c97e4e128cf11ad1620242e38f74649fbe4ddda0
[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
11 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
12
13 #include "HsVersions.h"
14
15 import HsSyn
16 import TcBinds
17 import TcTyClsDecls
18 import TcClassDcl( tcClassDecl2, tcATDefault,
19 HsSigFun, lookupHsSig, mkHsSigFun,
20 findMethodBind, instantiateMethod )
21 import TcPat ( TcIdSigInfo, addInlinePrags, completeIdSigPolyId, lookupPragEnv, emptyPragEnv )
22 import TcRnMonad
23 import TcValidity
24 import TcMType
25 import TcType
26 import BuildTyCl
27 import Inst
28 import InstEnv
29 import FamInst
30 import FamInstEnv
31 import TcDeriv
32 import TcEnv
33 import TcHsType
34 import TcUnify
35 import MkCore ( nO_METHOD_BINDING_ERROR_ID )
36 import Type
37 import TcEvidence
38 import TyCon
39 import CoAxiom
40 import DataCon
41 import Class
42 import Var
43 import VarEnv
44 import VarSet
45 import PrelNames ( typeableClassName, genericClassNames )
46 -- , knownNatClassName, knownSymbolClassName )
47 import Bag
48 import BasicTypes
49 import DynFlags
50 import ErrUtils
51 import FastString
52 import HscTypes ( isHsBoot )
53 import Id
54 import MkId
55 import Name
56 import NameSet
57 import Outputable
58 import SrcLoc
59 import Util
60 import BooleanFormula ( isUnsatisfied, pprBooleanFormulaNice )
61
62 import Control.Monad
63 import Maybes
64 import Data.List ( partition )
65
66 {-
67 Typechecking instance declarations is done in two passes. The first
68 pass, made by @tcInstDecls1@, collects information to be used in the
69 second pass.
70
71 This pre-processed info includes the as-yet-unprocessed bindings
72 inside the instance declaration. These are type-checked in the second
73 pass, when the class-instance envs and GVE contain all the info from
74 all the instance and value decls. Indeed that's the reason we need
75 two passes over the instance decls.
76
77
78 Note [How instance declarations are translated]
79 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
80 Here is how we translate instance declarations into Core
81
82 Running example:
83 class C a where
84 op1, op2 :: Ix b => a -> b -> b
85 op2 = <dm-rhs>
86
87 instance C a => C [a]
88 {-# INLINE [2] op1 #-}
89 op1 = <rhs>
90 ===>
91 -- Method selectors
92 op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
93 op1 = ...
94 op2 = ...
95
96 -- Default methods get the 'self' dictionary as argument
97 -- so they can call other methods at the same type
98 -- Default methods get the same type as their method selector
99 $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
100 $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
101 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
102 -- Note [Tricky type variable scoping]
103
104 -- A top-level definition for each instance method
105 -- Here op1_i, op2_i are the "instance method Ids"
106 -- The INLINE pragma comes from the user pragma
107 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
108 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
109 op1_i = /\a. \(d:C a).
110 let this :: C [a]
111 this = df_i a d
112 -- Note [Subtle interaction of recursion and overlap]
113
114 local_op1 :: forall b. Ix b => [a] -> b -> b
115 local_op1 = <rhs>
116 -- Source code; run the type checker on this
117 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
118 -- Note [Tricky type variable scoping]
119
120 in local_op1 a d
121
122 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
123
124 -- The dictionary function itself
125 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
126 df_i :: forall a. C a -> C [a]
127 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
128 -- But see Note [Default methods in instances]
129 -- We can't apply the type checker to the default-method call
130
131 -- Use a RULE to short-circuit applications of the class ops
132 {-# RULE "op1@C[a]" forall a, d:C a.
133 op1 [a] (df_i d) = op1_i a d #-}
134
135 Note [Instances and loop breakers]
136 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
137 * Note that df_i may be mutually recursive with both op1_i and op2_i.
138 It's crucial that df_i is not chosen as the loop breaker, even
139 though op1_i has a (user-specified) INLINE pragma.
140
141 * Instead the idea is to inline df_i into op1_i, which may then select
142 methods from the MkC record, and thereby break the recursion with
143 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
144 the same type, it won't mention df_i, so there won't be recursion in
145 the first place.)
146
147 * If op1_i is marked INLINE by the user there's a danger that we won't
148 inline df_i in it, and that in turn means that (since it'll be a
149 loop-breaker because df_i isn't), op1_i will ironically never be
150 inlined. But this is OK: the recursion breaking happens by way of
151 a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
152 unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
153
154 Note [ClassOp/DFun selection]
155 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
156 One thing we see a lot is stuff like
157 op2 (df d1 d2)
158 where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
159 'op2' and 'df' to get
160 case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
161 MkD _ op2 _ _ _ -> op2
162 And that will reduce to ($cop2 d1 d2) which is what we wanted.
163
164 But it's tricky to make this work in practice, because it requires us to
165 inline both 'op2' and 'df'. But neither is keen to inline without having
166 seen the other's result; and it's very easy to get code bloat (from the
167 big intermediate) if you inline a bit too much.
168
169 Instead we use a cunning trick.
170 * We arrange that 'df' and 'op2' NEVER inline.
171
172 * We arrange that 'df' is ALWAYS defined in the sylised form
173 df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
174
175 * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
176 that lists its methods.
177
178 * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
179 a suitable constructor application -- inlining df "on the fly" as it
180 were.
181
182 * ClassOp rules: We give the ClassOp 'op2' a BuiltinRule that
183 extracts the right piece iff its argument satisfies
184 exprIsConApp_maybe. This is done in MkId mkDictSelId
185
186 * We make 'df' CONLIKE, so that shared uses still match; eg
187 let d = df d1 d2
188 in ...(op2 d)...(op1 d)...
189
190 Note [Single-method classes]
191 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
192 If the class has just one method (or, more accurately, just one element
193 of {superclasses + methods}), then we use a different strategy.
194
195 class C a where op :: a -> a
196 instance C a => C [a] where op = <blah>
197
198 We translate the class decl into a newtype, which just gives a
199 top-level axiom. The "constructor" MkC expands to a cast, as does the
200 class-op selector.
201
202 axiom Co:C a :: C a ~ (a->a)
203
204 op :: forall a. C a -> (a -> a)
205 op a d = d |> (Co:C a)
206
207 MkC :: forall a. (a->a) -> C a
208 MkC = /\a.\op. op |> (sym Co:C a)
209
210 The clever RULE stuff doesn't work now, because ($df a d) isn't
211 a constructor application, so exprIsConApp_maybe won't return
212 Just <blah>.
213
214 Instead, we simply rely on the fact that casts are cheap:
215
216 $df :: forall a. C a => C [a]
217 {-# INLINE df #-} -- NB: INLINE this
218 $df = /\a. \d. MkC [a] ($cop_list a d)
219 = $cop_list |> forall a. C a -> (sym (Co:C [a]))
220
221 $cop_list :: forall a. C a => [a] -> [a]
222 $cop_list = <blah>
223
224 So if we see
225 (op ($df a d))
226 we'll inline 'op' and '$df', since both are simply casts, and
227 good things happen.
228
229 Why do we use this different strategy? Because otherwise we
230 end up with non-inlined dictionaries that look like
231 $df = $cop |> blah
232 which adds an extra indirection to every use, which seems stupid. See
233 Trac #4138 for an example (although the regression reported there
234 wasn't due to the indirection).
235
236 There is an awkward wrinkle though: we want to be very
237 careful when we have
238 instance C a => C [a] where
239 {-# INLINE op #-}
240 op = ...
241 then we'll get an INLINE pragma on $cop_list but it's important that
242 $cop_list only inlines when it's applied to *two* arguments (the
243 dictionary and the list argument). So we must not eta-expand $df
244 above. We ensure that this doesn't happen by putting an INLINE
245 pragma on the dfun itself; after all, it ends up being just a cast.
246
247 There is one more dark corner to the INLINE story, even more deeply
248 buried. Consider this (Trac #3772):
249
250 class DeepSeq a => C a where
251 gen :: Int -> a
252
253 instance C a => C [a] where
254 gen n = ...
255
256 class DeepSeq a where
257 deepSeq :: a -> b -> b
258
259 instance DeepSeq a => DeepSeq [a] where
260 {-# INLINE deepSeq #-}
261 deepSeq xs b = foldr deepSeq b xs
262
263 That gives rise to these defns:
264
265 $cdeepSeq :: DeepSeq a -> [a] -> b -> b
266 -- User INLINE( 3 args )!
267 $cdeepSeq a (d:DS a) b (x:[a]) (y:b) = ...
268
269 $fDeepSeq[] :: DeepSeq a -> DeepSeq [a]
270 -- DFun (with auto INLINE pragma)
271 $fDeepSeq[] a d = $cdeepSeq a d |> blah
272
273 $cp1 a d :: C a => DeepSep [a]
274 -- We don't want to eta-expand this, lest
275 -- $cdeepSeq gets inlined in it!
276 $cp1 a d = $fDeepSep[] a (scsel a d)
277
278 $fC[] :: C a => C [a]
279 -- Ordinary DFun
280 $fC[] a d = MkC ($cp1 a d) ($cgen a d)
281
282 Here $cp1 is the code that generates the superclass for C [a]. The
283 issue is this: we must not eta-expand $cp1 either, or else $fDeepSeq[]
284 and then $cdeepSeq will inline there, which is definitely wrong. Like
285 on the dfun, we solve this by adding an INLINE pragma to $cp1.
286
287 Note [Subtle interaction of recursion and overlap]
288 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
289 Consider this
290 class C a where { op1,op2 :: a -> a }
291 instance C a => C [a] where
292 op1 x = op2 x ++ op2 x
293 op2 x = ...
294 instance C [Int] where
295 ...
