[kqr]

The : operator is the linked list data constructor. It takes an element and a list and creates a new linked list by linking the element to the existing list. It does the opposite when used in a pattern match: separates out the first element in a linked list.

It is also an operator, meaning it can be used with infix notation, as in (x : xs). Haskell has something called operator sections, where if one supplies only one of the arguments to an operator it will return a function expecting the other argument. In other words

    (x:) == \xs -> (x:xs)

and

    (:xs) == \x -> (x:xs)

This can be used as in this article, to create a function that prepends x to any list. Another common example is (1+) which increments any number it is given, or (:[]) which turns any value into a one-element list.

It can also be used much more cleverly -- especially considering that any two-argument function can be turned into an operator with backticks -- but then (in my opinion) readability starts to suffer.

[shakna]

So it's the equivalent of Lisp's cons then?

[djtango]

Yeah so prepend is just currying cons with the head

[sshine]

Yes, : is pronounced cons.