Tiny refactor plus comments
[ghc.git] / compiler / typecheck / TcDeriv.hs
1 {-
2 (c) The University of Glasgow 2006
3 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4
5
6 Handles @deriving@ clauses on @data@ declarations.
7 -}
8
9 {-# LANGUAGE CPP #-}
10
11 module TcDeriv ( tcDeriving ) where
12
13 #include "HsVersions.h"
14
15 import HsSyn
16 import DynFlags
17
18 import TcRnMonad
19 import FamInst
20 import TcErrors( reportAllUnsolved )
21 import TcValidity( validDerivPred )
22 import TcEnv
23 import TcTyClsDecls( tcFamTyPats, famTyConShape, tcAddDataFamInstCtxt, kcDataDefn )
24 import TcClassDcl( tcAddDeclCtxt ) -- Small helper
25 import TcGenDeriv -- Deriv stuff
26 import TcGenGenerics
27 import InstEnv
28 import Inst
29 import FamInstEnv
30 import TcHsType
31 import TcMType
32 import TcSimplify
33 import LoadIface( loadInterfaceForName )
34 import Module( getModule )
35
36 import RnNames( extendGlobalRdrEnvRn )
37 import RnBinds
38 import RnEnv
39 import RnSource ( addTcgDUs )
40 import HscTypes
41 import Avail
42
43 import Unify( tcUnifyTy )
44 import Class
45 import Type
46 import ErrUtils
47 import DataCon
48 import Maybes
49 import RdrName
50 import Name
51 import NameSet
52 import TyCon
53 import TcType
54 import Var
55 import VarSet
56 import PrelNames
57 import SrcLoc
58 import Util
59 import Outputable
60 import FastString
61 import Bag
62 import Pair
63
64 import Control.Monad
65 import Data.List
66
67 {-
68 ************************************************************************
69 * *
70 Overview
71 * *
72 ************************************************************************
73
74 Overall plan
75 ~~~~~~~~~~~~
76 1. Convert the decls (i.e. data/newtype deriving clauses,
77 plus standalone deriving) to [EarlyDerivSpec]
78
79 2. Infer the missing contexts for the InferTheta's
80
81 3. Add the derived bindings, generating InstInfos
82 -}
83
84 -- DerivSpec is purely local to this module
85 data DerivSpec theta = DS { ds_loc :: SrcSpan
86 , ds_name :: Name -- DFun name
87 , ds_tvs :: [TyVar]
88 , ds_theta :: theta
89 , ds_cls :: Class
90 , ds_tys :: [Type]
91 , ds_tc :: TyCon
92 , ds_tc_args :: [Type]
93 , ds_overlap :: Maybe OverlapMode
94 , ds_newtype :: Bool }
95 -- This spec implies a dfun declaration of the form
96 -- df :: forall tvs. theta => C tys
97 -- The Name is the name for the DFun we'll build
98 -- The tyvars bind all the variables in the theta
99 -- For type families, the tycon in
100 -- in ds_tys is the *family* tycon
101 -- in ds_tc, ds_tc_args is the *representation* tycon
102 -- For non-family tycons, both are the same
103
104 -- the theta is either the given and final theta, in standalone deriving,
105 -- or the not-yet-simplified list of constraints together with their origin
106
107 -- ds_newtype = True <=> Generalised Newtype Deriving (GND)
108 -- False <=> Vanilla deriving
109
110 {-
111 Example:
112
113 newtype instance T [a] = MkT (Tree a) deriving( C s )
114 ==>
115 axiom T [a] = :RTList a
116 axiom :RTList a = Tree a
117
118 DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
119 , ds_tc = :RTList, ds_tc_args = [a]
120 , ds_newtype = True }
121 -}
122
123 type DerivContext = Maybe ThetaType
124 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
125 -- Just theta <=> Standalone deriving: context supplied by programmer
126
127 data PredOrigin = PredOrigin PredType CtOrigin
128 type ThetaOrigin = [PredOrigin]
129
130 mkPredOrigin :: CtOrigin -> PredType -> PredOrigin
131 mkPredOrigin origin pred = PredOrigin pred origin
132
133 mkThetaOrigin :: CtOrigin -> ThetaType -> ThetaOrigin
134 mkThetaOrigin origin = map (mkPredOrigin origin)
135
136 data EarlyDerivSpec = InferTheta (DerivSpec ThetaOrigin)
137 | GivenTheta (DerivSpec ThetaType)
138 -- InferTheta ds => the context for the instance should be inferred
139 -- In this case ds_theta is the list of all the constraints
140 -- needed, such as (Eq [a], Eq a), together with a suitable CtLoc
141 -- to get good error messages.
142 -- The inference process is to reduce this to a simpler form (e.g.
143 -- Eq a)
144 --
145 -- GivenTheta ds => the exact context for the instance is supplied
146 -- by the programmer; it is ds_theta
147
148 forgetTheta :: EarlyDerivSpec -> DerivSpec ()
149 forgetTheta (InferTheta spec) = spec { ds_theta = () }
150 forgetTheta (GivenTheta spec) = spec { ds_theta = () }
151
152 earlyDSLoc :: EarlyDerivSpec -> SrcSpan
153 earlyDSLoc (InferTheta spec) = ds_loc spec
154 earlyDSLoc (GivenTheta spec) = ds_loc spec
155
156 splitEarlyDerivSpec :: [EarlyDerivSpec] -> ([DerivSpec ThetaOrigin], [DerivSpec ThetaType])
157 splitEarlyDerivSpec [] = ([],[])
158 splitEarlyDerivSpec (InferTheta spec : specs) =
159 case splitEarlyDerivSpec specs of (is, gs) -> (spec : is, gs)
160 splitEarlyDerivSpec (GivenTheta spec : specs) =
161 case splitEarlyDerivSpec specs of (is, gs) -> (is, spec : gs)
162
163 pprDerivSpec :: Outputable theta => DerivSpec theta -> SDoc
164 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
165 ds_cls = c, ds_tys = tys, ds_theta = rhs })
166 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
167 <+> equals <+> ppr rhs)
168
169 instance Outputable theta => Outputable (DerivSpec theta) where
170 ppr = pprDerivSpec
171
172 instance Outputable EarlyDerivSpec where
173 ppr (InferTheta spec) = ppr spec <+> ptext (sLit "(Infer)")
174 ppr (GivenTheta spec) = ppr spec <+> ptext (sLit "(Given)")
175
176 instance Outputable PredOrigin where
177 ppr (PredOrigin ty _) = ppr ty -- The origin is not so interesting when debugging
178
179 {-
180 Inferring missing contexts
181 ~~~~~~~~~~~~~~~~~~~~~~~~~~
182 Consider
183
184 data T a b = C1 (Foo a) (Bar b)
185 | C2 Int (T b a)
186 | C3 (T a a)
187 deriving (Eq)
188
189 [NOTE: See end of these comments for what to do with
190 data (C a, D b) => T a b = ...
191 ]
192
193 We want to come up with an instance declaration of the form
194
195 instance (Ping a, Pong b, ...) => Eq (T a b) where
196 x == y = ...
197
198 It is pretty easy, albeit tedious, to fill in the code "...". The
199 trick is to figure out what the context for the instance decl is,
200 namely @Ping@, @Pong@ and friends.
201
202 Let's call the context reqd for the T instance of class C at types
203 (a,b, ...) C (T a b). Thus:
204
205 Eq (T a b) = (Ping a, Pong b, ...)
206
207 Now we can get a (recursive) equation from the @data@ decl:
208
209 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
210 u Eq (T b a) u Eq Int -- From C2
211 u Eq (T a a) -- From C3
212
213 Foo and Bar may have explicit instances for @Eq@, in which case we can
214 just substitute for them. Alternatively, either or both may have
215 their @Eq@ instances given by @deriving@ clauses, in which case they
216 form part of the system of equations.
217
218 Now all we need do is simplify and solve the equations, iterating to
219 find the least fixpoint. Notice that the order of the arguments can
220 switch around, as here in the recursive calls to T.
221
222 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
223
224 We start with:
225
226 Eq (T a b) = {} -- The empty set
227
228 Next iteration:
229 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
230 u Eq (T b a) u Eq Int -- From C2
231 u Eq (T a a) -- From C3
232
233 After simplification:
234 = Eq a u Ping b u {} u {} u {}
235 = Eq a u Ping b
236
237 Next iteration:
238
239 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
240 u Eq (T b a) u Eq Int -- From C2
241 u Eq (T a a) -- From C3
242
243 After simplification:
244 = Eq a u Ping b
245 u (Eq b u Ping a)
246 u (Eq a u Ping a)
247
248 = Eq a u Ping b u Eq b u Ping a
249
250 The next iteration gives the same result, so this is the fixpoint. We
251 need to make a canonical form of the RHS to ensure convergence. We do
252 this by simplifying the RHS to a form in which
253
254 - the classes constrain only tyvars
255 - the list is sorted by tyvar (major key) and then class (minor key)
256 - no duplicates, of course
257
258 So, here are the synonyms for the ``equation'' structures:
259
260
261 Note [Data decl contexts]
262 ~~~~~~~~~~~~~~~~~~~~~~~~~
263 Consider
264
265 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
266
267 We will need an instance decl like:
268
269 instance (Read a, RealFloat a) => Read (Complex a) where
270 ...
271
272 The RealFloat in the context is because the read method for Complex is bound
273 to construct a Complex, and doing that requires that the argument type is
274 in RealFloat.
275
276 But this ain't true for Show, Eq, Ord, etc, since they don't construct
277 a Complex; they only take them apart.
278
279 Our approach: identify the offending classes, and add the data type
280 context to the instance decl. The "offending classes" are
281
282 Read, Enum?
283
284 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
285 pattern matching against a constructor from a data type with a context
286 gives rise to the constraints for that context -- or at least the thinned
287 version. So now all classes are "offending".
288
289 Note [Newtype deriving]
290 ~~~~~~~~~~~~~~~~~~~~~~~
291 Consider this:
292 class C a b
293 instance C [a] Char
294 newtype T = T Char deriving( C [a] )
295
296 Notice the free 'a' in the deriving. We have to fill this out to
297 newtype T = T Char deriving( forall a. C [a] )
298
299 And then translate it to:
300 instance C [a] Char => C [a] T where ...
301
302
303 Note [Newtype deriving superclasses]
304 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
305 (See also Trac #1220 for an interesting exchange on newtype
306 deriving and superclasses.)
307
308 The 'tys' here come from the partial application in the deriving
309 clause. The last arg is the new instance type.
310
311 We must pass the superclasses; the newtype might be an instance
312 of them in a different way than the representation type
313 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
314 Then the Show instance is not done via Coercible; it shows
315 Foo 3 as "Foo 3"
316 The Num instance is derived via Coercible, but the Show superclass
317 dictionary must the Show instance for Foo, *not* the Show dictionary
318 gotten from the Num dictionary. So we must build a whole new dictionary
319 not just use the Num one. The instance we want is something like:
320 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
321 (+) = ((+)@a)
322 ...etc...
323 There may be a coercion needed which we get from the tycon for the newtype
324 when the dict is constructed in TcInstDcls.tcInstDecl2
325
326
327 Note [Unused constructors and deriving clauses]
328 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
329 See Trac #3221. Consider
330 data T = T1 | T2 deriving( Show )
331 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
332 both of them. So we gather defs/uses from deriving just like anything else.