296
297 When type-checking the C [a] instance, we need a C [a] dictionary (for
298 the call of op2). If we look up in the instance environment, we find
299 an overlap. And in *general* the right thing is to complain (see Note
300 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
301 complain, because we just want to delegate to the op2 of this same
302 instance.
303
304 Why is this justified? Because we generate a (C [a]) constraint in
305 a context in which 'a' cannot be instantiated to anything that matches
306 other overlapping instances, or else we would not be executing this
307 version of op1 in the first place.
308
309 It might even be a bit disguised:
310
311 nullFail :: C [a] => [a] -> [a]
312 nullFail x = op2 x ++ op2 x
313
314 instance C a => C [a] where
315 op1 x = nullFail x
316
317 Precisely this is used in package 'regex-base', module Context.hs.
318 See the overlapping instances for RegexContext, and the fact that they
319 call 'nullFail' just like the example above. The DoCon package also
320 does the same thing; it shows up in module Fraction.hs.
321
322 Conclusion: when typechecking the methods in a C [a] instance, we want to
323 treat the 'a' as an *existential* type variable, in the sense described
324 by Note [Binding when looking up instances]. That is why isOverlappableTyVar
325 responds True to an InstSkol, which is the kind of skolem we use in
326 tcInstDecl2.
327
328
329 Note [Tricky type variable scoping]
330 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
331 In our example
332 class C a where
333 op1, op2 :: Ix b => a -> b -> b
334 op2 = <dm-rhs>
335
336 instance C a => C [a]
337 {-# INLINE [2] op1 #-}
338 op1 = <rhs>
339
340 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
341 in scope in <rhs>. In particular, we must make sure that 'b' is in
342 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
343 which brings appropriate tyvars into scope. This happens for both
344 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
345 complained if 'b' is mentioned in <rhs>.
346
347
348
349 ************************************************************************
350 * *
351 \subsection{Extracting instance decls}
352 * *
353 ************************************************************************
354
355 Gather up the instance declarations from their various sources
356 -}
357
358 tcInstDecls1 -- Deal with both source-code and imported instance decls
359 :: [TyClGroup Name] -- For deriving stuff
360 -> [LInstDecl Name] -- Source code instance decls
361 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
362 -> TcM (TcGblEnv, -- The full inst env
363 [InstInfo Name], -- Source-code instance decls to process;
364 -- contains all dfuns for this module
365 HsValBinds Name) -- Supporting bindings for derived instances
366
367 tcInstDecls1 tycl_decls inst_decls deriv_decls
368 = checkNoErrs $
369 do { -- Stop if addInstInfos etc discovers any errors
370 -- (they recover, so that we get more than one error each
371 -- round)
372
373 -- Do class and family instance declarations
374 ; stuff <- mapAndRecoverM tcLocalInstDecl inst_decls
375 ; let (local_infos_s, fam_insts_s, datafam_deriv_infos) = unzip3 stuff
376 fam_insts = concat fam_insts_s
377 local_infos' = concat local_infos_s
378 -- Handwritten instances of the poly-kinded Typeable class are
379 -- forbidden, so we handle those separately
380 (typeable_instances, local_infos)
381 = partition bad_typeable_instance local_infos'
382
383 ; addClsInsts local_infos $
384 addFamInsts fam_insts $
385 do { -- Compute instances from "deriving" clauses;
386 -- This stuff computes a context for the derived instance
387 -- decl, so it needs to know about all the instances possible
388 -- NB: class instance declarations can contain derivings as
389 -- part of associated data type declarations
390 failIfErrsM -- If the addInsts stuff gave any errors, don't
391 -- try the deriving stuff, because that may give
392 -- more errors still
393
394 ; traceTc "tcDeriving" Outputable.empty
395 ; th_stage <- getStage -- See Note [Deriving inside TH brackets ]
396 ; (gbl_env, deriv_inst_info, deriv_binds)
397 <- if isBrackStage th_stage
398 then do { gbl_env <- getGblEnv
399 ; return (gbl_env, emptyBag, emptyValBindsOut) }
400 else do { data_deriv_infos <- mkDerivInfos tycl_decls
401 ; let deriv_infos = concat datafam_deriv_infos ++
402 data_deriv_infos
403 ; tcDeriving deriv_infos deriv_decls }
404
405 -- Fail if there are any handwritten instance of poly-kinded Typeable
406 ; mapM_ typeable_err typeable_instances
407
408 -- Check that if the module is compiled with -XSafe, there are no
409 -- hand written instances of old Typeable as then unsafe casts could be
410 -- performed. Derived instances are OK.
411 ; dflags <- getDynFlags
412 ; when (safeLanguageOn dflags) $ forM_ local_infos $ \x -> case x of
413 _ | genInstCheck x -> addErrAt (getSrcSpan $ iSpec x) (genInstErr x)
414 _ -> return ()
415
416 -- As above but for Safe Inference mode.
417 ; when (safeInferOn dflags) $ forM_ local_infos $ \x -> case x of
418 _ | genInstCheck x -> recordUnsafeInfer emptyBag
419 _ -> return ()
420
421 ; return ( gbl_env
422 , bagToList deriv_inst_info ++ local_infos
423 , deriv_binds )
424 }}
425 where
426 -- Separate the Typeable instances from the rest
427 bad_typeable_instance i
428 = typeableClassName == is_cls_nm (iSpec i)
429
430 -- Check for hand-written Generic instances (disallowed in Safe Haskell)
431 genInstCheck ty = is_cls_nm (iSpec ty) `elem` genericClassNames
432 genInstErr i = hang (ptext (sLit $ "Generic instances can only be "
433 ++ "derived in Safe Haskell.") $+$
434 ptext (sLit "Replace the following instance:"))
435 2 (pprInstanceHdr (iSpec i))
436
437 -- Report an error or a warning for a `Typeable` instances.
438 -- If we are working on an .hs-boot file, we just report a warning,
439 -- and ignore the instance. We do this, to give users a chance to fix
440 -- their code.
441 typeable_err i =
442 setSrcSpan (getSrcSpan (iSpec i)) $
443 do env <- getGblEnv
444 if isHsBoot (tcg_src env)
445 then
446 do warn <- woptM Opt_WarnDerivingTypeable
447 when warn $ addWarnTc $ vcat
448 [ ptext (sLit "`Typeable` instances in .hs-boot files are ignored.")
449 , ptext (sLit "This warning will become an error in future versions of the compiler.")
450 ]
451 else addErrTc $ ptext (sLit "Class `Typeable` does not support user-specified instances.")
452
453 addClsInsts :: [InstInfo Name] -> TcM a -> TcM a
454 addClsInsts infos thing_inside
455 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
456
457 addFamInsts :: [FamInst] -> TcM a -> TcM a
458 -- Extend (a) the family instance envt
459 -- (b) the type envt with stuff from data type decls
460 addFamInsts fam_insts thing_inside
461 = tcExtendLocalFamInstEnv fam_insts $
462 tcExtendGlobalEnv things $
463 do { traceTc "addFamInsts" (pprFamInsts fam_insts)
464 ; tcg_env <- tcAddImplicits things
465 ; setGblEnv tcg_env thing_inside }
466 where
467 axioms = map (toBranchedAxiom . famInstAxiom) fam_insts
468 tycons = famInstsRepTyCons fam_insts
469 things = map ATyCon tycons ++ map ACoAxiom axioms
470
471 {-
472 Note [Deriving inside TH brackets]
473 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
474 Given a declaration bracket
475 [d| data T = A | B deriving( Show ) |]
476
477 there is really no point in generating the derived code for deriving(
478 Show) and then type-checking it. This will happen at the call site
479 anyway, and the type check should never fail! Moreover (Trac #6005)
480 the scoping of the generated code inside the bracket does not seem to
481 work out.
482
483 The easy solution is simply not to generate the derived instances at
484 all. (A less brutal solution would be to generate them with no
485 bindings.) This will become moot when we shift to the new TH plan, so
486 the brutal solution will do.
487 -}
488
489 tcLocalInstDecl :: LInstDecl Name
490 -> TcM ([InstInfo Name], [FamInst], [DerivInfo])
491 -- A source-file instance declaration
492 -- Type-check all the stuff before the "where"
493 --
494 -- We check for respectable instance type, and context
495 tcLocalInstDecl (L loc (TyFamInstD { tfid_inst = decl }))
496 = do { fam_inst <- tcTyFamInstDecl Nothing (L loc decl)
497 ; return ([], [fam_inst], []) }
498
499 tcLocalInstDecl (L loc (DataFamInstD { dfid_inst = decl }))
500 = do { (fam_inst, m_deriv_info) <- tcDataFamInstDecl Nothing (L loc decl)
501 ; return ([], [fam_inst], maybeToList m_deriv_info) }
502
503 tcLocalInstDecl (L loc (ClsInstD { cid_inst = decl }))
504 = do { (insts, fam_insts, deriv_infos) <- tcClsInstDecl (L loc decl)
505 ; return (insts, fam_insts, deriv_infos) }
506
507 tcClsInstDecl :: LClsInstDecl Name
508 -> TcM ([InstInfo Name], [FamInst], [DerivInfo])
509 -- the returned DerivInfos are for any associated data families
510 tcClsInstDecl (L loc (ClsInstDecl { cid_poly_ty = poly_ty, cid_binds = binds
511 , cid_sigs = uprags, cid_tyfam_insts = ats
512 , cid_overlap_mode = overlap_mode
513 , cid_datafam_insts = adts }))
514 = setSrcSpan loc $
515 addErrCtxt (instDeclCtxt1 poly_ty) $
516 do { is_boot <- tcIsHsBootOrSig
517 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
518 badBootDeclErr
519
520 ; (tyvars, theta, clas, inst_tys) <- tcHsInstHead InstDeclCtxt poly_ty
521 ; let mini_env = mkVarEnv (classTyVars clas `zip` inst_tys)
522 mini_subst = mkTvSubst (mkInScopeSet (mkVarSet tyvars)) mini_env
523 mb_info = Just (clas, mini_env)
524
525 -- Next, process any associated types.
