473cc061c1d065d120b5498e97150335a8725de5
[ghc.git] / compiler / typecheck / TcInstDcls.hs
1 {-
2 (c) The University of Glasgow 2006
3 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4
5
6 TcInstDecls: Typechecking instance declarations
7 -}
8
9 {-# LANGUAGE CPP #-}
10 {-# LANGUAGE FlexibleContexts #-}
11 {-# LANGUAGE TypeFamilies #-}
12
13 module TcInstDcls ( tcInstDecls1, tcInstDeclsDeriv, tcInstDecls2 ) where
14
15 #include "HsVersions.h"
16
17 import GhcPrelude
18
19 import HsSyn
20 import TcBinds
21 import TcTyClsDecls
22 import TcTyDecls ( addTyConsToGblEnv )
23 import TcClassDcl( tcClassDecl2, tcATDefault,
24 HsSigFun, mkHsSigFun,
25 findMethodBind, instantiateMethod )
26 import TcSigs
27 import TcRnMonad
28 import TcValidity
29 import TcHsSyn ( zonkTyBndrsX, emptyZonkEnv
30 , zonkTcTypeToTypes, zonkTcTypeToType )
31 import TcMType
32 import TcType
33 import BuildTyCl
34 import Inst
35 import InstEnv
36 import FamInst
37 import FamInstEnv
38 import TcDeriv
39 import TcEnv
40 import TcHsType
41 import TcUnify
42 import CoreSyn ( Expr(..), mkApps, mkVarApps, mkLams )
43 import MkCore ( nO_METHOD_BINDING_ERROR_ID )
44 import CoreUnfold ( mkInlineUnfoldingWithArity, mkDFunUnfolding )
45 import Type
46 import TcEvidence
47 import TyCon
48 import CoAxiom
49 import DataCon
50 import ConLike
51 import Class
52 import Var
53 import VarEnv
54 import VarSet
55 import Bag
56 import BasicTypes
57 import DynFlags
58 import ErrUtils
59 import FastString
60 import Id
61 import ListSetOps
62 import MkId
63 import Name
64 import NameSet
65 import Outputable
66 import SrcLoc
67 import Util
68 import BooleanFormula ( isUnsatisfied, pprBooleanFormulaNice )
69 import qualified GHC.LanguageExtensions as LangExt
70
71 import Control.Monad
72 import Maybes
73
74
75 {-
76 Typechecking instance declarations is done in two passes. The first
77 pass, made by @tcInstDecls1@, collects information to be used in the
78 second pass.
79
80 This pre-processed info includes the as-yet-unprocessed bindings
81 inside the instance declaration. These are type-checked in the second
82 pass, when the class-instance envs and GVE contain all the info from
83 all the instance and value decls. Indeed that's the reason we need
84 two passes over the instance decls.
85
86
87 Note [How instance declarations are translated]
88 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
89 Here is how we translate instance declarations into Core
90
91 Running example:
92 class C a where
93 op1, op2 :: Ix b => a -> b -> b
94 op2 = <dm-rhs>
95
96 instance C a => C [a]
97 {-# INLINE [2] op1 #-}
98 op1 = <rhs>
99 ===>
100 -- Method selectors
101 op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
102 op1 = ...
103 op2 = ...
104
105 -- Default methods get the 'self' dictionary as argument
106 -- so they can call other methods at the same type
107 -- Default methods get the same type as their method selector
108 $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
109 $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
110 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
111 -- Note [Tricky type variable scoping]
112
113 -- A top-level definition for each instance method
114 -- Here op1_i, op2_i are the "instance method Ids"
115 -- The INLINE pragma comes from the user pragma
116 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
117 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
118 op1_i = /\a. \(d:C a).
119 let this :: C [a]
120 this = df_i a d
121 -- Note [Subtle interaction of recursion and overlap]
122
123 local_op1 :: forall b. Ix b => [a] -> b -> b
124 local_op1 = <rhs>
125 -- Source code; run the type checker on this
126 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
127 -- Note [Tricky type variable scoping]
128
129 in local_op1 a d
130
131 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
132
133 -- The dictionary function itself
134 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
135 df_i :: forall a. C a -> C [a]
136 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
137 -- But see Note [Default methods in instances]
138 -- We can't apply the type checker to the default-method call
139
140 -- Use a RULE to short-circuit applications of the class ops
141 {-# RULE "op1@C[a]" forall a, d:C a.
142 op1 [a] (df_i d) = op1_i a d #-}
143
144 Note [Instances and loop breakers]
145 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
146 * Note that df_i may be mutually recursive with both op1_i and op2_i.
147 It's crucial that df_i is not chosen as the loop breaker, even
148 though op1_i has a (user-specified) INLINE pragma.
149
150 * Instead the idea is to inline df_i into op1_i, which may then select
151 methods from the MkC record, and thereby break the recursion with
152 df_i, leaving a *self*-recursive op1_i. (If op1_i doesn't call op at
153 the same type, it won't mention df_i, so there won't be recursion in
154 the first place.)
155
156 * If op1_i is marked INLINE by the user there's a danger that we won't
157 inline df_i in it, and that in turn means that (since it'll be a
158 loop-breaker because df_i isn't), op1_i will ironically never be
159 inlined. But this is OK: the recursion breaking happens by way of
160 a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
161 unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
162
163 Note [ClassOp/DFun selection]
164 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
165 One thing we see a lot is stuff like
166 op2 (df d1 d2)
167 where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
168 'op2' and 'df' to get
169 case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
170 MkD _ op2 _ _ _ -> op2
171 And that will reduce to ($cop2 d1 d2) which is what we wanted.
172
173 But it's tricky to make this work in practice, because it requires us to
174 inline both 'op2' and 'df'. But neither is keen to inline without having
175 seen the other's result; and it's very easy to get code bloat (from the
176 big intermediate) if you inline a bit too much.
177
178 Instead we use a cunning trick.
179 * We arrange that 'df' and 'op2' NEVER inline.
180
181 * We arrange that 'df' is ALWAYS defined in the sylised form
182 df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
183
184 * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
185 that lists its methods.
186
187 * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
188 a suitable constructor application -- inlining df "on the fly" as it
189 were.
190
191 * ClassOp rules: We give the ClassOp 'op2' a BuiltinRule that
192 extracts the right piece iff its argument satisfies
193 exprIsConApp_maybe. This is done in MkId mkDictSelId
194
195 * We make 'df' CONLIKE, so that shared uses still match; eg
196 let d = df d1 d2
197 in ...(op2 d)...(op1 d)...
198
199 Note [Single-method classes]
200 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
201 If the class has just one method (or, more accurately, just one element
202 of {superclasses + methods}), then we use a different strategy.
203
204 class C a where op :: a -> a
205 instance C a => C [a] where op = <blah>
206
207 We translate the class decl into a newtype, which just gives a
208 top-level axiom. The "constructor" MkC expands to a cast, as does the
209 class-op selector.
210
211 axiom Co:C a :: C a ~ (a->a)
212
213 op :: forall a. C a -> (a -> a)
214 op a d = d |> (Co:C a)
215
216 MkC :: forall a. (a->a) -> C a
217 MkC = /\a.\op. op |> (sym Co:C a)
218
219 The clever RULE stuff doesn't work now, because ($df a d) isn't
220 a constructor application, so exprIsConApp_maybe won't return
221 Just <blah>.
222
223 Instead, we simply rely on the fact that casts are cheap:
224
225 $df :: forall a. C a => C [a]
226 {-# INLINE df #-} -- NB: INLINE this
227 $df = /\a. \d. MkC [a] ($cop_list a d)
228 = $cop_list |> forall a. C a -> (sym (Co:C [a]))
229
230 $cop_list :: forall a. C a => [a] -> [a]
231 $cop_list = <blah>
232
233 So if we see
234 (op ($df a d))
235 we'll inline 'op' and '$df', since both are simply casts, and
236 good things happen.
237
238 Why do we use this different strategy? Because otherwise we
239 end up with non-inlined dictionaries that look like
240 $df = $cop |> blah
241 which adds an extra indirection to every use, which seems stupid. See
242 Trac #4138 for an example (although the regression reported there
243 wasn't due to the indirection).
244
245 There is an awkward wrinkle though: we want to be very
246 careful when we have
247 instance C a => C [a] where
248 {-# INLINE op #-}
249 op = ...
250 then we'll get an INLINE pragma on $cop_list but it's important that
251 $cop_list only inlines when it's applied to *two* arguments (the
252 dictionary and the list argument). So we must not eta-expand $df
253 above. We ensure that this doesn't happen by putting an INLINE
254 pragma on the dfun itself; after all, it ends up being just a cast.
255
256 There is one more dark corner to the INLINE story, even more deeply
257 buried. Consider this (Trac #3772):
258
259 class DeepSeq a => C a where
260 gen :: Int -> a
261
262 instance C a => C [a] where
263 gen n = ...
264
265 class DeepSeq a where
266 deepSeq :: a -> b -> b
267
268 instance DeepSeq a => DeepSeq [a] where
269 {-# INLINE deepSeq #-}
270 deepSeq xs b = foldr deepSeq b xs
271
272 That gives rise to these defns:
273
274 $cdeepSeq :: DeepSeq a -> [a] -> b -> b
275 -- User INLINE( 3 args )!
276 $cdeepSeq a (d:DS a) b (x:[a]) (y:b) = ...
277
278 $fDeepSeq[] :: DeepSeq a -> DeepSeq [a]
279 -- DFun (with auto INLINE pragma)
280 $fDeepSeq[] a d = $cdeepSeq a d |> blah
281
282 $cp1 a d :: C a => DeepSep [a]
283 -- We don't want to eta-expand this, lest
284 -- $cdeepSeq gets inlined in it!
285 $cp1 a d = $fDeepSep[] a (scsel a d)
286
287 $fC[] :: C a => C [a]
288 -- Ordinary DFun
289 $fC[] a d = MkC ($cp1 a d) ($cgen a d)
290
291 Here $cp1 is the code that generates the superclass for C [a]. The
292 issue is this: we must not eta-expand $cp1 either, or else $fDeepSeq[]
293 and then $cdeepSeq will inline there, which is definitely wrong. Like
294 on the dfun, we solve this by adding an INLINE pragma to $cp1.
295
296 Note [Subtle interaction of recursion and overlap]
297 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
298 Consider this
299 class C a where { op1,op2 :: a -> a }
300 instance C a => C [a] where
301 op1 x = op2 x ++ op2 x
302 op2 x = ...
303 instance C [Int] where
304 ...
305
306 When type-checking the C [a] instance, we need a C [a] dictionary (for
307 the call of op2). If we look up in the instance environment, we find
308 an overlap. And in *general* the right thing is to complain (see Note
309 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
310 complain, because we just want to delegate to the op2 of this same
311 instance.
