-- ds_newtype = True <=> Newtype deriving
-- False <=> Vanilla deriving
+type DerivContext = Maybe ThetaType
+ -- Nothing <=> Vanilla deriving; infer the context of the instance decl
+ -- Just theta <=> Standalone deriving: context supplied by programmer
+
type EarlyDerivSpec = Either DerivSpec DerivSpec
-- Left ds => the context for the instance should be inferred
-- In this case ds_theta is the list of all the
; let aux_binds = listToBag $ map (genAuxBind loc) $
rm_dups [] $ concat deriv_aux_binds
; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv (ValBindsIn aux_binds [])
- ; let aux_names = map unLoc (collectHsValBinders rn_aux_lhs)
+ ; let aux_names = collectHsValBinders rn_aux_lhs
; bindLocalNames aux_names $
do { (rn_aux, dus_aux) <- rnTopBindsRHS (mkNameSet aux_names) rn_aux_lhs
| otherwise = rm_dups (b:acc) bs
- rn_inst_info (InstInfo { iSpec = inst, iBinds = NewTypeDerived co })
- = return (InstInfo { iSpec = inst, iBinds = NewTypeDerived co }, emptyFVs)
+ rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
+ rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
+ = return ( info { iBinds = NewTypeDerived coi tc }
+ , mkFVs (map dataConName (tyConDataCons tc)))
+ -- See Note [Newtype deriving and unused constructors]
rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
= -- Bring the right type variables into
; let binds' = VanillaInst rn_binds [] standalone_deriv
; return (InstInfo { iSpec = inst, iBinds = binds' }, fvs) }
where
- (tyvars,_,clas,_) = instanceHead inst
- clas_nm = className clas
+ (tyvars,_, clas,_) = instanceHead inst
+ clas_nm = className clas
-----------------------------------------
mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
-- The predicate tyConHasGenerics finds both of these
\end{code}
+Note [Newtype deriving and unused constructors]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this (see Trac #1954):
+
+ module Bug(P) where
+ newtype P a = MkP (IO a) deriving Monad
+
+If you compile with -fwarn-unused-binds you do not expect the warning
+"Defined but not used: data consructor MkP". Yet the newtype deriving
+code does not explicitly mention MkP, but it should behave as if you
+had written
+ instance Monad P where
+ return x = MkP (return x)
+ ...etc...
+
+So we want to signal a user of the data constructor 'MkP'. That's
+what we do in rn_inst_info, and it's the only reason we have the TyCon
+stored in NewTypeDerived.
+
%************************************************************************
%* *
all_tydata :: [(LHsType Name, LTyClDecl Name)]
-- Derived predicate paired with its data type declaration
- all_tydata = extractTyDataPreds tycl_decls ++
- [ pd -- Traverse assoc data families
- | L _ (InstDecl _ _ _ ats) <- inst_decls
- , pd <- extractTyDataPreds ats ]
+ all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
deriv_locs = map (getLoc . snd) all_tydata
++ map getLoc deriv_decls
<+> text "tvs:" <+> ppr tvs
<+> text "theta:" <+> ppr theta
<+> text "tau:" <+> ppr tau)
- ; (cls, inst_tys) <- checkValidInstHead tau
- ; checkValidInstance tvs theta cls inst_tys
+ ; (cls, inst_tys) <- checkValidInstance deriv_ty tvs theta tau
-- C.f. TcInstDcls.tcLocalInstDecl1
; let cls_tys = take (length inst_tys - 1) inst_tys
newtype S a = MkS [a]
-- :CoS :: S ~ [] -- Eta-reduced
- instance Eq [a] => Eq (S a) -- by coercion sym (Eq (coMkS a)) : Eq [a] ~ Eq (S a)
- instance Monad [] => Monad S -- by coercion sym (Monad coMkS) : Monad [] ~ Monad S
+ instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
+ instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
When type familes are involved it's trickier:
\begin{code}
mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
- -> Maybe ThetaType -- Just => context supplied (standalone deriving)
+ -> DerivContext -- Just => context supplied (standalone deriving)
-- Nothing => context inferred (deriving on data decl)
-> TcRn EarlyDerivSpec
-- Make the EarlyDerivSpec for an instance
mkNewTypeEqn orig dflags tvs cls cls_tys
tycon tc_args rep_tc rep_tc_args mtheta }
| otherwise
- = failWithTc (derivingThingErr cls cls_tys tc_app
+ = failWithTc (derivingThingErr False cls cls_tys tc_app
(ptext (sLit "The last argument of the instance must be a data or newtype application")))
\end{code}
-> [Var] -- Universally quantified type variables in the instance
-> Class -- Class for which we need to derive an instance
-> [Type] -- Other parameters to the class except the last
- -> TyCon -- Type constructor for which the instance is requested (last parameter to the type class)
+ -> TyCon -- Type constructor for which the instance is requested
+ -- (last parameter to the type class)
-> [Type] -- Parameters to the type constructor
-> TyCon -- rep of the above (for type families)
-> [Type] -- rep of the above
- -> Maybe ThetaType -- Context of the instance, for standalone deriving
+ -> DerivContext -- Context of the instance, for standalone deriving
-> TcRn EarlyDerivSpec -- Return 'Nothing' if error