526 ; traceTc "tcLocalInstDecl" (ppr poly_ty)
527 ; tyfam_insts0 <- tcExtendTyVarEnv tyvars $
528 mapAndRecoverM (tcTyFamInstDecl mb_info) ats
529 ; datafam_stuff <- tcExtendTyVarEnv tyvars $
530 mapAndRecoverM (tcDataFamInstDecl mb_info) adts
531 ; let (datafam_insts, m_deriv_infos) = unzip datafam_stuff
532 deriv_infos = catMaybes m_deriv_infos
533
534 -- Check for missing associated types and build them
535 -- from their defaults (if available)
536 ; let defined_ats = mkNameSet (map (tyFamInstDeclName . unLoc) ats)
537 `unionNameSet`
538 mkNameSet (map (unLoc . dfid_tycon . unLoc) adts)
539 ; tyfam_insts1 <- mapM (tcATDefault True loc mini_subst defined_ats)
540 (classATItems clas)
541
542 -- Finally, construct the Core representation of the instance.
543 -- (This no longer includes the associated types.)
544 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
545 -- Dfun location is that of instance *header*
546
547 ; ispec <- newClsInst (fmap unLoc overlap_mode) dfun_name tyvars theta
548 clas inst_tys
549 ; let inst_info = InstInfo { iSpec = ispec
550 , iBinds = InstBindings
551 { ib_binds = binds
552 , ib_tyvars = map Var.varName tyvars -- Scope over bindings
553 , ib_pragmas = uprags
554 , ib_extensions = []
555 , ib_derived = False } }
556
557 ; return ( [inst_info], tyfam_insts0 ++ concat tyfam_insts1 ++ datafam_insts
558 , deriv_infos ) }
559
560
561 {-
562 ************************************************************************
563 * *
564 Type checking family instances
565 * *
566 ************************************************************************
567
568 Family instances are somewhat of a hybrid. They are processed together with
569 class instance heads, but can contain data constructors and hence they share a
570 lot of kinding and type checking code with ordinary algebraic data types (and
571 GADTs).
572 -}
573
574 tcFamInstDeclCombined :: Maybe ClsInfo
575 -> Located Name -> TcM TyCon
576 tcFamInstDeclCombined mb_clsinfo fam_tc_lname
577 = do { -- Type family instances require -XTypeFamilies
578 -- and can't (currently) be in an hs-boot file
579 ; traceTc "tcFamInstDecl" (ppr fam_tc_lname)
580 ; type_families <- xoptM Opt_TypeFamilies
581 ; is_boot <- tcIsHsBootOrSig -- Are we compiling an hs-boot file?
582 ; checkTc type_families $ badFamInstDecl fam_tc_lname
583 ; checkTc (not is_boot) $ badBootFamInstDeclErr
584
585 -- Look up the family TyCon and check for validity including
586 -- check that toplevel type instances are not for associated types.
587 ; fam_tc <- tcLookupLocatedTyCon fam_tc_lname
588 ; when (isNothing mb_clsinfo && -- Not in a class decl
589 isTyConAssoc fam_tc) -- but an associated type
590 (addErr $ assocInClassErr fam_tc_lname)
591
592 ; return fam_tc }
593
594 tcTyFamInstDecl :: Maybe ClsInfo
595 -> LTyFamInstDecl Name -> TcM FamInst
596 -- "type instance"
597 tcTyFamInstDecl mb_clsinfo (L loc decl@(TyFamInstDecl { tfid_eqn = eqn }))
598 = setSrcSpan loc $
599 tcAddTyFamInstCtxt decl $
600 do { let fam_lname = tfe_tycon (unLoc eqn)
601 ; fam_tc <- tcFamInstDeclCombined mb_clsinfo fam_lname
602
603 -- (0) Check it's an open type family
604 ; checkTc (isFamilyTyCon fam_tc) (notFamily fam_tc)
605 ; checkTc (isTypeFamilyTyCon fam_tc) (wrongKindOfFamily fam_tc)
606 ; checkTc (isOpenTypeFamilyTyCon fam_tc) (notOpenFamily fam_tc)
607
608 -- (1) do the work of verifying the synonym group
609 ; co_ax_branch <- tcTyFamInstEqn (famTyConShape fam_tc) mb_clsinfo eqn
610
611 -- (2) check for validity
612 ; checkValidCoAxBranch mb_clsinfo fam_tc co_ax_branch
613
614 -- (3) construct coercion axiom
615 ; rep_tc_name <- newFamInstAxiomName loc (unLoc fam_lname)
616 [co_ax_branch]
617 ; let axiom = mkUnbranchedCoAxiom rep_tc_name fam_tc co_ax_branch
618 ; newFamInst SynFamilyInst axiom }
619
620 tcDataFamInstDecl :: Maybe ClsInfo
621 -> LDataFamInstDecl Name -> TcM (FamInst, Maybe DerivInfo)
622 -- "newtype instance" and "data instance"
623 tcDataFamInstDecl mb_clsinfo
624 (L loc decl@(DataFamInstDecl
625 { dfid_pats = pats
626 , dfid_tycon = fam_tc_name
627 , dfid_defn = defn@HsDataDefn { dd_ND = new_or_data, dd_cType = cType
628 , dd_ctxt = ctxt, dd_cons = cons
629 , dd_derivs = derivs } }))
630 = setSrcSpan loc $
631 tcAddDataFamInstCtxt decl $
632 do { fam_tc <- tcFamInstDeclCombined mb_clsinfo fam_tc_name
633
634 -- Check that the family declaration is for the right kind
635 ; checkTc (isFamilyTyCon fam_tc) (notFamily fam_tc)
636 ; checkTc (isAlgTyCon fam_tc) (wrongKindOfFamily fam_tc)
637
638 -- Kind check type patterns
639 ; tcFamTyPats (famTyConShape fam_tc) mb_clsinfo pats
640 (kcDataDefn defn) $
641 \tvs' pats' res_kind -> do
642
643 { -- Check that left-hand side contains no type family applications
644 -- (vanilla synonyms are fine, though, and we checked for
645 -- foralls earlier)
646 checkValidFamPats fam_tc tvs' pats'
647 -- Check that type patterns match class instance head, if any
648 ; checkConsistentFamInst mb_clsinfo fam_tc tvs' pats'
649
650 -- Result kind must be '*' (otherwise, we have too few patterns)
651 ; checkTc (isLiftedTypeKind res_kind) $ tooFewParmsErr (tyConArity fam_tc)
652
653 ; stupid_theta <- tcHsContext ctxt
654 ; gadt_syntax <- dataDeclChecks (tyConName fam_tc) new_or_data stupid_theta cons
655
656 -- Construct representation tycon
657 ; rep_tc_name <- newFamInstTyConName fam_tc_name pats'
658 ; axiom_name <- newImplicitBinder rep_tc_name mkInstTyCoOcc
659 ; let orig_res_ty = mkTyConApp fam_tc pats'
660
661 ; (rep_tc, fam_inst) <- fixM $ \ ~(rec_rep_tc, _) ->
662 do { data_cons <- tcConDecls new_or_data rec_rep_tc
663 (tvs', orig_res_ty) cons
664 ; tc_rhs <- case new_or_data of
665 DataType -> return (mkDataTyConRhs data_cons)
666 NewType -> ASSERT( not (null data_cons) )
667 mkNewTyConRhs rep_tc_name rec_rep_tc (head data_cons)
668 -- freshen tyvars
669 ; let (eta_tvs, eta_pats) = eta_reduce tvs' pats'
670 axiom = mkSingleCoAxiom Representational
671 axiom_name eta_tvs fam_tc eta_pats
672 (mkTyConApp rep_tc (mkTyVarTys eta_tvs))
673 parent = FamInstTyCon axiom fam_tc pats'
674 roles = map (const Nominal) tvs'
675
676 -- NB: Use the tvs' from the pats. See bullet toward
677 -- the end of Note [Data type families] in TyCon
678 rep_tc = buildAlgTyCon rep_tc_name tvs' roles
679 (fmap unLoc cType) stupid_theta
680 tc_rhs
681 Recursive
682 False -- No promotable to the kind level
683 gadt_syntax parent
684 -- We always assume that indexed types are recursive. Why?
685 -- (1) Due to their open nature, we can never be sure that a
686 -- further instance might not introduce a new recursive
687 -- dependency. (2) They are always valid loop breakers as
688 -- they involve a coercion.