312
313 Why is this justified? Because we generate a (C [a]) constraint in
314 a context in which 'a' cannot be instantiated to anything that matches
315 other overlapping instances, or else we would not be executing this
316 version of op1 in the first place.
317
318 It might even be a bit disguised:
319
320 nullFail :: C [a] => [a] -> [a]
321 nullFail x = op2 x ++ op2 x
322
323 instance C a => C [a] where
324 op1 x = nullFail x
325
326 Precisely this is used in package 'regex-base', module Context.hs.
327 See the overlapping instances for RegexContext, and the fact that they
328 call 'nullFail' just like the example above. The DoCon package also
329 does the same thing; it shows up in module Fraction.hs.
330
331 Conclusion: when typechecking the methods in a C [a] instance, we want to
332 treat the 'a' as an *existential* type variable, in the sense described
333 by Note [Binding when looking up instances]. That is why isOverlappableTyVar
334 responds True to an InstSkol, which is the kind of skolem we use in
335 tcInstDecl2.
336
337
338 Note [Tricky type variable scoping]
339 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
340 In our example
341 class C a where
342 op1, op2 :: Ix b => a -> b -> b
343 op2 = <dm-rhs>
344
345 instance C a => C [a]
346 {-# INLINE [2] op1 #-}
347 op1 = <rhs>
348
349 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
350 in scope in <rhs>. In particular, we must make sure that 'b' is in
351 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
352 which brings appropriate tyvars into scope. This happens for both
353 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
354 complained if 'b' is mentioned in <rhs>.
355
356
357
358 ************************************************************************
359 * *
360 \subsection{Extracting instance decls}
361 * *
362 ************************************************************************
363
364 Gather up the instance declarations from their various sources
365 -}
366
367 tcInstDecls1 -- Deal with both source-code and imported instance decls
368 :: [LInstDecl GhcRn] -- Source code instance decls
369 -> TcM (TcGblEnv, -- The full inst env
370 [InstInfo GhcRn], -- Source-code instance decls to process;
371 -- contains all dfuns for this module
372 [DerivInfo]) -- From data family instances
373
374 tcInstDecls1 inst_decls
375 = do { -- Do class and family instance declarations
376 ; stuff <- mapAndRecoverM tcLocalInstDecl inst_decls
377
378 ; let (local_infos_s, fam_insts_s, datafam_deriv_infos) = unzip3 stuff
379 fam_insts = concat fam_insts_s
380 local_infos = concat local_infos_s
381
382 ; gbl_env <- addClsInsts local_infos $
383 addFamInsts fam_insts $
384 getGblEnv
385
386 ; return ( gbl_env
387 , local_infos
388 , concat datafam_deriv_infos ) }
389
390 -- | Use DerivInfo for data family instances (produced by tcInstDecls1),
391 -- datatype declarations (TyClDecl), and standalone deriving declarations
392 -- (DerivDecl) to check and process all derived class instances.
393 tcInstDeclsDeriv
394 :: [DerivInfo]
395 -> [LTyClDecl GhcRn]
396 -> [LDerivDecl GhcRn]
397 -> TcM (TcGblEnv, [InstInfo GhcRn], HsValBinds GhcRn)
398 tcInstDeclsDeriv datafam_deriv_infos tyclds derivds
399 = do th_stage <- getStage -- See Note [Deriving inside TH brackets]
400 if isBrackStage th_stage
401 then do { gbl_env <- getGblEnv
402 ; return (gbl_env, bagToList emptyBag, emptyValBindsOut) }
403 else do { data_deriv_infos <- mkDerivInfos tyclds
404 ; let deriv_infos = datafam_deriv_infos ++ data_deriv_infos
405 ; (tcg_env, info_bag, valbinds) <- tcDeriving deriv_infos derivds
406 ; return (tcg_env, bagToList info_bag, valbinds) }
407
408 addClsInsts :: [InstInfo GhcRn] -> TcM a -> TcM a
409 addClsInsts infos thing_inside
410 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
411
412 addFamInsts :: [FamInst] -> TcM a -> TcM a
413 -- Extend (a) the family instance envt
414 -- (b) the type envt with stuff from data type decls
415 addFamInsts fam_insts thing_inside
416 = tcExtendLocalFamInstEnv fam_insts $
417 tcExtendGlobalEnv axioms $
418 do { traceTc "addFamInsts" (pprFamInsts fam_insts)
419 ; gbl_env <- addTyConsToGblEnv data_rep_tycons
420 -- Does not add its axiom; that comes
421 -- from adding the 'axioms' above
422 ; setGblEnv gbl_env thing_inside }
423 where
424 axioms = map (ACoAxiom . toBranchedAxiom . famInstAxiom) fam_insts
425 data_rep_tycons = famInstsRepTyCons fam_insts
426 -- The representation tycons for 'data instances' declarations
427
428 {-
429 Note [Deriving inside TH brackets]
430 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
431 Given a declaration bracket
432 [d| data T = A | B deriving( Show ) |]
433
434 there is really no point in generating the derived code for deriving(
435 Show) and then type-checking it. This will happen at the call site
436 anyway, and the type check should never fail! Moreover (Trac #6005)
437 the scoping of the generated code inside the bracket does not seem to
438 work out.
439
440 The easy solution is simply not to generate the derived instances at
441 all. (A less brutal solution would be to generate them with no
442 bindings.) This will become moot when we shift to the new TH plan, so
443 the brutal solution will do.
444 -}
445
446 tcLocalInstDecl :: LInstDecl GhcRn
447 -> TcM ([InstInfo GhcRn], [FamInst], [DerivInfo])
448 -- A source-file instance declaration
449 -- Type-check all the stuff before the "where"
450 --
451 -- We check for respectable instance type, and context
452 tcLocalInstDecl (L loc (TyFamInstD { tfid_inst = decl }))
453 = do { fam_inst <- tcTyFamInstDecl Nothing (L loc decl)
454 ; return ([], [fam_inst], []) }
455
456 tcLocalInstDecl (L loc (DataFamInstD { dfid_inst = decl }))
457 = do { (fam_inst, m_deriv_info) <- tcDataFamInstDecl Nothing (L loc decl)
458 ; return ([], [fam_inst], maybeToList m_deriv_info) }
459
460 tcLocalInstDecl (L loc (ClsInstD { cid_inst = decl }))
461 = do { (insts, fam_insts, deriv_infos) <- tcClsInstDecl (L loc decl)
462 ; return (insts, fam_insts, deriv_infos) }
463
464 tcLocalInstDecl (L _ (XInstDecl _)) = panic "tcLocalInstDecl"
465
466 tcClsInstDecl :: LClsInstDecl GhcRn
467 -> TcM ([InstInfo GhcRn], [FamInst], [DerivInfo])
468 -- The returned DerivInfos are for any associated data families
469 tcClsInstDecl (L loc (ClsInstDecl { cid_poly_ty = poly_ty, cid_binds = binds
470 , cid_sigs = uprags, cid_tyfam_insts = ats
471 , cid_overlap_mode = overlap_mode
472 , cid_datafam_insts = adts }))
473 = setSrcSpan loc $
474 addErrCtxt (instDeclCtxt1 poly_ty) $
475 do { (tyvars, theta, clas, inst_tys)
476 <- tcHsClsInstType (InstDeclCtxt False) poly_ty
477 -- NB: tcHsClsInstType does checkValidInstance
478
479 ; let mini_env = mkVarEnv (classTyVars clas `zip` inst_tys)
480 mini_subst = mkTvSubst (mkInScopeSet (mkVarSet tyvars)) mini_env
481 mb_info = Just (clas, tyvars, mini_env)
482
483 -- Next, process any associated types.
484 ; traceTc "tcLocalInstDecl" (ppr poly_ty)
485 ; tyfam_insts0 <- scopeTyVars InstSkol tyvars $
486 mapAndRecoverM (tcTyFamInstDecl mb_info) ats
487 ; datafam_stuff <- scopeTyVars InstSkol tyvars $
488 mapAndRecoverM (tcDataFamInstDecl mb_info) adts
489 ; let (datafam_insts, m_deriv_infos) = unzip datafam_stuff
490 deriv_infos = catMaybes m_deriv_infos
491
492 -- Check for missing associated types and build them
493 -- from their defaults (if available)
494 ; let defined_ats = mkNameSet (map (tyFamInstDeclName . unLoc) ats)
495 `unionNameSet`
496 mkNameSet (map (unLoc . feqn_tycon
497 . hsib_body
498 . dfid_eqn
499 . unLoc) adts)
500 ; tyfam_insts1 <- mapM (tcATDefault loc mini_subst defined_ats)
501 (classATItems clas)
502
503 -- Finally, construct the Core representation of the instance.
504 -- (This no longer includes the associated types.)
505 ; dfun_name <- newDFunName clas inst_tys (getLoc (hsSigType poly_ty))
506 -- Dfun location is that of instance *header*
507
508 ; ispec <- newClsInst (fmap unLoc overlap_mode) dfun_name tyvars theta
509 clas inst_tys
510
511 ; let inst_info = InstInfo { iSpec = ispec
512 , iBinds = InstBindings
513 { ib_binds = binds
514 , ib_tyvars = map Var.varName tyvars -- Scope over bindings
515 , ib_pragmas = uprags
516 , ib_extensions = []
517 , ib_derived = False } }
518
519 -- In hs-boot files there should be no bindings
520 ; is_boot <- tcIsHsBootOrSig
521 ; let no_binds = isEmptyLHsBinds binds && null uprags
522 ; failIfTc (is_boot && not no_binds) badBootDeclErr
523
524 ; return ( [inst_info], tyfam_insts0 ++ concat tyfam_insts1 ++ datafam_insts
525 , deriv_infos ) }
526 tcClsInstDecl (L _ (XClsInstDecl _)) = panic "tcClsInstDecl"
527
528 {-
529 ************************************************************************
530 * *
531 Type checking family instances
532 * *
533 ************************************************************************
534
535 Family instances are somewhat of a hybrid. They are processed together with
536 class instance heads, but can contain data constructors and hence they share a
537 lot of kinding and type checking code with ordinary algebraic data types (and
538 GADTs).
539 -}
540
541 tcFamInstDeclCombined :: Maybe ClsInstInfo
542 -> Located Name -> TcM TyCon
543 tcFamInstDeclCombined mb_clsinfo fam_tc_lname
544 = do { -- Type family instances require -XTypeFamilies
545 -- and can't (currently) be in an hs-boot file
546 ; traceTc "tcFamInstDecl" (ppr fam_tc_lname)
547 ; type_families <- xoptM LangExt.TypeFamilies
548 ; is_boot <- tcIsHsBootOrSig -- Are we compiling an hs-boot file?
549 ; checkTc type_families $ badFamInstDecl fam_tc_lname
550 ; checkTc (not is_boot) $ badBootFamInstDeclErr
551
552 -- Look up the family TyCon and check for validity including
553 -- check that toplevel type instances are not for associated types.
