
166 that lists its methods.
168 * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
169 a suitable constructor application -- inlining df "on the fly" as it
172 * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece
173 iff its argument satisfies exprIsConApp_maybe. This is done in
176 * We make 'df' CONLIKE, so that shared uses stil match; eg
178 in ...(op2 d)...(op1 d)...
180 Note [Single-method classes]
181 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
182 If the class has just one method (or, more accurately, just one elemen
183 of {superclasses + methods}), then we want a different strategy.
185 class C a where op :: a -> a
186 instance C a => C [a] where op = <blah>
188 We translate the class decl into a newtype, which just gives
191 axiom Co:C a :: C a ~ (a->a)
193 op :: forall a. C a -> (a -> a)
194 op a d = d |> (Co:C a)
196 df :: forall a. C a => C [a]
198 df = $cop_list |> (forall a. C a -> (sym (Co:C a))
200 $cop_list :: forall a. C a => a -> a
203 So the ClassOp is just a cast; and so is the dictionary function.
204 (The latter doesn't even have any lambdas.) We can inline both freely.
205 No need for fancy BuiltIn rules. Indeed the BuiltinRule stuff does
206 not work well for newtypes because it uses exprIsConApp_maybe.
208 The INLINE on df is vital, else $cop_list occurs just once and is inlined,
209 which is a disaster if $cop_list *itself* has an INLINE pragma.
212 Note [Subtle interaction of recursion and overlap]
213 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
215 class C a where { op1,op2 :: a -> a }
216 instance C a => C [a] where
217 op1 x = op2 x ++ op2 x
219 intance C [Int] where
222 When type-checking the C [a] instance, we need a C [a] dictionary (for
223 the call of op2). If we look up in the instance environment, we find
224 an overlap. And in *general* the right thing is to complain (see Note
225 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
226 complain, because we just want to delegate to the op2 of this same
229 Why is this justified? Because we generate a (C [a]) constraint in
230 a context in which 'a' cannot be instantiated to anything that matches
231 other overlapping instances, or else we would not be excecuting this
232 version of op1 in the first place.
234 It might even be a bit disguised:
236 nullFail :: C [a] => [a] -> [a]
237 nullFail x = op2 x ++ op2 x
239 instance C a => C [a] where
242 Precisely this is used in package 'regex-base', module Context.hs.
243 See the overlapping instances for RegexContext, and the fact that they
244 call 'nullFail' just like the example above. The DoCon package also
245 does the same thing; it shows up in module Fraction.hs
247 Conclusion: when typechecking the methods in a C [a] instance, we want
248 to have C [a] available. That is why we have the strange local
249 definition for 'this' in the definition of op1_i in the example above.
250 We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck
251 we supply 'this' as a given dictionary. Only needed, though, if there
252 are some type variables involved; otherwise there can be no overlap and
255 Note [Tricky type variable scoping]
256 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
259 op1, op2 :: Ix b => a -> b -> b
262 instance C a => C [a]
263 {-# INLINE [2] op1 #-}
266 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
267 in scope in <rhs>. In particular, we must make sure that 'b' is in
268 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
269 which brings appropriate tyvars into scope. This happens for both
270 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
271 complained if 'b' is mentioned in <rhs>.
