How do i check if a string can be parsed to a certain type in Haskell?

Question

So I have my own data type in haskell defined like this:

data Token = Num Double | Op String

I want to make a function that converts a list of strings into a list of tokens. E.g.

toTokenList ["2","+","3"]
> [Num 2.0, Op "+", Num 3.0]

How would I go about doing this?

I have implemented a function that converts a type Double into a Token type and another one that converts a String to a Token type. Can these be used for toTokenList or no?

I am new to Haskell relatively and if you need further clarification about the question please let me know in the comments.

Thanks in advance!

Answer 1

我们可以实现一个乐观的算法，它首先旨在将其解析为 a Double，如果失败，我们Op为该字符串返回 an ，例如：

import Text.Read(readMaybe)

toTokenList :: [String] -> [Token]
toTokenList = map (\x -> maybe (Op x) Num (readMaybe x))

或无点：

toTokenList :: [String] -> [Token]
toTokenList = map (flip maybe Num . Op <*> readMaybe)

我们在这里利用readMaybe :: Read a => String -> Maybe a, 并maybe :: b -> (a -> b) -> Maybe a -> b提供后备和后处理该值。

例如：

Prelude Data.Maybe Text.Read> toTokenList ["2","+","3"]
[Num 2.0,Op "+",Num 3.0]