554 ; fam_tc <- tcLookupLocatedTyCon fam_tc_lname
555 ; when (isNothing mb_clsinfo && -- Not in a class decl
556 isTyConAssoc fam_tc) -- but an associated type
557 (addErr $ assocInClassErr fam_tc_lname)
558
559 ; return fam_tc }
560
561 tcTyFamInstDecl :: Maybe ClsInstInfo
562 -> LTyFamInstDecl GhcRn -> TcM FamInst
563 -- "type instance"
564 tcTyFamInstDecl mb_clsinfo (L loc decl@(TyFamInstDecl { tfid_eqn = eqn }))
565 = setSrcSpan loc $
566 tcAddTyFamInstCtxt decl $
567 do { let fam_lname = feqn_tycon (hsib_body eqn)
568 ; fam_tc <- tcFamInstDeclCombined mb_clsinfo fam_lname
569
570 -- (0) Check it's an open type family
571 ; checkTc (isFamilyTyCon fam_tc) (notFamily fam_tc)
572 ; checkTc (isTypeFamilyTyCon fam_tc) (wrongKindOfFamily fam_tc)
573 ; checkTc (isOpenTypeFamilyTyCon fam_tc) (notOpenFamily fam_tc)
574
575 -- (1) do the work of verifying the synonym group
576 ; co_ax_branch <- tcTyFamInstEqn fam_tc mb_clsinfo
577 (L (getLoc fam_lname) eqn)
578
579 -- (2) check for validity
580 ; checkValidCoAxBranch mb_clsinfo fam_tc co_ax_branch
581
582 -- (3) construct coercion axiom
583 ; rep_tc_name <- newFamInstAxiomName fam_lname [coAxBranchLHS co_ax_branch]
584 ; let axiom = mkUnbranchedCoAxiom rep_tc_name fam_tc co_ax_branch
585 ; newFamInst SynFamilyInst axiom }
586
587 tcDataFamInstDecl :: Maybe ClsInstInfo
588 -> LDataFamInstDecl GhcRn -> TcM (FamInst, Maybe DerivInfo)
589 -- "newtype instance" and "data instance"
590 tcDataFamInstDecl mb_clsinfo
591 (L loc decl@(DataFamInstDecl { dfid_eqn = HsIB { hsib_ext = tv_names
592 , hsib_body =
593 FamEqn { feqn_pats = pats
594 , feqn_tycon = fam_tc_name
595 , feqn_fixity = fixity
596 , feqn_rhs = HsDataDefn { dd_ND = new_or_data
597 , dd_cType = cType
598 , dd_ctxt = ctxt
599 , dd_cons = cons
600 , dd_kindSig = m_ksig
601 , dd_derivs = derivs } }}}))
602 = setSrcSpan loc $
603 tcAddDataFamInstCtxt decl $
604 do { fam_tc <- tcFamInstDeclCombined mb_clsinfo fam_tc_name
605
606 -- Check that the family declaration is for the right kind
607 ; checkTc (isFamilyTyCon fam_tc) (notFamily fam_tc)
608 ; checkTc (isDataFamilyTyCon fam_tc) (wrongKindOfFamily fam_tc)
609
610 -- Kind check type patterns
611 ; let mb_kind_env = thdOf3 <$> mb_clsinfo
612 ; tcFamTyPats fam_tc mb_clsinfo tv_names pats
613 (kcDataDefn mb_kind_env decl) $
614 \tvs pats res_kind ->
615 do { stupid_theta <- solveEqualities $ tcHsContext ctxt
616
617 -- Zonk the patterns etc into the Type world
618 ; (ze, tvs') <- zonkTyBndrsX emptyZonkEnv tvs
619 ; pats' <- zonkTcTypeToTypes ze pats
620 ; res_kind' <- zonkTcTypeToType ze res_kind
621 ; stupid_theta' <- zonkTcTypeToTypes ze stupid_theta
622
623 ; gadt_syntax <- dataDeclChecks (tyConName fam_tc) new_or_data stupid_theta' cons
624
625 -- Construct representation tycon
626 ; rep_tc_name <- newFamInstTyConName fam_tc_name pats'
627 ; axiom_name <- newFamInstAxiomName fam_tc_name [pats']
628
629 ; let (eta_pats, etad_tvs) = eta_reduce pats'
630 eta_tvs = filterOut (`elem` etad_tvs) tvs'
631 -- NB: the "extra" tvs from tcDataKindSig would always be eta-reduced
632
633 full_tcbs = mkTyConBindersPreferAnon (eta_tvs ++ etad_tvs) res_kind'
634 -- Put the eta-removed tyvars at the end
635 -- Remember, tvs' is in arbitrary order (except kind vars are
636 -- first, so there is no reason to suppose that the etad_tvs
637 -- (obtained from the pats) are at the end (Trac #11148)
638
639 -- Deal with any kind signature.
640 -- See also Note [Arity of data families] in FamInstEnv
641 ; (extra_tcbs, final_res_kind) <- tcDataKindSig full_tcbs res_kind'
642 ; checkTc (tcIsLiftedTypeKind final_res_kind) (badKindSig True res_kind')
643
644 ; let extra_pats = map (mkTyVarTy . binderVar) extra_tcbs
645 all_pats = pats' `chkAppend` extra_pats
646 orig_res_ty = mkTyConApp fam_tc all_pats
647
648 ; (rep_tc, axiom) <- fixM $ \ ~(rec_rep_tc, _) ->
649 do { let ty_binders = full_tcbs `chkAppend` extra_tcbs
650 ; data_cons <- tcConDecls rec_rep_tc
651 (ty_binders, orig_res_ty) cons
652 ; tc_rhs <- case new_or_data of
653 DataType -> return (mkDataTyConRhs data_cons)
654 NewType -> ASSERT( not (null data_cons) )
655 mkNewTyConRhs rep_tc_name rec_rep_tc (head data_cons)
656 -- freshen tyvars
657 ; let axiom = mkSingleCoAxiom Representational
658 axiom_name eta_tvs [] fam_tc eta_pats
659 (mkTyConApp rep_tc (mkTyVarTys eta_tvs))
660 parent = DataFamInstTyCon axiom fam_tc all_pats
661
662
663 -- NB: Use the full ty_binders from the pats. See bullet toward
664 -- the end of Note [Data type families] in TyCon
665 rep_tc = mkAlgTyCon rep_tc_name
666 ty_binders liftedTypeKind
667 (map (const Nominal) ty_binders)
668 (fmap unLoc cType) stupid_theta
669 tc_rhs parent
670 gadt_syntax
671 -- We always assume that indexed types are recursive. Why?
672 -- (1) Due to their open nature, we can never be sure that a
673 -- further instance might not introduce a new recursive
674 -- dependency. (2) They are always valid loop breakers as
675 -- they involve a coercion.
676 ; return (rep_tc, axiom) }
677
678 -- Remember to check validity; no recursion to worry about here
679 -- Check that left-hand sides are ok (mono-types, no type families,
680 -- consistent instantiations, etc)
681 ; checkValidFamPats mb_clsinfo fam_tc tvs' [] pats' extra_pats pp_hs_pats
682
683 -- Result kind must be '*' (otherwise, we have too few patterns)
684 ; checkTc (tcIsLiftedTypeKind final_res_kind) $
685 tooFewParmsErr (tyConArity fam_tc)
686
687 ; checkValidTyCon rep_tc
688
689 ; let m_deriv_info = case derivs of
690 L _ [] -> Nothing
691 L _ preds ->
692 Just $ DerivInfo { di_rep_tc = rep_tc
693 , di_clauses = preds
694 , di_ctxt = tcMkDataFamInstCtxt decl }
695
696 ; fam_inst <- newFamInst (DataFamilyInst rep_tc) axiom
697 ; return (fam_inst, m_deriv_info) } }
698 where
699 eta_reduce :: [Type] -> ([Type], [TyVar])
700 -- See Note [Eta reduction for data families] in FamInstEnv
701 -- Splits the incoming patterns into two: the [TyVar]
702 -- are the patterns that can be eta-reduced away.
703 -- e.g. T [a] Int a d c ==> (T [a] Int a, [d,c])
704 --
705 -- NB: quadratic algorithm, but types are small here
706 eta_reduce pats
707 = go (reverse pats) []
708 go (pat:pats) etad_tvs
709 | Just tv <- getTyVar_maybe pat
710 , not (tv `elemVarSet` tyCoVarsOfTypes pats)
711 = go pats (tv : etad_tvs)
712 go pats etad_tvs = (reverse pats, etad_tvs)
713
714 pp_hs_pats = pprFamInstLHS fam_tc_name pats fixity (unLoc ctxt) m_ksig
715
716 tcDataFamInstDecl _
717 (L _ (DataFamInstDecl
718 { dfid_eqn = HsIB { hsib_body = FamEqn { feqn_rhs = XHsDataDefn _ }}}))
719 = panic "tcDataFamInstDecl"
720 tcDataFamInstDecl _ (L _ (DataFamInstDecl (XHsImplicitBndrs _)))
721 = panic "tcDataFamInstDecl"
722 tcDataFamInstDecl _ (L _ (DataFamInstDecl (HsIB _ (XFamEqn _))))
723 = panic "tcDataFamInstDecl"
724
725
726 {- *********************************************************************
727 * *
728 Type-checking instance declarations, pass 2
729 * *
730 ********************************************************************* -}
731
732 tcInstDecls2 :: [LTyClDecl GhcRn] -> [InstInfo GhcRn]
733 -> TcM (LHsBinds GhcTc)
734 -- (a) From each class declaration,
735 -- generate any default-method bindings
736 -- (b) From each instance decl
737 -- generate the dfun binding
738
739 tcInstDecls2 tycl_decls inst_decls
740 = do { -- (a) Default methods from class decls
741 let class_decls = filter (isClassDecl . unLoc) tycl_decls
742 ; dm_binds_s <- mapM tcClassDecl2 class_decls
743 ; let dm_binds = unionManyBags dm_binds_s
744
745 -- (b) instance declarations
746 ; let dm_ids = collectHsBindsBinders dm_binds
747 -- Add the default method Ids (again)
748 -- (they were arready added in TcTyDecls.tcAddImplicits)
749 -- See Note [Default methods in the type environment]
750 ; inst_binds_s <- tcExtendGlobalValEnv dm_ids $
751 mapM tcInstDecl2 inst_decls
752
753 -- Done
754 ; return (dm_binds `unionBags` unionManyBags inst_binds_s) }
755
756 {- Note [Default methods in the type environment]
757 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
758 The default method Ids are already in the type environment (see Note
759 [Default method Ids and Template Haskell] in TcTyDcls), BUT they
760 don't have their InlinePragmas yet. Usually that would not matter,
761 because the simplifier propagates information from binding site to
762 use. But, unusually, when compiling instance decls we *copy* the
763 INLINE pragma from the default method to the method for that
764 particular operation (see Note [INLINE and default methods] below).
