Haskell function definition and arguments list

Question

I have the following function definitions (which are equivalent):

reverseWords :: String -> String
reverseWords wds = unwords . map reverse . words $ wds

and

reverseWords :: String -> String
reverseWords = unwords . map reverse . words

I understand that they are equivalent as they yield the same result but I'm a bit confused about the second form.

If I call a function like:

reverseWords abc

How does haskell decide where to place this 'abc' parameter?

Answer 1

Haskell does not need to decide where to place abc . When you call reverseWords in your second example it will simply return unwords . map reverse . words unwords . map reverse . words unwords . map reverse . words .

And then abc gets applied to that with which you will end up with the same result as in your first example.

This is also called Point-free style as you can read more about it here: https://wiki.haskell.org/Point-free