j      :: (Ord a) => [(a,a)] -> a -> [a]
j xs x =   [ v  | (u,v) <- xs, u == x]

This code displays a list of the second elements of every tuple in xs based on the first element of the tuple (x).

For my assignment, I should define this in terms of higher order functions map and/or filter.

I came up with

j xs x = map snd . filter (\(a,_) -> a == x). xs

But I get errors regarding the type signature.

Can you help me modify this into the type signature mentioned above and any other changes? Thank you

CodePudding user response：

You can solve this by using parthesis and remove the last dot (.) before the xs:

j :: Eq a => [(a, b)] -> a -> [b]
j xs x = (map snd . filter ((a,_) -> a == x)) xs

here we thus construct a function between the parenthesis, and we use xs as argument. If you use f . x, then because of (.) :: (b -> c) -> (a -> b) -> a -> c, it expects x to be a function and it constructs a new function, but in your case xs is an argument, not another function to a use in the "chain".

The (a, _) -> a == x, can be replaced by (x ==) . fst:

j :: Eq a => [(a, b)] -> a -> [b]
j xs x = (map snd . filter ((x ==) . fst)) xs