a program that returns true if a string contains only digits

Question

I want to make a program that returns true if a string contains only digits in Haskell.

This is my attempt:

checkNum :: String -> Bool
checkNum xs = ((length (filter isDigit xs )) == length (xs))

This is the error I get:

Not in scope: `isDigit'

What's wrong with my code?

Answer 1

This should work:

import Data.Char (isDigit)

checkNum :: String -> Bool
checkNum = all isDigit