如何构建此谓词以与Find一起使用？

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)

Answer 2

弄清楚了：

find (\ws -> and $ map isDigit ws) ws

Answer 3

You can handle that case, when String do not contain the year with fromMaybe .

getYear = fromMaybe "0" . find (all isDigit) . words

For example,

λ> fromMaybe "0" . find (all isDigit) $ words "sdf"
"0"

λ> fromMaybe "" . find (all isDigit) $ words "42139 213 f dsf 2"
"42139"

UPD : on the other hand, input String may have some digits, but hasYear return False . That why you shouldn't handle case, when find returns Nothing because it will never happened.

And than your getYear may looks like that:

getYear str | hasYear str = fromJust . find (all isDigit) . words $ str
            | otherwise = "0"