function - "func::String -> [Int]; func = read "有什么问题 [3,5,7 ]""

Question

在一个非常简单的模块中test我有以下功能

func :: String -> [Int]
func = read "[3,5,7]"

由于我有明确的类型注释，我希望得到 [3,5,7]当我加载模块 test并调用 func在 ghci 中。然而，我得到了

    • No instance for (Read (String -> [Int]))
        arising from a use of ‘read’
        (maybe you haven't applied a function to enough arguments?)
    • In the expression: read "[3,5,7]"
      In an equation for ‘func’: func = read "[3,5,7]"
   |
11 | func = read "[3,5,7]"
   |        ^^^^^^^^^^^^^^

但是当我做 read "[3,5,7]" :: [Int] , [3,5,7]按预期返回。为什么在我加载模块时会引发错误？

Answer 1

您正在尝试将字符串作为 String -> [Int] 类型的函数读取。 ，而不是列表 [Int] .但是，read无法将字符串转换为函数。

试试这个:

myList :: [Int]
myList = read "[3,5,7]"