'Monad(Writer String)'的非法实例声明

Question

我试图创建自己的Writer类型,然后我也为它创建了一个实例.无论如何,我继续得到这个错误:

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

所有实例类型必须是(T a1 ... an)形式

......意思是,你可以写

instance Monad (Writer a) where ...

但不是

instance Monad (Writer String) where ...

因为String它不是一个类型变量.

这只是Haskell自Haskell98以来的一个愚蠢的限制标准.显然,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,这已经在GHC中使用了很长时间并且禁用了限制.

{-# 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)不能正常工作,因为您需要能够拥有空日志并连接日志.标准解决方案是为可连接类型调用泛型类:

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,您必须首先实例化Applicative.