mkDataTypeEqn orig dflags tvs cls cls_tys
tycon tc_args rep_tc rep_tc_args mtheta
- | isJust mtheta = go_for_it -- Do not test side conditions for standalone deriving
- | otherwise = case checkSideConditions dflags cls cls_tys rep_tc of
- -- NB: pass the *representation* tycon to checkSideConditions
- CanDerive -> go_for_it
- NonDerivableClass -> bale_out (nonStdErr cls)
- DerivableClassError msg -> bale_out msg
+ = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
+ -- NB: pass the *representation* tycon to checkSideConditions
+ CanDerive -> go_for_it
+ NonDerivableClass -> bale_out (nonStdErr cls)
+ DerivableClassError msg -> bale_out msg
where
go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
- bale_out msg = failWithTc (derivingThingErr cls cls_tys (mkTyConApp tycon tc_args) msg)
+ bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
mk_data_eqn, mk_typeable_eqn
:: InstOrigin -> [TyVar] -> Class
- -> TyCon -> [TcType] -> TyCon -> [TcType] -> Maybe ThetaType
+ -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
-> TcM EarlyDerivSpec
mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
| getName cls `elem` typeableClassNames
-- Generate a sufficiently large set of constraints that typechecking the
-- generated method definitions should succeed. This set will be simplified
-- before being used in the instance declaration
-inferConstraints tvs cls inst_tys rep_tc rep_tc_args
+inferConstraints _ cls inst_tys rep_tc rep_tc_args
= ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
stupid_constraints ++ extra_constraints
++ sc_constraints ++ con_arg_constraints
stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
- -- Extra constraints
+ -- Extra Data constraints
-- The Data class (only) requires that for
- -- instance (...) => Data (T a b)
- -- then (Data a, Data b) are among the (...) constraints
- -- Reason: that's what you need to typecheck the method
- -- dataCast1 f = gcast1 f
+ -- instance (...) => Data (T t1 t2)
+ -- IF t1:*, t2:*
+ -- THEN (Data t1, Data t2) are among the (...) constraints
+ -- Reason: when the IF holds, we generate a method
+ -- dataCast2 f = gcast2 f
+ -- and we need the Data constraints to typecheck the method
extra_constraints
- | cls `hasKey` dataClassKey = [mkClassPred cls [mkTyVarTy tv] | tv <- tvs]
- | otherwise = []
+ | cls `hasKey` dataClassKey
+ , all (isLiftedTypeKind . typeKind) rep_tc_args
+ = [mkClassPred cls [ty] | ty <- rep_tc_args]
+ | otherwise
+ = []
------------------------------------------------------------------
-- Check side conditions that dis-allow derivability for particular classes
| DerivableClassError SDoc -- Standard class, but can't do it
| NonDerivableClass -- Non-standard class
-checkSideConditions :: DynFlags -> Class -> [TcType] -> TyCon -> DerivStatus
-checkSideConditions dflags cls cls_tys rep_tc
- | Just cond <- sideConditions cls
+checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
+checkSideConditions dflags mtheta cls cls_tys rep_tc
+ | Just cond <- sideConditions mtheta cls
= case (cond (dflags, rep_tc)) of
Just err -> DerivableClassError err -- Class-specific error
- Nothing | null cls_tys -> CanDerive
+ Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
+ -- cls_tys (the type args other than last)
+ -- should be null
| otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
| otherwise = NonDerivableClass -- Not a standard class
where
nonStdErr :: Class -> SDoc
nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
-sideConditions :: Class -> Maybe Condition
-sideConditions cls
+sideConditions :: DerivContext -> Class -> Maybe Condition
+sideConditions mtheta cls
| cls_key == eqClassKey = Just cond_std
| cls_key == ordClassKey = Just cond_std
| cls_key == showClassKey = Just cond_std
| otherwise = Nothing
where
cls_key = getUnique cls
+ cond_std = cond_stdOK mtheta
type Condition = (DynFlags, TyCon) -> Maybe SDoc
-- first Bool is whether or not we are allowed to derive Data and Typeable
Nothing -> c2 tc -- c1 succeeds
Just x -> Just x -- c1 fails
-cond_std :: Condition
-cond_std (_, rep_tc)
- | null data_cons = Just no_cons_why
- | not (null con_whys) = Just (vcat con_whys)
+cond_stdOK :: DerivContext -> Condition
+cond_stdOK (Just _) _
+ = Nothing -- Don't check these conservative conditions for
+ -- standalone deriving; just generate the code
+cond_stdOK Nothing (_, rep_tc)
+ | null data_cons = Just (no_cons_why $$ suggestion)
+ | not (null con_whys) = Just (vcat con_whys $$ suggestion)
| otherwise = Nothing
where
- data_cons = tyConDataCons rep_tc
- no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
- ptext (sLit "has no data constructors")
+ suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
+ data_cons = tyConDataCons rep_tc
+ no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
+ ptext (sLit "has no data constructors")
con_whys = mapCatMaybes check_con data_cons
functions = ptext (sLit "contains function types")
wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
-checkFlag :: DynFlag -> Condition