333
334 ************************************************************************
335 * *
336 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
337 * *
338 ************************************************************************
339 -}
340
341 tcDeriving :: [LTyClDecl Name] -- All type constructors
342 -> [LInstDecl Name] -- All instance declarations
343 -> [LDerivDecl Name] -- All stand-alone deriving declarations
344 -> TcM (TcGblEnv, Bag (InstInfo Name), HsValBinds Name)
345 tcDeriving tycl_decls inst_decls deriv_decls
346 = recoverM (do { g <- getGblEnv
347 ; return (g, emptyBag, emptyValBindsOut)}) $
348 do { -- Fish the "deriving"-related information out of the TcEnv
349 -- And make the necessary "equations".
350 is_boot <- tcIsHsBootOrSig
351 ; traceTc "tcDeriving" (ppr is_boot)
352
353 ; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
354 ; traceTc "tcDeriving 1" (ppr early_specs)
355
356 -- for each type, determine the auxliary declarations that are common
357 -- to multiple derivations involving that type (e.g. Generic and
358 -- Generic1 should use the same TcGenGenerics.MetaTyCons)
359 ; (commonAuxs, auxDerivStuff) <- commonAuxiliaries $ map forgetTheta early_specs
360
361 ; let (infer_specs, given_specs) = splitEarlyDerivSpec early_specs
362 ; insts1 <- mapM (genInst commonAuxs) given_specs
363
364 -- the stand-alone derived instances (@insts1@) are used when inferring
365 -- the contexts for "deriving" clauses' instances (@infer_specs@)
366 ; final_specs <- extendLocalInstEnv (map (iSpec . fstOf3) insts1) $
367 inferInstanceContexts infer_specs
368
369 ; insts2 <- mapM (genInst commonAuxs) final_specs
370
371 ; let (inst_infos, deriv_stuff, maybe_fvs) = unzip3 (insts1 ++ insts2)
372 ; loc <- getSrcSpanM
373 ; let (binds, newTyCons, famInsts, extraInstances) =
374 genAuxBinds loc (unionManyBags (auxDerivStuff : deriv_stuff))
375
376 ; dflags <- getDynFlags
377
378 ; (inst_info, rn_binds, rn_dus) <-
379 renameDeriv is_boot (inst_infos ++ (bagToList extraInstances)) binds
380
381 ; unless (isEmptyBag inst_info) $
382 liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
383 (ddump_deriving inst_info rn_binds newTyCons famInsts))
384
385 ; let all_tycons = map ATyCon (bagToList newTyCons)
386 ; gbl_env <- tcExtendGlobalEnv all_tycons $
387 tcExtendGlobalEnvImplicit (concatMap implicitTyThings all_tycons) $
388 tcExtendLocalFamInstEnv (bagToList famInsts) $
389 tcExtendLocalInstEnv (map iSpec (bagToList inst_info)) getGblEnv
390 ; let all_dus = rn_dus `plusDU` usesOnly (mkFVs $ catMaybes maybe_fvs)
391 ; return (addTcgDUs gbl_env all_dus, inst_info, rn_binds) }
392 where
393 ddump_deriving :: Bag (InstInfo Name) -> HsValBinds Name
394 -> Bag TyCon -- ^ Empty data constructors
395 -> Bag FamInst -- ^ Rep type family instances
396 -> SDoc
397 ddump_deriving inst_infos extra_binds repMetaTys repFamInsts
398 = hang (ptext (sLit "Derived instances:"))
399 2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") (bagToList inst_infos))
400 $$ ppr extra_binds)
401 $$ hangP "Generic representation:" (
402 hangP "Generated datatypes for meta-information:"
403 (vcat (map ppr (bagToList repMetaTys)))
404 $$ hangP "Representation types:"
405 (vcat (map pprRepTy (bagToList repFamInsts))))
406
407 hangP s x = text "" $$ hang (ptext (sLit s)) 2 x
408
409 {-
410 genTypeableTyConReps :: DynFlags ->
411 [LTyClDecl Name] ->
412 [LInstDecl Name] ->
413 TcM (Bag (LHsBind RdrName, LSig RdrName))
414 genTypeableTyConReps dflags decls insts =
415 do tcs1 <- mapM tyConsFromDecl decls
416 tcs2 <- mapM tyConsFromInst insts
417 return $ listToBag [ genTypeableTyConRep dflags loc tc
418 | (loc,tc) <- concat (tcs1 ++ tcs2) ]
419 where
420
421 tyConFromDataCon (L l n) = do dc <- tcLookupDataCon n
422 return (do tc <- promoteDataCon_maybe dc
423 return (l,tc))
424
425 -- Promoted data constructors from a data declaration, or
426 -- a data-family instance.
427 tyConsFromDataRHS = fmap catMaybes
428 . mapM tyConFromDataCon
429 . concatMap (con_names . unLoc)
430 . dd_cons
431
432 -- Tycons from a data-family declaration; not promotable.
433 tyConFromDataFamDecl FamilyDecl { fdLName = L loc name } =
434 do tc <- tcLookupTyCon name
435 return (loc,tc)
436
437
438 -- tycons from a type-level declaration
439 tyConsFromDecl (L _ d)
440
441 -- data or newtype declaration: promoted tycon, tycon, promoted ctrs.
442 | isDataDecl d =
443 do let L loc name = tcdLName d
444 tc <- tcLookupTyCon name
445 promotedCtrs <- tyConsFromDataRHS (tcdDataDefn d)
446 let tyCons = (loc,tc) : promotedCtrs
447
448 return (case promotableTyCon_maybe tc of
449 Nothing -> tyCons
450 Just kc -> (loc,kc) : tyCons)
451
452 -- data family: just the type constructor; these are not promotable.
453 | isDataFamilyDecl d =
454 do res <- tyConFromDataFamDecl (tcdFam d)
455 return [res]
456
457 -- class: the type constructors of associated data families
458 | isClassDecl d =
459 let isData FamilyDecl { fdInfo = DataFamily } = True
460 isData _ = False
461
462 in mapM tyConFromDataFamDecl (filter isData (map unLoc (tcdATs d)))
463
464 | otherwise = return []
465
466
467 tyConsFromInst (L _ d) =
468 case d of
469 ClsInstD ci -> fmap concat
470 $ mapM (tyConsFromDataRHS . dfid_defn . unLoc)
471 $ cid_datafam_insts ci
472 DataFamInstD dfi -> tyConsFromDataRHS (dfid_defn dfi)
473 TyFamInstD {} -> return []
474 -}
475
476 -- Prints the representable type family instance
477 pprRepTy :: FamInst -> SDoc
478 pprRepTy fi@(FamInst { fi_tys = lhs })
479 = ptext (sLit "type") <+> ppr (mkTyConApp (famInstTyCon fi) lhs) <+>
480 equals <+> ppr rhs
481 where rhs = famInstRHS fi
482
483 -- As of 24 April 2012, this only shares MetaTyCons between derivations of
484 -- Generic and Generic1; thus the types and logic are quite simple.
485 type CommonAuxiliary = MetaTyCons
486 type CommonAuxiliaries = [(TyCon, CommonAuxiliary)] -- NSF what is a more efficient map type?
487
488 commonAuxiliaries :: [DerivSpec ()] -> TcM (CommonAuxiliaries, BagDerivStuff)
489 commonAuxiliaries = foldM snoc ([], emptyBag) where
490 snoc acc@(cas, stuff) (DS {ds_name = nm, ds_cls = cls, ds_tc = rep_tycon})
491 | getUnique cls `elem` [genClassKey, gen1ClassKey] =
492 extendComAux $ genGenericMetaTyCons rep_tycon (nameModule nm)
493 | otherwise = return acc
494 where extendComAux m -- don't run m if its already in the accumulator
495 | any ((rep_tycon ==) . fst) cas = return acc
496 | otherwise = do (ca, new_stuff) <- m
497 return $ ((rep_tycon, ca) : cas, stuff `unionBags` new_stuff)
498
499 renameDeriv :: Bool
500 -> [InstInfo RdrName]
501 -> Bag (LHsBind RdrName, LSig RdrName)
502 -> TcM (Bag (InstInfo Name), HsValBinds Name, DefUses)
503 renameDeriv is_boot inst_infos bagBinds
504 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
505 -- The inst-info bindings will all be empty, but it's easier to
506 -- just use rn_inst_info to change the type appropriately
507 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
508 ; return ( listToBag rn_inst_infos
509 , emptyValBindsOut, usesOnly (plusFVs fvs)) }
510
511 | otherwise
512 = discardWarnings $ -- Discard warnings about unused bindings etc
513 setXOptM Opt_EmptyCase $ -- Derived decls (for empty types) can have
514 -- case x of {}
515 setXOptM Opt_ScopedTypeVariables $ -- Derived decls (for newtype-deriving) can
516 setXOptM Opt_KindSignatures $ -- used ScopedTypeVariables & KindSignatures
517 do {
518 -- Bring the extra deriving stuff into scope
519 -- before renaming the instances themselves
520 ; (aux_binds, aux_sigs) <- mapAndUnzipBagM return bagBinds
521 ; let aux_val_binds = ValBindsIn aux_binds (bagToList aux_sigs)
522 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
523 ; let bndrs = collectHsValBinders rn_aux_lhs
524 ; envs <- extendGlobalRdrEnvRn (map Avail bndrs) emptyFsEnv ;
525 ; setEnvs envs $
526 do { (rn_aux, dus_aux) <- rnValBindsRHS (TopSigCtxt (mkNameSet bndrs)) rn_aux_lhs
527 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
528 ; return (listToBag rn_inst_infos, rn_aux,
529 dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
530
531 where
532 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
533 rn_inst_info
534 inst_info@(InstInfo { iSpec = inst
535 , iBinds = InstBindings
536 { ib_binds = binds
537 , ib_tyvars = tyvars
538 , ib_pragmas = sigs
539 , ib_extensions = exts -- Only for type-checking
540 , ib_derived = sa } })
541 = ASSERT( null sigs )
542 bindLocalNamesFV tyvars $
543 do { (rn_binds, fvs) <- rnMethodBinds (is_cls_nm inst) (\_ -> []) binds
544 ; let binds' = InstBindings { ib_binds = rn_binds
545 , ib_tyvars = tyvars
546 , ib_pragmas = []
547 , ib_extensions = exts
548 , ib_derived = sa }
549 ; return (inst_info { iBinds = binds' }, fvs) }
550
551 {-
552 Note [Newtype deriving and unused constructors]
553 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
554 Consider this (see Trac #1954):
555
556 module Bug(P) where
557 newtype P a = MkP (IO a) deriving Monad
558
559 If you compile with -fwarn-unused-binds you do not expect the warning
560 "Defined but not used: data consructor MkP". Yet the newtype deriving
561 code does not explicitly mention MkP, but it should behave as if you
562 had written
563 instance Monad P where
564 return x = MkP (return x)
565 ...etc...
566
567 So we want to signal a user of the data constructor 'MkP'.
568 This is the reason behind the (Maybe Name) part of the return type
569 of genInst.
