超类都来自泛型-sop

Question

我正在尝试使用generics-sop中的All来约束类型列表。

class (Typeable a) => TestClass (a :: k)
instance (Typeable a) => TestClass a

foo :: (All Typeable xs) => NP f xs -> z
foo = undefined

bar :: (All TestClass xs) => NP f xs -> z
bar = foo 

这会产生错误

Could not deduce: Generics.SOP.Constraint.AllF Typeable xs
  arising from a use of ‘foo’
  from the context: All TestClass xs

generics-sop文档指出

"All Eq‘Int，Bool，Char等同于约束(Eq Int，Eq Bool，Eq Char)

但在这种情况下，情况似乎并非如此，因为

foo2 :: (Typeable a, Typeable b) => NP f '[a,b] -> z
foo2 = undefined

bar2 :: (TestClass a, TestClass b) => NP f '[a,b] -> z
bar2 = foo2

编译正常。

我的问题

1)这是预期的行为吗? 2)如果是，有什么解决方法吗？

我的用例是，我希望传递一个类型级别的类型列表，该列表由单个类名(如class (Typeable a, Eq a, Show a) => MyClass a)下的一组不同的类约束，但也可以调用不太专业的函数，这些函数只需要这些类的一些子集。

搜索结果是superclasses aren't considered，但我不认为这是这里的问题-我认为这与generics-sop中All约束的组合方式有关。

Answer 1

All f xs实际上等同于(AllF f xs, SListI xs)。AllF是一个类型族：

type family AllF (c :: k -> Constraint) (xs :: [k]) :: Constraint where
  AllF _ '[] = ()
  AllF c (x:xs) = (c x, All c xs)

你会发现它不能减少，除非xs在WHNF中，所以它就卡在你的情况下了。您可以使用mapAll

import Generics.SOP.Dict

mapAll :: forall c d xs.
          (forall a. Dict c a -> Dict d a) ->
          Dict (All c) xs -> Dict (All d) xs
-- ::ish forall f g xs. (forall a. f a -> g a) -> All f xs -> All g xs

-- stores a constraint in a manipulatable way
data Dict (f :: k -> Constraint) (a :: k) where
     Dict :: f a => Dict f a

bar :: forall xs f z. (All TestClass xs) => NP f xs -> z
bar = case mapAll @TestClass @Typeable @xs (\Dict -> Dict) Dict of
           Dict -> foo

-- TestClass a -> Typeable a pretty trivially:
--   match Dict to reveal TestClass a
--   put the Typeable part of the TestClass instance into another Dict
-- We already know All TestClass xs; place that into a Dict
-- mapAll magic makes a Dict (All Typeable) xs
-- match on it to reveal
-- foo's constraint is satisfied