689 ; fam_inst <- newFamInst (DataFamilyInst rep_tc) axiom
690 ; return (rep_tc, fam_inst) }
691
692 -- Remember to check validity; no recursion to worry about here
693 ; checkValidTyCon rep_tc
694
695 ; let m_deriv_info = case derivs of
696 Nothing -> Nothing
697 Just (L _ preds) ->
698 Just $ DerivInfo { di_rep_tc = rep_tc
699 , di_preds = preds
700 , di_ctxt = tcMkDataFamInstCtxt decl }
701
702 ; return (fam_inst, m_deriv_info) } }
703 where
704 -- See Note [Eta reduction for data family axioms]
705 -- [a,b,c,d].T [a] c Int c d ==> [a,b,c]. T [a] c Int c
706 eta_reduce tvs pats = go (reverse tvs) (reverse pats)
707 go (tv:tvs) (pat:pats)
708 | Just tv' <- getTyVar_maybe pat
709 , tv == tv'
710 , not (tv `elemVarSet` tyVarsOfTypes pats)
711 = go tvs pats
712 go tvs pats = (reverse tvs, reverse pats)
713
714 {-
715 Note [Eta reduction for data family axioms]
716 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
717 Consider this
718 data family T a b :: *
719 newtype instance T Int a = MkT (IO a) deriving( Monad )
720 We'd like this to work. From the 'newtype instance' you might
721 think we'd get:
722 newtype TInt a = MkT (IO a)
723 axiom ax1 a :: T Int a ~ TInt a -- The type-instance part
724 axiom ax2 a :: TInt a ~ IO a -- The newtype part
725
726 But now what can we do? We have this problem
727 Given: d :: Monad IO
728 Wanted: d' :: Monad (T Int) = d |> ????
729 What coercion can we use for the ???
730
731 Solution: eta-reduce both axioms, thus:
732 axiom ax1 :: T Int ~ TInt
733 axiom ax2 :: TInt ~ IO
734 Now
735 d' = d |> Monad (sym (ax2 ; ax1))
736
737 This eta reduction happens both for data instances and newtype instances.
738
739 See Note [Newtype eta] in TyCon.
740
741
742
743 ************************************************************************
744 * *
745 Type-checking instance declarations, pass 2
746 * *
747 ************************************************************************
748 -}
749
750 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
751 -> TcM (LHsBinds Id)
752 -- (a) From each class declaration,
753 -- generate any default-method bindings
754 -- (b) From each instance decl
755 -- generate the dfun binding
756
757 tcInstDecls2 tycl_decls inst_decls
758 = do { -- (a) Default methods from class decls
759 let class_decls = filter (isClassDecl . unLoc) tycl_decls
760 ; dm_binds_s <- mapM tcClassDecl2 class_decls
761 ; let dm_binds = unionManyBags dm_binds_s
762
763 -- (b) instance declarations
764 ; let dm_ids = collectHsBindsBinders dm_binds
765 -- Add the default method Ids (again)
766 -- See Note [Default methods and instances]
767 ; inst_binds_s <- tcExtendLetEnv TopLevel dm_ids $
768 mapM tcInstDecl2 inst_decls
769
770 -- Done
771 ; return (dm_binds `unionBags` unionManyBags inst_binds_s) }
772
773 {-
774 See Note [Default methods and instances]
775 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
776 The default method Ids are already in the type environment (see Note
777 [Default method Ids and Template Haskell] in TcTyClsDcls), BUT they
778 don't have their InlinePragmas yet. Usually that would not matter,
779 because the simplifier propagates information from binding site to
780 use. But, unusually, when compiling instance decls we *copy* the
781 INLINE pragma from the default method to the method for that
782 particular operation (see Note [INLINE and default methods] below).
783
784 So right here in tcInstDecls2 we must re-extend the type envt with
785 the default method Ids replete with their INLINE pragmas. Urk.
786 -}
787
788 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
789 -- Returns a binding for the dfun
790 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
791 = recoverM (return emptyLHsBinds) $
792 setSrcSpan loc $
793 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
794 do { -- Instantiate the instance decl with skolem constants
795 ; (inst_tyvars, dfun_theta, inst_head) <- tcSkolDFunType (idType dfun_id)
796 ; dfun_ev_vars <- newEvVars dfun_theta
797 -- We instantiate the dfun_id with superSkolems.
798 -- See Note [Subtle interaction of recursion and overlap]
799 -- and Note [Binding when looking up instances]
800
801 ; let (clas, inst_tys) = tcSplitDFunHead inst_head
802 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
803 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys) sc_theta
804
805 ; traceTc "tcInstDecl2" (vcat [ppr inst_tyvars, ppr inst_tys, ppr dfun_theta, ppr sc_theta'])
806
807 -- Deal with 'SPECIALISE instance' pragmas
808 -- See Note [SPECIALISE instance pragmas]
809 ; spec_inst_info@(spec_inst_prags,_) <- tcSpecInstPrags dfun_id ibinds
810
811 -- Typecheck superclasses and methods
812 -- See Note [Typechecking plan for instance declarations]
813 ; dfun_ev_binds_var <- newTcEvBinds
814 ; let dfun_ev_binds = TcEvBinds dfun_ev_binds_var
815 ; ((sc_meth_ids, sc_meth_binds, sc_meth_implics), tclvl)
816 <- pushTcLevelM $
817 do { fam_envs <- tcGetFamInstEnvs
818 ; (sc_ids, sc_binds, sc_implics)
819 <- tcSuperClasses dfun_id clas inst_tyvars dfun_ev_vars
820 inst_tys dfun_ev_binds fam_envs
821 sc_theta'
822
823 -- Typecheck the methods
824 ; (meth_ids, meth_binds, meth_implics)
825 <- tcMethods dfun_id clas inst_tyvars dfun_ev_vars
826 inst_tys dfun_ev_binds spec_inst_info
827 op_items ibinds
828
829 ; return ( sc_ids ++ meth_ids
830 , sc_binds `unionBags` meth_binds
831 , sc_implics `unionBags` meth_implics ) }
832
833 ; env <- getLclEnv
834 ; emitImplication $ Implic { ic_tclvl = tclvl
835 , ic_skols = inst_tyvars
836 , ic_no_eqs = False
837 , ic_given = dfun_ev_vars
838 , ic_wanted = addImplics emptyWC sc_meth_implics
839 , ic_status = IC_Unsolved
840 , ic_binds = dfun_ev_binds_var
841 , ic_env = env
842 , ic_info = InstSkol }
843
844 -- Create the result bindings
845 ; self_dict <- newDict clas inst_tys
846 ; let class_tc = classTyCon clas
847 [dict_constr] = tyConDataCons class_tc
848 dict_bind = mkVarBind self_dict (L loc con_app_args)
849
850 -- We don't produce a binding for the dict_constr; instead we
851 -- rely on the simplifier to unfold this saturated application
852 -- We do this rather than generate an HsCon directly, because
853 -- it means that the special cases (e.g. dictionary with only one
854 -- member) are dealt with by the common MkId.mkDataConWrapId
855 -- code rather than needing to be repeated here.
856 -- con_app_tys = MkD ty1 ty2
857 -- con_app_scs = MkD ty1 ty2 sc1 sc2
858 -- con_app_args = MkD ty1 ty2 sc1 sc2 op1 op2
859 con_app_tys = wrapId (mkWpTyApps inst_tys)
860 (dataConWrapId dict_constr)
861 con_app_args = foldl app_to_meth con_app_tys sc_meth_ids
862
863 app_to_meth :: HsExpr Id -> Id -> HsExpr Id
864 app_to_meth fun meth_id = L loc fun `HsApp` L loc (wrapId arg_wrapper meth_id)
865
866 inst_tv_tys = mkTyVarTys inst_tyvars
867 arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps inst_tv_tys
868
869 -- Do not inline the dfun; instead give it a magic DFunFunfolding
870 dfun_spec_prags
871 | isNewTyCon class_tc = SpecPrags []
872 -- Newtype dfuns just inline unconditionally,
873 -- so don't attempt to specialise them
874 | otherwise
875 = SpecPrags spec_inst_prags
876
877 export = ABE { abe_wrap = idHsWrapper, abe_poly = dfun_id
878 , abe_mono = self_dict, abe_prags = dfun_spec_prags }
879 -- NB: see Note [SPECIALISE instance pragmas]
880 main_bind = AbsBinds { abs_tvs = inst_tyvars
881 , abs_ev_vars = dfun_ev_vars
882 , abs_exports = [export]
883 , abs_ev_binds = []
884 , abs_binds = unitBag dict_bind }
885
886 ; return (unitBag (L loc main_bind) `unionBags` sc_meth_binds)
887 }
888 where
889 dfun_id = instanceDFunId ispec
890 loc = getSrcSpan dfun_id
891
892 wrapId :: HsWrapper -> id -> HsExpr id
893 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
894
895 {- Note [Typechecking plan for instance declarations]
896 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
897 For intance declarations we generate the following bindings and implication
898 constraints. Example:
899
900 instance Ord a => Ord [a] where compare = <compare-rhs>
901
902 generates this:
903
904 Bindings:
905 -- Method bindings
906 $ccompare :: forall a. Ord a => a -> a -> Ordering
907 $ccompare = /\a \(d:Ord a). let <meth-ev-binds> in ...
908
909 -- Superclass bindings
910 $cp1Ord :: forall a. Ord a => Eq [a]
911 $cp1Ord = /\a \(d:Ord a). let <sc-ev-binds>
912 in dfEqList (dw :: Eq a)
913
914 Constraints:
915 forall a. Ord a =>
916 -- Method constraint
917 (forall. (empty) => <constraints from compare-rhs>)
918 -- Superclass constraint
919 /\ (forall. (empty) => dw :: Eq a)
920
921 Notice that
922
923 * Per-meth/sc implication. There is one inner implication per
924 superclass or method, with no skolem variables or givens. The only
925 reason for this one is to gather the evidence bindings privately
926 for this superclass or method. This implication is generated
927 by checkInstConstraints.
928
929 * Overall instance implication. There is an overall enclosing
930 implication for the whole instance declaratation, with the expected
931 skolems and givens. We need this to get the correct "redundant
932 constraint" warnings, gathering all the uses from all the methods
933 and superclasses. See TcSimplify Note [Tracking redundant
934 constraints]
935
936 * The given constraints in the outer implication may generate
937 evidence, notably by superclass selection. Since the method and
938 superclass bindings are top-level, we want that evidence copied
939 into *every* method or superclass definition. (Some of it will
940 be usused in some, but dead-code elimination will drop it.)
941
942 We achieve this by putting the the evidence variable for the overall
943 instance implicaiton into the AbsBinds for each method/superclass.
944 Hence the 'dfun_ev_binds' passed into tcMethods and tcSuperClasses.
945 (And that in turn is why the abs_ev_binds field of AbBinds is a
946 [TcEvBinds] rather than simply TcEvBinds.
