Question

I want to define a Haskell function that removes from a list of strings any string contained in a pair of strings and returns only a list that contains all the remaining strings. So an example would be:

function ["football","basketball","soccer"] ("football", "basketball") = ["soccer"]

I know you can use the filter function to filter list that satisfy the predicate given. I know that I can filter out a list in this manner:

function' xs s = filter (/=s) xs

But I can’t figure out how to get this to work with tuples. I keep getting errors when I run the code. Any idea how to do this? Thanks

Answer 1

myfunc :: Eq a => [a] -> (a, a) -> [a]
myfunc xs (a,b) = filter (\x -> (x /= a) && (x /= b)) xs

Lets see type of filter

filter :: (a -> Bool) -> [a] -> [a]

When you do filter f xs,
it just applies the function f of type a -> Bool on all the elements of the list removing those for which this function returns False.

I have pattern matched on elements of the tuple as (a,b) and I just defined function f as \x -> (x /= a) && (x /= b). This is an anonymous function which takes an element of the list and returns True only when it is neither equal to first element of the tuple nor equal to the second element. && is boolean and so it returns True only when both its arguments are True.