The double colon (::) has two uses. Both could be subscribed as “being of type”:

1. In binaries, it is used to explain what size(bit/byte/multiple bytes) an element in the binary is. An example, taken from the Parsing UDP with Binary Pattern Matching blog post:

    <<
      _header        :: size(240), # 30 bytes * 8 = 240 bits
      priority_code  :: bitstring-size(8),
      agent_number   :: little-unsigned-integer-size(32),
      message        :: bitstring-size(320)
    >> = data

When pattern matching with binaries, a size has to be specified for all but the last element to-be-matched element.

2.In specifying what return type a function has, when writing @spec, @type or @callback attributes:

@spec fibonacci(int) :: int
@type num_or_str :: number | string
@callback read_description(Entity.t) :: iodata

Here it is in the docs.

It’s not an operator. Think of it as saying “is of size” So title
is-of-size 30 bits.

It looks like that is binary pattern matching, so the value after the double colon specifies the size of the value before it.

Ok, that makes sense when looking at the code. I wonder if I came across this before, maybe in Introducing Elixir or Programming Elixir 1.2 . Thus far, I’ve not come across an explanation in The Little Elixir and OTP Guidebook. And I had a heck of a time trying to search for it on Google. You guys rock!