765
766 So right here in tcInstDecls2 we must re-extend the type envt with
767 the default method Ids replete with their INLINE pragmas. Urk.
768 -}
769
770 tcInstDecl2 :: InstInfo GhcRn -> TcM (LHsBinds GhcTc)
771 -- Returns a binding for the dfun
772 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
773 = recoverM (return emptyLHsBinds) $
774 setSrcSpan loc $
775 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
776 do { -- Instantiate the instance decl with skolem constants
777 ; (inst_tyvars, dfun_theta, inst_head) <- tcSkolDFunType dfun_id
778 ; dfun_ev_vars <- newEvVars dfun_theta
779 -- We instantiate the dfun_id with superSkolems.
780 -- See Note [Subtle interaction of recursion and overlap]
781 -- and Note [Binding when looking up instances]
782
783 ; let (clas, inst_tys) = tcSplitDFunHead inst_head
784 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
785 sc_theta' = substTheta (zipTvSubst class_tyvars inst_tys) sc_theta
786
787 ; traceTc "tcInstDecl2" (vcat [ppr inst_tyvars, ppr inst_tys, ppr dfun_theta, ppr sc_theta'])
788
789 -- Deal with 'SPECIALISE instance' pragmas
790 -- See Note [SPECIALISE instance pragmas]
791 ; spec_inst_info@(spec_inst_prags,_) <- tcSpecInstPrags dfun_id ibinds
792
793 -- Typecheck superclasses and methods
794 -- See Note [Typechecking plan for instance declarations]
795 ; dfun_ev_binds_var <- newTcEvBinds
796 ; let dfun_ev_binds = TcEvBinds dfun_ev_binds_var
797 ; ((sc_meth_ids, sc_meth_binds, sc_meth_implics), tclvl)
798 <- pushTcLevelM $
799 do { (sc_ids, sc_binds, sc_implics)
800 <- tcSuperClasses dfun_id clas inst_tyvars dfun_ev_vars
801 inst_tys dfun_ev_binds
802 sc_theta'
803
804 -- Typecheck the methods
805 ; (meth_ids, meth_binds, meth_implics)
806 <- tcMethods dfun_id clas inst_tyvars dfun_ev_vars
807 inst_tys dfun_ev_binds spec_inst_info
808 op_items ibinds
809
810 ; return ( sc_ids ++ meth_ids
811 , sc_binds `unionBags` meth_binds
812 , sc_implics `unionBags` meth_implics ) }
813
814 ; imp <- newImplication
815 ; emitImplication $
816 imp { ic_tclvl = tclvl
817 , ic_skols = inst_tyvars
818 , ic_given = dfun_ev_vars
819 , ic_wanted = mkImplicWC sc_meth_implics
820 , ic_binds = dfun_ev_binds_var
821 , ic_info = InstSkol }
822
823 -- Create the result bindings
824 ; self_dict <- newDict clas inst_tys
825 ; let class_tc = classTyCon clas
826 [dict_constr] = tyConDataCons class_tc
827 dict_bind = mkVarBind self_dict (L loc con_app_args)
828
829 -- We don't produce a binding for the dict_constr; instead we
830 -- rely on the simplifier to unfold this saturated application
831 -- We do this rather than generate an HsCon directly, because
832 -- it means that the special cases (e.g. dictionary with only one
833 -- member) are dealt with by the common MkId.mkDataConWrapId
834 -- code rather than needing to be repeated here.
835 -- con_app_tys = MkD ty1 ty2
836 -- con_app_scs = MkD ty1 ty2 sc1 sc2
837 -- con_app_args = MkD ty1 ty2 sc1 sc2 op1 op2
838 con_app_tys = mkHsWrap (mkWpTyApps inst_tys)
839 (HsConLikeOut noExt (RealDataCon dict_constr))
840 -- NB: We *can* have covars in inst_tys, in the case of
841 -- promoted GADT constructors.
842
843 con_app_args = foldl' app_to_meth con_app_tys sc_meth_ids
844
845 app_to_meth :: HsExpr GhcTc -> Id -> HsExpr GhcTc
846 app_to_meth fun meth_id = HsApp noExt (L loc fun)
847 (L loc (wrapId arg_wrapper meth_id))
848
849 inst_tv_tys = mkTyVarTys inst_tyvars
850 arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps inst_tv_tys
851
852 is_newtype = isNewTyCon class_tc
853 dfun_id_w_prags = addDFunPrags dfun_id sc_meth_ids
854 dfun_spec_prags
855 | is_newtype = SpecPrags []
856 | otherwise = SpecPrags spec_inst_prags
857 -- Newtype dfuns just inline unconditionally,
858 -- so don't attempt to specialise them
859
860 export = ABE { abe_ext = noExt
861 , abe_wrap = idHsWrapper
862 , abe_poly = dfun_id_w_prags
863 , abe_mono = self_dict
864 , abe_prags = dfun_spec_prags }
865 -- NB: see Note [SPECIALISE instance pragmas]
866 main_bind = AbsBinds { abs_ext = noExt
867 , abs_tvs = inst_tyvars
868 , abs_ev_vars = dfun_ev_vars
869 , abs_exports = [export]
870 , abs_ev_binds = []
871 , abs_binds = unitBag dict_bind
872 , abs_sig = True }
873
874 ; return (unitBag (L loc main_bind) `unionBags` sc_meth_binds)
875 }
876 where
877 dfun_id = instanceDFunId ispec
878 loc = getSrcSpan dfun_id
879
880 addDFunPrags :: DFunId -> [Id] -> DFunId
881 -- DFuns need a special Unfolding and InlinePrag
882 -- See Note [ClassOp/DFun selection]
883 -- and Note [Single-method classes]
884 -- It's easiest to create those unfoldings right here, where
885 -- have all the pieces in hand, even though we are messing with
886 -- Core at this point, which the typechecker doesn't usually do
887 -- However we take care to build the unfolding using the TyVars from
888 -- the DFunId rather than from the skolem pieces that the typechecker
889 -- is messing with.
890 addDFunPrags dfun_id sc_meth_ids
891 | is_newtype
892 = dfun_id `setIdUnfolding` mkInlineUnfoldingWithArity 0 con_app
893 `setInlinePragma` alwaysInlinePragma { inl_sat = Just 0 }
894 | otherwise
895 = dfun_id `setIdUnfolding` mkDFunUnfolding dfun_bndrs dict_con dict_args
896 `setInlinePragma` dfunInlinePragma
897 where
898 con_app = mkLams dfun_bndrs $
899 mkApps (Var (dataConWrapId dict_con)) dict_args
900 -- mkApps is OK because of the checkForLevPoly call in checkValidClass
901 -- See Note [Levity polymorphism checking] in DsMonad
902 dict_args = map Type inst_tys ++
903 [mkVarApps (Var id) dfun_bndrs | id <- sc_meth_ids]
904
905 (dfun_tvs, dfun_theta, clas, inst_tys) = tcSplitDFunTy (idType dfun_id)
906 ev_ids = mkTemplateLocalsNum 1 dfun_theta
907 dfun_bndrs = dfun_tvs ++ ev_ids
908 clas_tc = classTyCon clas
909 [dict_con] = tyConDataCons clas_tc
910 is_newtype = isNewTyCon clas_tc
911
912 wrapId :: HsWrapper -> IdP (GhcPass id) -> HsExpr (GhcPass id)
913 wrapId wrapper id = mkHsWrap wrapper (HsVar noExt (noLoc id))
914
915 {- Note [Typechecking plan for instance declarations]
916 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
917 For instance declarations we generate the following bindings and implication
918 constraints. Example:
919
920 instance Ord a => Ord [a] where compare = <compare-rhs>
921
922 generates this:
923
924 Bindings:
925 -- Method bindings
926 $ccompare :: forall a. Ord a => a -> a -> Ordering
927 $ccompare = /\a \(d:Ord a). let <meth-ev-binds> in ...
928
929 -- Superclass bindings
930 $cp1Ord :: forall a. Ord a => Eq [a]
931 $cp1Ord = /\a \(d:Ord a). let <sc-ev-binds>
932 in dfEqList (dw :: Eq a)
933
934 Constraints:
935 forall a. Ord a =>
936 -- Method constraint
937 (forall. (empty) => <constraints from compare-rhs>)
938 -- Superclass constraint
939 /\ (forall. (empty) => dw :: Eq a)
940
941 Notice that
942
943 * Per-meth/sc implication. There is one inner implication per
944 superclass or method, with no skolem variables or givens. The only
945 reason for this one is to gather the evidence bindings privately
946 for this superclass or method. This implication is generated
947 by checkInstConstraints.
948
949 * Overall instance implication. There is an overall enclosing
950 implication for the whole instance declaration, with the expected
951 skolems and givens. We need this to get the correct "redundant
952 constraint" warnings, gathering all the uses from all the methods
953 and superclasses. See TcSimplify Note [Tracking redundant
954 constraints]
955
956 * The given constraints in the outer implication may generate
957 evidence, notably by superclass selection. Since the method and
958 superclass bindings are top-level, we want that evidence copied
959 into *every* method or superclass definition. (Some of it will
960 be usused in some, but dead-code elimination will drop it.)
961
962 We achieve this by putting the evidence variable for the overall
963 instance implication into the AbsBinds for each method/superclass.
964 Hence the 'dfun_ev_binds' passed into tcMethods and tcSuperClasses.
965 (And that in turn is why the abs_ev_binds field of AbBinds is a
966 [TcEvBinds] rather than simply TcEvBinds.
967
968 This is a bit of a hack, but works very nicely in practice.
969
970 * Note that if a method has a locally-polymorphic binding, there will
971 be yet another implication for that, generated by tcPolyCheck
972 in tcMethodBody. E.g.
973 class C a where
974 foo :: forall b. Ord b => blah
975
976
977 ************************************************************************
978 * *
979 Type-checking superclasses
980 * *
981 ************************************************************************
982 -}
983
984 tcSuperClasses :: DFunId -> Class -> [TcTyVar] -> [EvVar] -> [TcType]
985 -> TcEvBinds
986 -> TcThetaType
987 -> TcM ([EvVar], LHsBinds GhcTc, Bag Implication)
988 -- Make a new top-level function binding for each superclass,
989 -- something like
990 -- $Ordp1 :: forall a. Ord a => Eq [a]
991 -- $Ordp1 = /\a \(d:Ord a). dfunEqList a (sc_sel d)
992 --
993 -- See Note [Recursive superclasses] for why this is so hard!
994 -- In effect, we build a special-purpose solver for the first step
995 -- of solving each superclass constraint
996 tcSuperClasses dfun_id cls tyvars dfun_evs inst_tys dfun_ev_binds sc_theta
997 = do { (ids, binds, implics) <- mapAndUnzip3M tc_super (zip sc_theta [fIRST_TAG..])
