2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Typechecking class declarations
9 module TcClassDcl ( tcClassSigs, tcClassDecl2,
10 findMethodBind, instantiateMethod, tcInstanceMethodBody,
12 tcAddDeclCtxt, badMethodErr, badATErr, omittedATWarn
15 #include "HsVersions.h"
22 import TcPat( addInlinePrags )
29 import BuildTyCl( TcMethInfo )
57 Every class implicitly declares a new data type, corresponding to dictionaries
58 of that class. So, for example:
60 class (D a) => C a where
62 op2 :: forall b. Ord b => a -> b -> b
64 would implicitly declare
66 data CDict a = CDict (D a)
68 (forall b. Ord b => a -> b -> b)
70 (We could use a record decl, but that means changing more of the existing apparatus.
73 For classes with just one superclass+method, we use a newtype decl instead:
76 op :: forallb. a -> b -> b
80 newtype CDict a = CDict (forall b. a -> b -> b)
82 Now DictTy in Type is just a form of type synomym:
83 DictTy c t = TyConTy CDict `AppTy` t
85 Death to "ExpandingDicts".
88 %************************************************************************
90 Type-checking the class op signatures
92 %************************************************************************
95 tcClassSigs :: Name -- Name of the class
98 -> TcM [TcMethInfo] -- One for each method
100 tcClassSigs clas sigs def_methods
101 = do { -- Check that all def_methods are in the class
102 ; op_info <- mapM (addLocM tc_sig) [sig | sig@(L _ (TypeSig _ _)) <- sigs]
103 ; let op_names = [ n | (n,_,_) <- op_info ]
105 ; sequence_ [ failWithTc (badMethodErr clas n)
106 | n <- dm_bind_names, not (n `elem` op_names) ]
107 -- Value binding for non class-method (ie no TypeSig)
109 ; sequence_ [ failWithTc (badGenericMethod clas n)
110 | n <- genop_names, not (n `elem` dm_bind_names) ]
111 -- Generic signature without value binding
115 dm_bind_names :: [Name] -- These ones have a value binding in the class decl
116 dm_bind_names = [op | L _ (FunBind {fun_id = L _ op}) <- bagToList def_methods]
118 genop_names :: [Name] -- These ones have a generic signature
119 genop_names = [n | L _ (GenericSig (L _ n) _) <- sigs]
121 tc_sig (TypeSig (L _ op_name) op_hs_ty)
122 = do { op_ty <- tcHsKindedType op_hs_ty -- Class tyvars already in scope
123 ; let dm | op_name `elem` genop_names = GenericDM
124 | op_name `elem` dm_bind_names = VanillaDM
126 ; return (op_name, dm, op_ty) }
127 tc_sig sig = pprPanic "tc_cls_sig" (ppr sig)
131 %************************************************************************
135 %************************************************************************
138 tcClassDecl2 :: LTyClDecl Name -- The class declaration
141 tcClassDecl2 (L loc (ClassDecl {tcdLName = class_name, tcdSigs = sigs,
142 tcdMeths = default_binds}))
143 = recoverM (return emptyLHsBinds) $
145 do { clas <- tcLookupLocatedClass class_name
147 -- We make a separate binding for each default method.
148 -- At one time I used a single AbsBinds for all of them, thus
149 -- AbsBind [d] [dm1, dm2, dm3] { dm1 = ...; dm2 = ...; dm3 = ... }
150 -- But that desugars into
151 -- ds = \d -> (..., ..., ...)