275 %************************************************************************
277 \subsection{Extracting instance decls}
279 %************************************************************************
281 Gather up the instance declarations from their various sources
284 tcInstDecls1 -- Deal with both source-code and imported instance decls
285 :: [LTyClDecl Name] -- For deriving stuff
286 -> [LInstDecl Name] -- Source code instance decls
287 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
288 -> TcM (TcGblEnv, -- The full inst env
289 [InstInfo Name], -- Source-code instance decls to process;
290 -- contains all dfuns for this module
291 HsValBinds Name) -- Supporting bindings for derived instances
293 tcInstDecls1 tycl_decls inst_decls deriv_decls
295 do { -- Stop if addInstInfos etc discovers any errors
296 -- (they recover, so that we get more than one error each
299 -- (1) Do class and family instance declarations
300 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
301 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
302 ; idx_tycons <- mapAndRecoverM tcIdxTyInstDeclTL idxty_decls
305 at_tycons_s) = unzip local_info_tycons
306 ; at_idx_tycons = concat at_tycons_s ++ idx_tycons
307 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
308 ; implicit_things = concatMap implicitTyThings at_idx_tycons
309 ; aux_binds = mkAuxBinds at_idx_tycons
312 -- (2) Add the tycons of indexed types and their implicit
313 -- tythings to the global environment
314 ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {
316 -- (3) Instances from generic class declarations
317 ; generic_inst_info <- getGenericInstances clas_decls
319 -- Next, construct the instance environment so far, consisting
321 -- a) local instance decls
322 -- b) generic instances
323 -- c) local family instance decls
324 ; addInsts local_info $
325 addInsts generic_inst_info $
326 addFamInsts at_idx_tycons $ do {
328 -- (4) Compute instances from "deriving" clauses;
329 -- This stuff computes a context for the derived instance
330 -- decl, so it needs to know about all the instances possible
331 -- NB: class instance declarations can contain derivings as
332 -- part of associated data type declarations
333 failIfErrsM -- If the addInsts stuff gave any errors, don't
334 -- try the deriving stuff, becuase that may give
336 ; (deriv_inst_info, deriv_binds, deriv_dus)
337 <- tcDeriving tycl_decls inst_decls deriv_decls
338 ; gbl_env <- addInsts deriv_inst_info getGblEnv
339 ; return ( addTcgDUs gbl_env deriv_dus,
340 generic_inst_info ++ deriv_inst_info ++ local_info,
341 aux_binds `plusHsValBinds` deriv_binds)
344 -- Make sure that toplevel type instance are not for associated types.
345 -- !!!TODO: Need to perform this check for the TyThing of type functions,
347 tcIdxTyInstDeclTL ldecl@(L loc decl) =
348 do { tything <- tcFamInstDecl ldecl
350 when (isAssocFamily tything) $
351 addErr $ assocInClassErr (tcdName decl)
354 isAssocFamily (ATyCon tycon) =
355 case tyConFamInst_maybe tycon of
356 Nothing -> panic "isAssocFamily: no family?!?"
357 Just (fam, _) -> isTyConAssoc fam
358 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
360 assocInClassErr :: Name -> SDoc
361 assocInClassErr name =
362 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
363 ptext (sLit "must be inside a class instance")
365 addInsts :: [InstInfo Name] -> TcM a -> TcM a
366 addInsts infos thing_inside
367 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
369 addFamInsts :: [TyThing] -> TcM a -> TcM a
370 addFamInsts tycons thing_inside
371 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
373 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
374 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
379 tcLocalInstDecl1 :: LInstDecl Name
380 -> TcM (InstInfo Name, [TyThing])
381 -- A source-file instance declaration
382 -- Type-check all the stuff before the "where"
384 -- We check for respectable instance type, and context
385 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
387 addErrCtxt (instDeclCtxt1 poly_ty) $
389 do { is_boot <- tcIsHsBoot
390 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
393 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
395 -- Now, check the validity of the instance.
396 ; (clas, inst_tys) <- checkValidInstHead tau
397 ; checkValidInstance tyvars theta clas inst_tys
399 -- Next, process any associated types.
400 ; idx_tycons <- recoverM (return []) $
401 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
402 ; checkValidAndMissingATs clas (tyvars, inst_tys)
404 ; return idx_tycons }
406 -- Finally, construct the Core representation of the instance.
407 -- (This no longer includes the associated types.)
