Common HOFs
Besides foldr and dropWhile, Haskell’s standard library contains a number of useful and frequently used HOF.
Taking instead of dropping
The dropWhile function has an opposite that takes rather than drops elements from the front of a list:
Prelude
takeWhile :: (a -> Bool) -> [a] -> [a]
takeWhile p [] = []
takeWhile p (x:xs)
| p x = x : takeWhile p xs
| otherwise = [] As long as the elements satisfy the predicate, they are returned. Yet, as soon as a first element is found that does not satisfy the predicate, no more elements are returned.
For example, by passing isDigit from the Data.Char module to takeWhile, we can take the leading digits of a string:
*Main> takeWhile isDigit...