编译时出现Haskell类型错误 [英] Haskell type error on compilation

问题描述

我不确定为什么以下代码会导致以下错误。

代码：

  type Symbol = Char 
 
 symbols :: [Symbol] 
 symbols = ['a'..'f'] 
 
类型代码= [Symbol] 
 
成员::代码 - > Bool 
会员xs =和[b | x < -  xs，b < -  map（elem x）symbols] 
  

汇编错误：

 无法将类型'Char'与't0符号'匹配
预期类型： [t0符号] 
实际类型：[符号] 
•在'map'的第二个参数中，即'符号'
在表达式中：map（elem x）符号
在一个列表解析中：b < -  map（elem x）symbols 
  

解决方案

1. 由于@FramkSchmitt提到了一个参数<$ c $您尝试将 elem x 映射到列表上 - 这将需要一个名为
   

这里是我猜你想要的东西。

  members :: Code  - > Bool 
成员xs =和[x`elem`符号| x <-xs] 
  

可以写得更简洁一些（我相信像hlint这样的工具会甚至建议这种简化）。

  members':: Code  - > Bool 
 members'=全部（`elem`符号）
  

I'm not sure why the following code is causing the following error.

Code:

type Symbol = Char

symbols :: [Symbol]
symbols = ['a'..'f']

type Code = [Symbol]

members :: Code -> Bool
members xs = and [ b | x <- xs, b <- map (elem x) symbols ]

Compilation Error:

Couldn't match type ‘Char’ with ‘t0 Symbol’
     Expected type: [t0 Symbol]
       Actual type: [Symbol]
   • In the second argument of ‘map’, namely ‘symbols’
     In the expression: map (elem x) symbols
     In a stmt of a list comprehension: b <- map (elem x) symbols

解决方案

The code you gave has a few errors.

1. As @FramkSchmitt mentioned there is a parameter xs missing.
2. you try to map elem x over a list - which would need a list of lists to be correct.

here is what I would guess what you intended.

members :: Code -> Bool
members xs = and [ x `elem` symbols  | x <- xs ]

which can be written a bit more concise (I believe tools like hlint would even suggest this simplification).

members' :: Code -> Bool
members' = all (`elem` symbols)

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