This post originated from an RSS feed registered with Java Buzz by Elliotte Rusty Harold. Original Post: foldr Starts at the Head Feed Title: Mokka mit Schlag Feed URL: http://www.elharo.com/blog/feed/atom/? Feed Description: Ranting and Raving Latest Java Buzz Posts Latest Java Buzz Posts by Elliotte Rusty Harold Latest Posts From Mokka mit Schlag

It took me long enough to realize that foldr still moves from the beginning to the end of a list. Somehow I thought it started at the right (i.e. the tail) of the list. Once I realized that Exercise 7 was easy:

Write your own definition of the standard takeWhile function, first using explicit recursion, and then foldr.

myTakeWhile :: (a -> Bool) -> [a] -> [a]
myTakeWhile _ [] = []
myTakeWhile f (x:xs) = if (f x)
                       then x : (myTakeWhile f xs)
                       else []

fMyTakeWhile :: (a -> Bool) -> [a] -> [a]
fMyTakeWhile f (xs) = foldr step [] xs
     where step x ys | f x = x : ys
                     | otherwise = []