在Haskell中读取int

问题描述

我一直在研究Haskell，更具体的是

解决方案

这是一个从字符串 ：

  import Data.List（unfoldr，tails）
 import Data.Maybe（listToMaybe）
 
 readMany ::阅读a =>字符串 - > [a] 
 readMany = unfoldr $ listToMaybe。 concatMap读取。尾巴
  

例如：

 > readMany我喜欢数字7,11和42. :: [Int] 
 [7,11,42] 
  

to jozefg的功能 getNumber ：

  justOne :: [a]  - > ;也许是
 justOne [x] =只是x 
 justOne _ = Nothing 
 
 getNumber :: String  - >也许Int 
 getNumber = justOne。 readMany 
  

或者您可以稍微宽松些，并且在指定多个时选择第一个数字：

  getNumber = listToMaybe。 readMany 
  

I've been studying Haskell, more specifically the IO monad, and I would like to know how can i do the following:

Let's say I have this function signature:

getNumber :: String −> (Int −> Bool) −> IO Int

and this text:

"My name is Gary, and I'm 21 years old"

If I want to read only the number "21" from this sentence, how would I do it in Haskell ?

解决方案

Here is a function that extracts multiple readable things from a String:

import Data.List (unfoldr, tails)
import Data.Maybe (listToMaybe)

readMany :: Read a => String -> [a]
readMany = unfoldr $ listToMaybe . concatMap reads . tails

So for example:

> readMany "I like the numbers 7, 11, and 42." :: [Int]
[7,11,42]

You can easily specialize this to jozefg's function getNumber:

justOne :: [a] -> Maybe a
justOne [x] = Just x
justOne _   = Nothing

getNumber :: String -> Maybe Int
getNumber = justOne . readMany

Or you can be a little more lenient and pick the first number when more than one is specified:

getNumber = listToMaybe . readMany

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