947
948 This is a bit of a hack, but works very nicely in practice.
949
950 * Note that if a method has a locally-polymorphic binding, there will
951 be yet another implication for that, generated by tcPolyCheck
952 in tcMethodBody. E.g.
953 class C a where
954 foo :: forall b. Ord b => blah
955
956
957 ************************************************************************
958 * *
959 Type-checking superclases
960 * *
961 ************************************************************************
962 -}
963
964 tcSuperClasses :: DFunId -> Class -> [TcTyVar] -> [EvVar] -> [TcType]
965 -> TcEvBinds -> FamInstEnvs
966 -> TcThetaType
967 -> TcM ([EvVar], LHsBinds Id, Bag Implication)
968 -- Make a new top-level function binding for each superclass,
969 -- something like
970 -- $Ordp1 :: forall a. Ord a => Eq [a]
971 -- $Ordp1 = /\a \(d:Ord a). dfunEqList a (sc_sel d)
972 --
973 -- See Note [Recursive superclasses] for why this is so hard!
974 -- In effect, be build a special-purpose solver for the first step
975 -- of solving each superclass constraint
976 tcSuperClasses dfun_id cls tyvars dfun_evs inst_tys dfun_ev_binds _fam_envs sc_theta
977 = do { (ids, binds, implics) <- mapAndUnzip3M tc_super (zip sc_theta [fIRST_TAG..])
978 ; return (ids, listToBag binds, listToBag implics) }
979 where
980 loc = getSrcSpan dfun_id
981 size = sizeTypes inst_tys
982 tc_super (sc_pred, n)
983 = do { (sc_implic, sc_ev_id) <- checkInstConstraints $ \_ ->
984 emitWanted (ScOrigin size) sc_pred
985
986 ; sc_top_name <- newName (mkSuperDictAuxOcc n (getOccName cls))
987 ; let sc_top_ty = mkForAllTys tyvars (mkPiTypes dfun_evs sc_pred)
988 sc_top_id = mkLocalId sc_top_name sc_top_ty
989 export = ABE { abe_wrap = idHsWrapper, abe_poly = sc_top_id
990 , abe_mono = sc_ev_id
991 , abe_prags = SpecPrags [] }
992 local_ev_binds = TcEvBinds (ic_binds sc_implic)
993 bind = AbsBinds { abs_tvs = tyvars
994 , abs_ev_vars = dfun_evs
995 , abs_exports = [export]
996 , abs_ev_binds = [dfun_ev_binds, local_ev_binds]
997 , abs_binds = emptyBag }
998 ; return (sc_top_id, L loc bind, sc_implic) }
999
1000 -------------------
1001 checkInstConstraints :: (EvBindsVar -> TcM result)
1002 -> TcM (Implication, result)
1003 -- See Note [Typechecking plan for instance declarations]
1004 -- The thing_inside is also passed the EvBindsVar,
1005 -- so that emit_sc_pred can add evidence for the superclass
1006 -- (not used for methods)
1007 checkInstConstraints thing_inside
1008 = do { ev_binds_var <- newTcEvBinds
1009 ; env <- getLclEnv
1010 ; (result, tclvl, wanted) <- pushLevelAndCaptureConstraints $
1011 thing_inside ev_binds_var
1012
1013 ; let implic = Implic { ic_tclvl = tclvl
1014 , ic_skols = []
1015 , ic_no_eqs = False
1016 , ic_given = []
1017 , ic_wanted = wanted
1018 , ic_status = IC_Unsolved
1019 , ic_binds = ev_binds_var
1020 , ic_env = env
1021 , ic_info = InstSkol }
1022
1023 ; return (implic, result) }
1024
1025 {-
1026 Note [Recursive superclasses]
1027 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1028 See Trac #3731, #4809, #5751, #5913, #6117, #6161, which all
1029 describe somewhat more complicated situations, but ones
1030 encountered in practice.
1031
1032 See also tests tcrun020, tcrun021, tcrun033
1033
1034 ----- THE PROBLEM --------
1035 The problem is that it is all too easy to create a class whose
1036 superclass is bottom when it should not be.
1037
1038 Consider the following (extreme) situation:
1039 class C a => D a where ...
1040 instance D [a] => D [a] where ... (dfunD)
1041 instance C [a] => C [a] where ... (dfunC)
1042 Although this looks wrong (assume D [a] to prove D [a]), it is only a
1043 more extreme case of what happens with recursive dictionaries, and it
1044 can, just about, make sense because the methods do some work before
1045 recursing.
1046
1047 To implement the dfunD we must generate code for the superclass C [a],
1048 which we had better not get by superclass selection from the supplied
1049 argument:
1050 dfunD :: forall a. D [a] -> D [a]
1051 dfunD = \d::D [a] -> MkD (scsel d) ..
1052
1053 Otherwise if we later encounter a situation where
1054 we have a [Wanted] dw::D [a] we might solve it thus:
1055 dw := dfunD dw
1056 Which is all fine except that now ** the superclass C is bottom **!
1057
1058 The instance we want is:
1059 dfunD :: forall a. D [a] -> D [a]
1060 dfunD = \d::D [a] -> MkD (dfunC (scsel d)) ...
1061
1062 ----- THE SOLUTION --------
1063 The basic solution is simple: be very careful about using superclass
1064 selection to generate a superclass witness in a dictionary function
1065 definition. More precisely:
1066
1067 Superclass Invariant: in every class dictionary,
1068 every superclass dictionary field
1069 is non-bottom
1070
1071 To achieve the Superclass Invariant, in a dfun definition we can
1072 generate a guaranteed-non-bottom superclass witness from:
1073 (sc1) one of the dictionary arguments itself (all non-bottom)
1074 (sc2) an immediate superclass of a smaller dictionary
1075 (sc3) a call of a dfun (always returns a dictionary constructor)
1076
1077 The tricky case is (sc2). We proceed by induction on the size of
1078 the (type of) the dictionary, defined by TcValidity.sizeTypes.
1079 Let's suppose we are building a dictionary of size 3, and
1080 suppose the Superclass Invariant holds of smaller dictionaries.
1081 Then if we have a smaller dictionary, its immediate superclasses
1082 will be non-bottom by induction.
1083
1084 What does "we have a smaller dictionary" mean? It might be
1085 one of the arguments of the instance, or one of its superclasses.
1086 Here is an example, taken from CmmExpr:
1087 class Ord r => UserOfRegs r a where ...
1088 (i1) instance UserOfRegs r a => UserOfRegs r (Maybe a) where
1089 (i2) instance (Ord r, UserOfRegs r CmmReg) => UserOfRegs r CmmExpr where
1090
1091 For (i1) we can get the (Ord r) superclass by selection from (UserOfRegs r a),
1092 since it is smaller than the thing we are building (UserOfRegs r (Maybe a).
1093
1094 But for (i2) that isn't the case, so we must add an explicit, and
1095 perhaps surprising, (Ord r) argument to the instance declaration.
1096
1097 Here's another example from Trac #6161:
1098
1099 class Super a => Duper a where ...
1100 class Duper (Fam a) => Foo a where ...
1101 (i3) instance Foo a => Duper (Fam a) where ...
1102 (i4) instance Foo Float where ...
1103
1104 It would be horribly wrong to define
1105 dfDuperFam :: Foo a -> Duper (Fam a) -- from (i3)
1106 dfDuperFam d = MkDuper (sc_sel1 (sc_sel2 d)) ...
1107
1108 dfFooFloat :: Foo Float -- from (i4)
1109 dfFooFloat = MkFoo (dfDuperFam dfFooFloat) ...
1110
1111 Now the Super superclass of Duper is definitely bottom!
1112
1113 This won't happen because when processing (i3) we can use the
1114 superclasses of (Foo a), which is smaller, namely Duper (Fam a). But
1115 that is *not* smaller than the target so we can't take *its*
1116 superclasses. As a result the program is rightly rejected, unless you
1117 add (Super (Fam a)) to the context of (i3).
1118
1119 Note [Solving superclass constraints]
1120 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1121 How do we ensure that every superclass witness is generated by
1122 one of (sc1) (sc2) or (sc3) in Note [Recursive superclases].
1123 Answer:
1124
1125 * Superclass "wanted" constraints have CtOrigin of (ScOrigin size)
1126 where 'size' is the size of the instance declaration. e.g.
1127 class C a => D a where...
1128 instance blah => D [a] where ...
1129 The wanted superclass constraint for C [a] has origin
1130 ScOrigin size, where size = size( D [a] ).
1131
1132 * (sc1) When we rewrite such a wanted constraint, it retains its
1133 origin. But if we apply an instance declaration, we can set the
1134 origin to (ScOrigin infinity), thus lifting any restrictions by
1135 making prohibitedSuperClassSolve return False.
1136
1137 * (sc2) ScOrigin wanted constraints can't be solved from a
1138 superclass selection, except at a smaller type. This test is
1139 implemented by TcInteract.prohibitedSuperClassSolve
1140
1141 * The "given" constraints of an instance decl have CtOrigin
1142 GivenOrigin InstSkol.
1143
1144 * When we make a superclass selection from InstSkol we use
1145 a SkolemInfo of (InstSC size), where 'size' is the size of
1146 the constraint whose superclass we are taking. An similarly
1147 when taking the superclass of an InstSC. This is implemented
1148 in TcCanonical.newSCWorkFromFlavored
1149
1150 Note [Silent superclass arguments] (historical interest only)
1151 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1152 NB1: this note describes our *old* solution to the
1153 recursive-superclass problem. I'm keeping the Note
1154 for now, just as institutional memory.
