普遍适用的约束

Question

这是一个非常有用的课程:

class Foo f a where
  foo :: f a

它让我为很多类型制作默认值.事实上,我甚至可能不需要知道它是什么a.

instance Foo Maybe a where
  foo = Nothing

现在我有一个Maybe a为所有人a,我可以在以后专门.

specialize :: (forall a. f a) -> f Int
specialize x = x

fooMaybe :: Maybe Int
fooMaybe = specialize foo

嗯......这fooMaybe看起来确实非常具体.让我们看看我是否可以使用上下文来概括它:

fooAll :: (Foo f a) => f Int
fooAll = specialize foo

哎呀!可能不会.

Foo.hs:18:21:
    Could not deduce (Foo * f a1) arising from a use of `foo'
    from the context (Foo * f a)
      bound by the type signature for fooAll :: Foo * f a => f Int
      at Foo.hs:17:11-28

所以,我的问题是,我怎么写fooAll,广义版fooMaybe？或者,更一般地说,我如何普遍推广类型类约束？

Answer 1

目前Haskell没有好办法.该约束包提供了一个类型,可能你需要什么,尽管我最近想出如何打破它.仍然可能是你最好的选择,因为如果有人知道如何做到这一点它可能会更新

fooAll :: Forall (Foo f) => f Int
fooAll = specialize foo_f where
  foo_f :: forall a. f a
  foo_f = foo \\ (inst `trans` (Sub Dict) :: () :- Foo f a)