408 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
409 -- Dfun location is that of instance *header*
410 ; overlap_flag <- getOverlapFlag
411 ; let (eq_theta,dict_theta) = partition isEqPred theta
412 theta' = eq_theta ++ dict_theta
413 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
414 ispec = mkLocalInstance dfun overlap_flag
416 ; return (InstInfo { iSpec = ispec,
417 iBinds = VanillaInst binds uprags False },
421 -- We pass in the source form and the type checked form of the ATs. We
422 -- really need the source form only to be able to produce more informative
424 checkValidAndMissingATs :: Class
425 -> ([TyVar], [TcType]) -- instance types
426 -> [(LTyClDecl Name, -- source form of AT
427 TyThing)] -- Core form of AT
429 checkValidAndMissingATs clas inst_tys ats
430 = do { -- Issue a warning for each class AT that is not defined in this
432 ; let class_ats = map tyConName (classATs clas)
433 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
434 omitted = filterOut (`elemNameSet` defined_ats) class_ats
435 ; warn <- doptM Opt_WarnMissingMethods
436 ; mapM_ (warnTc warn . omittedATWarn) omitted
438 -- Ensure that all AT indexes that correspond to class parameters
439 -- coincide with the types in the instance head. All remaining
440 -- AT arguments must be variables. Also raise an error for any
441 -- type instances that are not associated with this class.
442 ; mapM_ (checkIndexes clas inst_tys) ats
445 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
446 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
447 = checkIndexes' clas inst_tys hsAT
449 snd . fromJust . tyConFamInst_maybe $ tycon)
450 checkIndexes _ _ _ = panic "checkIndexes"
452 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
453 = let atName = tcdName . unLoc $ hsAT
455 setSrcSpan (getLoc hsAT) $
456 addErrCtxt (atInstCtxt atName) $
457 case find ((atName ==) . tyConName) (classATs clas) of
458 Nothing -> addErrTc $ badATErr clas atName -- not in this class
460 case assocTyConArgPoss_maybe atycon of
461 Nothing -> panic "checkIndexes': AT has no args poss?!?"
464 -- The following is tricky! We need to deal with three
465 -- complications: (1) The AT possibly only uses a subset of
466 -- the class parameters as indexes and those it uses may be in
467 -- a different order; (2) the AT may have extra arguments,
468 -- which must be type variables; and (3) variables in AT and
469 -- instance head will be different `Name's even if their
470 -- source lexemes are identical.
472 -- e.g. class C a b c where
473 -- data D b a :: * -> * -- NB (1) b a, omits c
474 -- instance C [x] Bool Char where
475 -- data D Bool [x] v = MkD x [v] -- NB (2) v
476 -- -- NB (3) the x in 'instance C...' have differnt
477 -- -- Names to x's in 'data D...'
479 -- Re (1), `poss' contains a permutation vector to extract the
480 -- class parameters in the right order.
482 -- Re (2), we wrap the (permuted) class parameters in a Maybe
483 -- type and use Nothing for any extra AT arguments. (First
484 -- equation of `checkIndex' below.)
486 -- Re (3), we replace any type variable in the AT parameters
487 -- that has the same source lexeme as some variable in the
488 -- instance types with the instance type variable sharing its
491 let relevantInstTys = map (instTys !!) poss
492 instArgs = map Just relevantInstTys ++
493 repeat Nothing -- extra arguments
494 renaming = substSameTyVar atTvs instTvs
496 zipWithM_ checkIndex (substTys renaming atTys) instArgs
498 checkIndex ty Nothing
499 | isTyVarTy ty = return ()
500 | otherwise = addErrTc $ mustBeVarArgErr ty
501 checkIndex ty (Just instTy)
502 | ty `tcEqType` instTy = return ()
503 | otherwise = addErrTc $ wrongATArgErr ty instTy
505 listToNameSet = addListToNameSet emptyNameSet
507 substSameTyVar [] _ = emptyTvSubst
508 substSameTyVar (tv:tvs) replacingTvs =
509 let replacement = case find (tv `sameLexeme`) replacingTvs of
510 Nothing -> mkTyVarTy tv
511 Just rtv -> mkTyVarTy rtv
513 tv1 `sameLexeme` tv2 =
514 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
516 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
520 %************************************************************************
522 Type-checking instance declarations, pass 2
524 %************************************************************************
527 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
528 -> TcM (LHsBinds Id, TcLclEnv)
529 -- (a) From each class declaration,
530 -- generate any default-method bindings
531 -- (b) From each instance decl
532 -- generate the dfun binding
534 tcInstDecls2 tycl_decls inst_decls
535 = do { -- (a) Default methods from class decls
536 let class_decls = filter (isClassDecl . unLoc) tycl_decls
537 ; (dm_ids_s, dm_binds_s) <- mapAndUnzipM tcClassDecl2 class_decls
539 ; tcExtendIdEnv (concat dm_ids_s) $ do
541 -- (b) instance declarations
542 { inst_binds_s <- mapM tcInstDecl2 inst_decls
545 ; let binds = unionManyBags dm_binds_s `unionBags`
546 unionManyBags inst_binds_s
547 ; tcl_env <- getLclEnv -- Default method Ids in here
548 ; return (binds, tcl_env) } }
550 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
551 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
552 = recoverM (return emptyLHsBinds) $
554 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
555 tc_inst_decl2 dfun_id ibinds
557 dfun_id = instanceDFunId ispec
558 loc = getSrcSpan dfun_id
563 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
564 -- Returns a binding for the dfun
566 ------------------------
567 -- Derived newtype instances; surprisingly tricky!
