Question

I want to return a String in a list that contains only Digits. I am trying to use the find function to achieve this but am having trouble creating the predicate. Here is what I have attempted:

getYear str | hasYear str = fromJust $ find (map and $ (map.map) isDigit ws) ws
            | otherwise = "0"
                where ws = words str

I have noticed that my predicate

map and $ (map.map) isDigit ws

returns a [Bool] with True if one of the Strings contains only digits but isn’t designed correctly for use in this instance.

Answer 1

There is also the handy

all :: (a -> Bool) -> [a] -> Bool

which can be used to write your function

find (all isDigit)