不存在量化方法时重叠实例错误的原因

Question

给定以下代码，从 go 类型中删除 forall a r 失败并返回 "Overlapping instances for Typeable (D r)"。我想知道为什么？

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
module M where

import Data.Typeable (Proxy, Typeable, cast)

class C r where
    data D r :: *

deriving instance Typeable D

data A = forall r . A (D r)

go :: forall r a . (Typeable a, Typeable (D r)) => a -> Proxy r -> A
go a _ = case cast a of
  Just (b :: D r) -> A b
  Nothing -> error "fail to cast"

错误还显示 "The choice depends on the instantiation of r" - 但不是由提供的 Proxy r 固定的吗？

Answer 1

这就是作用域类型变量在 Haskell 中的工作方式。请注意，您在这里重复使用 r：

go :: forall r a . (Typeable a, Typeable (D r)) => a -> Proxy r -> A
go a _ = case cast a of
  Just (b :: D r) -> A b  -- this r is the same as the r above

如果没有显式 forall，类型变量将被解释为签名的局部变量。也就是说，您的代码被读取为：

go :: (Typeable a1, Typeable (D r1)) => a1 -> Proxy r1 -> A  -- renaming local variables
go a _ = case cast a of
  Just (b :: D r) -> A b   -- r bears no relation to r1

因此出现类型错误。

（不过是令人困惑的 Overlapping instances 错误。）

不存在量化方法时重叠实例错误的原因

Reason for overlapping instances error when not quantifying method existentially

haskell

type-families

overlapping-instances