570
571 ************************************************************************
572 * *
573 From HsSyn to DerivSpec
574 * *
575 ************************************************************************
576
577 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
578 -}
579
580 makeDerivSpecs :: Bool
581 -> [LTyClDecl Name]
582 -> [LInstDecl Name]
583 -> [LDerivDecl Name]
584 -> TcM [EarlyDerivSpec]
585 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
586 = do { eqns1 <- concatMapM (recoverM (return []) . deriveTyDecl) tycl_decls
587 ; eqns2 <- concatMapM (recoverM (return []) . deriveInstDecl) inst_decls
588 ; eqns3 <- concatMapM (recoverM (return []) . deriveStandalone) deriv_decls
589 ; let eqns = eqns1 ++ eqns2 ++ eqns3
590
591 ; if is_boot then -- No 'deriving' at all in hs-boot files
592 do { unless (null eqns) (add_deriv_err (head eqns))
593 ; return [] }
594 else return eqns }
595 where
596 add_deriv_err eqn
597 = setSrcSpan (earlyDSLoc eqn) $
598 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
599 2 (ptext (sLit "Use an instance declaration instead")))
600
601 ------------------------------------------------------------------
602 deriveTyDecl :: LTyClDecl Name -> TcM [EarlyDerivSpec]
603 deriveTyDecl (L _ decl@(DataDecl { tcdLName = L _ tc_name
604 , tcdDataDefn = HsDataDefn { dd_derivs = preds } }))
605 = tcAddDeclCtxt decl $
606 do { tc <- tcLookupTyCon tc_name
607 ; let tvs = tyConTyVars tc
608 tys = mkTyVarTys tvs
609
610 ; case preds of
611 Just (L _ preds') -> concatMapM (deriveTyData tvs tc tys) preds'
612 Nothing -> return [] }
613
614 deriveTyDecl _ = return []
615
616 ------------------------------------------------------------------
617 deriveInstDecl :: LInstDecl Name -> TcM [EarlyDerivSpec]
618 deriveInstDecl (L _ (TyFamInstD {})) = return []
619 deriveInstDecl (L _ (DataFamInstD { dfid_inst = fam_inst }))
620 = deriveFamInst fam_inst
621 deriveInstDecl (L _ (ClsInstD { cid_inst = ClsInstDecl { cid_datafam_insts = fam_insts } }))
622 = concatMapM (deriveFamInst . unLoc) fam_insts
623
624 ------------------------------------------------------------------
625 deriveFamInst :: DataFamInstDecl Name -> TcM [EarlyDerivSpec]
626 deriveFamInst decl@(DataFamInstDecl
627 { dfid_tycon = L _ tc_name, dfid_pats = pats
628 , dfid_defn
629 = defn@(HsDataDefn { dd_derivs = Just (L _ preds) }) })
630 = tcAddDataFamInstCtxt decl $
631 do { fam_tc <- tcLookupTyCon tc_name
632 ; tcFamTyPats (famTyConShape fam_tc) pats (kcDataDefn defn) $
633 -- kcDataDefn defn: see Note [Finding the LHS patterns]
634 \ tvs' pats' _ ->
635 concatMapM (deriveTyData tvs' fam_tc pats') preds }
636
637 deriveFamInst _ = return []
638
639 {-
640 Note [Finding the LHS patterns]
641 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
642 When kind polymorphism is in play, we need to be careful. Here is
643 Trac #9359:
644 data Cmp a where
645 Sup :: Cmp a
646 V :: a -> Cmp a
647
648 data family CmpInterval (a :: Cmp k) (b :: Cmp k) :: *
649 data instance CmpInterval (V c) Sup = Starting c deriving( Show )
650
651 So CmpInterval is kind-polymorphic, but the data instance is not
652 CmpInterval :: forall k. Cmp k -> Cmp k -> *
653 data instance CmpInterval * (V (c::*)) Sup = Starting c deriving( Show )
654
655 Hence, when deriving the type patterns in deriveFamInst, we must kind
656 check the RHS (the data constructor 'Starting c') as well as the LHS,
657 so that we correctly see the instantiation to *.
658 -}
659
660 ------------------------------------------------------------------
661 deriveStandalone :: LDerivDecl Name -> TcM [EarlyDerivSpec]
662 -- Standalone deriving declarations
663 -- e.g. deriving instance Show a => Show (T a)
664 -- Rather like tcLocalInstDecl
665 deriveStandalone (L loc (DerivDecl deriv_ty overlap_mode))
666 = setSrcSpan loc $
667 addErrCtxt (standaloneCtxt deriv_ty) $
668 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
669 ; (tvs, theta, cls, inst_tys) <- tcHsInstHead TcType.InstDeclCtxt deriv_ty
670 ; traceTc "Standalone deriving;" $ vcat
671 [ text "tvs:" <+> ppr tvs
672 , text "theta:" <+> ppr theta
673 , text "cls:" <+> ppr cls
674 , text "tys:" <+> ppr inst_tys ]
675 -- C.f. TcInstDcls.tcLocalInstDecl1
676 ; checkTc (not (null inst_tys)) derivingNullaryErr
677
678 ; let cls_tys = take (length inst_tys - 1) inst_tys
679 inst_ty = last inst_tys
680 ; traceTc "Standalone deriving:" $ vcat
681 [ text "class:" <+> ppr cls
682 , text "class types:" <+> ppr cls_tys
683 , text "type:" <+> ppr inst_ty ]
684
685 ; case tcSplitTyConApp_maybe inst_ty of
686 Just (tc, tc_args)
687 | className cls == typeableClassName
688 -> do warn <- woptM Opt_WarnDerivingTypeable
689 when warn
690 $ addWarnTc
691 $ text "Standalone deriving `Typeable` has no effect."
692 return []
693
694 | isAlgTyCon tc -- All other classes
695 -> do { spec <- mkEqnHelp (fmap unLoc overlap_mode)
696 tvs cls cls_tys tc tc_args (Just theta)
697 ; return [spec] }
698
699 _ -> -- Complain about functions, primitive types, etc,
700 failWithTc $ derivingThingErr False cls cls_tys inst_ty $
701 ptext (sLit "The last argument of the instance must be a data or newtype application")
702 }
703
704
705 ------------------------------------------------------------------
706 deriveTyData :: [TyVar] -> TyCon -> [Type] -- LHS of data or data instance
707 -- Can be a data instance, hence [Type] args
708 -> LHsType Name -- The deriving predicate
709 -> TcM [EarlyDerivSpec]
710 -- The deriving clause of a data or newtype declaration
711 -- I.e. not standalone deriving
712 deriveTyData tvs tc tc_args (L loc deriv_pred)
713 = setSrcSpan loc $ -- Use the location of the 'deriving' item
714 do { (deriv_tvs, cls, cls_tys, cls_arg_kind)
715 <- tcExtendTyVarEnv tvs $
716 tcHsDeriv deriv_pred
717 -- Deriving preds may (now) mention
718 -- the type variables for the type constructor, hence tcExtendTyVarenv
719 -- The "deriv_pred" is a LHsType to take account of the fact that for
720 -- newtype deriving we allow deriving (forall a. C [a]).
721
722 -- Typeable is special, because Typeable :: forall k. k -> Constraint
723 -- so the argument kind 'k' is not decomposable by splitKindFunTys
724 -- as is the case for all other derivable type classes
725 ; if className cls == typeableClassName
726 then do warn <- woptM Opt_WarnDerivingTypeable
727 when warn
728 $ addWarnTc
729 $ text "Deriving `Typeable` has no effect."
730 return []
731 else
732
733 do { -- Given data T a b c = ... deriving( C d ),
734 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
735 let (arg_kinds, _) = splitKindFunTys cls_arg_kind
736 n_args_to_drop = length arg_kinds
737 n_args_to_keep = tyConArity tc - n_args_to_drop
738 args_to_drop = drop n_args_to_keep tc_args
739 tc_args_to_keep = take n_args_to_keep tc_args
740 inst_ty_kind = typeKind (mkTyConApp tc tc_args_to_keep)
741 dropped_tvs = tyVarsOfTypes args_to_drop
742
743 -- Match up the kinds, and apply the resulting kind substitution
744 -- to the types. See Note [Unify kinds in deriving]
745 -- We are assuming the tycon tyvars and the class tyvars are distinct
746 mb_match = tcUnifyTy inst_ty_kind cls_arg_kind
747 Just kind_subst = mb_match
748 (univ_kvs, univ_tvs) = partition isKindVar $ varSetElems $
749 mkVarSet deriv_tvs `unionVarSet`
750 tyVarsOfTypes tc_args_to_keep
751 univ_kvs' = filter (`notElemTvSubst` kind_subst) univ_kvs
752 (subst', univ_tvs') = mapAccumL substTyVarBndr kind_subst univ_tvs
753 final_tc_args = substTys subst' tc_args_to_keep
754 final_cls_tys = substTys subst' cls_tys
755
756 ; traceTc "derivTyData1" (vcat [ pprTvBndrs tvs, ppr tc, ppr tc_args, ppr deriv_pred
757 , pprTvBndrs (varSetElems $ tyVarsOfTypes tc_args)
758 , ppr n_args_to_keep, ppr n_args_to_drop
759 , ppr inst_ty_kind, ppr cls_arg_kind, ppr mb_match
760 , ppr final_tc_args, ppr final_cls_tys ])
761
762 -- Check that the result really is well-kinded
763 ; checkTc (n_args_to_keep >= 0 && isJust mb_match)
764 (derivingKindErr tc cls cls_tys cls_arg_kind)
765
766 ; traceTc "derivTyData2" (vcat [ ppr univ_tvs ])
767
768 ; checkTc (allDistinctTyVars args_to_drop && -- (a) and (b)
769 not (any (`elemVarSet` dropped_tvs) univ_tvs)) -- (c)
770 (derivingEtaErr cls final_cls_tys (mkTyConApp tc final_tc_args))
771 -- Check that
772 -- (a) The args to drop are all type variables; eg reject:
773 -- data instance T a Int = .... deriving( Monad )
774 -- (b) The args to drop are all *distinct* type variables; eg reject:
775 -- class C (a :: * -> * -> *) where ...
776 -- data instance T a a = ... deriving( C )
777 -- (c) The type class args, or remaining tycon args,
778 -- do not mention any of the dropped type variables
779 -- newtype T a s = ... deriving( ST s )
780 -- newtype K a a = ... deriving( Monad )
781
782 ; spec <- mkEqnHelp Nothing (univ_kvs' ++ univ_tvs')
783 cls final_cls_tys tc final_tc_args Nothing
784 ; return [spec] } }
785
786
787 {-
788 Note [Unify kinds in deriving]
789 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
790 Consider (Trac #8534)
791 data T a b = MkT a deriving( Functor )
792 -- where Functor :: (*->*) -> Constraint
793
794 So T :: forall k. * -> k -> *. We want to get
795 instance Functor (T * (a:*)) where ...
796 Notice the '*' argument to T.
797
798 Moreover, as well as instantiating T's kind arguments, we may need to instantiate
799 C's kind args. Consider (Trac #8865):
800 newtype T a b = MkT (Either a b) deriving( Category )
801 where
802 Category :: forall k. (k -> k -> *) -> Constraint
803 We need to generate the instance
804 instance Category * (Either a) where ...
805 Notice the '*' argument to Category.
806
807 So we need to
808 * drop arguments from (T a b) to match the number of
809 arrows in the (last argument of the) class;
810 * and then *unify* kind of the remaining type against the
811 expected kind, to figure out how to instantiate C's and T's
812 kind arguments.
813
814 In the two examples,
815 * we unify kind-of( T k (a:k) ) ~ kind-of( Functor )
816 i.e. (k -> *) ~ (* -> *) to find k:=*.