569 -- class Show a => Foo a b where ...
570 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
572 -- The newtype gives an FC axiom looking like
573 -- axiom CoN a :: N a ~ Tree [a]
574 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
576 -- So all need is to generate a binding looking like:
577 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
578 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
579 -- case df `cast` (Foo Int (sym (CoN a))) of
580 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
582 -- If there are no superclasses, matters are simpler, because we don't need the case
583 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
585 tc_inst_decl2 dfun_id (NewTypeDerived coi)
586 = do { let rigid_info = InstSkol
587 origin = SigOrigin rigid_info
588 inst_ty = idType dfun_id
589 inst_tvs = fst (tcSplitForAllTys inst_ty)
590 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
591 -- inst_head_ty is a PredType
593 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
594 (class_tyvars, sc_theta, _, _) = classBigSig cls
595 cls_tycon = classTyCon cls
596 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
597 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
601 IdCo -> (last_ty, idHsWrapper)
602 ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
604 co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
605 -- NB: the free variable of coi are bound by the
606 -- universally quantified variables of the dfun_id
607 -- This is weird, and maybe we should make NewTypeDerived
608 -- carry a type-variable list too; but it works fine
610 -----------------------
612 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
613 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
614 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
615 -- where rep_ty is the (eta-reduced) type rep of T
616 -- So we just replace T with CoT, and insert a 'sym'
617 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
619 mk_full_coercion co = mkTyConApp cls_tycon
620 (initial_cls_inst_tys ++ [mkSymCoercion co])
621 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
623 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
624 -- In our example, rep_pred is (Foo Int (Tree [a]))
626 ; sc_loc <- getInstLoc InstScOrigin
627 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
628 ; inst_loc <- getInstLoc origin
629 ; dfun_dicts <- newDictBndrs inst_loc theta
630 ; rep_dict <- newDictBndr inst_loc rep_pred
631 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
633 -- Figure out bindings for the superclass context from dfun_dicts
634 -- Don't include this_dict in the 'givens', else
635 -- sc_dicts get bound by just selecting from this_dict!!
636 ; sc_binds <- addErrCtxt superClassCtxt $
637 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
640 -- It's possible that the superclass stuff might unified something
641 -- in the envt with one of the clas_tyvars
642 ; checkSigTyVars inst_tvs'
644 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
646 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
647 ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
649 ; return (unitBag $ noLoc $
650 AbsBinds inst_tvs' (map instToVar dfun_dicts)
651 [(inst_tvs', dfun_id, instToId this_dict, [])]
652 (dict_bind `consBag` sc_binds)) }
654 -----------------------
655 -- (make_body C tys scs coreced_rep_dict)
657 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
658 -- But if there are no superclasses, it returns just coerced_rep_dict
659 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
661 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
662 | null sc_dicts -- Case (a)
663 = return coerced_rep_dict
664 | otherwise -- Case (b)
665 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
666 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
667 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
668 pat_dicts = dummy_sc_dict_ids,
669 pat_binds = emptyLHsBinds,
670 pat_args = PrefixCon (map nlVarPat op_ids),
672 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
673 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
674 map HsVar (sc_dict_ids ++ op_ids)
676 -- Warning: this HsCase scrutinises a value with a PredTy, which is
677 -- never otherwise seen in Haskell source code. It'd be
678 -- nicer to generate Core directly!
