验证字符串列表

问题描述

这是我之前的问题的后续操作.
假设我需要编写一个函数validate，以确保给定的字符串列表由"a"，"b"和一个或多个"c"组成.

This is a follow up to my previous question.
Suppose I need to write a function validate in order to make sure that a given list of strings consists of "a", "b", and one or more "c".

def validate(ss: List[String]): Either[NonEmptyList[MyError], Unit] = ???

假设我有三个函数来检查给定的字符串是"a"，"b"还是"c":

Suppose I have three functions to check if a given string is "a", "b", or "c":

def validateA(str: String): Either[MyError, Unit] = ???
def validateB(str: String): Either[MyError, Unit] = ???
def validateC(str: String): Either[MyError, Unit] = ???

如何编写这些函数以实现validate?

How to compose those functions to implement validate ?

一种解决方案是解析器组合器"方法.为类型Validator = Either[NonEmptyList[MyError], List[String]]定义一个monad实例，类似于解析器组合器等类似oneOrMore的组合器.

One solution is a "parser combinators" approach. Define a monad instance for type Validator = Either[NonEmptyList[MyError], List[String]], combinators like oneOrMore similar to parser combinators etc.

我想知道是否有更简单的解决方案.

I am wondering whether there is an easier solution.

推荐答案

我建议您利用cats Validated.

如果您确实不想更改您的validateT方法签名，请定义一些辅助方法:

Let's define some helper methods, if you really don't want to change your validateT methods signatures:

def validateA_(str: String): ValidatedNel[MyError, Unit] = validateA(str).toValidatedNel
def validateB_(str: String): ValidatedNel[MyError, Unit] = validateB(str).toValidatedNel
def validateC_(str: String): ValidatedNel[MyError, Unit] = validateC(str).toValidatedNel

然后，您可以实现validate_辅助函数:

Then you can implement a validate_ helper function:

import cats.data.Validated.{ invalidNel, valid }

def validate_(ss: List[String]): ValidatedNel[MyError, Unit] = ss match {
  case a :: b :: c if c.nonEmpty =>
    validateA_(a) combine validateB_(b) combine c.traverseU_(validateC_)
  case _ => invalidNel(MyError(???)) //List too short
}

最后将您的validate函数实现为:

And finally implement your validate function as:

def validate(ss: List[String]): Either[NonEmptyList[MyError], Unit] =
  validate_(ss).toEither

假设:对输入列表进行了排序，如果输入列表短于3元素，则可以接受特定错误(例如，列表太短).

Assumptions: the input list is sorted and if it is shorter than 3 elements a specific error (e.g. list too short) is acceptable.

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