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

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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.

elixir rabbitmq erlang
2 Answers

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.)

user_1138480

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.

user_116979360
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