679 ; return (HsCase (noLoc coerced_rep_dict) $
680 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
682 sc_dict_ids = map instToId sc_dicts
683 pat_ty = mkTyConApp cls_tycon cls_inst_tys
684 cls_data_con = head (tyConDataCons cls_tycon)
685 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
686 op_tys = dropList sc_dict_ids cls_arg_tys
688 ------------------------
689 -- Ordinary instances
691 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
692 = do { let rigid_info = InstSkol
693 inst_ty = idType dfun_id
694 loc = getSrcSpan dfun_id
696 -- Instantiate the instance decl with skolem constants
697 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
698 -- These inst_tyvars' scope over the 'where' part
699 -- Those tyvars are inside the dfun_id's type, which is a bit
700 -- bizarre, but OK so long as you realise it!
702 (clas, inst_tys') = tcSplitDFunHead inst_head'
703 (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
705 -- Instantiate the super-class context with inst_tys
706 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
707 origin = SigOrigin rigid_info
709 -- Create dictionary Ids from the specified instance contexts.
710 ; inst_loc <- getInstLoc origin
711 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
712 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
713 -- Default-method Ids may be mentioned in synthesised RHSs,
714 -- but they'll already be in the environment.
717 -- Cook up a binding for "this = df d1 .. dn",
718 -- to use in each method binding
719 -- Need to clone the dict in case it is floated out, and
720 -- then clashes with its friends
721 ; cloned_this <- cloneDict this_dict
722 ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
723 L loc $ wrapId app_wrapper dfun_id
724 app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
725 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
727 | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
728 | otherwise = (cloned_this, unitBag cloned_this_bind)
730 -- Deal with 'SPECIALISE instance' pragmas
731 -- See Note [SPECIALISE instance pragmas]
732 ; let spec_inst_sigs = filter isSpecInstLSig uprags
733 -- The filter removes the pragmas for methods
734 ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
736 -- Typecheck the methods
737 ; let prag_fn = mkPragFun uprags
738 tc_meth = tcInstanceMethod loc standalone_deriv
742 prag_fn spec_inst_prags monobinds
744 ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
745 mapAndUnzipM tc_meth op_items
747 -- Figure out bindings for the superclass context
748 ; sc_loc <- getInstLoc InstScOrigin
749 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
750 ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
751 ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
753 -- It's possible that the superclass stuff might unified
754 -- something in the envt with one of the inst_tyvars'
755 ; checkSigTyVars inst_tyvars'
757 -- Create the result bindings
758 ; let dict_constr = classDataCon clas
759 this_dict_id = instToId this_dict
760 dict_bind = mkVarBind this_dict_id dict_rhs
761 dict_rhs = foldl mk_app inst_constr (sc_ids ++ meth_ids)
762 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
763 (dataConWrapId dict_constr)
764 -- We don't produce a binding for the dict_constr; instead we
765 -- rely on the simplifier to unfold this saturated application
766 -- We do this rather than generate an HsCon directly, because
767 -- it means that the special cases (e.g. dictionary with only one
768 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
769 -- than needing to be repeated here.
771 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
772 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
773 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
775 dfun_id_w_fun | isNewTyCon (classTyCon clas)
776 = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
777 `setInlinePragma` alwaysInlinePragma
779 = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
780 -- See Note [ClassOp/DFun selection]
781 `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
782 `setInlinePragma` dfunInlinePragma
784 main_bind = noLoc $ AbsBinds
787 [(inst_tyvars', dfun_id_w_fun, this_dict_id, spec_inst_prags)]
790 ; showLIE (text "instance")
791 ; return (unitBag main_bind `unionBags`
792 listToBag meth_binds `unionBags`
793 listToBag sc_binds) }
796 ------------------------------
797 tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
798 -> (Inst, LHsBinds Id)
799 -> (Id, Inst) -> TcM (Id, LHsBind Id)
800 -- Build a top level decl like
801 -- sc_op = /\a \d. let this = ... in
804 -- The "this" part is just-in-case (discarded if not used)
805 -- See Note [Recursive superclasses]
806 tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
808 = addErrCtxt superClassCtxt $
809 do { sc_binds <- tcSimplifySuperClasses inst_loc
810 this_dict dicts [sc_dict]
811 -- Don't include this_dict in the 'givens', else
812 -- sc_dicts get bound by just selecting from this_dict!!
