What is the meaning of comma separated names in brackets - as a binding?

Question

In a document (https://riptutorial.com/Download/haskell-language.pdf), in Chapter 5: Arrows, I encountered the following example:

spaceAround :: Double -> [Double] -> Double
spaceAround x ys = minimum greater - maximum smaller  where (greater, smaller) = partition (>x) ys

What does the expression (greater, smaller) mean?

Answer 1

The partition function returns a pair or 2-element tuple as its result. Assigning that tuple to (greater, smaller) assigns values of the first and second elements of the pair to the two variables greater and smaller respectively.

If you are familiar with the let... in syntax, another way of writing this function would be as follows:

spaceAround2 :: Double -> [Double] -> Double
spaceAround2 x ys = 
    let myPair = partition (>x) ys
        greater = fst myPair
        smaller = snd myPair
    in minimum greater - maximum smaller

Answer 2

The mechanism is just called pattern matching.

It is eplained here:

Syntax in Functions

Pattern matching

...

Where!?

...

You can also use where bindings to pattern match! We could have rewritten the where section of our previous function as:

  ...
  where bmi = weight / height ^ 2  
  (skinny, normal, fat) = (18.5, 25.0, 30.0)

...

Source: http://learnyouahaskell.com/syntax-in-functions

Hence, in the example that is provided with the question, the result of the function partition is split into the elements by mattern mathching.

Further tested example:

main :: IO ()
main = do
        print e1
        print e2
        print e3

(e1, e2, e3) = (1, "Hello", "World!")

...it prints:

1
"Hello"
"World!"