haskell, chaining filters

Question

to write "map f (map g xs)" as a single call to map you could write

example xs = map (f.g) xs

but how would you write "filter p (filter q xs)" as a single call to filter? the dot operator doesnt seem to work for filter as it does for maps. guessing you'd use something else for predicates?

Answer 1

If you defined a function both that looked like this:

both :: (a -> Bool) -> (a -> Bool) -> a -> Bool
both f g x = f x && g x

Then you could write:

example xs = filter (both p q) xs

I'm not sure if there's a standard function that does this for you...