815 ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
816 (mkPredTy (dictPred sc_dict))
817 sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
819 sc_op_id = mkLocalId sc_op_name sc_op_ty
820 sc_id = instToVar sc_dict
821 sc_op_bind = AbsBinds tyvars
822 (map instToVar dicts)
823 [(tyvars, sc_op_id, sc_id, [])]
824 (this_bind `unionBags` sc_binds)
826 ; return (sc_op_id, noLoc sc_op_bind) }
829 Note [Recursive superclasses]
830 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
831 See Trac #1470 for why we would *like* to add "this_dict" to the
832 available instances here. But we can't do so because then the superclases
833 get satisfied by selection from this_dict, and that leads to an immediate
834 loop. What we need is to add this_dict to Avails without adding its
835 superclasses, and we currently have no way to do that.
837 Note [SPECIALISE instance pragmas]
838 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
841 instance (Ix a, Ix b) => Ix (a,b) where
842 {-# SPECIALISE instance Ix (Int,Int) #-}
845 We do *not* want to make a specialised version of the dictionary
846 function. Rather, we want specialised versions of each method.
847 Thus we should generate something like this:
849 $dfIx :: (Ix a, Ix x) => Ix (a,b)
850 {- DFUN [$crange, ...] -}
851 $dfIx da db = Ix ($crange da db) (...other methods...)
853 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
854 {- DFUN [$crangePair, ...] -}
855 $dfIxPair = Ix ($crangePair da db) (...other methods...)
857 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
858 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
859 $crange da db = <blah>
861 {-# RULE range ($dfIx da db) = $crange da db #-}
865 * The RULE is unaffected by the specialisation. We don't want to
866 specialise $dfIx, because then it would need a specialised RULE
867 which is a pain. The single RULE works fine at all specialisations.
868 See Note [How instance declarations are translated] above
870 * Instead, we want to specialise the *method*, $crange
872 In practice, rather than faking up a SPECIALISE pragama for each
873 method (which is painful, since we'd have to figure out its
874 specialised type), we call tcSpecPrag *as if* were going to specialise
875 $dfIx -- you can see that in the call to tcSpecInst. That generates a
876 SpecPrag which, as it turns out, can be used unchanged for each method.
877 The "it turns out" bit is delicate, but it works fine!
880 tcSpecInst :: Id -> Sig Name -> TcM SpecPrag
881 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
882 = addErrCtxt (spec_ctxt prag) $
883 do { let name = idName dfun_id
884 ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
885 ; let spec_ty = mkSigmaTy tyvars theta tau
886 ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
887 ; return (SpecPrag co_fn defaultInlinePragma) }
889 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
891 tcSpecInst _ _ = panic "tcSpecInst"
894 %************************************************************************
896 Type-checking an instance method
898 %************************************************************************
901 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
902 - Remembering to use fresh Name (the instance method Name) as the binder
903 - Bring the instance method Ids into scope, for the benefit of tcInstSig
904 - Use sig_fn mapping instance method Name -> instance tyvars
906 - Use tcValBinds to do the checking
909 tcInstanceMethod :: SrcSpan -> Bool -> Class -> [TcTyVar] -> [Inst]
911 -> (Inst, LHsBinds Id) -- "This" and its binding
912 -> TcPragFun -- Local prags
913 -> [LSpecPrag] -- Arising from 'SPECLALISE instance'
916 -> TcM (Id, LHsBind Id)
917 -- The returned inst_meth_ids all have types starting
918 -- forall tvs. theta => ...