998 ; return (ids, listToBag binds, listToBag implics) }
999 where
1000 loc = getSrcSpan dfun_id
1001 size = sizeTypes inst_tys
1002 tc_super (sc_pred, n)
1003 = do { (sc_implic, ev_binds_var, sc_ev_tm)
1004 <- checkInstConstraints $ emitWanted (ScOrigin size) sc_pred
1005
1006 ; sc_top_name <- newName (mkSuperDictAuxOcc n (getOccName cls))
1007 ; sc_ev_id <- newEvVar sc_pred
1008 ; addTcEvBind ev_binds_var $ mkWantedEvBind sc_ev_id sc_ev_tm
1009 ; let sc_top_ty = mkInvForAllTys tyvars (mkLamTypes dfun_evs sc_pred)
1010 sc_top_id = mkLocalId sc_top_name sc_top_ty
1011 export = ABE { abe_ext = noExt
1012 , abe_wrap = idHsWrapper
1013 , abe_poly = sc_top_id
1014 , abe_mono = sc_ev_id
1015 , abe_prags = noSpecPrags }
1016 local_ev_binds = TcEvBinds ev_binds_var
1017 bind = AbsBinds { abs_ext = noExt
1018 , abs_tvs = tyvars
1019 , abs_ev_vars = dfun_evs
1020 , abs_exports = [export]
1021 , abs_ev_binds = [dfun_ev_binds, local_ev_binds]
1022 , abs_binds = emptyBag
1023 , abs_sig = False }
1024 ; return (sc_top_id, L loc bind, sc_implic) }
1025
1026 -------------------
1027 checkInstConstraints :: TcM result
1028 -> TcM (Implication, EvBindsVar, result)
1029 -- See Note [Typechecking plan for instance declarations]
1030 checkInstConstraints thing_inside
1031 = do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints $
1032 thing_inside
1033
1034 ; ev_binds_var <- newTcEvBinds
1035 ; implic <- newImplication
1036 ; let implic' = implic { ic_tclvl = tclvl
1037 , ic_wanted = wanted
1038 , ic_binds = ev_binds_var
1039 , ic_info = InstSkol }
1040
1041 ; return (implic', ev_binds_var, result) }
1042
1043 {-
1044 Note [Recursive superclasses]
1045 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1046 See Trac #3731, #4809, #5751, #5913, #6117, #6161, which all
1047 describe somewhat more complicated situations, but ones
1048 encountered in practice.
1049
1050 See also tests tcrun020, tcrun021, tcrun033, and Trac #11427.
1051
1052 ----- THE PROBLEM --------
1053 The problem is that it is all too easy to create a class whose
1054 superclass is bottom when it should not be.
1055
1056 Consider the following (extreme) situation:
1057 class C a => D a where ...
1058 instance D [a] => D [a] where ... (dfunD)
1059 instance C [a] => C [a] where ... (dfunC)
1060 Although this looks wrong (assume D [a] to prove D [a]), it is only a
1061 more extreme case of what happens with recursive dictionaries, and it
1062 can, just about, make sense because the methods do some work before
1063 recursing.
1064
1065 To implement the dfunD we must generate code for the superclass C [a],
1066 which we had better not get by superclass selection from the supplied
1067 argument:
1068 dfunD :: forall a. D [a] -> D [a]
1069 dfunD = \d::D [a] -> MkD (scsel d) ..
1070
1071 Otherwise if we later encounter a situation where
1072 we have a [Wanted] dw::D [a] we might solve it thus:
1073 dw := dfunD dw
1074 Which is all fine except that now ** the superclass C is bottom **!
1075
1076 The instance we want is:
1077 dfunD :: forall a. D [a] -> D [a]
1078 dfunD = \d::D [a] -> MkD (dfunC (scsel d)) ...
1079
1080 ----- THE SOLUTION --------
1081 The basic solution is simple: be very careful about using superclass
1082 selection to generate a superclass witness in a dictionary function
1083 definition. More precisely:
1084
1085 Superclass Invariant: in every class dictionary,
1086 every superclass dictionary field
1087 is non-bottom
1088
1089 To achieve the Superclass Invariant, in a dfun definition we can
1090 generate a guaranteed-non-bottom superclass witness from:
1091 (sc1) one of the dictionary arguments itself (all non-bottom)
1092 (sc2) an immediate superclass of a smaller dictionary
1093 (sc3) a call of a dfun (always returns a dictionary constructor)
1094
1095 The tricky case is (sc2). We proceed by induction on the size of
1096 the (type of) the dictionary, defined by TcValidity.sizeTypes.
1097 Let's suppose we are building a dictionary of size 3, and
1098 suppose the Superclass Invariant holds of smaller dictionaries.
1099 Then if we have a smaller dictionary, its immediate superclasses
1100 will be non-bottom by induction.
1101
1102 What does "we have a smaller dictionary" mean? It might be
1103 one of the arguments of the instance, or one of its superclasses.
1104 Here is an example, taken from CmmExpr:
1105 class Ord r => UserOfRegs r a where ...
1106 (i1) instance UserOfRegs r a => UserOfRegs r (Maybe a) where
1107 (i2) instance (Ord r, UserOfRegs r CmmReg) => UserOfRegs r CmmExpr where
1108
1109 For (i1) we can get the (Ord r) superclass by selection from (UserOfRegs r a),
1110 since it is smaller than the thing we are building (UserOfRegs r (Maybe a).
1111
1112 But for (i2) that isn't the case, so we must add an explicit, and
1113 perhaps surprising, (Ord r) argument to the instance declaration.
1114
1115 Here's another example from Trac #6161:
1116
1117 class Super a => Duper a where ...
1118 class Duper (Fam a) => Foo a where ...
1119 (i3) instance Foo a => Duper (Fam a) where ...
1120 (i4) instance Foo Float where ...
1121
1122 It would be horribly wrong to define
1123 dfDuperFam :: Foo a -> Duper (Fam a) -- from (i3)
1124 dfDuperFam d = MkDuper (sc_sel1 (sc_sel2 d)) ...
1125
1126 dfFooFloat :: Foo Float -- from (i4)
1127 dfFooFloat = MkFoo (dfDuperFam dfFooFloat) ...
1128
1129 Now the Super superclass of Duper is definitely bottom!
1130
1131 This won't happen because when processing (i3) we can use the
1132 superclasses of (Foo a), which is smaller, namely Duper (Fam a). But
1133 that is *not* smaller than the target so we can't take *its*
1134 superclasses. As a result the program is rightly rejected, unless you
1135 add (Super (Fam a)) to the context of (i3).
1136
1137 Note [Solving superclass constraints]
1138 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1139 How do we ensure that every superclass witness is generated by
1140 one of (sc1) (sc2) or (sc3) in Note [Recursive superclasses].
1141 Answer:
1142
1143 * Superclass "wanted" constraints have CtOrigin of (ScOrigin size)
1144 where 'size' is the size of the instance declaration. e.g.
1145 class C a => D a where...
1146 instance blah => D [a] where ...
1147 The wanted superclass constraint for C [a] has origin
1148 ScOrigin size, where size = size( D [a] ).
1149
1150 * (sc1) When we rewrite such a wanted constraint, it retains its
1151 origin. But if we apply an instance declaration, we can set the
1152 origin to (ScOrigin infinity), thus lifting any restrictions by
1153 making prohibitedSuperClassSolve return False.
1154
1155 * (sc2) ScOrigin wanted constraints can't be solved from a
1156 superclass selection, except at a smaller type. This test is
1157 implemented by TcInteract.prohibitedSuperClassSolve
1158
1159 * The "given" constraints of an instance decl have CtOrigin
1160 GivenOrigin InstSkol.
1161
1162 * When we make a superclass selection from InstSkol we use
1163 a SkolemInfo of (InstSC size), where 'size' is the size of
1164 the constraint whose superclass we are taking. A similarly
1165 when taking the superclass of an InstSC. This is implemented
1166 in TcCanonical.newSCWorkFromFlavored
1167
1168 Note [Silent superclass arguments] (historical interest only)
1169 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1170 NB1: this note describes our *old* solution to the
1171 recursive-superclass problem. I'm keeping the Note
1172 for now, just as institutional memory.
1173 However, the code for silent superclass arguments
1174 was removed in late Dec 2014
1175
1176 NB2: the silent-superclass solution introduced new problems
1177 of its own, in the form of instance overlap. Tests
1178 SilentParametersOverlapping, T5051, and T7862 are examples
1179
1180 NB3: the silent-superclass solution also generated tons of
1181 extra dictionaries. For example, in monad-transformer
1182 code, when constructing a Monad dictionary you had to pass
1183 an Applicative dictionary; and to construct that you neede
1184 a Functor dictionary. Yet these extra dictionaries were
1185 often never used. Test T3064 compiled *far* faster after
1186 silent superclasses were eliminated.
1187
1188 Our solution to this problem "silent superclass arguments". We pass
1189 to each dfun some ``silent superclass arguments’’, which are the
1190 immediate superclasses of the dictionary we are trying to
1191 construct. In our example:
1192 dfun :: forall a. C [a] -> D [a] -> D [a]
1193 dfun = \(dc::C [a]) (dd::D [a]) -> DOrd dc ...
1194 Notice the extra (dc :: C [a]) argument compared to the previous version.
1195
1196 This gives us:
1197
1198 -----------------------------------------------------------
1199 DFun Superclass Invariant
1200 ~~~~~~~~~~~~~~~~~~~~~~~~
1201 In the body of a DFun, every superclass argument to the
1202 returned dictionary is
1203 either * one of the arguments of the DFun,
1204 or * constant, bound at top level
1205 -----------------------------------------------------------
1206
1207 This net effect is that it is safe to treat a dfun application as
1208 wrapping a dictionary constructor around its arguments (in particular,
1209 a dfun never picks superclasses from the arguments under the
1210 dictionary constructor). No superclass is hidden inside a dfun
1211 application.
1212
1213 The extra arguments required to satisfy the DFun Superclass Invariant
1214 always come first, and are called the "silent" arguments. You can
1215 find out how many silent arguments there are using Id.dfunNSilent;
1216 and then you can just drop that number of arguments to see the ones
1217 that were in the original instance declaration.
1218
1219 DFun types are built (only) by MkId.mkDictFunId, so that is where we
1220 decide what silent arguments are to be added.
1221 -}
1222
1223 {-
1224 ************************************************************************
1225 * *
1226 Type-checking an instance method
1227 * *
1228 ************************************************************************
1229
1230 tcMethod
1231 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
1232 - Remembering to use fresh Name (the instance method Name) as the binder
1233 - Bring the instance method Ids into scope, for the benefit of tcInstSig
1234 - Use sig_fn mapping instance method Name -> instance tyvars
1235 - Ditto prag_fn
1236 - Use tcValBinds to do the checking
1237 -}
1238
1239 tcMethods :: DFunId -> Class
1240 -> [TcTyVar] -> [EvVar]
1241 -> [TcType]
1242 -> TcEvBinds
1243 -> ([Located TcSpecPrag], TcPragEnv)
1244 -> [ClassOpItem]
1245 -> InstBindings GhcRn
1246 -> TcM ([Id], LHsBinds GhcTc, Bag Implication)
1247 -- The returned inst_meth_ids all have types starting
1248 -- forall tvs. theta => ...
