Fold functions.md

Fold functions.md

Fold functions

Use fold-family functions and some auxiliary functions, such as the map, all and any defined in class slides, and your own, to define the following functions. In other words, you cannot use direct recursion to define them.

a. Write a function maxl xs using that generates an error "empty list" if xs==[] and otherwise returns the largest element of xs

maxl :: (Ord a) => [a] -> a
maxl xs = ...
>maxl [1,3,2,5,6,4]
6
>maxl []
Program error: empty list

b. Write a function member x ys which returns True if x is an element of ys, False otherwise. (No direct recursion)

member :: (Eq a) => [a] -> a -> Bool
member xs s = ...
> member [1,2,3] 4
False
> member [1,2,3,4.0,5] 4
True
> member [’a’,’b’,’x’,’z’] ’b’
True

maxl.hs

maxl :: (Ord a) => [a] -> a  
maxl = foldr1 (\x acc -> if x > acc then x else acc) 

main = do
        print(maxl [1,3,6,10,2,8])

member.hs

member :: (Eq a) => [a] -> a -> Bool  
member xs x = foldl (\acc y -> if x == y then True else acc) False xs  

main =  do
        print(member [1,3,6,10,2,8] 11)
        print(member [1,3,6,10,2,8] 10)