In erlang, how do I interpret the `+` operator in `fun erlang:'+'/2`

Question

I'm new to Erlang but have some experience with Elixir . As I've been trying to learn Erlang while experimenting with the RabbitMQ implementation of RAFT, ra , I've come across a line in erlang Machine = {simple, fun erlang:'+'/2, 0},

Machine = {simple, fun erlang:'+'/2, 0},

So, in {simple, fun erlang:'+'/2, 0}, , this looks like it is creating a tuple. The first item in the tuple is an atom named simple , the next function and the last an integer :

{atom, function, integer}

I don't understand what the function fun erlang:'+'/2 is doing in this context. The /2 means it should take 2 params. Is the '+' just an addition operator? If so, is this a simple sum function and I am overthinking it? The erlang docs say "An atom is to be enclosed in single quotes (') if it does not begin with a lower-case letter or if it contains other characters than alphanumeric characters, underscore (_), or @."

In the given context that I'm seeing this code, it states State machine that implements the logic , which is leading me to understand this state machine as being named with the atom simple , performs addition, and saves the result in the last item of the tuple.

Is it equivalent of doing &:erlang.+/2 in elixir? Doc Reference

Any context would really help.

Answer 1

You got it exactly right - this function is just the addition operator, and it is enclosed in single quotes because it doesn't start with a lowercase letter. fun erlang:'+'/2 is equivalent to Elixir's &:erlang.+/2 .

You can call it using function syntax instead of operator syntax:

> erlang:'+'(1,2).
3

And you can use it as a higher-order function:

> lists:foldl(fun erlang:'+'/2, 0, [1, 2, 3]).
6

(Of course, you'd usually use lists:sum/1 instead of the latter example.)

Answer 2

As per the Erlang documentation(s), this particular usage is one of the forms for specifying Fun expression , and this one is fun Module:Name/Arity .