Question

我尝试创建自己的Writer类型，然后我还为其创建了一个实例。无论如何，我一直在收到此错误：

Illegal instance declaration for ‘Monad (Writer String)’
  (All instance types must be of the form (T a1 ... an)
   where a1 ... an are *distinct type variables*,
   and each type variable appears at most once in the instance head.
   Use FlexibleInstances if you want to disable this.)
In the instance declaration for ‘Monad (Writer String)’

这是我的代码：

newtype Writer log a = Writer {runWriter :: (a,log)} 
instance Monad (Writer String) where
  return a = Writer (a, "")
  ma >>= k = let (a, log1) = runWriter ma 
                 (b, log2) = runWriter (k a)
             in Writer (b, log1 ++ log2)

Answer 1

所有实例类型的格式都必须为（T a1 ... an）

...意思是，你可以写

instance Monad (Writer a) where ...

但不是

instance Monad (Writer String) where ...

因为String不是类型变量。

这只是Haskell98以来Haskell的愚蠢限制标准。显然，该限制使编写编译器更加容易，我不知道。每个人都使用FlexibleInstances扩展名，该扩展名已在GHC中使用了很长时间，并禁用了该限制。

{-# LANGUAGE FlexibleInstances #-}
newtype Writer log a = Writer {runWriter :: (a,log)} 
instance Monad (Writer String) where
  ...

或者，您可以使用更多多态的实例，但是Monad (Writer a)不能完全正常工作，因为您需要能够具有空日志和连接日志。标准的解决方案是为可连接类型调用通用类：

import Data.Monoid

instance Monoid a => Monad (Writer a) where
  return a = Writer (a, mempty)
  ma >>= k = let (a, log1) = runWriter ma 
                 (b, log2) = runWriter (k a)
             in Writer (b, log1 <> log2)

另一方面，要拥有一个Monad实例，您还必须首先实例化Applicative。

'Monad（Writer String）'的非法实例声明

1 个答案: