haskell - Haskell类型错误编译

Question

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']

type Code = [Symbol]

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

编译错误:

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

Answer 1

您提供的代码有一些错误。

如@FramkSchmitt所述，缺少xs参数。

您尝试将elem x映射到列表上-这将需要一个列表列表才能正确。

这就是我想你想要的。

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

可以写得更简洁一些(我相信像hlint这样的工具甚至可以建议简化)。

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