817 yielding k:=*
818
819 * we unify kind-of( Either ) ~ kind-of( Category )
820 i.e. (* -> * -> *) ~ (k -> k -> k)
821 yielding k:=*
822
823 Now we get a kind substitution. We then need to:
824
825 1. Remove the substituted-out kind variables from the quantified kind vars
826
827 2. Apply the substitution to the kinds of quantified *type* vars
828 (and extend the substitution to reflect this change)
829
830 3. Apply that extended substitution to the non-dropped args (types and
831 kinds) of the type and class
832
833 Forgetting step (2) caused Trac #8893:
834 data V a = V [a] deriving Functor
835 data P (x::k->*) (a:k) = P (x a) deriving Functor
836 data C (x::k->*) (a:k) = C (V (P x a)) deriving Functor
837
838 When deriving Functor for P, we unify k to *, but we then want
839 an instance $df :: forall (x:*->*). Functor x => Functor (P * (x:*->*))
840 and similarly for C. Notice the modified kind of x, both at binding
841 and occurrence sites.
842 -}
843
844 mkEqnHelp :: Maybe OverlapMode
845 -> [TyVar]
846 -> Class -> [Type]
847 -> TyCon -> [Type]
848 -> DerivContext -- Just => context supplied (standalone deriving)
849 -- Nothing => context inferred (deriving on data decl)
850 -> TcRn EarlyDerivSpec
851 -- Make the EarlyDerivSpec for an instance
852 -- forall tvs. theta => cls (tys ++ [ty])
853 -- where the 'theta' is optional (that's the Maybe part)
854 -- Assumes that this declaration is well-kinded
855
856 mkEqnHelp overlap_mode tvs cls cls_tys tycon tc_args mtheta
857 = do { -- Find the instance of a data family
858 -- Note [Looking up family instances for deriving]
859 fam_envs <- tcGetFamInstEnvs
860 ; let (rep_tc, rep_tc_args, _co) = tcLookupDataFamInst fam_envs tycon tc_args
861
862 -- If it's still a data family, the lookup failed; i.e no instance exists
863 ; when (isDataFamilyTyCon rep_tc)
864 (bale_out (ptext (sLit "No family instance for") <+> quotes (pprTypeApp tycon tc_args)))
865
866 -- For standalone deriving (mtheta /= Nothing),
867 -- check that all the data constructors are in scope.
868 ; rdr_env <- getGlobalRdrEnv
869 ; let data_con_names = map dataConName (tyConDataCons rep_tc)
870 hidden_data_cons = not (isWiredInName (tyConName rep_tc)) &&
871 (isAbstractTyCon rep_tc ||
872 any not_in_scope data_con_names)
873 not_in_scope dc = null (lookupGRE_Name rdr_env dc)
874
875 -- Make a Qual RdrName that will do for each DataCon
876 -- so we can report it as used (Trac #7969)
877 data_con_rdrs = [ greUsedRdrName gre
878 | dc_name <- data_con_names
879 , gre : _ <- [lookupGRE_Name rdr_env dc_name]
880 , not (isLocalGRE gre) ]
881
882 ; addUsedRdrNames data_con_rdrs
883 ; unless (isNothing mtheta || not hidden_data_cons)
884 (bale_out (derivingHiddenErr tycon))
885
886 ; dflags <- getDynFlags
887 ; if isDataTyCon rep_tc then
888 mkDataTypeEqn dflags overlap_mode tvs cls cls_tys
889 tycon tc_args rep_tc rep_tc_args mtheta
890 else
891 mkNewTypeEqn dflags overlap_mode tvs cls cls_tys
892 tycon tc_args rep_tc rep_tc_args mtheta }
893 where
894 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
895
896 {-
897 Note [Looking up family instances for deriving]
898 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
899 tcLookupFamInstExact is an auxiliary lookup wrapper which requires
900 that looked-up family instances exist. If called with a vanilla
901 tycon, the old type application is simply returned.
902
903 If we have
904 data instance F () = ... deriving Eq
905 data instance F () = ... deriving Eq
906 then tcLookupFamInstExact will be confused by the two matches;
907 but that can't happen because tcInstDecls1 doesn't call tcDeriving
908 if there are any overlaps.
909
910 There are two other things that might go wrong with the lookup.
911 First, we might see a standalone deriving clause
912 deriving Eq (F ())
913 when there is no data instance F () in scope.
914
915 Note that it's OK to have
916 data instance F [a] = ...
917 deriving Eq (F [(a,b)])
918 where the match is not exact; the same holds for ordinary data types
919 with standalone deriving declarations.
920
921 Note [Deriving, type families, and partial applications]
922 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
923 When there are no type families, it's quite easy:
924
925 newtype S a = MkS [a]
926 -- :CoS :: S ~ [] -- Eta-reduced
927
928 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
929 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
930
931 When type familes are involved it's trickier:
932
933 data family T a b
934 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
935 -- :RT is the representation type for (T Int a)
936 -- :Co:RT :: :RT ~ [] -- Eta-reduced!
937 -- :CoF:RT a :: T Int a ~ :RT a -- Also eta-reduced!
938
939 instance Eq [a] => Eq (T Int a) -- easy by coercion
940 -- d1 :: Eq [a]
941 -- d2 :: Eq (T Int a) = d1 |> Eq (sym (:Co:RT a ; :coF:RT a))
942
943 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
944 -- d1 :: Monad []
945 -- d2 :: Monad (T Int) = d1 |> Monad (sym (:Co:RT ; :coF:RT))
946
947 Note the need for the eta-reduced rule axioms. After all, we can
948 write it out
949 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
950 return x = MkT [x]
951 ... etc ...
952
953 See Note [Eta reduction for data family axioms] in TcInstDcls.
954
955
956 ************************************************************************
957 * *
958 Deriving data types
959 * *
960 ************************************************************************
961 -}
962
963 mkDataTypeEqn :: DynFlags
964 -> Maybe OverlapMode
965 -> [Var] -- Universally quantified type variables in the instance
966 -> Class -- Class for which we need to derive an instance
967 -> [Type] -- Other parameters to the class except the last
968 -> TyCon -- Type constructor for which the instance is requested
969 -- (last parameter to the type class)
970 -> [Type] -- Parameters to the type constructor
971 -> TyCon -- rep of the above (for type families)
972 -> [Type] -- rep of the above
973 -> DerivContext -- Context of the instance, for standalone deriving
974 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
975
976 mkDataTypeEqn dflags overlap_mode tvs cls cls_tys
977 tycon tc_args rep_tc rep_tc_args mtheta
978 = case checkSideConditions dflags mtheta cls cls_tys rep_tc rep_tc_args of
979 -- NB: pass the *representation* tycon to checkSideConditions
980 NonDerivableClass msg -> bale_out (nonStdErr cls $$ msg)
981 DerivableClassError msg -> bale_out msg
982 CanDerive -> go_for_it
983 DerivableViaInstance -> go_for_it
984 where
985 go_for_it = mk_data_eqn overlap_mode tvs cls tycon tc_args rep_tc rep_tc_args mtheta
986 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
987
988 mk_data_eqn :: Maybe OverlapMode -> [TyVar] -> Class
989 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
990 -> TcM EarlyDerivSpec
991 mk_data_eqn overlap_mode tvs cls tycon tc_args rep_tc rep_tc_args mtheta
992 = do loc <- getSrcSpanM
993 dfun_name <- new_dfun_name cls tycon
994 case mtheta of
995 Nothing -> do --Infer context
996 inferred_constraints <- inferConstraints cls inst_tys rep_tc rep_tc_args
997 return $ InferTheta $ DS
998 { ds_loc = loc
999 , ds_name = dfun_name, ds_tvs = tvs
1000 , ds_cls = cls, ds_tys = inst_tys
1001 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
1002 , ds_theta = inferred_constraints
1003 , ds_overlap = overlap_mode
1004 , ds_newtype = False }
1005 Just theta -> do -- Specified context
1006 return $ GivenTheta $ DS
1007 { ds_loc = loc
1008 , ds_name = dfun_name, ds_tvs = tvs
1009 , ds_cls = cls, ds_tys = inst_tys
1010 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
1011 , ds_theta = theta
1012 , ds_overlap = overlap_mode
1013 , ds_newtype = False }
1014 where
1015 inst_tys = [mkTyConApp tycon tc_args]
1016
1017 ----------------------
1018
1019 inferConstraints :: Class -> [TcType]
1020 -> TyCon -> [TcType]
1021 -> TcM ThetaOrigin
1022 -- Generate a sufficiently large set of constraints that typechecking the
1023 -- generated method definitions should succeed. This set will be simplified
1024 -- before being used in the instance declaration
1025 inferConstraints cls inst_tys rep_tc rep_tc_args
1026 | cls `hasKey` genClassKey -- Generic constraints are easy
1027 = return []
1028
1029 | cls `hasKey` gen1ClassKey -- Gen1 needs Functor
1030 = ASSERT(length rep_tc_tvs > 0) -- See Note [Getting base classes]
1031 do { functorClass <- tcLookupClass functorClassName
1032 ; return (con_arg_constraints (get_gen1_constraints functorClass)) }
1033
1034 | otherwise -- The others are a bit more complicated
1035 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
1036 do { traceTc "inferConstraints" (vcat [ppr cls <+> ppr inst_tys, ppr arg_constraints])
1037 ; return (stupid_constraints ++ extra_constraints
1038 ++ sc_constraints
1039 ++ arg_constraints) }
1040 where
1041 arg_constraints = con_arg_constraints get_std_constrained_tys
1042
1043 -- Constraints arising from the arguments of each constructor
1044 con_arg_constraints :: (CtOrigin -> Type -> [PredOrigin]) -> [PredOrigin]
1045 con_arg_constraints get_arg_constraints
1046 = [ pred
1047 | data_con <- tyConDataCons rep_tc
1048 , (arg_n, arg_ty) <- ASSERT( isVanillaDataCon data_con )
1049 zip [1..] $ -- ASSERT is precondition of dataConInstOrigArgTys
1050 dataConInstOrigArgTys data_con all_rep_tc_args
1051 , not (isUnLiftedType arg_ty)
1052 , let orig = DerivOriginDC data_con arg_n
1053 , pred <- get_arg_constraints orig arg_ty ]
1054
1055 -- No constraints for unlifted types
1056 -- See Note [Deriving and unboxed types]
1057
1058 -- For functor-like classes, two things are different
1059 -- (a) We recurse over argument types to generate constraints
1060 -- See Functor examples in TcGenDeriv
1061 -- (b) The rep_tc_args will be one short
1062 is_functor_like = getUnique cls `elem` functorLikeClassKeys
1063 || onlyOneAndTypeConstr inst_tys
1064 onlyOneAndTypeConstr [inst_ty] = typeKind inst_ty `tcEqKind` a2a_kind
1065 onlyOneAndTypeConstr _ = False
1066
1067 a2a_kind = mkArrowKind liftedTypeKind liftedTypeKind
1068
1069 get_gen1_constraints functor_cls orig ty
1070 = mk_functor_like_constraints orig functor_cls $
1071 get_gen1_constrained_tys last_tv ty
1072
1073 get_std_constrained_tys :: CtOrigin -> Type -> [PredOrigin]
1074 get_std_constrained_tys orig ty
1075 | is_functor_like = mk_functor_like_constraints orig cls $
1076 deepSubtypesContaining last_tv ty
1077 | otherwise = [mkPredOrigin orig (mkClassPred cls [ty])]
1078
1079 mk_functor_like_constraints :: CtOrigin -> Class -> [Type] -> [PredOrigin]
1080 -- 'cls' is Functor or Traversable etc
1081 -- For each type, generate two constraints: (cls ty, kind(ty) ~ (*->*))
1082 -- The second constraint checks that the first is well-kinded.
