functional programming in haskell

4. (a) What is the type of the following unknown5 function? Give the most general type and include any required class constraints.

> unknown5 p = foldr (\x xs -> if p x then x:xs else xs) [ ]

(b) Give a simple description of what the unknown5 function in part (a) does. Describe what it does, not how it works.

(c) Give a non-trivial test case of the function unknown5 and show the expected results.