(arg_kinds, _) = splitKindFunTys kind
n_args_to_drop = length arg_kinds
n_args_to_keep = tyConArity tc - n_args_to_drop
- inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
- inst_ty_kind = typeKind inst_ty
-
+ args_to_drop = drop n_args_to_keep tc_args
+ inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
+ inst_ty_kind = typeKind inst_ty
+ dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
+ univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
+ `minusVarSet` dropped_tvs
+
-- Check that the result really is well-kinded
; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
(derivingKindErr tc cls cls_tys kind)
+ ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
+ tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
+ (derivingEtaErr cls cls_tys inst_ty)
+ -- Check that
+ -- (a) The data type can be eta-reduced; eg reject:
+ -- data instance T a a = ... deriving( Monad )
+ -- (b) The type class args do not mention any of the dropped type
+ -- variables
+ -- newtype T a s = ... deriving( ST s )
+
-- Type families can't be partially applied
- -- e.g. newtype instance T Int a = ... deriving( Monad )
+ -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
+ -- Note [Deriving, type families, and partial applications]
; checkTc (not (isOpenTyCon tc) || n_args_to_drop == 0)
(typeFamilyPapErr tc cls cls_tys inst_ty)
- ; mkEqnHelp DerivOrigin (tvs++deriv_tvs) cls cls_tys inst_ty Nothing } }
+ ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
where
-- Tiresomely we must figure out the "lhs", which is awkward for type families
-- E.g. data T a b = .. deriving( Eq )
deriveTyData _other
= panic "derivTyData" -- Caller ensures that only TyData can happen
+\end{code}
-------------------------------------------------------------------
+Note [Deriving, type families, and partial applications]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When there are no type families, it's quite easy:
+
+ 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
+
+When type familes are involved it's trickier:
+
+ data family T a b
+ newtype instance T Int a = MkT [a] deriving( Eq, Monad )
+ -- :RT is the representation type for (T Int a)
+ -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
+ -- :Co:R1T :: :RT ~ [] -- Eta-reduced
+
+ instance Eq [a] => Eq (T Int a) -- easy by coercion
+ instance Monad [] => Monad (T Int) -- only if we can eta reduce???
+
+The "???" bit is that we don't build the :CoF thing in eta-reduced form
+Henc the current typeFamilyPapErr, even though the instance makes sense.
+After all, we can write it out
+ instance Monad [] => Monad (T Int) -- only if we can eta reduce???
+ return x = MkT [x]
+ ... etc ...
+
+\begin{code}
mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
-> Maybe ThetaType -- Just => context supplied (standalone deriving)
-- Nothing => context inferred (deriving on data decl)
-- family tycon (with indexes) in error messages.
data DerivStatus = CanDerive
- | NonDerivableClass
- | DerivableClassError SDoc
+ | DerivableClassError SDoc -- Standard class, but can't do it
+ | NonDerivableClass -- Non-standard class
checkSideConditions :: Bool -> Class -> [TcType] -> TyCon -> DerivStatus
checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tc
- | notNull cls_tys
- = DerivableClassError ty_args_why -- e.g. deriving( Foo s )
- | otherwise
- = case sideConditions cls of
- Nothing -> NonDerivableClass
- Just cond -> case (cond (mayDeriveDataTypeable, rep_tc)) of
- Nothing -> CanDerive
- Just err -> DerivableClassError err
+ | Just cond <- sideConditions cls
+ = case (cond (mayDeriveDataTypeable, rep_tc)) of
+ Just err -> DerivableClassError err -- Class-specific error
+ Nothing | null cls_tys -> CanDerive
+ | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
+ | otherwise = NonDerivableClass -- Not a standard class
where
ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
-> TcRn EarlyDerivSpec
mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving tvs
cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
+-- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
| can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
= do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
; dfun_name <- new_dfun_name cls tycon
-- See Note [Newtype deriving superclasses] above
cls_tyvars = classTyVars cls
- dfun_tvs = tyVarsOfTypes tc_args
+ dfun_tvs = tyVarsOfTypes inst_tys
inst_ty = mkTyConApp tycon tc_args
inst_tys = cls_tys ++ [inst_ty]
sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
-- recursive newtypes too
-- Check that eta reduction is OK
- eta_ok = (nt_eta_arity <= length rep_tc_args)
- -- (a) the newtype can be eta-reduced to match the number
+ eta_ok = nt_eta_arity <= length rep_tc_args
+ -- The newtype can be eta-reduced to match the number
-- of type argument actually supplied
-- newtype T a b = MkT (S [a] b) deriving( Monad )
-- Here the 'b' must be the same in the rep type (S [a] b)
-- And the [a] must not mention 'b'. That's all handled
-- by nt_eta_rity.
- && (tyVarsOfTypes cls_tys `subVarSet` dfun_tvs)
- -- (c) the type class args do not mention any of the dropped type
- -- variables
- -- newtype T a b = ... deriving( Monad b )
-
cant_derive_err = vcat [ptext (sLit "even with cunning newtype deriving:"),
if isRecursiveTyCon tycon then
ptext (sLit "the newtype may be recursive")
2 (ptext (sLit "Class") <+> quotes (ppr cls)
<+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
+derivingEtaErr :: Class -> [Type] -> Type -> Message
+derivingEtaErr cls cls_tys inst_ty
+ = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
+ nest 2 (ptext (sLit "instance (...) =>")
+ <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
+
typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
typeFamilyPapErr tc cls cls_tys inst_ty
= hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
projects / ghc-hetmet.git / commitdiff