1083 -- Lacking that, as Trac #10561 showed, we can just generate an
1084 -- ill-kinded instance.
1085 mk_functor_like_constraints orig cls tys
1086 = [ mkPredOrigin orig pred
1087 | ty <- tys
1088 , pred <- [ mkClassPred cls [ty]
1089 , mkEqPred (typeKind ty) a2a_kind] ]
1090
1091 rep_tc_tvs = tyConTyVars rep_tc
1092 last_tv = last rep_tc_tvs
1093 all_rep_tc_args | cls `hasKey` gen1ClassKey || is_functor_like
1094 = rep_tc_args ++ [mkTyVarTy last_tv]
1095 | otherwise = rep_tc_args
1096
1097 -- Constraints arising from superclasses
1098 -- See Note [Superclasses of derived instance]
1099 sc_constraints = mkThetaOrigin DerivOrigin $
1100 substTheta (zipOpenTvSubst (classTyVars cls) inst_tys) (classSCTheta cls)
1101
1102 -- Stupid constraints
1103 stupid_constraints = mkThetaOrigin DerivOrigin $
1104 substTheta subst (tyConStupidTheta rep_tc)
1105 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
1106
1107 -- Extra Data constraints
1108 -- The Data class (only) requires that for
1109 -- instance (...) => Data (T t1 t2)
1110 -- IF t1:*, t2:*
1111 -- THEN (Data t1, Data t2) are among the (...) constraints
1112 -- Reason: when the IF holds, we generate a method
1113 -- dataCast2 f = gcast2 f
1114 -- and we need the Data constraints to typecheck the method
1115 extra_constraints
1116 | cls `hasKey` dataClassKey
1117 , all (isLiftedTypeKind . typeKind) rep_tc_args
1118 = [mkPredOrigin DerivOrigin (mkClassPred cls [ty]) | ty <- rep_tc_args]
1119 | otherwise
1120 = []
1121
1122 {-
1123 Note [Getting base classes]
1124 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1125 Functor and Typeable are defined in package 'base', and that is not available
1126 when compiling 'ghc-prim'. So we must be careful that 'deriving' for stuff in
1127 ghc-prim does not use Functor or Typeable implicitly via these lookups.
1128
1129 Note [Deriving and unboxed types]
1130 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1131 We have some special hacks to support things like
1132 data T = MkT Int# deriving ( Show )
1133
1134 Specifically, we use TcGenDeriv.box to box the Int# into an Int
1135 (which we know how to show), and append a '#'. Parenthesis are not required
1136 for unboxed values (`MkT -3#` is a valid expression).
1137
1138 Note [Deriving any class]
1139 ~~~~~~~~~~~~~~~~~~~~~~~~~
1140 Classic uses of a deriving clause, or a standalone-deriving declaration, are
1141 for:
1142 * a built-in class like Eq or Show, for which GHC knows how to generate
1143 the instance code
1144 * a newtype, via the mechanism enabled by GeneralizedNewtypeDeriving
1145
1146 The DeriveAnyClass extension adds a third way to derive instances, based on
1147 empty instance declarations.
1148
1149 The canonical use case is in combination with GHC.Generics and default method
1150 signatures. These allow us have have instance declarations be empty, but still
1151 useful, e.g.
1152
1153 data T a = ...blah..blah... deriving( Generic )
1154 instance C a => C (T a) -- No 'where' clause
1155
1156 where C is some "random" user-defined class.
1157
1158 This boilerplate code can be replaced by the more compact
1159
1160 data T a = ...blah..blah... deriving( Generic, C )
1161
1162 if DeriveAnyClass is enabled.
1163
1164 This is not restricted to Generics; any class can be derived, simply giving
1165 rise to an empty instance.
1166
1167 Unfortunately, it is not clear how to determine the context (in case of
1168 standard deriving; in standalone deriving, the user provides the context).
1169 GHC uses the same heuristic for figuring out the class context that it uses for
1170 Eq in the case of *-kinded classes, and for Functor in the case of
1171 * -> *-kinded classes. That may not be optimal or even wrong. But in such
1172 cases, standalone deriving can still be used.
1173 -}
1174
1175 ------------------------------------------------------------------
1176 -- Check side conditions that dis-allow derivability for particular classes
1177 -- This is *apart* from the newtype-deriving mechanism
1178 --
1179 -- Here we get the representation tycon in case of family instances as it has
1180 -- the data constructors - but we need to be careful to fall back to the
1181 -- family tycon (with indexes) in error messages.
1182
1183 data DerivStatus = CanDerive
1184 | DerivableClassError SDoc -- Standard class, but can't do it
1185 | DerivableViaInstance -- See Note [Deriving any class]
1186 | NonDerivableClass SDoc -- Non-standard class
1187
1188 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType]
1189 -> TyCon -> [Type] -- tycon and its parameters
1190 -> DerivStatus
1191 checkSideConditions dflags mtheta cls cls_tys rep_tc rep_tc_args
1192 | Just cond <- sideConditions mtheta cls
1193 = case (cond (dflags, rep_tc, rep_tc_args)) of
1194 NotValid err -> DerivableClassError err -- Class-specific error
1195 IsValid | null cls_tys -> CanDerive -- All derivable classes are unary, so
1196 -- cls_tys (the type args other than last)
1197 -- should be null
1198 | otherwise -> DerivableClassError (classArgsErr cls cls_tys) -- e.g. deriving( Eq s )
1199 | otherwise = maybe DerivableViaInstance NonDerivableClass
1200 (canDeriveAnyClass dflags rep_tc cls)
1201
1202 classArgsErr :: Class -> [Type] -> SDoc
1203 classArgsErr cls cls_tys = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
1204
1205 nonStdErr :: Class -> SDoc
1206 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
1207
1208 sideConditions :: DerivContext -> Class -> Maybe Condition
1209 sideConditions mtheta cls
1210 | cls_key == eqClassKey = Just (cond_std `andCond` cond_args cls)
1211 | cls_key == ordClassKey = Just (cond_std `andCond` cond_args cls)
1212 | cls_key == showClassKey = Just (cond_std `andCond` cond_args cls)
1213 | cls_key == readClassKey = Just (cond_std `andCond` cond_args cls)
1214 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
1215 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct cls)
1216 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct cls)
1217 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
1218 cond_std `andCond`
1219 cond_args cls)
1220 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
1221 cond_vanilla `andCond`
1222 cond_functorOK True)
1223 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
1224 cond_vanilla `andCond`
1225 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
1226 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
1227 cond_vanilla `andCond`
1228 cond_functorOK False)
1229 | cls_key == genClassKey = Just (checkFlag Opt_DeriveGeneric `andCond`
1230 cond_vanilla `andCond`
1231 cond_RepresentableOk)
1232 | cls_key == gen1ClassKey = Just (checkFlag Opt_DeriveGeneric `andCond`
1233 cond_vanilla `andCond`
1234 cond_Representable1Ok)
1235 | otherwise = Nothing
1236 where
1237 cls_key = getUnique cls
1238 cond_std = cond_stdOK mtheta False -- Vanilla data constructors, at least one,
1239 -- and monotype arguments
1240 cond_vanilla = cond_stdOK mtheta True -- Vanilla data constructors but
1241 -- allow no data cons or polytype arguments
1242
1243 type Condition = (DynFlags, TyCon, [Type]) -> Validity
1244 -- first Bool is whether or not we are allowed to derive Data and Typeable
1245 -- second Bool is whether or not we are allowed to derive Functor
1246 -- TyCon is the *representation* tycon if the data type is an indexed one
1247 -- [Type] are the type arguments to the (representation) TyCon
1248 -- Nothing => OK
1249
1250 orCond :: Condition -> Condition -> Condition
1251 orCond c1 c2 tc
1252 = case (c1 tc, c2 tc) of
1253 (IsValid, _) -> IsValid -- c1 succeeds
1254 (_, IsValid) -> IsValid -- c21 succeeds
1255 (NotValid x, NotValid y) -> NotValid (x $$ ptext (sLit " or") $$ y)
1256 -- Both fail
1257
1258 andCond :: Condition -> Condition -> Condition
1259 andCond c1 c2 tc = c1 tc `andValid` c2 tc
1260
1261 cond_stdOK :: DerivContext -- Says whether this is standalone deriving or not;
1262 -- if standalone, we just say "yes, go for it"
1263 -> Bool -- True <=> permissive: allow higher rank
1264 -- args and no data constructors
1265 -> Condition
1266 cond_stdOK (Just _) _ _
1267 = IsValid -- Don't check these conservative conditions for
1268 -- standalone deriving; just generate the code
1269 -- and let the typechecker handle the result
1270 cond_stdOK Nothing permissive (_, rep_tc, _)
1271 | null data_cons
1272 , not permissive = NotValid (no_cons_why rep_tc $$ suggestion)
1273 | not (null con_whys) = NotValid (vcat con_whys $$ suggestion)
1274 | otherwise = IsValid
1275 where
1276 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
1277 data_cons = tyConDataCons rep_tc
1278 con_whys = getInvalids (map check_con data_cons)
1279
1280 check_con :: DataCon -> Validity
1281 check_con con
1282 | not (isVanillaDataCon con)
1283 = NotValid (badCon con (ptext (sLit "has existentials or constraints in its type")))
1284 | not (permissive || all isTauTy (dataConOrigArgTys con))
1285 = NotValid (badCon con (ptext (sLit "has a higher-rank type")))
1286 | otherwise
1287 = IsValid
1288
1289 no_cons_why :: TyCon -> SDoc
1290 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
1291 ptext (sLit "must have at least one data constructor")
1292
1293 cond_RepresentableOk :: Condition
1294 cond_RepresentableOk (_, tc, tc_args) = canDoGenerics tc tc_args
1295
1296 cond_Representable1Ok :: Condition
1297 cond_Representable1Ok (_, tc, tc_args) = canDoGenerics1 tc tc_args
1298
1299 cond_enumOrProduct :: Class -> Condition
1300 cond_enumOrProduct cls = cond_isEnumeration `orCond`
1301 (cond_isProduct `andCond` cond_args cls)
1302
1303 cond_args :: Class -> Condition
1304 -- For some classes (eg Eq, Ord) we allow unlifted arg types
1305 -- by generating specialised code. For others (eg Data) we don't.
1306 cond_args cls (_, tc, _)
1307 = case bad_args of
1308 [] -> IsValid
1309 (ty:_) -> NotValid (hang (ptext (sLit "Don't know how to derive") <+> quotes (ppr cls))
1310 2 (ptext (sLit "for type") <+> quotes (ppr ty)))
1311 where
1312 bad_args = [ arg_ty | con <- tyConDataCons tc
1313 , arg_ty <- dataConOrigArgTys con
1314 , isUnLiftedType arg_ty
1315 , not (ok_ty arg_ty) ]
1316
1317 cls_key = classKey cls
1318 ok_ty arg_ty
1319 | cls_key == eqClassKey = check_in arg_ty ordOpTbl
1320 | cls_key == ordClassKey = check_in arg_ty ordOpTbl
1321 | cls_key == showClassKey = check_in arg_ty boxConTbl
1322 | otherwise = False -- Read, Ix etc
1323
1324 check_in :: Type -> [(Type,a)] -> Bool
1325 check_in arg_ty tbl = any (eqType arg_ty . fst) tbl
1326
1327
1328 cond_isEnumeration :: Condition
1329 cond_isEnumeration (_, rep_tc, _)
1330 | isEnumerationTyCon rep_tc = IsValid
1331 | otherwise = NotValid why
1332 where
1333 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
1334 ptext (sLit "must be an enumeration type")
1335 , ptext (sLit "(an enumeration consists of one or more nullary, non-GADT constructors)") ]
1336 -- See Note [Enumeration types] in TyCon
1337
1338 cond_isProduct :: Condition
1339 cond_isProduct (_, rep_tc, _)
1340 | isProductTyCon rep_tc = IsValid
1341 | otherwise = NotValid why
1342 where
1343 why = quotes (pprSourceTyCon rep_tc) <+>
1344 ptext (sLit "must have precisely one constructor")
1345
1346 functorLikeClassKeys :: [Unique]
1347 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
1348
1349 cond_functorOK :: Bool -> Condition
1350 -- OK for Functor/Foldable/Traversable class
1351 -- Currently: (a) at least one argument
1352 -- (b) don't use argument contravariantly
1353 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
1354 -- (d) optionally: don't use function types
1355 -- (e) no "stupid context" on data type
1356 cond_functorOK allowFunctions (_, rep_tc, _)
1357 | null tc_tvs
1358 = NotValid (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
1359 <+> ptext (sLit "must have some type parameters"))
1360
1361 | not (null bad_stupid_theta)
1362 = NotValid (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
1363 <+> ptext (sLit "must not have a class context:") <+> pprTheta bad_stupid_theta)
1364
1365 | otherwise
1366 = allValid (map check_con data_cons)
1367 where
1368 tc_tvs = tyConTyVars rep_tc
1369 Just (_, last_tv) = snocView tc_tvs
1370 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
1371 is_bad pred = last_tv `elemVarSet` tyVarsOfType pred
1372
1373 data_cons = tyConDataCons rep_tc
1374 check_con con = allValid (check_universal con : foldDataConArgs (ft_check con) con)
1375
1376 check_universal :: DataCon -> Validity
1377 check_universal con
1378 | Just tv <- getTyVar_maybe (last (tyConAppArgs (dataConOrigResTy con)))
1379 , tv `elem` dataConUnivTyVars con
1380 , not (tv `elemVarSet` tyVarsOfTypes (dataConTheta con))
1381 = IsValid -- See Note [Check that the type variable is truly universal]
1382 | otherwise
1383 = NotValid (badCon con existential)
1384
1385 ft_check :: DataCon -> FFoldType Validity
1386 ft_check con = FT { ft_triv = IsValid, ft_var = IsValid
1387 , ft_co_var = NotValid (badCon con covariant)
1388 , ft_fun = \x y -> if allowFunctions then x `andValid` y
1389 else NotValid (badCon con functions)
1390 , ft_tup = \_ xs -> allValid xs
1391 , ft_ty_app = \_ x -> x
1392 , ft_bad_app = NotValid (badCon con wrong_arg)
1393 , ft_forall = \_ x -> x }
1394
1395 existential = ptext (sLit "must be truly polymorphic in the last argument of the data type")
1396 covariant = ptext (sLit "must not use the type variable in a function argument")
1397 functions = ptext (sLit "must not contain function types")
1398 wrong_arg = ptext (sLit "must use the type variable only as the last argument of a data type")
1399
1400 checkFlag :: ExtensionFlag -> Condition
1401 checkFlag flag (dflags, _, _)
1402 | xopt flag dflags = IsValid
1403 | otherwise = NotValid why
1404 where
1405 why = ptext (sLit "You need ") <> text flag_str
1406 <+> ptext (sLit "to derive an instance for this class")
1407 flag_str = case [ flagSpecName f | f <- xFlags , flagSpecFlag f == flag ] of
1408 [s] -> s
1409 other -> pprPanic "checkFlag" (ppr other)
1410
1411 std_class_via_coercible :: Class -> Bool
1412 -- These standard classes can be derived for a newtype
1413 -- using the coercible trick *even if no -XGeneralizedNewtypeDeriving
1414 -- because giving so gives the same results as generating the boilerplate
1415 std_class_via_coercible clas
1416 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
1417 -- Not Read/Show because they respect the type
1418 -- Not Enum, because newtypes are never in Enum
1419
1420
1421 non_coercible_class :: Class -> Bool
1422 -- *Never* derive Read, Show, Typeable, Data, Generic, Generic1 by Coercible,
1423 -- even with -XGeneralizedNewtypeDeriving
1424 -- Also, avoid Traversable, as the Coercible-derived instance and the "normal"-derived
1425 -- instance behave differently if there's a non-lawful Applicative out there.
1426 -- Besides, with roles, Coercible-deriving Traversable is ill-roled.
1427 non_coercible_class cls
1428 = classKey cls `elem` ([ readClassKey, showClassKey, dataClassKey
1429 , genClassKey, gen1ClassKey, typeableClassKey
1430 , traversableClassKey ])
1431
1432 new_dfun_name :: Class -> TyCon -> TcM Name
1433 new_dfun_name clas tycon -- Just a simple wrapper
1434 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1435 ; newDFunName clas [mkTyConApp tycon []] loc }
1436 -- The type passed to newDFunName is only used to generate
1437 -- a suitable string; hence the empty type arg list
1438
1439 badCon :: DataCon -> SDoc -> SDoc
1440 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1441
1442 {-
1443 Note [Check that the type variable is truly universal]
1444 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1445 For Functor, Foldable, Traversable, we must check that the *last argument*
1446 of the type constructor is used truly universally quantified. Example
1447
1448 data T a b where
1449 T1 :: a -> b -> T a b -- Fine! Vanilla H-98
1450 T2 :: b -> c -> T a b -- Fine! Existential c, but we can still map over 'b'
1451 T3 :: b -> T Int b -- Fine! Constraint 'a', but 'b' is still polymorphic
1452 T4 :: Ord b => b -> T a b -- No! 'b' is constrained
1453 T5 :: b -> T b b -- No! 'b' is constrained
1454 T6 :: T a (b,b) -- No! 'b' is constrained
1455
1456 Notice that only the first of these constructors is vanilla H-98. We only
1457 need to take care about the last argument (b in this case). See Trac #8678.
1458 Eg. for T1-T3 we can write
1459
1460 fmap f (T1 a b) = T1 a (f b)
1461 fmap f (T2 b c) = T2 (f b) c
1462 fmap f (T3 x) = T3 (f x)
1463
1464
1465 Note [Superclasses of derived instance]
1466 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1467 In general, a derived instance decl needs the superclasses of the derived
1468 class too. So if we have
1469 data T a = ...deriving( Ord )
1470 then the initial context for Ord (T a) should include Eq (T a). Often this is
1471 redundant; we'll also generate an Ord constraint for each constructor argument,
1472 and that will probably generate enough constraints to make the Eq (T a) constraint
1473 be satisfied too. But not always; consider:
1474
1475 data S a = S
1476 instance Eq (S a)
1477 instance Ord (S a)
1478
1479 data T a = MkT (S a) deriving( Ord )
1480 instance Num a => Eq (T a)
1481
1482 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1483 Similarly consider:
1484 data T a = MkT deriving( Data, Typeable )
1485 Here there *is* no argument field, but we must nevertheless generate
1486 a context for the Data instances:
1487 instance Typable a => Data (T a) where ...
1488
1489
1490 ************************************************************************
1491 * *
1492 Deriving newtypes
1493 * *
1494 ************************************************************************
1495 -}
1496
1497 mkNewTypeEqn :: DynFlags -> Maybe OverlapMode -> [Var] -> Class
1498 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1499 -> DerivContext
1500 -> TcRn EarlyDerivSpec
1501 mkNewTypeEqn dflags overlap_mode tvs
1502 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1503 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1504 | ASSERT( length cls_tys + 1 == classArity cls )
1505 might_derive_via_coercible && ((newtype_deriving && not deriveAnyClass)
1506 || std_class_via_coercible cls)
1507 = do traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1508 dfun_name <- new_dfun_name cls tycon
1509 loc <- getSrcSpanM
1510 case mtheta of
1511 Just theta -> return $ GivenTheta $ DS
1512 { ds_loc = loc
1513 , ds_name = dfun_name, ds_tvs = varSetElemsKvsFirst dfun_tvs
1514 , ds_cls = cls, ds_tys = inst_tys
1515 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1516 , ds_theta = theta
1517 , ds_overlap = overlap_mode
1518 , ds_newtype = True }
1519 Nothing -> return $ InferTheta $ DS
1520 { ds_loc = loc
1521 , ds_name = dfun_name, ds_tvs = varSetElemsKvsFirst dfun_tvs
1522 , ds_cls = cls, ds_tys = inst_tys
1523 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1524 , ds_theta = all_preds
1525 , ds_overlap = overlap_mode
1526 , ds_newtype = True }
1527 | otherwise
1528 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon rep_tc_args of
1529 -- Error with standard class
1530 DerivableClassError msg
1531 | might_derive_via_coercible -> bale_out (msg $$ suggest_nd)
1532 | otherwise -> bale_out msg
1533 -- Must use newtype deriving or DeriveAnyClass
1534 NonDerivableClass _msg
1535 -- Too hard, even with newtype deriving
1536 | newtype_deriving -> bale_out cant_derive_err
1537 -- Try newtype deriving!
1538 | might_derive_via_coercible -> bale_out (non_std $$ suggest_nd)
1539 | otherwise -> bale_out non_std
1540 -- CanDerive/DerivableViaInstance
1541 _ -> do when (newtype_deriving && deriveAnyClass) $
1542 addWarnTc (sep [ ptext (sLit "Both DeriveAnyClass and GeneralizedNewtypeDeriving are enabled")
1543 , ptext (sLit "Defaulting to the DeriveAnyClass strategy for instantiating") <+> ppr cls ])
1544 go_for_it
1545 where
1546 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1547 deriveAnyClass = xopt Opt_DeriveAnyClass dflags
1548 go_for_it = mk_data_eqn overlap_mode tvs cls tycon tc_args
1549 rep_tycon rep_tc_args mtheta
1550 bale_out = bale_out' newtype_deriving
1551 bale_out' b = failWithTc . derivingThingErr b cls cls_tys inst_ty
1552
1553 non_std = nonStdErr cls
1554 suggest_nd = ptext (sLit "Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1555
1556 -- Here is the plan for newtype derivings. We see
1557 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1558 -- where t is a type,
1559 -- ak+1...an is a suffix of a1..an, and are all tyars
1560 -- ak+1...an do not occur free in t, nor in the s1..sm
1561 -- (C s1 ... sm) is a *partial applications* of class C
1562 -- with the last parameter missing
1563 -- (T a1 .. ak) matches the kind of C's last argument
1564 -- (and hence so does t)
1565 -- The latter kind-check has been done by deriveTyData already,
1566 -- and tc_args are already trimmed
1567 --
1568 -- We generate the instance
1569 -- instance forall ({a1..ak} u fvs(s1..sm)).
1570 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1571 -- where T a1...ap is the partial application of
1572 -- the LHS of the correct kind and p >= k
1573 --
1574 -- NB: the variables below are:
1575 -- tc_tvs = [a1, ..., an]
1576 -- tyvars_to_keep = [a1, ..., ak]
1577 -- rep_ty = t ak .. an
1578 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1579 -- tys = [s1, ..., sm]
1580 -- rep_fn' = t
1581 --
1582 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1583 -- We generate the instance
1584 -- instance Monad (ST s) => Monad (T s) where
1585
1586 nt_eta_arity = newTyConEtadArity rep_tycon
1587 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1588 -- eta-reduces the representation type, so we know that
1589 -- T a ~ S a a
1590 -- That's convenient here, because we may have to apply
1591 -- it to fewer than its original complement of arguments
1592
1593 -- Note [Newtype representation]
1594 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1595 -- Need newTyConRhs (*not* a recursive representation finder)
1596 -- to get the representation type. For example
1597 -- newtype B = MkB Int
1598 -- newtype A = MkA B deriving( Num )
1599 -- We want the Num instance of B, *not* the Num instance of Int,
1600 -- when making the Num instance of A!