152 -- dm1 = \d -> case ds d of (a,b,c) -> a
153 -- And since ds is big, it doesn't get inlined, so we don't get good
154 -- default methods. Better to make separate AbsBinds for each
156 (tyvars, _, _, op_items) = classBigSig clas
157 prag_fn = mkPragFun sigs default_binds
158 sig_fn = mkSigFun sigs
159 clas_tyvars = tcSuperSkolTyVars tyvars
160 pred = mkClassPred clas (mkTyVarTys clas_tyvars)
161 ; this_dict <- newEvVar pred
163 ; traceTc "TIM2" (ppr sigs)
164 ; let tc_dm = tcDefMeth clas clas_tyvars
165 this_dict default_binds sigs
168 ; dm_binds <- tcExtendTyVarEnv clas_tyvars $
171 ; return (unionManyBags dm_binds) }
173 tcClassDecl2 d = pprPanic "tcClassDecl2" (ppr d)
175 tcDefMeth :: Class -> [TyVar] -> EvVar -> LHsBinds Name -> [LSig Name]
176 -> SigFun -> PragFun -> ClassOpItem
177 -> TcM (LHsBinds TcId)
178 -- Generate code for polymorphic default methods only (hence DefMeth)
179 -- (Generic default methods have turned into instance decls by now.)
180 -- This is incompatible with Hugs, which expects a polymorphic
181 -- default method for every class op, regardless of whether or not
182 -- the programmer supplied an explicit default decl for the class.
183 -- (If necessary we can fix that, but we don't have a convenient Id to hand.)
184 tcDefMeth clas tyvars this_dict binds_in sigs sig_fn prag_fn (sel_id, dm_info)
186 NoDefMeth -> do { mapM_ (addLocM (badDmPrag sel_id)) prags
188 DefMeth dm_name -> tc_dm dm_name (instantiateMethod clas sel_id (mkTyVarTys tyvars))
189 GenDefMeth dm_name -> do { tau <- tc_genop_ty (findGenericSig sigs sel_name)
190 ; tc_dm dm_name tau }
191 -- In the case of a generic default, we have to get the type from the signature
192 -- Otherwise we can get it by instantiating the method selector
194 sel_name = idName sel_id
195 prags = prag_fn sel_name
196 dm_sig_fn _ = sig_fn sel_name
197 dm_bind = findMethodBind sel_name binds_in
198 `orElse` pprPanic "tcDefMeth" (ppr sel_id)
200 -- Eg. class C a where
201 -- op :: forall b. Eq b => a -> [b] -> a
203 -- generic gen_op :: D a => a -> a
204 -- The "local_dm_ty" is precisely the type in the above
205 -- type signatures, ie with no "forall a. C a =>" prefix
207 tc_dm dm_name local_dm_ty
208 = do { local_dm_name <- newLocalName sel_name
209 -- Base the local_dm_name on the selector name, because
210 -- type errors from tcInstanceMethodBody come from here
212 ; let dm_ty = mkSigmaTy tyvars [mkClassPred clas (mkTyVarTys tyvars)] local_dm_ty
213 dm_id = mkExportedLocalId dm_name dm_ty
214 local_dm_id = mkLocalId local_dm_name local_dm_ty
216 ; dm_id_w_inline <- addInlinePrags dm_id prags
217 ; spec_prags <- tcSpecPrags dm_id prags
219 ; warnTc (not (null spec_prags))
220 (ptext (sLit "Ignoring SPECIALISE pragmas on default method")
221 <+> quotes (ppr sel_name))
223 ; tc_bind <- tcInstanceMethodBody (ClsSkol clas) tyvars [this_dict]
224 dm_id_w_inline local_dm_id dm_sig_fn
225 IsDefaultMethod dm_bind
227 ; return (unitBag tc_bind) }
229 tc_genop_ty :: LHsType Name -> TcM Type
231 = setSrcSpan (getLoc hs_ty) $
232 do { tau <- tcHsKindedType hs_ty
233 ; checkValidType (FunSigCtxt sel_name) tau
236 findGenericSig :: [LSig Name] -> Name -> LHsType Name
237 -- Find the 'generic op :: ty' signature among the sigs
238 -- If dm_info is GenDefMeth, the corresponding signature
239 -- should jolly well exist! Hence the panic
240 findGenericSig sigs sel_name
241 = case [lty | L _ (GenericSig (L _ n) lty) <- sigs
244 _ -> pprPanic "tcDefMeth" (ppr sel_name $$ ppr sigs)
