-- 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
\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
| 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
\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
+ = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
CanDerive -> go_for_it -- Use the standard H98 method
DerivableClassError msg -> bale_out msg -- Error with standard class
NonDerivableClass -- Must use newtype deriving
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 $$
ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
-- so for 'data' instance decls
cant_derive_err
- = vcat [ ptext (sLit "even with cunning newtype deriving:")
- , ppUnless arity_ok arity_msg
+ = vcat [ ppUnless arity_ok arity_msg
, ppUnless eta_ok eta_msg
, ppUnless ats_ok ats_msg ]
arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
= hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
-derivingThingErr :: Class -> [Type] -> Type -> Message -> Message
-derivingThingErr clas tys ty why
- = sep [hsep [ptext (sLit "Can't make a derived instance of"),
- quotes (ppr pred)],
- nest 2 (parens why)]
+derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
+derivingThingErr newtype_deriving clas tys ty why
+ = sep [(hang (ptext (sLit "Can't make a derived instance of"))
+ 2 (quotes (ppr pred))
+ $$ 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
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