'Monad (Writer String)' 的非法实例声明

Question

我尝试创建自己的 Writer 类型，之后我也为它创建了一个实例。不管怎样，我一直收到这个错误：

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

All instance types must be of the form (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 Haskell 自 Haskell98 以来的愚蠢限制标准。显然，该限制使编写编译器变得更容易，我不知道。每个人都使用 FlexibleInstances 扩展，它已经在 GHC 中使用了很长时间并禁用了限制。

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

或者，您可以使用更多的多态实例，但 Monad (Writer a) 不太有效，因为您需要能够拥有空日志和连接日志。标准解决方案是为可连接类型调用泛型 class：

import Data.Monoid

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

另一方面，要获得 Monad 实例，您还必须首先实例化 Applicative。

'Monad (Writer String)' 的非法实例声明

Illegal instance declaration for 'Monad (Writer String)'

monads

haskell