1249 tcMethods dfun_id clas tyvars dfun_ev_vars inst_tys
1250 dfun_ev_binds (spec_inst_prags, prag_fn) op_items
1251 (InstBindings { ib_binds = binds
1252 , ib_tyvars = lexical_tvs
1253 , ib_pragmas = sigs
1254 , ib_extensions = exts
1255 , ib_derived = is_derived })
1256 -- tcExtendTyVarEnv (not scopeTyVars) is OK because the TcLevel is pushed
1257 -- in checkInstConstraints
1258 = tcExtendNameTyVarEnv (lexical_tvs `zip` tyvars) $
1259 -- The lexical_tvs scope over the 'where' part
1260 do { traceTc "tcInstMeth" (ppr sigs $$ ppr binds)
1261 ; checkMinimalDefinition
1262 ; checkMethBindMembership
1263 ; (ids, binds, mb_implics) <- set_exts exts $
1264 unset_warnings_deriving $
1265 mapAndUnzip3M tc_item op_items
1266 ; return (ids, listToBag binds, listToBag (catMaybes mb_implics)) }
1267 where
1268 set_exts :: [LangExt.Extension] -> TcM a -> TcM a
1269 set_exts es thing = foldr setXOptM thing es
1270
1271 -- See Note [Avoid -Winaccessible-code when deriving]
1272 unset_warnings_deriving :: TcM a -> TcM a
1273 unset_warnings_deriving
1274 | is_derived = unsetWOptM Opt_WarnInaccessibleCode
1275 | otherwise = id
1276
1277 hs_sig_fn = mkHsSigFun sigs
1278 inst_loc = getSrcSpan dfun_id
1279
1280 ----------------------
1281 tc_item :: ClassOpItem -> TcM (Id, LHsBind GhcTc, Maybe Implication)
1282 tc_item (sel_id, dm_info)
1283 | Just (user_bind, bndr_loc, prags) <- findMethodBind (idName sel_id) binds prag_fn
1284 = tcMethodBody clas tyvars dfun_ev_vars inst_tys
1285 dfun_ev_binds is_derived hs_sig_fn
1286 spec_inst_prags prags
1287 sel_id user_bind bndr_loc
1288 | otherwise
1289 = do { traceTc "tc_def" (ppr sel_id)
1290 ; tc_default sel_id dm_info }
1291
1292 ----------------------
1293 tc_default :: Id -> DefMethInfo
1294 -> TcM (TcId, LHsBind GhcTc, Maybe Implication)
1295
1296 tc_default sel_id (Just (dm_name, _))
1297 = do { (meth_bind, inline_prags) <- mkDefMethBind clas inst_tys sel_id dm_name
1298 ; tcMethodBody clas tyvars dfun_ev_vars inst_tys
1299 dfun_ev_binds is_derived hs_sig_fn
1300 spec_inst_prags inline_prags
1301 sel_id meth_bind inst_loc }
1302
1303 tc_default sel_id Nothing -- No default method at all
1304 = do { traceTc "tc_def: warn" (ppr sel_id)
1305 ; (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars
1306 inst_tys sel_id
1307 ; dflags <- getDynFlags
1308 ; let meth_bind = mkVarBind meth_id $
1309 mkLHsWrap lam_wrapper (error_rhs dflags)
1310 ; return (meth_id, meth_bind, Nothing) }
1311 where
1312 error_rhs dflags = L inst_loc $ HsApp noExt error_fun (error_msg dflags)
1313 error_fun = L inst_loc $
1314 wrapId (mkWpTyApps
1315 [ getRuntimeRep meth_tau, meth_tau])
1316 nO_METHOD_BINDING_ERROR_ID
1317 error_msg dflags = L inst_loc (HsLit noExt (HsStringPrim NoSourceText
1318 (unsafeMkByteString (error_string dflags))))
1319 meth_tau = funResultTy (piResultTys (idType sel_id) inst_tys)
1320 error_string dflags = showSDoc dflags
1321 (hcat [ppr inst_loc, vbar, ppr sel_id ])
1322 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_ev_vars
1323
1324 ----------------------
1325 -- Check if one of the minimal complete definitions is satisfied
1326 checkMinimalDefinition
1327 = whenIsJust (isUnsatisfied methodExists (classMinimalDef clas)) $
1328 warnUnsatisfiedMinimalDefinition
1329
1330 methodExists meth = isJust (findMethodBind meth binds prag_fn)
1331
1332 ----------------------
1333 -- Check if any method bindings do not correspond to the class.
1334 -- See Note [Mismatched class methods and associated type families].
1335 checkMethBindMembership
1336 = let bind_nms = map unLoc $ collectMethodBinders binds
1337 cls_meth_nms = map (idName . fst) op_items
1338 mismatched_meths = bind_nms `minusList` cls_meth_nms
1339 in forM_ mismatched_meths $ \mismatched_meth ->
1340 addErrTc $ hsep
1341 [ text "Class", quotes (ppr (className clas))
1342 , text "does not have a method", quotes (ppr mismatched_meth)]
1343
1344 {-
1345 Note [Mismatched class methods and associated type families]
1346 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1347 It's entirely possible for someone to put methods or associated type family
1348 instances inside of a class in which it doesn't belong. For instance, we'd
1349 want to fail if someone wrote this:
1350
1351 instance Eq () where
1352 type Rep () = Maybe
1353 compare = undefined
1354
1355 Since neither the type family `Rep` nor the method `compare` belong to the
1356 class `Eq`. Normally, this is caught in the renamer when resolving RdrNames,
1357 since that would discover that the parent class `Eq` is incorrect.
1358
1359 However, there is a scenario in which the renamer could fail to catch this:
1360 if the instance was generated through Template Haskell, as in #12387. In that
1361 case, Template Haskell will provide fully resolved names (e.g.,
1362 `GHC.Classes.compare`), so the renamer won't notice the sleight-of-hand going
1363 on. For this reason, we also put an extra validity check for this in the
1364 typechecker as a last resort.
1365
1366 Note [Avoid -Winaccessible-code when deriving]
1367 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1368 -Winaccessible-code can be particularly noisy when deriving instances for
1369 GADTs. Consider the following example (adapted from #8128):
1370
1371 data T a where
1372 MkT1 :: Int -> T Int
1373 MkT2 :: T Bool
1374 MkT3 :: T Bool
1375 deriving instance Eq (T a)
1376 deriving instance Ord (T a)
1377
1378 In the derived Ord instance, GHC will generate the following code:
1379
1380 instance Ord (T a) where
1381 compare x y
1382 = case x of
1383 MkT2
1384 -> case y of
1385 MkT1 {} -> GT
1386 MkT2 -> EQ
1387 _ -> LT
1388 ...
1389
1390 However, that MkT1 is unreachable, since the type indices for MkT1 and MkT2
1391 differ, so if -Winaccessible-code is enabled, then deriving this instance will
1392 result in unwelcome warnings.
1393
1394 One conceivable approach to fixing this issue would be to change `deriving Ord`
1395 such that it becomes smarter about not generating unreachable cases. This,
1396 however, would be a highly nontrivial refactor, as we'd have to propagate
1397 through typing information everywhere in the algorithm that generates Ord
1398 instances in order to determine which cases were unreachable. This seems like
1399 a lot of work for minimal gain, so we have opted not to go for this approach.
1400
1401 Instead, we take the much simpler approach of always disabling
1402 -Winaccessible-code for derived code. To accomplish this, we do the following:
1403
1404 1. In tcMethods (which typechecks method bindings), disable
1405 -Winaccessible-code.
1406 2. When creating Implications during typechecking, record the Env
1407 (through ic_env) at the time of creation. Since the Env also stores
1408 DynFlags, this will remember that -Winaccessible-code was disabled over
1409 the scope of that implication.
1410 3. After typechecking comes error reporting, where GHC must decide how to
1411 report inaccessible code to the user, on an Implication-by-Implication
1412 basis. If an Implication's DynFlags indicate that -Winaccessible-code was
1413 disabled, then don't bother reporting it. That's it!