920 tcInstanceMethod loc standalone_deriv clas tyvars dfun_dicts inst_tys
921 (this_dict, this_dict_bind)
922 prag_fn spec_inst_prags binds_in (sel_id, dm_info)
923 = do { uniq <- newUnique
924 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
925 ; local_meth_name <- newLocalName sel_name
926 -- Base the local_meth_name on the selector name, becuase
927 -- type errors from tcInstanceMethodBody come from here
929 ; let local_meth_ty = instantiateMethod clas sel_id inst_tys
930 meth_ty = mkSigmaTy tyvars (map dictPred dfun_dicts) local_meth_ty
931 meth_id = mkLocalId meth_name meth_ty
932 local_meth_id = mkLocalId local_meth_name local_meth_ty
936 = add_meth_ctxt rn_bind $
937 do { (meth_id1, spec_prags) <- tcPrags NonRecursive False True
938 meth_id (prag_fn sel_name)
939 ; tcInstanceMethodBody (instLoc this_dict)
941 ([this_dict], this_dict_bind)
942 meth_id1 local_meth_id
944 (spec_inst_prags ++ spec_prags)
948 tc_default :: DefMeth -> TcM (Id, LHsBind Id)
949 -- The user didn't supply a method binding, so we have to make
950 -- up a default binding, in a way depending on the default-method info
952 tc_default NoDefMeth -- No default method at all
953 = do { warnMissingMethod sel_id
954 ; return (meth_id, mkVarBind meth_id $
955 mkLHsWrap lam_wrapper error_rhs) }
957 tc_default GenDefMeth -- Derivable type classes stuff
958 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
959 ; tc_body meth_bind }
961 tc_default DefMeth -- An polymorphic default method
962 = do { -- Build the typechecked version directly,
963 -- without calling typecheck_method;
964 -- see Note [Default methods in instances]
965 -- Generate /\as.\ds. let this = df as ds
966 -- in $dm inst_tys this
967 -- The 'let' is necessary only because HsSyn doesn't allow
968 -- you to apply a function to a dictionary *expression*.
969 dm_name <- lookupGlobalOccRn (mkDefMethRdrName sel_name)
970 -- Might not be imported, but will be an OrigName
971 ; dm_id <- tcLookupId dm_name
972 ; inline_id <- tcLookupId inlineIdName
973 ; let dm_inline_prag = idInlinePragma dm_id
974 dm_app = HsWrap (WpApp (instToId this_dict) <.> mkWpTyApps inst_tys) $
976 rhs | isInlinePragma dm_inline_prag -- See Note [INLINE and default methods]
977 = HsApp (L loc (HsWrap (WpTyApp local_meth_ty) (HsVar inline_id)))
981 meth_bind = L loc $ VarBind { var_id = local_meth_id
982 , var_rhs = L loc rhs
983 , var_inline = False }
984 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
985 -- Copy the inline pragma (if any) from the default
986 -- method to this version. Note [INLINE and default methods]
988 bind = AbsBinds { abs_tvs = tyvars, abs_dicts = dfun_lam_vars
989 , abs_exports = [( tyvars, meth_id1
990 , local_meth_id, spec_inst_prags)]
991 , abs_binds = this_dict_bind `unionBags` unitBag meth_bind }
992 -- Default methods in an instance declaration can't have their own
993 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
994 -- currently they are rejected with
995 -- "INLINE pragma lacks an accompanying binding"
997 ; return (meth_id1, L loc bind) }
999 ; case findMethodBind sel_name local_meth_name binds_in of
1000 Just user_bind -> tc_body user_bind -- User-supplied method binding
1001 Nothing -> tc_default dm_info -- None supplied
1004 sel_name = idName sel_id
1006 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
1007 -- But there are no scoped type variables from local_method_id
1008 -- Only the ones from the instance decl itself, which are already
1009 -- in scope. Example:
1010 -- class C a where { op :: forall b. Eq b => ... }
1011 -- instance C [c] where { op = <rhs> }
1012 -- In <rhs>, 'c' is scope but 'b' is not!
