Haskell中的类型安全的"read" [英] Type-safe `read` in Haskell

问题描述

了解Haskell 讨论以下数据类型:

Learn You a Haskell discusses the following data type:

data Day = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday   
           deriving (Eq, Ord, Show, Read, Bounded, Enum)  

这本书演示了如何使用 read 将String解析为 Day 类型.

The book demonstrates how to use read to parse a String into a Day type.

$ read "Saturday" :: Day
Saturday

但是，我可以传递非Day值，从而导致异常.

However, I can pass in a non-Day value, resulting in an exception.

$ read "foo" :: Day
*** Exception: Prelude.read: no parse

在上面的示例中，使用 read 的类型安全的方法是什么?

What's a type-safe way to use read in the above example?

推荐答案

除了@JonPurdy提到的旧标准函数 reads 外，还有最近添加的

In addition to the old standard function reads mentioned by @JonPurdy, there's also the more recently added

Text.Read.readMaybe :: Read a => String -> Maybe a

当字符串仅包含一个要解析的值时，使用起来更简单.

which is simpler to use when the string contains just one value to parse.

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