::

	std::cout

This would tell you that the output stream instance cout belonged to the namespace std.

18.09.01: Thanks to Wicker808 for the factual corrections!

length :: [a] -> Integer

Example:

fish:water::worm:?

1. bird
2. earth
3. dog
4. sidewalk

Example:
::smiles::
::opens his backpack and pulls out a small lantern::
::hugs LadyFyr497::

The first place I ever encountered double-colons was in AOL's Red Dragon Inn, back in 1995 or so, when I first got a modem. I still use them in chatrooms today, well, except ones that recognize /me. Maybe I'm just a sucker for mid-to-late 90's alternative culture.

Easily much classier than emoticons.

One more example of forgotten netiquette.

In SML, :: is called "cons", and is the infix operator for adding an element at the front of a list. For example, 5::[6,7] results in the list [5,6,7].

:: is right associative, so the expression 1::2::3::nil makes sense and means 1::(2::(3::nil)).

When performing pattern matching, :: is used to decompose a list into the first element (the head) and the rest of the list (the tail). Here is a small function that returns the sum of all elements in a list:

fun sum [] = 0
 |  sum (x::xs) = x + sum xs;

If x::xs is matched with a list then x is the first element in the list, and xs the rest of the list.

:: is used on the HP 48 series and HP 49 series calculators as the opening mark for a "secondary object", which is closed with ;. Secondaries are usually used to group a number of commands and functions for a SystemRPL program, but can be used to hold any other objects together as well, like a list (which on the calculator is delimited by { and }). Lists and secondaries are both examples of composite objects, and thus share many commands on the calculator. The :: and ; are similar to { and } in C, Java, and quite a few other languages.