1014 error_rhs = L loc $ HsApp error_fun error_msg
1015 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
1016 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
1017 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
1018 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
1020 dfun_lam_vars = map instToVar dfun_dicts
1021 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_lam_vars
1023 -- For instance decls that come from standalone deriving clauses
1024 -- we want to print out the full source code if there's an error
1025 -- because otherwise the user won't see the code at all
1026 add_meth_ctxt rn_bind thing
1027 | standalone_deriv = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing
1030 wrapId :: HsWrapper -> id -> HsExpr id
1031 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
1033 derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc
1034 derivBindCtxt clas tys bind
1035 = vcat [ ptext (sLit "When typechecking a standalone-derived method for")
1036 <+> quotes (pprClassPred clas tys) <> colon
1037 , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
1039 warnMissingMethod :: Id -> TcM ()
1040 warnMissingMethod sel_id
1041 = do { warn <- doptM Opt_WarnMissingMethods
1042 ; warnTc (warn -- Warn only if -fwarn-missing-methods
1043 && not (startsWithUnderscore (getOccName sel_id)))
1044 -- Don't warn about _foo methods
1045 (ptext (sLit "No explicit method nor default method for")
1046 <+> quotes (ppr sel_id)) }
1049 Note [Export helper functions]
1050 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1051 We arrange to export the "helper functions" of an instance declaration,
1052 so that they are not subject to preInlineUnconditionally, even if their
1053 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1054 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1055 non-variable for them.
1057 We could change this by making DFunUnfoldings have CoreExprs, but it
1058 seems a bit simpler this way.
1060 Note [Default methods in instances]
1061 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1068 instance Baz Int Int
1070 From the class decl we get
1072 $dmfoo :: forall v x. Baz v x => x -> x
1075 Notice that the type is ambiguous. That's fine, though. The instance decl generates
1077 $dBazIntInt = MkBaz fooIntInt
1078 fooIntInt = $dmfoo Int Int $dBazIntInt
1080 BUT this does mean we must generate the dictionary translation of
1081 fooIntInt directly, rather than generating source-code and
1082 type-checking it. That was the bug in Trac #1061. In any case it's
1083 less work to generate the translated version!
1085 Note [INLINE and default methods]
1086 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1087 We *copy* any INLINE pragma from the default method to the instance.
1090 op1, op2 :: Bool -> a -> a
1093 op1 b x = op2 (not b) x
1095 instance Foo Int where
1100 {-# INLINE $dmop1 #-}
1101 $dmop1 d b x = op2 d (not b) x
1103 $fFooInt = MkD $cop1 $cop2
1105 {-# INLINE $cop1 #-}
1106 $cop1 = inline $dmop1 $fFooInt
1111 a) We copy $dmop1's inline pragma to $cop1. Otherwise
1112 we'll just inline the former in the latter and stop, which
1113 isn't what the user expected
1115 b) We use the magic 'inline' Id to ensure that $dmop1 really is
1116 inlined in $cop1, even though the latter itself has an INLINE pragma
1117 That is important to allow the mutual recursion between $fooInt and
1120 This is all regrettably delicate.
1123 %************************************************************************
1125 \subsection{Error messages}
1127 %************************************************************************
1130 instDeclCtxt1 :: LHsType Name -> SDoc
1131 instDeclCtxt1 hs_inst_ty
1132 = inst_decl_ctxt (case unLoc hs_inst_ty of
1133 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1134 HsPredTy pred -> ppr pred
1135 _ -> ppr hs_inst_ty) -- Don't expect this
1136 instDeclCtxt2 :: Type -> SDoc
1137 instDeclCtxt2 dfun_ty
1138 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1140 (_,cls,tys) = tcSplitDFunTy dfun_ty
1142 inst_decl_ctxt :: SDoc -> SDoc
1143 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1145 superClassCtxt :: SDoc
1146 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
1148 atInstCtxt :: Name -> SDoc
1149 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1152 mustBeVarArgErr :: Type -> SDoc
1153 mustBeVarArgErr ty =
1154 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1155 ptext (sLit "must be variables")
1156 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1159 wrongATArgErr :: Type -> Type -> SDoc
1160 wrongATArgErr ty instTy =
1161 sep [ ptext (sLit "Type indexes must match class instance head")
1162 , ptext (sLit "Found") <+> quotes (ppr ty)
1163 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)