非负整数 [英] Non-negative integers

问题描述

说我有一个函数原型，如下:

Say i have a function prototype as follows:

func :: [Int] -> [Int]

如何仅强制将非负整数列表作为输入参数?我必须将参数类型从[Int]更改为什么.在这个公平的时刻，它可以与func [-1，-2]一起使用，我只希望它与[1,2]一起使用，即由解释器发出错误消息.

How is it possible to enforce only a non-negative list of integers as input arguments? I would have to change the param type from [Int] to what.. ? At this fair moment it works with func [-1,-2], i only want it to work with [1,2] i.e. with the interpreter spewing the error message.

推荐答案

newtype NonNegative a = NonNegative a

toNonNegative :: (Num a, Ord a) => a -> NonNegative a
toNonNegative x
  | x < 0 = error "Only non-negative values are allowed."
  | otherwise = NonNegative x

fromNonNegative :: NonNegative a -> a
fromNonNegative (NonNegative x) = x

请小心不要直接使用NonNegative构造函数.如果将其放在单独的模块中并且不导出，则将更加容易.

Just be careful to never use the NonNegative constructor directly. This will be easier if you put this in a separate module and don't export it.

此外，现在您可以使用(映射到NonNegative)来懒惰地变换数字列表.

Also, now you can use (map toNonNegative) to lazily transform a list of numbers.

无论您在何处注入原始数字，这仍然需要运行时检查.

This will still require a runtime check wherever you inject raw numbers.

或者，您可以使用Data.Word.

Alternatively, you can use Data.Word.

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