+checkFlag :: ExtensionFlag -> Condition
checkFlag flag (dflags, _)
| dopt flag dflags = Nothing
| otherwise = Just why
other -> pprPanic "checkFlag" (ppr other)
std_class_via_iso :: Class -> Bool
-std_class_via_iso clas -- These standard classes can be derived for a newtype
- -- using the isomorphism trick *even if no -fglasgow-exts*
+-- These standard classes can be derived for a newtype
+-- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
+-- because giving so gives the same results as generating the boilerplate
+std_class_via_iso clas
= classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
-- Not Read/Show because they respect the type
-- Not Enum, because newtypes are never in Enum
+non_iso_class :: Class -> Bool
+-- *Never* derive Read,Show,Typeable,Data by isomorphism,
+-- even with -XGeneralizedNewtypeDeriving
+non_iso_class cls
+ = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
+ typeableClassKeys)
+
+typeableClassKeys :: [Unique]
+typeableClassKeys = map getUnique typeableClassNames
+
new_dfun_name :: Class -> TyCon -> TcM Name
new_dfun_name clas tycon -- Just a simple wrapper
= do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
\begin{code}
mkNewTypeEqn :: InstOrigin -> DynFlags -> [Var] -> Class
-> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
- -> Maybe ThetaType
+ -> DerivContext
-> TcRn EarlyDerivSpec
mkNewTypeEqn orig dflags tvs
cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
; return (if isJust mtheta then Right spec
else Left spec) }
- | isJust mtheta = go_for_it -- Do not check side conditions for standalone deriving
| otherwise
- = case checkSideConditions dflags cls cls_tys rep_tycon of
- CanDerive -> go_for_it -- Use the standard H98 method
- DerivableClassError msg -> bale_out msg -- Error with standard class
+ = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
+ CanDerive -> go_for_it -- Use the standard H98 method
+ DerivableClassError msg -- Error with standard class
+ | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
+ | otherwise -> bale_out msg
NonDerivableClass -- Must use newtype deriving
- | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
- | otherwise -> bale_out non_std_err -- Try newtype deriving!
+ | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
+ | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
+ | otherwise -> bale_out non_std
where
newtype_deriving = dopt Opt_GeneralizedNewtypeDeriving dflags
go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
- bale_out msg = failWithTc (derivingThingErr cls cls_tys inst_ty msg)
+ bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
- non_std_err = nonStdErr cls $$
- ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
+ non_std = nonStdErr cls
+ suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
-- Here is the plan for newtype derivings. We see
-- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
&& ats_ok
-- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
- -- Never derive Read,Show,Typeable,Data by isomorphism
- non_iso_class cls = className cls `elem` ([readClassName, showClassName, dataClassName] ++
- typeableClassNames)
-
arity_ok = length cls_tys + 1 == classArity cls
-- Well kinded; eg not: newtype T ... deriving( ST )
-- because ST needs *2* type params
-- so for 'data' instance decls
cant_derive_err
- = vcat [ ptext (sLit "even with cunning newtype deriving:")
- , if arity_ok then empty else arity_msg
- , if eta_ok then empty else eta_msg
- , if ats_ok then empty else ats_msg ]
+ = vcat [ ppUnless arity_ok arity_msg
+ , ppUnless eta_ok eta_msg
+ , ppUnless ats_ok ats_msg ]
arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
ats_msg = ptext (sLit "the class has associated types")
weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
; mapM_ (addErrTc . badDerivedPred) weird_preds
+ ; traceTc (text "TcDeriv" <+> (ppr deriv_rhs $$ ppr theta))
-- Claim: the result instance declaration is guaranteed valid
-- Hence no need to call:
-- checkValidInstance tyvars theta clas inst_tys
genInst standalone_deriv oflag spec
| ds_newtype spec
= return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
- , iBinds = NewTypeDerived co }, [])
+ , iBinds = NewTypeDerived co rep_tycon }, [])
| otherwise
= do { let loc = getSrcSpan (ds_name spec)
= hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
-derivingThingErr :: Class -> [Type] -> Type -> Message -> Message
-derivingThingErr clas tys ty why
- = sep [hsep [ptext (sLit "Can't make a derived instance of"),
- quotes (ppr pred)],
- nest 2 (parens why)]
+derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
+derivingThingErr newtype_deriving clas tys ty why
+ = sep [(hang (ptext (sLit "Can't make a derived instance of"))
+ 2 (quotes (ppr pred))
+ $$ nest 2 extra) <> colon,
+ nest 2 why]
where
+ extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
+ | otherwise = empty
pred = mkClassPred clas (tys ++ [ty])
derivingHiddenErr :: TyCon -> SDoc
projects / ghc-hetmet.git / blobdiff