247 tcInstanceMethodBody :: SkolemInfo -> [TcTyVar] -> [EvVar]
249 -> SigFun -> TcSpecPrags -> LHsBind Name
251 tcInstanceMethodBody skol_info tyvars dfun_ev_vars
252 meth_id local_meth_id
255 = do { -- Typecheck the binding, first extending the envt
256 -- so that when tcInstSig looks up the local_meth_id to find
257 -- its signature, we'll find it in the environment
258 let lm_bind = L loc (bind { fun_id = L loc (idName local_meth_id) })
259 -- Substitute the local_meth_name for the binder
260 -- NB: the binding is always a FunBind
261 ; traceTc "TIM" (ppr local_meth_id $$ ppr (meth_sig_fn (idName local_meth_id)))
262 ; (ev_binds, (tc_bind, _))
263 <- checkConstraints skol_info tyvars dfun_ev_vars $
264 tcExtendIdEnv [local_meth_id] $
265 tcPolyBinds TopLevel meth_sig_fn no_prag_fn
266 NonRecursive NonRecursive
269 ; let full_bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
270 , abs_exports = [(tyvars, meth_id, local_meth_id, specs)]
271 , abs_ev_binds = ev_binds
272 , abs_binds = tc_bind }
274 ; return (L loc full_bind) }
276 no_prag_fn _ = [] -- No pragmas for local_meth_id;
277 -- they are all for meth_id
281 instantiateMethod :: Class -> Id -> [TcType] -> TcType
282 -- Take a class operation, say
283 -- op :: forall ab. C a => forall c. Ix c => (b,c) -> a
284 -- Instantiate it at [ty1,ty2]
285 -- Return the "local method type":
286 -- forall c. Ix x => (ty2,c) -> ty1
287 instantiateMethod clas sel_id inst_tys
288 = ASSERT( ok_first_pred ) local_meth_ty
290 (sel_tyvars,sel_rho) = tcSplitForAllTys (idType sel_id)
291 rho_ty = ASSERT( length sel_tyvars == length inst_tys )
292 substTyWith sel_tyvars inst_tys sel_rho
294 (first_pred, local_meth_ty) = tcSplitPredFunTy_maybe rho_ty
295 `orElse` pprPanic "tcInstanceMethod" (ppr sel_id)
297 ok_first_pred = case getClassPredTys_maybe first_pred of
298 Just (clas1, _tys) -> clas == clas1
300 -- The first predicate should be of form (C a b)
301 -- where C is the class in question
304 ---------------------------
305 findMethodBind :: Name -- Selector name
306 -> LHsBinds Name -- A group of bindings
307 -> Maybe (LHsBind Name) -- The binding
308 findMethodBind sel_name binds
309 = foldlBag mplus Nothing (mapBag f binds)
311 f bind@(L _ (FunBind { fun_id = L _ op_name }))
312 | op_name == sel_name
317 Note [Polymorphic methods]
318 ~~~~~~~~~~~~~~~~~~~~~~~~~~
321 op :: forall b. Ord b => a -> b -> b -> b
322 instance Foo c => Foo [c] where
325 When typechecking the binding 'op = e', we'll have a meth_id for op
327 op :: forall c. Foo c => forall b. Ord b => [c] -> b -> b -> b
329 So tcPolyBinds must be capable of dealing with nested polytypes;
330 and so it is. See TcBinds.tcMonoBinds (with type-sig case).
332 Note [Silly default-method bind]
333 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
334 When we pass the default method binding to the type checker, it must
337 otherwise the "$dm" stuff comes out error messages. But we want the
338 "$dm" to come out in the interface file. So we typecheck the former,
339 and wrap it in a let, thus
340 $dmop2 = let op2 = e in op2
341 This makes the error messages right.
344 %************************************************************************
346 Extracting generic instance declaration from class declarations
348 %************************************************************************
350 @getGenericInstances@ extracts the generic instance declarations from a class
351 declaration. For exmaple
356 op{ x+y } (Inl v) = ...