1155 However, the code for silent superclass arguments
1156 was removed in late Dec 2014
1157
1158 NB2: the silent-superclass solution introduced new problems
1159 of its own, in the form of instance overlap. Tests
1160 SilentParametersOverlapping, T5051, and T7862 are examples
1161
1162 NB3: the silent-superclass solution also generated tons of
1163 extra dictionaries. For example, in monad-transformer
1164 code, when constructing a Monad dictionary you had to pass
1165 an Applicative dictionary; and to construct that you neede
1166 a Functor dictionary. Yet these extra dictionaries were
1167 often never used. Test T3064 compiled *far* faster after
1168 silent superclasses were eliminated.
1169
1170 Our solution to this problem "silent superclass arguments". We pass
1171 to each dfun some ``silent superclass arguments’’, which are the
1172 immediate superclasses of the dictionary we are trying to
1173 construct. In our example:
1174 dfun :: forall a. C [a] -> D [a] -> D [a]
1175 dfun = \(dc::C [a]) (dd::D [a]) -> DOrd dc ...
1176 Notice the extra (dc :: C [a]) argument compared to the previous version.
1177
1178 This gives us:
1179
1180 -----------------------------------------------------------
1181 DFun Superclass Invariant
1182 ~~~~~~~~~~~~~~~~~~~~~~~~
1183 In the body of a DFun, every superclass argument to the
1184 returned dictionary is
1185 either * one of the arguments of the DFun,
1186 or * constant, bound at top level
1187 -----------------------------------------------------------
1188
1189 This net effect is that it is safe to treat a dfun application as
1190 wrapping a dictionary constructor around its arguments (in particular,
1191 a dfun never picks superclasses from the arguments under the
1192 dictionary constructor). No superclass is hidden inside a dfun
1193 application.
1194
1195 The extra arguments required to satisfy the DFun Superclass Invariant
1196 always come first, and are called the "silent" arguments. You can
1197 find out how many silent arguments there are using Id.dfunNSilent;
1198 and then you can just drop that number of arguments to see the ones
1199 that were in the original instance declaration.
1200
1201 DFun types are built (only) by MkId.mkDictFunId, so that is where we
1202 decide what silent arguments are to be added.
1203 -}
1204
1205 {-
1206 ************************************************************************
1207 * *
1208 Type-checking an instance method
1209 * *
1210 ************************************************************************
1211
1212 tcMethod
1213 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
1214 - Remembering to use fresh Name (the instance method Name) as the binder
1215 - Bring the instance method Ids into scope, for the benefit of tcInstSig
1216 - Use sig_fn mapping instance method Name -> instance tyvars
1217 - Ditto prag_fn
1218 - Use tcValBinds to do the checking
1219 -}
1220
1221 tcMethods :: DFunId -> Class
1222 -> [TcTyVar] -> [EvVar]
1223 -> [TcType]
1224 -> TcEvBinds
1225 -> ([Located TcSpecPrag], TcPragEnv)
1226 -> [(Id, DefMeth)]
1227 -> InstBindings Name
1228 -> TcM ([Id], LHsBinds Id, Bag Implication)
1229 -- The returned inst_meth_ids all have types starting
1230 -- forall tvs. theta => ...
1231 tcMethods dfun_id clas tyvars dfun_ev_vars inst_tys
1232 dfun_ev_binds prags@(spec_inst_prags,_) op_items
1233 (InstBindings { ib_binds = binds
1234 , ib_tyvars = lexical_tvs
1235 , ib_pragmas = sigs
1236 , ib_extensions = exts
1237 , ib_derived = is_derived })
1238 = tcExtendTyVarEnv2 (lexical_tvs `zip` tyvars) $
1239 -- The lexical_tvs scope over the 'where' part
1240 do { traceTc "tcInstMeth" (ppr sigs $$ ppr binds)
1241 ; checkMinimalDefinition
1242 ; (ids, binds, mb_implics) <- set_exts exts $
1243 mapAndUnzip3M tc_item op_items
1244 ; return (ids, listToBag binds, listToBag (catMaybes mb_implics)) }
1245 where
1246 set_exts :: [ExtensionFlag] -> TcM a -> TcM a
1247 set_exts es thing = foldr setXOptM thing es
1248
1249 hs_sig_fn = mkHsSigFun sigs
1250 inst_loc = getSrcSpan dfun_id
1251
1252 ----------------------
1253 tc_item :: (Id, DefMeth) -> TcM (Id, LHsBind Id, Maybe Implication)
1254 tc_item (sel_id, dm_info)
1255 | Just (user_bind, bndr_loc) <- findMethodBind (idName sel_id) binds
1256 = tcMethodBody clas tyvars dfun_ev_vars inst_tys
1257 dfun_ev_binds is_derived hs_sig_fn prags
1258 sel_id user_bind bndr_loc
1259 | otherwise
1260 = do { traceTc "tc_def" (ppr sel_id)
1261 ; tc_default sel_id dm_info }
1262
1263 ----------------------
1264 tc_default :: Id -> DefMeth -> TcM (TcId, LHsBind Id, Maybe Implication)
1265
1266 tc_default sel_id (GenDefMeth dm_name)
1267 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id dm_name
1268 ; tcMethodBody clas tyvars dfun_ev_vars inst_tys
1269 dfun_ev_binds is_derived hs_sig_fn prags
1270 sel_id meth_bind inst_loc }
1271
1272 tc_default sel_id NoDefMeth -- No default method at all
1273 = do { traceTc "tc_def: warn" (ppr sel_id)
1274 ; (meth_id, _, _) <- mkMethIds hs_sig_fn clas tyvars dfun_ev_vars
1275 inst_tys sel_id
1276 ; dflags <- getDynFlags
1277 ; let meth_bind = mkVarBind meth_id $
1278 mkLHsWrap lam_wrapper (error_rhs dflags)
1279 ; return (meth_id, meth_bind, Nothing) }
1280 where
1281 error_rhs dflags = L inst_loc $ HsApp error_fun (error_msg dflags)
1282 error_fun = L inst_loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
1283 error_msg dflags = L inst_loc (HsLit (HsStringPrim ""
1284 (unsafeMkByteString (error_string dflags))))
1285 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
1286 error_string dflags = showSDoc dflags (hcat [ppr inst_loc, text "|", ppr sel_id ])
1287 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_ev_vars
1288
1289 tc_default sel_id (DefMeth dm_name) -- A polymorphic default method
1290 = do { -- Build the typechecked version directly,
1291 -- without calling typecheck_method;
1292 -- see Note [Default methods in instances]
1293 -- Generate /\as.\ds. let self = df as ds
1294 -- in $dm inst_tys self
1295 -- The 'let' is necessary only because HsSyn doesn't allow
1296 -- you to apply a function to a dictionary *expression*.
1297
1298 ; self_dict <- newDict clas inst_tys
1299 ; let self_ev_bind = mkWantedEvBind self_dict
1300 (EvDFunApp dfun_id (mkTyVarTys tyvars) dfun_ev_vars)
1301
1302 ; (meth_id, local_meth_sig, hs_wrap)
1303 <- mkMethIds hs_sig_fn clas tyvars dfun_ev_vars inst_tys sel_id
1304 ; dm_id <- tcLookupId dm_name
1305 ; let dm_inline_prag = idInlinePragma dm_id
1306 rhs = HsWrap (mkWpEvVarApps [self_dict] <.> mkWpTyApps inst_tys) $
1307 HsVar dm_id
1308
1309 -- A method always has a complete type signature,
1310 -- hence it is safe to call completeIdSigPolyId
1311 local_meth_id = completeIdSigPolyId local_meth_sig
1312 meth_bind = mkVarBind local_meth_id (L inst_loc rhs)
1313 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
1314 -- Copy the inline pragma (if any) from the default
1315 -- method to this version. Note [INLINE and default methods]
1316
1317
1318 export = ABE { abe_wrap = hs_wrap, abe_poly = meth_id1
1319 , abe_mono = local_meth_id
1320 , abe_prags = mk_meth_spec_prags meth_id1 spec_inst_prags [] }
1321 bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
1322 , abs_exports = [export]
1323 , abs_ev_binds = [EvBinds (unitBag self_ev_bind)]
1324 , abs_binds = unitBag meth_bind }
1325 -- Default methods in an instance declaration can't have their own
1326 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
1327 -- currently they are rejected with
1328 -- "INLINE pragma lacks an accompanying binding"
1329
1330 ; return (meth_id1, L inst_loc bind, Nothing) }
1331
1332 ----------------------
1333 -- Check if one of the minimal complete definitions is satisfied
1334 checkMinimalDefinition
1335 = whenIsJust (isUnsatisfied methodExists (classMinimalDef clas)) $
1336 warnUnsatisfiedMinimalDefinition
1337 where
1338 methodExists meth = isJust (findMethodBind meth binds)
1339
1340 ------------------------
1341 tcMethodBody :: Class -> [TcTyVar] -> [EvVar] -> [TcType]
1342 -> TcEvBinds -> Bool
1343 -> HsSigFun
1344 -> ([LTcSpecPrag], TcPragEnv)
1345 -> Id -> LHsBind Name -> SrcSpan
1346 -> TcM (TcId, LHsBind Id, Maybe Implication)
1347 tcMethodBody clas tyvars dfun_ev_vars inst_tys
1348 dfun_ev_binds is_derived
1349 sig_fn (spec_inst_prags, prag_fn)
1350 sel_id (L bind_loc meth_bind) bndr_loc
1351 = add_meth_ctxt $
1352 do { traceTc "tcMethodBody" (ppr sel_id <+> ppr (idType sel_id))
1353 ; (global_meth_id, local_meth_sig, hs_wrap)
1354 <- setSrcSpan bndr_loc $
1355 mkMethIds sig_fn clas tyvars dfun_ev_vars
1356 inst_tys sel_id
1357
1358 ; let prags = lookupPragEnv prag_fn (idName sel_id)
