如何正确使用代理？

Question

为什么这段代码不起作用？

class Foo a where 
    foo :: Proxy a -> Int

bar :: Foo a => a -> Int
bar _ = foo (Proxy :: Proxy a)

无法使用以下消息进行编译：

Could not deduce (Foo a0) arising from a use of `foo'
from the context (Foo a)
  bound by the type signature for bar :: Foo a => a -> Int
The type variable `a0' is ambiguous
In the expression: foo (Proxy :: Proxy a)
In an equation for `bar': bar _ = foo (Proxy :: Proxy a)

我尝试使用和不使用ScopedTypeVariables扩展

Answer 1

get a scoped type variable:

需要ScopedTypeVariables 和 forall简介

{-# LANGUAGE ScopedTypeVariables #-}

bar :: forall a. Foo a => a -> Int
bar _ = foo (Proxy :: Proxy a)