Commit 895eccb0 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com

Tidy up deriving error messages

I did this in response to a suggestion in Trac #3702
parent 0f3dd21e
......@@ -92,6 +92,10 @@ data DerivSpec = DS { ds_loc :: SrcSpan
-- 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
......@@ -549,7 +553,7 @@ After all, we can write it out
\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
......@@ -584,7 +588,7 @@ mkEqnHelp orig tvs cls cls_tys tc_app mtheta
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}
......@@ -643,28 +647,28 @@ mkDataTypeEqn :: InstOrigin
-> [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
......@@ -781,12 +785,14 @@ data DerivStatus = CanDerive
| 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
......@@ -795,8 +801,8 @@ checkSideConditions dflags cls cls_tys rep_tc
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
......@@ -816,6 +822,7 @@ sideConditions cls
| 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
......@@ -838,15 +845,19 @@ andCond c1 c2 tc = case c1 tc of
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
......@@ -1007,7 +1018,7 @@ a context for the Data instances:
\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
......@@ -1025,9 +1036,8 @@ mkNewTypeEqn orig dflags tvs
; 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
......@@ -1036,7 +1046,7 @@ mkNewTypeEqn orig dflags tvs
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 $$
ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
......@@ -1146,8 +1156,7 @@ mkNewTypeEqn orig dflags tvs
-- so for 'data' instance decls
cant_derive_err
= vcat [ ptext (sLit "even with cunning newtype deriving:")
, ppUnless arity_ok arity_msg
= vcat [ ppUnless arity_ok arity_msg
, ppUnless eta_ok eta_msg
, ppUnless ats_ok ats_msg ]
arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
......@@ -1440,12 +1449,15 @@ typeFamilyPapErr tc cls cls_tys inst_ty
= hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
derivingThingErr :: Class -> [Type] -> Type -> Message -> Message
derivingThingErr clas tys ty why
= sep [hsep [ptext (sLit "Can't make a derived instance of"),
quotes (ppr pred)],
nest 2 (parens why)]
derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
derivingThingErr newtype_deriving clas tys ty why
= sep [(hang (ptext (sLit "Can't make a derived instance of"))
2 (quotes (ppr pred))
$$ 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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