What are the prerequisites for a point-free function in Haskell

Question

I always thought that the prerequisites for a pointfree function were to get the function arguments to the end of the definition. E.g.

-- This can be made pointfree quite easily:    
let lengths x = map length x    
let lengths' = map length

-- However this cannot:
let lengthz x = length `map` x
-- let lengthz' = length `map` (parse error)

I originally came across this reading this question. There we have this example:

agreeLen :: (Eq a) => [a] -> [a] -> Int
agreeLen x y = length $ takeWhile id $ zipWith (==) x y
-- This may look like it can easily be made pointfree, however it cannot
-- agreeLen' :: (Eq a) => [a] -> [a] -> Int
-- agreeLen' = length $ takeWhile id $ zipWith (==) (zipWith is applied to too few arguments)

So why can my first example be made pointfree, but the other two cannot?

Answer 1

-- However this cannot:
let lengthz x = length `map` x
-- let lengthz' = length `map` (parse error)

\x -> length `map` x written point free is simply map length. The infix backticks are just syntactical sugar. (As chepner points out, if you really want it you can use a section, i.e. (length `map`).)

agreeLen :: (Eq a) => [a] -> [a] -> Int
agreeLen x y = length $ takeWhile id $ zipWith (==) x y
-- This may look like it can easily be made pointfree, however it cannot

The key word here is "easily". Pretty much anything can be made point-free if you try hard enough. In this case, omitting the y parameter is easy if we write agreeLen in terms of (.) rather than ($):

agreeLen x y = (length . takeWhile id . zipWith (==) x) y
agreeLen x = length . takeWhile id . zipWith (==) x

As for x, we can handle it by treating the use of (.) to compose zipWith (==) x with the other functions as just another case of a value being modified with a function:

agreeLen x = (.) (length . takeWhile id) (zipWith (==) x)
agreeLen x = ((length . takeWhile id) .) (zipWith (==) x) -- cosmetical change
agreeLen x = (((length . takeWhile id) .) . zipWith (==)) x
agreeLen = ((length . takeWhile id) .) . zipWith (==)

It is not something you'd actually want to do in practice, but it is certainly possible.