1359 -- A method always has a complete type signature,
1360 -- so it is safe to call cmpleteIdSigPolyId
1361 local_meth_id = completeIdSigPolyId local_meth_sig
1362 lm_bind = meth_bind { fun_id = L bndr_loc (idName local_meth_id) }
1363 -- Substitute the local_meth_name for the binder
1364 -- NB: the binding is always a FunBind
1365
1366 ; global_meth_id <- addInlinePrags global_meth_id prags
1367 ; spec_prags <- tcSpecPrags global_meth_id prags
1368 ; (meth_implic, (tc_bind, _))
1369 <- checkInstConstraints $ \ _ev_binds ->
1370 tcPolyCheck NonRecursive no_prag_fn local_meth_sig
1371 (L bind_loc lm_bind)
1372
1373 ; let specs = mk_meth_spec_prags global_meth_id spec_inst_prags spec_prags
1374 export = ABE { abe_poly = global_meth_id
1375 , abe_mono = local_meth_id
1376 , abe_wrap = hs_wrap
1377 , abe_prags = specs }
1378
1379 local_ev_binds = TcEvBinds (ic_binds meth_implic)
1380 full_bind = AbsBinds { abs_tvs = tyvars
1381 , abs_ev_vars = dfun_ev_vars
1382 , abs_exports = [export]
1383 , abs_ev_binds = [dfun_ev_binds, local_ev_binds]
1384 , abs_binds = tc_bind }
1385
1386 ; return (global_meth_id, L bind_loc full_bind, Just meth_implic) }
1387 where
1388 -- For instance decls that come from deriving clauses
1389 -- we want to print out the full source code if there's an error
1390 -- because otherwise the user won't see the code at all
1391 add_meth_ctxt thing
1392 | is_derived = addLandmarkErrCtxt (derivBindCtxt sel_id clas inst_tys) thing
1393 | otherwise = thing
1394
1395 no_prag_fn = emptyPragEnv -- No pragmas for local_meth_id;
1396 -- they are all for meth_id
1397
1398
1399 ------------------------
1400 mkMethIds :: HsSigFun -> Class -> [TcTyVar] -> [EvVar]
1401 -> [TcType] -> Id -> TcM (TcId, TcIdSigInfo, HsWrapper)
1402 mkMethIds sig_fn clas tyvars dfun_ev_vars inst_tys sel_id
1403 = do { poly_meth_name <- newName (mkClassOpAuxOcc sel_occ)
1404 ; local_meth_name <- newName sel_occ
1405 -- Base the local_meth_name on the selector name, because
1406 -- type errors from tcMethodBody come from here
1407 ; let poly_meth_id = mkLocalId poly_meth_name poly_meth_ty
1408 local_meth_id = mkLocalId local_meth_name local_meth_ty
1409
1410 ; case lookupHsSig sig_fn sel_name of
1411 Just lhs_ty -- There is a signature in the instance declaration
1412 -- See Note [Instance method signatures]
1413 -> setSrcSpan (getLoc lhs_ty) $
1414 do { inst_sigs <- xoptM Opt_InstanceSigs
1415 ; checkTc inst_sigs (misplacedInstSig sel_name lhs_ty)
1416 ; sig_ty <- tcHsSigType (FunSigCtxt sel_name False) lhs_ty
1417 ; let poly_sig_ty = mkSigmaTy tyvars theta sig_ty
1418 ctxt = FunSigCtxt sel_name True
1419 ; tc_sig <- instTcTySig ctxt lhs_ty sig_ty Nothing [] local_meth_name
1420 ; hs_wrap <- addErrCtxtM (methSigCtxt sel_name poly_sig_ty poly_meth_ty) $
1421 tcSubType ctxt poly_sig_ty poly_meth_ty
1422 ; return (poly_meth_id, tc_sig, hs_wrap) }
1423
1424 Nothing -- No type signature
1425 -> do { tc_sig <- instTcTySigFromId local_meth_id
1426 ; return (poly_meth_id, tc_sig, idHsWrapper) } }
1427 -- Absent a type sig, there are no new scoped type variables here
1428 -- Only the ones from the instance decl itself, which are already
1429 -- in scope. Example:
1430 -- class C a where { op :: forall b. Eq b => ... }
1431 -- instance C [c] where { op = <rhs> }
1432 -- In <rhs>, 'c' is scope but 'b' is not!
1433 where
1434 sel_name = idName sel_id
1435 sel_occ = nameOccName sel_name
1436 local_meth_ty = instantiateMethod clas sel_id inst_tys
1437 poly_meth_ty = mkSigmaTy tyvars theta local_meth_ty
1438 theta = map idType dfun_ev_vars
1439
1440 methSigCtxt :: Name -> TcType -> TcType -> TidyEnv -> TcM (TidyEnv, MsgDoc)
1441 methSigCtxt sel_name sig_ty meth_ty env0
1442 = do { (env1, sig_ty) <- zonkTidyTcType env0 sig_ty
1443 ; (env2, meth_ty) <- zonkTidyTcType env1 meth_ty
1444 ; let msg = hang (ptext (sLit "When checking that instance signature for") <+> quotes (ppr sel_name))
1445 2 (vcat [ ptext (sLit "is more general than its signature in the class")
1446 , ptext (sLit "Instance sig:") <+> ppr sig_ty
1447 , ptext (sLit " Class sig:") <+> ppr meth_ty ])
1448 ; return (env2, msg) }
1449
1450 misplacedInstSig :: Name -> LHsType Name -> SDoc
1451 misplacedInstSig name hs_ty
1452 = vcat [ hang (ptext (sLit "Illegal type signature in instance declaration:"))
1453 2 (hang (pprPrefixName name)
1454 2 (dcolon <+> ppr hs_ty))
1455 , ptext (sLit "(Use InstanceSigs to allow this)") ]
1456
1457 {-
1458 Note [Instance method signatures]
1459 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1460 With -XInstanceSigs we allow the user to supply a signature for the
1461 method in an instance declaration. Here is an artificial example:
1462
1463 data Age = MkAge Int
1464 instance Ord Age where
1465 compare :: a -> a -> Bool
1466 compare = error "You can't compare Ages"
1467
1468 The instance signature can be *more* polymorphic than the instantiated
1469 class method (in this case: Age -> Age -> Bool), but it cannot be less
1470 polymorphic. Moreover, if a signature is given, the implementation
1471 code should match the signature, and type variables bound in the
1472 singature should scope over the method body.
1473
1474 We achieve this by building a TcSigInfo for the method, whether or not
1475 there is an instance method signature, and using that to typecheck
1476 the declaration (in tcMethodBody). That means, conveniently,
1477 that the type variables bound in the signature will scope over the body.
1478
1479 What about the check that the instance method signature is more
1480 polymorphic than the instantiated class method type? We just do a
1481 tcSubType call in mkMethIds, and use the HsWrapper thus generated in
1482 the method AbsBind. It's very like the tcSubType impedance-matching
1483 call in mkExport. We have to pass the HsWrapper into
1484 tcMethodBody.
1485 -}
1486
1487 ----------------------
1488 mk_meth_spec_prags :: Id -> [LTcSpecPrag] -> [LTcSpecPrag] -> TcSpecPrags
1489 -- Adapt the 'SPECIALISE instance' pragmas to work for this method Id
1490 -- There are two sources:
1491 -- * spec_prags_for_me: {-# SPECIALISE op :: <blah> #-}
1492 -- * spec_prags_from_inst: derived from {-# SPECIALISE instance :: <blah> #-}
1493 -- These ones have the dfun inside, but [perhaps surprisingly]
1494 -- the correct wrapper.
1495 -- See Note [Handling SPECIALISE pragmas] in TcBinds
1496 mk_meth_spec_prags meth_id spec_inst_prags spec_prags_for_me
1497 = SpecPrags (spec_prags_for_me ++ spec_prags_from_inst)
1498 where
1499 spec_prags_from_inst
1500 | isInlinePragma (idInlinePragma meth_id)
1501 = [] -- Do not inherit SPECIALISE from the instance if the
1502 -- method is marked INLINE, because then it'll be inlined
1503 -- and the specialisation would do nothing. (Indeed it'll provoke
1504 -- a warning from the desugarer
1505 | otherwise
1506 = [ L inst_loc (SpecPrag meth_id wrap inl)
1507 | L inst_loc (SpecPrag _ wrap inl) <- spec_inst_prags]
1508
1509
1510 mkGenericDefMethBind :: Class -> [Type] -> Id -> Name -> TcM (LHsBind Name)
1511 mkGenericDefMethBind clas inst_tys sel_id dm_name
1512 = -- A generic default method
1513 -- If the method is defined generically, we only have to call the
1514 -- dm_name.
1515 do { dflags <- getDynFlags
1516 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Filling in method body"
1517 (vcat [ppr clas <+> ppr inst_tys,
1518 nest 2 (ppr sel_id <+> equals <+> ppr rhs)]))
1519
1520 ; return (noLoc $ mkTopFunBind Generated (noLoc (idName sel_id))
1521 [mkSimpleMatch [] rhs]) }
1522 where
1523 rhs = nlHsVar dm_name
1524
1525 ----------------------
1526 derivBindCtxt :: Id -> Class -> [Type ] -> SDoc
1527 derivBindCtxt sel_id clas tys
1528 = vcat [ ptext (sLit "When typechecking the code for") <+> quotes (ppr sel_id)
1529 , nest 2 (ptext (sLit "in a derived instance for")
1530 <+> quotes (pprClassPred clas tys) <> colon)
1531 , nest 2 $ ptext (sLit "To see the code I am typechecking, use -ddump-deriv") ]
1532
1533 warnUnsatisfiedMinimalDefinition :: ClassMinimalDef -> TcM ()
1534 warnUnsatisfiedMinimalDefinition mindef
1535 = do { warn <- woptM Opt_WarnMissingMethods
1536 ; warnTc warn message
1537 }
1538 where
1539 message = vcat [ptext (sLit "No explicit implementation for")
1540 ,nest 2 $ pprBooleanFormulaNice mindef
1541 ]
1542
1543 {-
1544 Note [Export helper functions]
1545 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1546 We arrange to export the "helper functions" of an instance declaration,
1547 so that they are not subject to preInlineUnconditionally, even if their
1548 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1549 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1550 non-variable for them.
