Question

There is a list of String [“f”, “1”, “h”, “6”, “b”, “7”].

How can I count Int in this list?

Now I have this algorithm but it’s not so good.

import Data.Char
let listOfStrings = ["f", "1", "h", "6", "b", "7"]
let convertedString = "f1h6b7"
let listOfInt = map (\x -> read [x]::Int) (filter (\x -> isDigit x) convertedString)
length listOfInt
Prelude> 3

Besides, I can’t convert listOfStrings to one string. This algorithm doesn’t even work properly

Can you help me with optimization?

Answer 1

1) Use reads :: Reads Int (this expression is just reads :: String -> [(Int, String)] in disguise) to test whether a string is a representation of an integer value:

isNumber :: String -> Bool
isNumber s = case (reads s) :: [(Int, String)] of
    [(_, "")] -> True
    _         -> False

Why reads? Because it returns additional information about parsing process from which we can conclude if it was successful. read :: Int would just throw an exception.

2) then filter a list of strings with it and take its length:

intsCount :: [String] -> Int 
intsCount = length . filter isNumber