Hey folks!

Let’s start talking about Function Application operator which is a function that help us to remove parentheses from our functions. While Function Application with spaces are left-associative the Function Application with $ is right-associative and because of that we can add functions without parentheses.

Function Application with spaces: f a b c => ((f a) b) c)
Function Application with $: f a b c => (f (a (b c)))

Let see some examples with Function Application operator:

sqrt 3 + 2 * 5
1.732050807568877

sqrt (3 + 2 * 5)
3.605551275463989

sqrt $ 3 + 2 * 5
3.605551275463989

We can see here that $ allowed us to remove the parentheses.

Another example:

foldl (+) 0 (tail (map (*2) [1,2,3,4,5,6,7]))
54

foldl (+) 0 $ tail $ map (*2) [1,2,3,4,5,6,7]
54

In the second example we can remove all parentheses.

Now let’s take a look in Function Composition which is the way that we can can compose functions. Function Composition is represented by a . (point) in Haskell. Sometimes Function Composition allowed us to remove some parentheses as Function Application, but its principal responsibility is compose functions.

map (\x -> negate (abs x)) [1,-2,3,-4,5,-6,7]
[-1,-2,-3,-4,-5,-6,-7]

map (negate .abs) [1,-2,3,-4,5,-6,7]
[-1,-2,-3,-4,-5,-6,-7]

See that we remove the anonymous function only leaving the function and we remove the parentheses. It happens because Function Composition just work with functions that are waiting for one parameter.

sumTail :: (Num a, Ord a) => [a] -> [a] -> a
sumTail xs ys = foldl (+) 0 . tail . map (negate . abs) $ zipWith max xs ys

sumTail [1,2,3] [4,5,6]
-11

And we can combine Function Application with Function Composition and get clean and readable functions.