1551
1552 We could change this by making DFunUnfoldings have CoreExprs, but it
1553 seems a bit simpler this way.
1554
1555 Note [Default methods in instances]
1556 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1557 Consider this
1558
1559 class Baz v x where
1560 foo :: x -> x
1561 foo y = <blah>
1562
1563 instance Baz Int Int
1564
1565 From the class decl we get
1566
1567 $dmfoo :: forall v x. Baz v x => x -> x
1568 $dmfoo y = <blah>
1569
1570 Notice that the type is ambiguous. That's fine, though. The instance
1571 decl generates
1572
1573 $dBazIntInt = MkBaz fooIntInt
1574 fooIntInt = $dmfoo Int Int $dBazIntInt
1575
1576 BUT this does mean we must generate the dictionary translation of
1577 fooIntInt directly, rather than generating source-code and
1578 type-checking it. That was the bug in Trac #1061. In any case it's
1579 less work to generate the translated version!
1580
1581 Note [INLINE and default methods]
1582 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1583 Default methods need special case. They are supposed to behave rather like
1584 macros. For exmample
1585
1586 class Foo a where
1587 op1, op2 :: Bool -> a -> a
1588
1589 {-# INLINE op1 #-}
1590 op1 b x = op2 (not b) x
1591
1592 instance Foo Int where
1593 -- op1 via default method
1594 op2 b x = <blah>
1595
1596 The instance declaration should behave
1597
1598 just as if 'op1' had been defined with the
1599 code, and INLINE pragma, from its original
1600 definition.
1601
1602 That is, just as if you'd written
1603
1604 instance Foo Int where
1605 op2 b x = <blah>
1606
1607 {-# INLINE op1 #-}
1608 op1 b x = op2 (not b) x
1609
1610 So for the above example we generate:
1611
1612 {-# INLINE $dmop1 #-}
1613 -- $dmop1 has an InlineCompulsory unfolding
1614 $dmop1 d b x = op2 d (not b) x
1615
1616 $fFooInt = MkD $cop1 $cop2
1617
1618 {-# INLINE $cop1 #-}
1619 $cop1 = $dmop1 $fFooInt
1620
1621 $cop2 = <blah>
1622
1623 Note carefully:
1624
1625 * We *copy* any INLINE pragma from the default method $dmop1 to the
1626 instance $cop1. Otherwise we'll just inline the former in the
1627 latter and stop, which isn't what the user expected
1628
1629 * Regardless of its pragma, we give the default method an
1630 unfolding with an InlineCompulsory source. That means
1631 that it'll be inlined at every use site, notably in
1632 each instance declaration, such as $cop1. This inlining
1633 must happen even though
1634 a) $dmop1 is not saturated in $cop1
1635 b) $cop1 itself has an INLINE pragma
1636
1637 It's vital that $dmop1 *is* inlined in this way, to allow the mutual
1638 recursion between $fooInt and $cop1 to be broken
1639
1640 * To communicate the need for an InlineCompulsory to the desugarer
1641 (which makes the Unfoldings), we use the IsDefaultMethod constructor
1642 in TcSpecPrags.
1643
1644
1645 ************************************************************************
1646 * *
1647 Specialise instance pragmas
1648 * *
1649 ************************************************************************
1650
1651 Note [SPECIALISE instance pragmas]
1652 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1653 Consider
1654
1655 instance (Ix a, Ix b) => Ix (a,b) where
1656 {-# SPECIALISE instance Ix (Int,Int) #-}
1657 range (x,y) = ...
1658
1659 We make a specialised version of the dictionary function, AND
1660 specialised versions of each *method*. Thus we should generate
1661 something like this:
1662
1663 $dfIxPair :: (Ix a, Ix b) => Ix (a,b)
1664 {-# DFUN [$crangePair, ...] #-}
1665 {-# SPECIALISE $dfIxPair :: Ix (Int,Int) #-}
1666 $dfIxPair da db = Ix ($crangePair da db) (...other methods...)
1667
1668 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
1669 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
1670 $crange da db = <blah>
1671
1672 The SPECIALISE pragmas are acted upon by the desugarer, which generate
1673
1674 dii :: Ix Int
1675 dii = ...
1676
1677 $s$dfIxPair :: Ix ((Int,Int),(Int,Int))
1678 {-# DFUN [$crangePair di di, ...] #-}
1679 $s$dfIxPair = Ix ($crangePair di di) (...)
1680
1681 {-# RULE forall (d1,d2:Ix Int). $dfIxPair Int Int d1 d2 = $s$dfIxPair #-}
1682
1683 $s$crangePair :: ((Int,Int),(Int,Int)) -> [(Int,Int)]
1684 $c$crangePair = ...specialised RHS of $crangePair...
1685
1686 {-# RULE forall (d1,d2:Ix Int). $crangePair Int Int d1 d2 = $s$crangePair #-}
1687
1688 Note that
1689
1690 * The specialised dictionary $s$dfIxPair is very much needed, in case we
1691 call a function that takes a dictionary, but in a context where the
1692 specialised dictionary can be used. See Trac #7797.
1693
1694 * The ClassOp rule for 'range' works equally well on $s$dfIxPair, because
1695 it still has a DFunUnfolding. See Note [ClassOp/DFun selection]
1696
1697 * A call (range ($dfIxPair Int Int d1 d2)) might simplify two ways:
1698 --> {ClassOp rule for range} $crangePair Int Int d1 d2
1699 --> {SPEC rule for $crangePair} $s$crangePair
1700 or thus:
1701 --> {SPEC rule for $dfIxPair} range $s$dfIxPair
1702 --> {ClassOpRule for range} $s$crangePair
1703 It doesn't matter which way.
1704
1705 * We want to specialise the RHS of both $dfIxPair and $crangePair,
1706 but the SAME HsWrapper will do for both! We can call tcSpecPrag
1707 just once, and pass the result (in spec_inst_info) to tcMethods.
1708 -}
1709
1710 tcSpecInstPrags :: DFunId -> InstBindings Name
1711 -> TcM ([Located TcSpecPrag], TcPragEnv)
1712 tcSpecInstPrags dfun_id (InstBindings { ib_binds = binds, ib_pragmas = uprags })
1713 = do { spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) $
1714 filter isSpecInstLSig uprags
1715 -- The filter removes the pragmas for methods
1716 ; return (spec_inst_prags, mkPragEnv uprags binds) }
1717
1718 ------------------------------
1719 tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag
1720 tcSpecInst dfun_id prag@(SpecInstSig _ hs_ty)
1721 = addErrCtxt (spec_ctxt prag) $
1722 do { (tyvars, theta, clas, tys) <- tcHsInstHead SpecInstCtxt hs_ty
1723 ; let spec_dfun_ty = mkDictFunTy tyvars theta clas tys
1724 ; co_fn <- tcSpecWrapper SpecInstCtxt (idType dfun_id) spec_dfun_ty
1725 ; return (SpecPrag dfun_id co_fn defaultInlinePragma) }
1726 where
1727 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
1728
1729 tcSpecInst _ _ = panic "tcSpecInst"
1730
1731 {-
1732 ************************************************************************
1733 * *
1734 \subsection{Error messages}
1735 * *
1736 ************************************************************************
1737 -}
1738
1739 instDeclCtxt1 :: LHsType Name -> SDoc
1740 instDeclCtxt1 hs_inst_ty
1741 = inst_decl_ctxt (case unLoc hs_inst_ty of
1742 HsForAllTy _ _ _ _ (L _ ty') -> ppr ty'
1743 _ -> ppr hs_inst_ty) -- Don't expect this
1744 instDeclCtxt2 :: Type -> SDoc
1745 instDeclCtxt2 dfun_ty
1746 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1747 where
1748 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
1749
1750 inst_decl_ctxt :: SDoc -> SDoc
1751 inst_decl_ctxt doc = hang (ptext (sLit "In the instance declaration for"))
1752 2 (quotes doc)
1753
1754 badBootFamInstDeclErr :: SDoc
1755 badBootFamInstDeclErr
1756 = ptext (sLit "Illegal family instance in hs-boot file")
1757
1758 notFamily :: TyCon -> SDoc
1759 notFamily tycon
1760 = vcat [ ptext (sLit "Illegal family instance for") <+> quotes (ppr tycon)
1761 , nest 2 $ parens (ppr tycon <+> ptext (sLit "is not an indexed type family"))]
1762
1763 tooFewParmsErr :: Arity -> SDoc
1764 tooFewParmsErr arity
1765 = ptext (sLit "Family instance has too few parameters; expected") <+>
1766 ppr arity
1767
1768 assocInClassErr :: Located Name -> SDoc
1769 assocInClassErr name
1770 = ptext (sLit "Associated type") <+> quotes (ppr name) <+>
1771 ptext (sLit "must be inside a class instance")
1772
1773 badFamInstDecl :: Located Name -> SDoc
1774 badFamInstDecl tc_name
1775 = vcat [ ptext (sLit "Illegal family instance for") <+>
1776 quotes (ppr tc_name)
1777 , nest 2 (parens $ ptext (sLit "Use TypeFamilies to allow indexed type families")) ]
1778
1779 notOpenFamily :: TyCon -> SDoc
1780 notOpenFamily tc
1781 = ptext (sLit "Illegal instance for closed family") <+> quotes (ppr tc)