357 op{ x+y } (Inr v) = ...
358 op{ x*y } (v :*: w) = ...
361 gives rise to the instance declarations
363 instance C (x+y) where
367 instance C (x*y) where
374 mkGenericDefMethBind :: Class -> [Type] -> Id -> Name -> TcM (LHsBind Name)
375 mkGenericDefMethBind clas inst_tys sel_id dm_name
376 = -- A generic default method
377 -- If the method is defined generically, we only have to call the
379 do { dflags <- getDOpts
380 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Filling in method body"
381 (vcat [ppr clas <+> ppr inst_tys,
382 nest 2 (ppr sel_id <+> equals <+> ppr rhs)]))
384 ; return (noLoc $ mkFunBind (noLoc (idName sel_id))
385 [mkSimpleMatch [] rhs]) }
387 rhs = nlHsVar dm_name
390 %************************************************************************
394 %************************************************************************
397 tcAddDeclCtxt :: TyClDecl Name -> TcM a -> TcM a
398 tcAddDeclCtxt decl thing_inside
399 = addErrCtxt ctxt thing_inside
401 thing | isClassDecl decl = "class"
402 | isTypeDecl decl = "type synonym" ++ maybeInst
403 | isDataDecl decl = if tcdND decl == NewType
404 then "newtype" ++ maybeInst
405 else "data type" ++ maybeInst
406 | isFamilyDecl decl = "family"
407 | otherwise = panic "tcAddDeclCtxt/thing"
409 maybeInst | isFamInstDecl decl = " instance"
412 ctxt = hsep [ptext (sLit "In the"), text thing,
413 ptext (sLit "declaration for"), quotes (ppr (tcdName decl))]
415 badMethodErr :: Outputable a => a -> Name -> SDoc
417 = hsep [ptext (sLit "Class"), quotes (ppr clas),
418 ptext (sLit "does not have a method"), quotes (ppr op)]
420 badGenericMethod :: Outputable a => a -> Name -> SDoc
421 badGenericMethod clas op
422 = hsep [ptext (sLit "Class"), quotes (ppr clas),
423 ptext (sLit "has a generic-default signature without a binding"), quotes (ppr op)]
425 badATErr :: Class -> Name -> SDoc
427 = hsep [ptext (sLit "Class"), quotes (ppr clas),
428 ptext (sLit "does not have an associated type"), quotes (ppr at)]
430 omittedATWarn :: Name -> SDoc
432 = ptext (sLit "No explicit AT declaration for") <+> quotes (ppr at)
434 badGenericInstanceType :: LHsBinds Name -> SDoc
435 badGenericInstanceType binds
436 = vcat [ptext (sLit "Illegal type pattern in the generic bindings"),
439 missingGenericInstances :: [Name] -> SDoc
440 missingGenericInstances missing
441 = ptext (sLit "Missing type patterns for") <+> pprQuotedList missing
443 dupGenericInsts :: [(TyCon, InstInfo a)] -> SDoc
444 dupGenericInsts tc_inst_infos
445 = vcat [ptext (sLit "More than one type pattern for a single generic type constructor:"),
446 nest 2 (vcat (map ppr_inst_ty tc_inst_infos)),
447 ptext (sLit "All the type patterns for a generic type constructor must be identical")
450 ppr_inst_ty (_,inst) = ppr (simpleInstInfoTy inst)
452 badDmPrag :: Id -> Sig Name -> TcM ()
453 badDmPrag sel_id prag
454 = addErrTc (ptext (sLit "The") <+> hsSigDoc prag <+> ptext (sLit "for default method")
455 <+> quotes (ppr sel_id)
456 <+> ptext (sLit "lacks an accompanying binding"))