1601 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1602 rep_tys = cls_tys ++ [rep_inst_ty]
1603 rep_pred = mkClassPred cls rep_tys
1604 rep_pred_o = mkPredOrigin DerivOrigin rep_pred
1605 -- rep_pred is the representation dictionary, from where
1606 -- we are gong to get all the methods for the newtype
1607 -- dictionary
1608
1609
1610 -- Next we figure out what superclass dictionaries to use
1611 -- See Note [Newtype deriving superclasses] above
1612
1613 cls_tyvars = classTyVars cls
1614 dfun_tvs = tyVarsOfTypes inst_tys
1615 inst_ty = mkTyConApp tycon tc_args
1616 inst_tys = cls_tys ++ [inst_ty]
1617 sc_theta =
1618 mkThetaOrigin DerivOrigin $
1619 substTheta (zipOpenTvSubst cls_tyvars inst_tys) (classSCTheta cls)
1620
1621
1622 -- Next we collect Coercible constraints between
1623 -- the Class method types, instantiated with the representation and the
1624 -- newtype type; precisely the constraints required for the
1625 -- calls to coercible that we are going to generate.
1626 coercible_constraints =
1627 [ let (Pair t1 t2) = mkCoerceClassMethEqn cls (varSetElemsKvsFirst dfun_tvs) inst_tys rep_inst_ty meth
1628 in mkPredOrigin (DerivOriginCoerce meth t1 t2) (mkCoerciblePred t1 t2)
1629 | meth <- classMethods cls ]
1630
1631 -- If there are no tyvars, there's no need
1632 -- to abstract over the dictionaries we need
1633 -- Example: newtype T = MkT Int deriving( C )
1634 -- We get the derived instance
1635 -- instance C T
1636 -- rather than
1637 -- instance C Int => C T
1638 all_preds = rep_pred_o : coercible_constraints ++ sc_theta -- NB: rep_pred comes first
1639
1640 -------------------------------------------------------------------
1641 -- Figuring out whether we can only do this newtype-deriving thing
1642
1643 -- See Note [Determining whether newtype-deriving is appropriate]
1644 might_derive_via_coercible
1645 = not (non_coercible_class cls)
1646 && eta_ok
1647 && ats_ok
1648 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1649
1650 -- Check that eta reduction is OK
1651 eta_ok = nt_eta_arity <= length rep_tc_args
1652 -- The newtype can be eta-reduced to match the number
1653 -- of type argument actually supplied
1654 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1655 -- Here the 'b' must be the same in the rep type (S [a] b)
1656 -- And the [a] must not mention 'b'. That's all handled
1657 -- by nt_eta_rity.
1658
1659 ats_ok = null (classATs cls)
1660 -- No associated types for the class, because we don't
1661 -- currently generate type 'instance' decls; and cannot do
1662 -- so for 'data' instance decls
1663
1664 cant_derive_err
1665 = vcat [ ppUnless eta_ok eta_msg
1666 , ppUnless ats_ok ats_msg ]
1667 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1668 ats_msg = ptext (sLit "the class has associated types")
1669
1670 {-
1671 Note [Recursive newtypes]
1672 ~~~~~~~~~~~~~~~~~~~~~~~~~
1673 Newtype deriving works fine, even if the newtype is recursive.
1674 e.g. newtype S1 = S1 [T1 ()]
1675 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1676 Remember, too, that type families are currently (conservatively) given
1677 a recursive flag, so this also allows newtype deriving to work
1678 for type famillies.
1679
1680 We used to exclude recursive types, because we had a rather simple
1681 minded way of generating the instance decl:
1682 newtype A = MkA [A]
1683 instance Eq [A] => Eq A -- Makes typechecker loop!
1684 But now we require a simple context, so it's ok.
1685
1686 Note [Determining whether newtype-deriving is appropriate]
1687 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1688 When we see
1689 newtype NT = MkNT Foo
1690 deriving C
1691 we have to decide how to perform the deriving. Do we do newtype deriving,
1692 or do we do normal deriving? In general, we prefer to do newtype deriving
1693 wherever possible. So, we try newtype deriving unless there's a glaring
1694 reason not to.
1695
1696 Note that newtype deriving might fail, even after we commit to it. This
1697 is because the derived instance uses `coerce`, which must satisfy its
1698 `Coercible` constraint. This is different than other deriving scenarios,
1699 where we're sure that the resulting instance will type-check.
1700
1701 ************************************************************************
1702 * *
1703 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1704 * *
1705 ************************************************************************
1706
1707 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1708 terms, which is the final correct RHS for the corresponding original
1709 equation.
1710 \begin{itemize}
1711 \item
1712 Each (k,TyVarTy tv) in a solution constrains only a type
1713 variable, tv.
1714
1715 \item
1716 The (k,TyVarTy tv) pairs in a solution are canonically
1717 ordered by sorting on type varible, tv, (major key) and then class, k,
1718 (minor key)
1719 \end{itemize}
1720 -}
1721
1722 inferInstanceContexts :: [DerivSpec ThetaOrigin] -> TcM [DerivSpec ThetaType]
1723
1724 inferInstanceContexts [] = return []
1725
1726 inferInstanceContexts infer_specs
1727 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1728 ; iterate_deriv 1 initial_solutions }
1729 where
1730 ------------------------------------------------------------------
1731 -- The initial solutions for the equations claim that each
1732 -- instance has an empty context; this solution is certainly
1733 -- in canonical form.
1734 initial_solutions :: [ThetaType]
1735 initial_solutions = [ [] | _ <- infer_specs ]
1736
1737 ------------------------------------------------------------------
1738 -- iterate_deriv calculates the next batch of solutions,
1739 -- compares it with the current one; finishes if they are the
1740 -- same, otherwise recurses with the new solutions.
1741 -- It fails if any iteration fails
1742 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec ThetaType]
1743 iterate_deriv n current_solns
1744 | n > 20 -- Looks as if we are in an infinite loop
1745 -- This can happen if we have -XUndecidableInstances
1746 -- (See TcSimplify.tcSimplifyDeriv.)
1747 = pprPanic "solveDerivEqns: probable loop"
1748 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1749 | otherwise
1750 = do { -- Extend the inst info from the explicit instance decls
1751 -- with the current set of solutions, and simplify each RHS
1752 inst_specs <- zipWithM newDerivClsInst current_solns infer_specs
1753 ; new_solns <- checkNoErrs $
1754 extendLocalInstEnv inst_specs $
1755 mapM gen_soln infer_specs
1756
1757 ; if (current_solns `eqSolution` new_solns) then
1758 return [ spec { ds_theta = soln }
1759 | (spec, soln) <- zip infer_specs current_solns ]
1760 else
1761 iterate_deriv (n+1) new_solns }
1762
1763 eqSolution = eqListBy (eqListBy eqType)
1764
1765 ------------------------------------------------------------------
1766 gen_soln :: DerivSpec ThetaOrigin -> TcM ThetaType
1767 gen_soln (DS { ds_loc = loc, ds_tvs = tyvars
1768 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1769 = setSrcSpan loc $
1770 addErrCtxt (derivInstCtxt the_pred) $
1771 do { theta <- simplifyDeriv the_pred tyvars deriv_rhs
1772 -- checkValidInstance tyvars theta clas inst_tys
1773 -- Not necessary; see Note [Exotic derived instance contexts]
1774
1775 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1776 -- Claim: the result instance declaration is guaranteed valid
1777 -- Hence no need to call:
1778 -- checkValidInstance tyvars theta clas inst_tys
1779 ; return (sortBy cmpType theta) } -- Canonicalise before returning the solution
1780 where
1781 the_pred = mkClassPred clas inst_tys
1782
1783 ------------------------------------------------------------------
1784 newDerivClsInst :: ThetaType -> DerivSpec theta -> TcM ClsInst
1785 newDerivClsInst theta (DS { ds_name = dfun_name, ds_overlap = overlap_mode
1786 , ds_tvs = tvs, ds_cls = clas, ds_tys = tys })
1787 = newClsInst overlap_mode dfun_name tvs theta clas tys
1788
1789 extendLocalInstEnv :: [ClsInst] -> TcM a -> TcM a
1790 -- Add new locally-defined instances; don't bother to check
1791 -- for functional dependency errors -- that'll happen in TcInstDcls
1792 extendLocalInstEnv dfuns thing_inside
1793 = do { env <- getGblEnv
1794 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1795 env' = env { tcg_inst_env = inst_env' }
1796 ; setGblEnv env' thing_inside }
1797
1798 {-
1799 ***********************************************************************************
1800 * *
1801 * Simplify derived constraints
1802 * *
1803 ***********************************************************************************
1804 -}
1805
1806 simplifyDeriv :: PredType
1807 -> [TyVar]
1808 -> ThetaOrigin -- Wanted
1809 -> TcM ThetaType -- Needed
1810 -- Given instance (wanted) => C inst_ty
1811 -- Simplify 'wanted' as much as possibles
1812 -- Fail if not possible
1813 simplifyDeriv pred tvs theta
1814 = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
1815 -- The constraint solving machinery
1816 -- expects *TcTyVars* not TyVars.
1817 -- We use *non-overlappable* (vanilla) skolems
1818 -- See Note [Overlap and deriving]
1819
1820 ; let skol_set = mkVarSet tvs_skols
1821 doc = ptext (sLit "deriving") <+> parens (ppr pred)
1822
1823 ; wanted <- mapM (\(PredOrigin t o) -> newWanted o (substTy skol_subst t)) theta
1824
1825 ; traceTc "simplifyDeriv" $
1826 vcat [ pprTvBndrs tvs $$ ppr theta $$ ppr wanted, doc ]
1827 ; residual_wanted <- solveWantedsTcM wanted
1828
1829 ; residual_simple <- zonkSimples (wc_simple residual_wanted)
1830 ; let (good, bad) = partitionBagWith get_good residual_simple
1831 -- See Note [Exotic derived instance contexts]
1832 get_good :: Ct -> Either PredType Ct
1833 get_good ct | validDerivPred skol_set p
1834 , isWantedCt ct = Left p
1835 -- NB: residual_wanted may contain unsolved
1836 -- Derived and we stick them into the bad set
1837 -- so that reportUnsolved may decide what to do with them
1838 | otherwise = Right ct
1839 where p = ctPred ct
1840
1841 ; traceTc "simplifyDeriv 2" $
1842 vcat [ ppr tvs_skols, ppr residual_simple, ppr good, ppr bad ]
1843
1844 -- If we are deferring type errors, simply ignore any insoluble
1845 -- constraints. They'll come up again when we typecheck the
1846 -- generated instance declaration
1847 ; defer <- goptM Opt_DeferTypeErrors
1848 ; unless defer (reportAllUnsolved (residual_wanted { wc_simple = bad }))
1849
1850 ; let min_theta = mkMinimalBySCs (bagToList good)
1851 subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
1852 -- The reverse substitution (sigh)
1853 ; return (substTheta subst_skol min_theta) }
1854
1855 {-
1856 Note [Overlap and deriving]
1857 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1858 Consider some overlapping instances:
1859 data Show a => Show [a] where ..