1414 -}
1415
1416 ------------------------
1417 tcMethodBody :: Class -> [TcTyVar] -> [EvVar] -> [TcType]
1418 -> TcEvBinds -> Bool
1419 -> HsSigFun
1420 -> [LTcSpecPrag] -> [LSig GhcRn]
1421 -> Id -> LHsBind GhcRn -> SrcSpan
1422 -> TcM (TcId, LHsBind GhcTc, Maybe Implication)
1423 tcMethodBody clas tyvars dfun_ev_vars inst_tys
1424 dfun_ev_binds is_derived
1425 sig_fn spec_inst_prags prags
1426 sel_id (L bind_loc meth_bind) bndr_loc
1427 = add_meth_ctxt $
1428 do { traceTc "tcMethodBody" (ppr sel_id <+> ppr (idType sel_id) $$ ppr bndr_loc)
1429 ; (global_meth_id, local_meth_id) <- setSrcSpan bndr_loc $
1430 mkMethIds clas tyvars dfun_ev_vars
1431 inst_tys sel_id
1432
1433 ; let lm_bind = meth_bind { fun_id = L bndr_loc (idName local_meth_id) }
1434 -- Substitute the local_meth_name for the binder
1435 -- NB: the binding is always a FunBind
1436
1437 -- taking instance signature into account might change the type of
1438 -- the local_meth_id
1439 ; (meth_implic, ev_binds_var, tc_bind)
1440 <- checkInstConstraints $
1441 tcMethodBodyHelp sig_fn sel_id local_meth_id (L bind_loc lm_bind)
1442
1443 ; global_meth_id <- addInlinePrags global_meth_id prags
1444 ; spec_prags <- tcSpecPrags global_meth_id prags
1445
1446 ; let specs = mk_meth_spec_prags global_meth_id spec_inst_prags spec_prags
1447 export = ABE { abe_ext = noExt
1448 , abe_poly = global_meth_id
1449 , abe_mono = local_meth_id
1450 , abe_wrap = idHsWrapper
1451 , abe_prags = specs }
1452
1453 local_ev_binds = TcEvBinds ev_binds_var
1454 full_bind = AbsBinds { abs_ext = noExt
1455 , abs_tvs = tyvars
1456 , abs_ev_vars = dfun_ev_vars
1457 , abs_exports = [export]
1458 , abs_ev_binds = [dfun_ev_binds, local_ev_binds]
1459 , abs_binds = tc_bind
1460 , abs_sig = True }
1461
1462 ; return (global_meth_id, L bind_loc full_bind, Just meth_implic) }
1463 where
1464 -- For instance decls that come from deriving clauses
1465 -- we want to print out the full source code if there's an error
1466 -- because otherwise the user won't see the code at all
1467 add_meth_ctxt thing
1468 | is_derived = addLandmarkErrCtxt (derivBindCtxt sel_id clas inst_tys) thing
1469 | otherwise = thing
1470
1471 tcMethodBodyHelp :: HsSigFun -> Id -> TcId
1472 -> LHsBind GhcRn -> TcM (LHsBinds GhcTcId)
1473 tcMethodBodyHelp hs_sig_fn sel_id local_meth_id meth_bind
1474 | Just hs_sig_ty <- hs_sig_fn sel_name
1475 -- There is a signature in the instance
1476 -- See Note [Instance method signatures]
1477 = do { let ctxt = FunSigCtxt sel_name True
1478 ; (sig_ty, hs_wrap)
1479 <- setSrcSpan (getLoc (hsSigType hs_sig_ty)) $
1480 do { inst_sigs <- xoptM LangExt.InstanceSigs
1481 ; checkTc inst_sigs (misplacedInstSig sel_name hs_sig_ty)
1482 ; sig_ty <- tcHsSigType (FunSigCtxt sel_name False) hs_sig_ty
1483 ; let local_meth_ty = idType local_meth_id
1484 ; hs_wrap <- addErrCtxtM (methSigCtxt sel_name sig_ty local_meth_ty) $
1485 tcSubType_NC ctxt sig_ty local_meth_ty
1486 ; return (sig_ty, hs_wrap) }
1487
1488 ; inner_meth_name <- newName (nameOccName sel_name)
1489 ; let inner_meth_id = mkLocalId inner_meth_name sig_ty
1490 inner_meth_sig = CompleteSig { sig_bndr = inner_meth_id
1491 , sig_ctxt = ctxt
1492 , sig_loc = getLoc (hsSigType hs_sig_ty) }
1493
1494
1495 ; (tc_bind, [inner_id]) <- tcPolyCheck no_prag_fn inner_meth_sig meth_bind
1496
1497 ; let export = ABE { abe_ext = noExt
1498 , abe_poly = local_meth_id
1499 , abe_mono = inner_id
1500 , abe_wrap = hs_wrap
1501 , abe_prags = noSpecPrags }
1502
1503 ; return (unitBag $ L (getLoc meth_bind) $
1504 AbsBinds { abs_ext = noExt, abs_tvs = [], abs_ev_vars = []
1505 , abs_exports = [export]
1506 , abs_binds = tc_bind, abs_ev_binds = []
1507 , abs_sig = True }) }
1508
1509 | otherwise -- No instance signature
1510 = do { let ctxt = FunSigCtxt sel_name False
1511 -- False <=> don't report redundant constraints
1512 -- The signature is not under the users control!
1513 tc_sig = completeSigFromId ctxt local_meth_id
1514 -- Absent a type sig, there are no new scoped type variables here
1515 -- Only the ones from the instance decl itself, which are already
1516 -- in scope. Example:
1517 -- class C a where { op :: forall b. Eq b => ... }
1518 -- instance C [c] where { op = <rhs> }
1519 -- In <rhs>, 'c' is scope but 'b' is not!
1520
1521 ; (tc_bind, _) <- tcPolyCheck no_prag_fn tc_sig meth_bind
1522 ; return tc_bind }
1523
1524 where
1525 sel_name = idName sel_id
1526 no_prag_fn = emptyPragEnv -- No pragmas for local_meth_id;
1527 -- they are all for meth_id
1528
1529
1530 ------------------------
1531 mkMethIds :: Class -> [TcTyVar] -> [EvVar]
1532 -> [TcType] -> Id -> TcM (TcId, TcId)
1533 -- returns (poly_id, local_id), but ignoring any instance signature
1534 -- See Note [Instance method signatures]
1535 mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id
1536 = do { poly_meth_name <- newName (mkClassOpAuxOcc sel_occ)
1537 ; local_meth_name <- newName sel_occ
1538 -- Base the local_meth_name on the selector name, because
1539 -- type errors from tcMethodBody come from here
1540 ; let poly_meth_id = mkLocalId poly_meth_name poly_meth_ty
1541 local_meth_id = mkLocalId local_meth_name local_meth_ty
1542
1543 ; return (poly_meth_id, local_meth_id) }
1544 where
1545 sel_name = idName sel_id
1546 sel_occ = nameOccName sel_name
1547 local_meth_ty = instantiateMethod clas sel_id inst_tys
1548 poly_meth_ty = mkSpecSigmaTy tyvars theta local_meth_ty
1549 theta = map idType dfun_ev_vars
1550
1551 methSigCtxt :: Name -> TcType -> TcType -> TidyEnv -> TcM (TidyEnv, MsgDoc)
1552 methSigCtxt sel_name sig_ty meth_ty env0
1553 = do { (env1, sig_ty) <- zonkTidyTcType env0 sig_ty
1554 ; (env2, meth_ty) <- zonkTidyTcType env1 meth_ty
1555 ; let msg = hang (text "When checking that instance signature for" <+> quotes (ppr sel_name))
1556 2 (vcat [ text "is more general than its signature in the class"
1557 , text "Instance sig:" <+> ppr sig_ty
1558 , text " Class sig:" <+> ppr meth_ty ])
1559 ; return (env2, msg) }
1560
1561 misplacedInstSig :: Name -> LHsSigType GhcRn -> SDoc
1562 misplacedInstSig name hs_ty
1563 = vcat [ hang (text "Illegal type signature in instance declaration:")
1564 2 (hang (pprPrefixName name)
1565 2 (dcolon <+> ppr hs_ty))
1566 , text "(Use InstanceSigs to allow this)" ]
1567
1568 {- Note [Instance method signatures]
1569 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1570 With -XInstanceSigs we allow the user to supply a signature for the
1571 method in an instance declaration. Here is an artificial example:
1572
1573 data T a = MkT a
1574 instance Ord a => Ord (T a) where
1575 (>) :: forall b. b -> b -> Bool
1576 (>) = error "You can't compare Ts"
1577
1578 The instance signature can be *more* polymorphic than the instantiated
1579 class method (in this case: Age -> Age -> Bool), but it cannot be less
1580 polymorphic. Moreover, if a signature is given, the implementation
1581 code should match the signature, and type variables bound in the
1582 singature should scope over the method body.
1583
1584 We achieve this by building a TcSigInfo for the method, whether or not
1585 there is an instance method signature, and using that to typecheck
1586 the declaration (in tcMethodBody). That means, conveniently,
1587 that the type variables bound in the signature will scope over the body.
1588
1589 What about the check that the instance method signature is more
1590 polymorphic than the instantiated class method type? We just do a
1591 tcSubType call in tcMethodBodyHelp, and generate a nested AbsBind, like
1592 this (for the example above
1593
1594 AbsBind { abs_tvs = [a], abs_ev_vars = [d:Ord a]
1595 , abs_exports
1596 = ABExport { (>) :: forall a. Ord a => T a -> T a -> Bool
1597 , gr_lcl :: T a -> T a -> Bool }
1598 , abs_binds
1599 = AbsBind { abs_tvs = [], abs_ev_vars = []
1600 , abs_exports = ABExport { gr_lcl :: T a -> T a -> Bool
1601 , gr_inner :: forall b. b -> b -> Bool }
1602 , abs_binds = AbsBind { abs_tvs = [b], abs_ev_vars = []
1603 , ..etc.. }
1604 } }
1605
1606 Wow! Three nested AbsBinds!
1607 * The outer one abstracts over the tyvars and dicts for the instance
1608 * The middle one is only present if there is an instance signature,
1609 and does the impedance matching for that signature
1610 * The inner one is for the method binding itself against either the
1611 signature from the class, or the instance signature.
1612 -}
1613
1614 ----------------------
1615 mk_meth_spec_prags :: Id -> [LTcSpecPrag] -> [LTcSpecPrag] -> TcSpecPrags
1616 -- Adapt the 'SPECIALISE instance' pragmas to work for this method Id
1617 -- There are two sources:
1618 -- * spec_prags_for_me: {-# SPECIALISE op :: <blah> #-}
1619 -- * spec_prags_from_inst: derived from {-# SPECIALISE instance :: <blah> #-}
1620 -- These ones have the dfun inside, but [perhaps surprisingly]
1621 -- the correct wrapper.
1622 -- See Note [Handling SPECIALISE pragmas] in TcBinds
1623 mk_meth_spec_prags meth_id spec_inst_prags spec_prags_for_me
1624 = SpecPrags (spec_prags_for_me ++ spec_prags_from_inst)
1625 where
1626 spec_prags_from_inst
1627 | isInlinePragma (idInlinePragma meth_id)
1628 = [] -- Do not inherit SPECIALISE from the instance if the
1629 -- method is marked INLINE, because then it'll be inlined
1630 -- and the specialisation would do nothing. (Indeed it'll provoke
1631 -- a warning from the desugarer
1632 | otherwise
1633 = [ L inst_loc (SpecPrag meth_id wrap inl)
1634 | L inst_loc (SpecPrag _ wrap inl) <- spec_inst_prags]
1635
1636
1637 mkDefMethBind :: Class -> [Type] -> Id -> Name
1638 -> TcM (LHsBind GhcRn, [LSig GhcRn])
1639 -- The is a default method (vanailla or generic) defined in the class
1640 -- So make a binding op = $dmop @t1 @t2
1641 -- where $dmop is the name of the default method in the class,
1642 -- and t1,t2 are the instance types.
