在Haskell中读取int [英] Reading int's in Haskell
问题描述
我一直在研究Haskell,更具体的是 IO
monad,我想知道我该怎么做:
假设我有这个函数签名:
getNumber :: String - > (Int→> Bool)→> IO Int
以及本文:
我的名字是加里,我21岁如果我只想从这句话中只读出数字21,我该如何做Haskell?
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 ?
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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