1860 data Show [Char] where ...
1861
1862 Now a data type with deriving:
1863 data T a = MkT [a] deriving( Show )
1864
1865 We want to get the derived instance
1866 instance Show [a] => Show (T a) where...
1867 and NOT
1868 instance Show a => Show (T a) where...
1869 so that the (Show (T Char)) instance does the Right Thing
1870
1871 It's very like the situation when we're inferring the type
1872 of a function
1873 f x = show [x]
1874 and we want to infer
1875 f :: Show [a] => a -> String
1876
1877 BOTTOM LINE: use vanilla, non-overlappable skolems when inferring
1878 the context for the derived instance.
1879 Hence tcInstSkolTyVars not tcInstSuperSkolTyVars
1880
1881 Note [Exotic derived instance contexts]
1882 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1883 In a 'derived' instance declaration, we *infer* the context. It's a
1884 bit unclear what rules we should apply for this; the Haskell report is
1885 silent. Obviously, constraints like (Eq a) are fine, but what about
1886 data T f a = MkT (f a) deriving( Eq )
1887 where we'd get an Eq (f a) constraint. That's probably fine too.
1888
1889 One could go further: consider
1890 data T a b c = MkT (Foo a b c) deriving( Eq )
1891 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1892
1893 Notice that this instance (just) satisfies the Paterson termination
1894 conditions. Then we *could* derive an instance decl like this:
1895
1896 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1897 even though there is no instance for (C Int a), because there just
1898 *might* be an instance for, say, (C Int Bool) at a site where we
1899 need the equality instance for T's.
1900
1901 However, this seems pretty exotic, and it's quite tricky to allow
1902 this, and yet give sensible error messages in the (much more common)
1903 case where we really want that instance decl for C.
1904
1905 So for now we simply require that the derived instance context
1906 should have only type-variable constraints.
1907
1908 Here is another example:
1909 data Fix f = In (f (Fix f)) deriving( Eq )
1910 Here, if we are prepared to allow -XUndecidableInstances we
1911 could derive the instance
1912 instance Eq (f (Fix f)) => Eq (Fix f)
1913 but this is so delicate that I don't think it should happen inside
1914 'deriving'. If you want this, write it yourself!
1915
1916 NB: if you want to lift this condition, make sure you still meet the
1917 termination conditions! If not, the deriving mechanism generates
1918 larger and larger constraints. Example:
1919 data Succ a = S a
1920 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1921
1922 Note the lack of a Show instance for Succ. First we'll generate
1923 instance (Show (Succ a), Show a) => Show (Seq a)
1924 and then
1925 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1926 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1927
1928 The bottom line
1929 ~~~~~~~~~~~~~~~
1930 Allow constraints which consist only of type variables, with no repeats.
1931
1932
1933 ************************************************************************
1934 * *
1935 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1936 * *
1937 ************************************************************************
1938
1939 After all the trouble to figure out the required context for the
1940 derived instance declarations, all that's left is to chug along to
1941 produce them. They will then be shoved into @tcInstDecls2@, which
1942 will do all its usual business.
1943
1944 There are lots of possibilities for code to generate. Here are
1945 various general remarks.
1946
1947 PRINCIPLES:
1948 \begin{itemize}
1949 \item
1950 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1951 ``you-couldn't-do-better-by-hand'' efficient.
1952
1953 \item
1954 Deriving @Show@---also pretty common--- should also be reasonable good code.
1955
1956 \item
1957 Deriving for the other classes isn't that common or that big a deal.
1958 \end{itemize}
1959
1960 PRAGMATICS:
1961
1962 \begin{itemize}
1963 \item
1964 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1965
1966 \item
1967 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1968
1969 \item
1970 We {\em normally} generate code only for the non-defaulted methods;
1971 there are some exceptions for @Eq@ and (especially) @Ord@...
1972
1973 \item
1974 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1975 constructor's numeric (@Int#@) tag. These are generated by
1976 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1977 these is around is given by @hasCon2TagFun@.
1978
1979 The examples under the different sections below will make this
1980 clearer.
1981
1982 \item
1983 Much less often (really just for deriving @Ix@), we use a
1984 @_tag2con_<tycon>@ function. See the examples.
1985
1986 \item
1987 We use the renamer!!! Reason: we're supposed to be
1988 producing @LHsBinds Name@ for the methods, but that means
1989 producing correctly-uniquified code on the fly. This is entirely
1990 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1991 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1992 the renamer. What a great hack!
1993 \end{itemize}
1994 -}
1995
1996 -- Generate the InstInfo for the required instance paired with the
1997 -- *representation* tycon for that instance,
1998 -- plus any auxiliary bindings required
1999 --
2000 -- Representation tycons differ from the tycon in the instance signature in
2001 -- case of instances for indexed families.
2002 --
2003 genInst :: CommonAuxiliaries
2004 -> DerivSpec ThetaType
2005 -> TcM (InstInfo RdrName, BagDerivStuff, Maybe Name)
2006 genInst comauxs
2007 spec@(DS { ds_tvs = tvs, ds_tc = rep_tycon, ds_tc_args = rep_tc_args
2008 , ds_theta = theta, ds_newtype = is_newtype, ds_tys = tys
2009 , ds_name = dfun_name, ds_cls = clas, ds_loc = loc })
2010 | is_newtype -- See Note [Bindings for Generalised Newtype Deriving]
2011 = do { inst_spec <- newDerivClsInst theta spec
2012 ; traceTc "genInst/is_newtype" (vcat [ppr loc, ppr clas, ppr tvs, ppr tys, ppr rhs_ty])
2013 ; return ( InstInfo
2014 { iSpec = inst_spec
2015 , iBinds = InstBindings
2016 { ib_binds = gen_Newtype_binds loc clas tvs tys rhs_ty
2017 , ib_tyvars = map Var.varName tvs -- Scope over bindings
2018 , ib_pragmas = []
2019 , ib_extensions = [ Opt_ImpredicativeTypes
2020 , Opt_RankNTypes ]
2021 , ib_derived = True } }
2022 , emptyBag
2023 , Just $ getName $ head $ tyConDataCons rep_tycon ) }
2024 -- See Note [Newtype deriving and unused constructors]
2025
2026 | otherwise
2027 = do { (meth_binds, deriv_stuff) <- genDerivStuff loc clas
2028 dfun_name rep_tycon
2029 (lookup rep_tycon comauxs)
2030 ; inst_spec <- newDerivClsInst theta spec
2031 ; traceTc "newder" (ppr inst_spec)
2032 ; let inst_info = InstInfo { iSpec = inst_spec
2033 , iBinds = InstBindings
2034 { ib_binds = meth_binds
2035 , ib_tyvars = map Var.varName tvs
2036 , ib_pragmas = []
2037 , ib_extensions = []
2038 , ib_derived = True } }
2039 ; return ( inst_info, deriv_stuff, Nothing ) }
2040 where
2041 rhs_ty = newTyConInstRhs rep_tycon rep_tc_args
2042
2043 genDerivStuff :: SrcSpan -> Class -> Name -> TyCon
2044 -> Maybe CommonAuxiliary
2045 -> TcM (LHsBinds RdrName, BagDerivStuff)
2046 genDerivStuff loc clas dfun_name tycon comaux_maybe
2047 | let ck = classKey clas
2048 , ck `elem` [genClassKey, gen1ClassKey] -- Special case because monadic
2049 = let gk = if ck == genClassKey then Gen0 else Gen1
2050 -- TODO NSF: correctly identify when we're building Both instead of One
2051 Just metaTyCons = comaux_maybe -- well-guarded by commonAuxiliaries and genInst
2052 in do
2053 (binds, faminst) <- gen_Generic_binds gk tycon metaTyCons (nameModule dfun_name)
2054 return (binds, unitBag (DerivFamInst faminst))
2055
2056 | otherwise -- Non-monadic generators
2057 = do { dflags <- getDynFlags
2058 ; fix_env <- getDataConFixityFun tycon
2059 ; return (genDerivedBinds dflags fix_env clas loc tycon) }
2060
2061 getDataConFixityFun :: TyCon -> TcM (Name -> Fixity)
2062 -- If the TyCon is locally defined, we want the local fixity env;
2063 -- but if it is imported (which happens for standalone deriving)
2064 -- we need to get the fixity env from the interface file
2065 -- c.f. RnEnv.lookupFixity, and Trac #9830
2066 getDataConFixityFun tc
2067 = do { this_mod <- getModule
2068 ; if nameIsLocalOrFrom this_mod name
2069 then do { fix_env <- getFixityEnv
2070 ; return (lookupFixity fix_env) }
2071 else do { iface <- loadInterfaceForName doc name
2072 -- Should already be loaded!
2073 ; return (mi_fix_fn iface . nameOccName) } }
2074 where
2075 name = tyConName tc
2076 doc = ptext (sLit "Data con fixities for") <+> ppr name
2077
2078 {-
2079 Note [Bindings for Generalised Newtype Deriving]
2080 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2081 Consider
2082 class Eq a => C a where
2083 f :: a -> a
2084 newtype N a = MkN [a] deriving( C )
2085 instance Eq (N a) where ...
2086
2087 The 'deriving C' clause generates, in effect
2088 instance (C [a], Eq a) => C (N a) where
2089 f = coerce (f :: [a] -> [a])
2090
2091 This generates a cast for each method, but allows the superclasse to
2092 be worked out in the usual way. In this case the superclass (Eq (N
2093 a)) will be solved by the explicit Eq (N a) instance. We do *not*
2094 create the superclasses by casting the superclass dictionaries for the
2095 representation type.
2096
2097 See the paper "Safe zero-cost coercions for Hsakell".
2098
2099
2100 ************************************************************************
2101 * *
2102 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
2103 * *
2104 ************************************************************************
2105 -}
2106
2107 derivingNullaryErr :: MsgDoc
2108 derivingNullaryErr = ptext (sLit "Cannot derive instances for nullary classes")
2109
2110 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> MsgDoc
2111 derivingKindErr tc cls cls_tys cls_kind
2112 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
2113 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
2114 2 (ptext (sLit "Class") <+> quotes (ppr cls)
2115 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
2116
2117 derivingEtaErr :: Class -> [Type] -> Type -> MsgDoc
2118 derivingEtaErr cls cls_tys inst_ty
2119 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
2120 nest 2 (ptext (sLit "instance (...) =>")
2121 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
2122
2123 derivingThingErr :: Bool -> Class -> [Type] -> Type -> MsgDoc -> MsgDoc
2124 derivingThingErr newtype_deriving clas tys ty why
2125 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
2126 2 (quotes (ppr pred))
2127 $$ nest 2 extra) <> colon,
2128 nest 2 why]
2129 where
2130 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
2131 | otherwise = Outputable.empty
2132 pred = mkClassPred clas (tys ++ [ty])
2133
2134 derivingHiddenErr :: TyCon -> SDoc
2135 derivingHiddenErr tc
2136 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
2137 2 (ptext (sLit "so you cannot derive an instance for it"))
2138
2139 standaloneCtxt :: LHsType Name -> SDoc
2140 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
2141 2 (quotes (ppr ty))
2142
2143 derivInstCtxt :: PredType -> MsgDoc
2144 derivInstCtxt pred
2145 = ptext (sLit "When deriving the instance for") <+> parens (ppr pred)