1643 -- See Note [Default methods in instances] for why we use
1644 -- visible type application here
1645 mkDefMethBind clas inst_tys sel_id dm_name
1646 = do { dflags <- getDynFlags
1647 ; dm_id <- tcLookupId dm_name
1648 ; let inline_prag = idInlinePragma dm_id
1649 inline_prags | isAnyInlinePragma inline_prag
1650 = [noLoc (InlineSig noExt fn inline_prag)]
1651 | otherwise
1652 = []
1653 -- Copy the inline pragma (if any) from the default method
1654 -- to this version. Note [INLINE and default methods]
1655
1656 fn = noLoc (idName sel_id)
1657 visible_inst_tys = [ ty | (tcb, ty) <- tyConBinders (classTyCon clas) `zip` inst_tys
1658 , tyConBinderArgFlag tcb /= Inferred ]
1659 rhs = foldl' mk_vta (nlHsVar dm_name) visible_inst_tys
1660 bind = noLoc $ mkTopFunBind Generated fn $
1661 [mkSimpleMatch (mkPrefixFunRhs fn) [] rhs]
1662
1663 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Filling in method body"
1664 (vcat [ppr clas <+> ppr inst_tys,
1665 nest 2 (ppr sel_id <+> equals <+> ppr rhs)]))
1666
1667 ; return (bind, inline_prags) }
1668 where
1669 mk_vta :: LHsExpr GhcRn -> Type -> LHsExpr GhcRn
1670 mk_vta fun ty = noLoc (HsAppType (mkEmptyWildCardBndrs $ nlHsParTy
1671 $ noLoc $ XHsType $ NHsCoreTy ty) fun)
1672 -- NB: use visible type application
1673 -- See Note [Default methods in instances]
1674
1675 ----------------------
1676 derivBindCtxt :: Id -> Class -> [Type ] -> SDoc
1677 derivBindCtxt sel_id clas tys
1678 = vcat [ text "When typechecking the code for" <+> quotes (ppr sel_id)
1679 , nest 2 (text "in a derived instance for"
1680 <+> quotes (pprClassPred clas tys) <> colon)
1681 , nest 2 $ text "To see the code I am typechecking, use -ddump-deriv" ]
1682
1683 warnUnsatisfiedMinimalDefinition :: ClassMinimalDef -> TcM ()
1684 warnUnsatisfiedMinimalDefinition mindef
1685 = do { warn <- woptM Opt_WarnMissingMethods
1686 ; warnTc (Reason Opt_WarnMissingMethods) warn message
1687 }
1688 where
1689 message = vcat [text "No explicit implementation for"
1690 ,nest 2 $ pprBooleanFormulaNice mindef
1691 ]
1692
1693 {-
1694 Note [Export helper functions]
1695 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1696 We arrange to export the "helper functions" of an instance declaration,
1697 so that they are not subject to preInlineUnconditionally, even if their
1698 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1699 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1700 non-variable for them.
1701
1702 We could change this by making DFunUnfoldings have CoreExprs, but it
1703 seems a bit simpler this way.
1704
1705 Note [Default methods in instances]
1706 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1707 Consider this
1708
1709 class Baz v x where
1710 foo :: x -> x
1711 foo y = <blah>
1712
1713 instance Baz Int Int
1714
1715 From the class decl we get
1716
1717 $dmfoo :: forall v x. Baz v x => x -> x
1718 $dmfoo y = <blah>
1719
1720 Notice that the type is ambiguous. So we use Visible Type Application
1721 to disambiguate:
1722
1723 $dBazIntInt = MkBaz fooIntInt
1724 fooIntInt = $dmfoo @Int @Int
1725
1726 Lacking VTA we'd get ambiguity errors involving the default method. This applies
1727 equally to vanilla default methods (Trac #1061) and generic default methods
1728 (Trac #12220).
1729
1730 Historical note: before we had VTA we had to generate
1731 post-type-checked code, which took a lot more code, and didn't work for
1732 generic default methods.
1733
1734 Note [INLINE and default methods]
1735 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1736 Default methods need special case. They are supposed to behave rather like
1737 macros. For example
1738
1739 class Foo a where
1740 op1, op2 :: Bool -> a -> a
1741
1742 {-# INLINE op1 #-}
1743 op1 b x = op2 (not b) x
1744
1745 instance Foo Int where
1746 -- op1 via default method
1747 op2 b x = <blah>
1748
1749 The instance declaration should behave
1750
1751 just as if 'op1' had been defined with the
1752 code, and INLINE pragma, from its original
1753 definition.
1754
1755 That is, just as if you'd written
1756
1757 instance Foo Int where
1758 op2 b x = <blah>
1759
1760 {-# INLINE op1 #-}
1761 op1 b x = op2 (not b) x
1762
1763 So for the above example we generate:
1764
1765 {-# INLINE $dmop1 #-}
1766 -- $dmop1 has an InlineCompulsory unfolding
1767 $dmop1 d b x = op2 d (not b) x
1768
1769 $fFooInt = MkD $cop1 $cop2
1770
1771 {-# INLINE $cop1 #-}
1772 $cop1 = $dmop1 $fFooInt
1773
1774 $cop2 = <blah>
1775
1776 Note carefully:
1777
1778 * We *copy* any INLINE pragma from the default method $dmop1 to the
1779 instance $cop1. Otherwise we'll just inline the former in the
1780 latter and stop, which isn't what the user expected
1781
1782 * Regardless of its pragma, we give the default method an
1783 unfolding with an InlineCompulsory source. That means
1784 that it'll be inlined at every use site, notably in
1785 each instance declaration, such as $cop1. This inlining
1786 must happen even though
1787 a) $dmop1 is not saturated in $cop1
1788 b) $cop1 itself has an INLINE pragma
1789
1790 It's vital that $dmop1 *is* inlined in this way, to allow the mutual
1791 recursion between $fooInt and $cop1 to be broken
1792
1793 * To communicate the need for an InlineCompulsory to the desugarer
1794 (which makes the Unfoldings), we use the IsDefaultMethod constructor
1795 in TcSpecPrags.
1796
1797
1798 ************************************************************************
1799 * *
1800 Specialise instance pragmas
1801 * *
1802 ************************************************************************
1803
1804 Note [SPECIALISE instance pragmas]
1805 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1806 Consider
1807
1808 instance (Ix a, Ix b) => Ix (a,b) where
1809 {-# SPECIALISE instance Ix (Int,Int) #-}
1810 range (x,y) = ...
1811
1812 We make a specialised version of the dictionary function, AND
1813 specialised versions of each *method*. Thus we should generate
1814 something like this:
1815
1816 $dfIxPair :: (Ix a, Ix b) => Ix (a,b)
1817 {-# DFUN [$crangePair, ...] #-}
1818 {-# SPECIALISE $dfIxPair :: Ix (Int,Int) #-}
1819 $dfIxPair da db = Ix ($crangePair da db) (...other methods...)
1820
1821 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
1822 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
1823 $crange da db = <blah>
1824
1825 The SPECIALISE pragmas are acted upon by the desugarer, which generate
1826
1827 dii :: Ix Int
1828 dii = ...
1829
1830 $s$dfIxPair :: Ix ((Int,Int),(Int,Int))
1831 {-# DFUN [$crangePair di di, ...] #-}
1832 $s$dfIxPair = Ix ($crangePair di di) (...)
1833
1834 {-# RULE forall (d1,d2:Ix Int). $dfIxPair Int Int d1 d2 = $s$dfIxPair #-}
1835
1836 $s$crangePair :: ((Int,Int),(Int,Int)) -> [(Int,Int)]
1837 $c$crangePair = ...specialised RHS of $crangePair...
1838
1839 {-# RULE forall (d1,d2:Ix Int). $crangePair Int Int d1 d2 = $s$crangePair #-}
1840
1841 Note that
1842
1843 * The specialised dictionary $s$dfIxPair is very much needed, in case we
1844 call a function that takes a dictionary, but in a context where the
1845 specialised dictionary can be used. See Trac #7797.
1846
1847 * The ClassOp rule for 'range' works equally well on $s$dfIxPair, because
1848 it still has a DFunUnfolding. See Note [ClassOp/DFun selection]
1849
1850 * A call (range ($dfIxPair Int Int d1 d2)) might simplify two ways:
1851 --> {ClassOp rule for range} $crangePair Int Int d1 d2
1852 --> {SPEC rule for $crangePair} $s$crangePair
1853 or thus:
1854 --> {SPEC rule for $dfIxPair} range $s$dfIxPair
1855 --> {ClassOpRule for range} $s$crangePair
1856 It doesn't matter which way.
1857
1858 * We want to specialise the RHS of both $dfIxPair and $crangePair,
1859 but the SAME HsWrapper will do for both! We can call tcSpecPrag
1860 just once, and pass the result (in spec_inst_info) to tcMethods.
1861 -}
1862
1863 tcSpecInstPrags :: DFunId -> InstBindings GhcRn
1864 -> TcM ([Located TcSpecPrag], TcPragEnv)
1865 tcSpecInstPrags dfun_id (InstBindings { ib_binds = binds, ib_pragmas = uprags })
1866 = do { spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) $
1867 filter isSpecInstLSig uprags
1868 -- The filter removes the pragmas for methods
1869 ; return (spec_inst_prags, mkPragEnv uprags binds) }
1870
1871 ------------------------------
1872 tcSpecInst :: Id -> Sig GhcRn -> TcM TcSpecPrag
1873 tcSpecInst dfun_id prag@(SpecInstSig _ _ hs_ty)
1874 = addErrCtxt (spec_ctxt prag) $
1875 do { (tyvars, theta, clas, tys) <- tcHsClsInstType SpecInstCtxt hs_ty
1876 ; let spec_dfun_ty = mkDictFunTy tyvars theta clas tys
1877 ; co_fn <- tcSpecWrapper SpecInstCtxt (idType dfun_id) spec_dfun_ty
1878 ; return (SpecPrag dfun_id co_fn defaultInlinePragma) }
1879 where
1880 spec_ctxt prag = hang (text "In the SPECIALISE pragma") 2 (ppr prag)
1881
1882 tcSpecInst _ _ = panic "tcSpecInst"
1883
1884 {-
1885 ************************************************************************
1886 * *
1887 \subsection{Error messages}
1888 * *
1889 ************************************************************************
1890 -}
1891
1892 instDeclCtxt1 :: LHsSigType GhcRn -> SDoc
1893 instDeclCtxt1 hs_inst_ty
1894 = inst_decl_ctxt (ppr (getLHsInstDeclHead hs_inst_ty))
1895
1896 instDeclCtxt2 :: Type -> SDoc
1897 instDeclCtxt2 dfun_ty
1898 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1899 where
1900 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
1901
1902 inst_decl_ctxt :: SDoc -> SDoc
1903 inst_decl_ctxt doc = hang (text "In the instance declaration for")
1904 2 (quotes doc)
1905
1906 badBootFamInstDeclErr :: SDoc
1907 badBootFamInstDeclErr
1908 = text "Illegal family instance in hs-boot file"
1909
1910 notFamily :: TyCon -> SDoc
1911 notFamily tycon
1912 = vcat [ text "Illegal family instance for" <+> quotes (ppr tycon)
1913 , nest 2 $ parens (ppr tycon <+> text "is not an indexed type family")]
1914
1915 tooFewParmsErr :: Arity -> SDoc
1916 tooFewParmsErr arity
1917 = text "Family instance has too few parameters; expected" <+>
1918 ppr arity
1919
1920 assocInClassErr :: Located Name -> SDoc
1921 assocInClassErr name
1922 = text "Associated type" <+> quotes (ppr name) <+>
1923 text "must be inside a class instance"
1924
1925 badFamInstDecl :: Located Name -> SDoc
1926 badFamInstDecl tc_name
1927 = vcat [ text "Illegal family instance for" <+>
1928 quotes (ppr tc_name)
1929 , nest 2 (parens $ text "Use TypeFamilies to allow indexed type families") ]
1930
1931 notOpenFamily :: TyCon -> SDoc
1932 notOpenFamily tc
1933 = text "Illegal instance for closed family" <+> quotes (ppr tc)