What does the console output represent, after defining an anonymous function?

I am trying to understand the output that is returned by the console when I define an anonymous function. I haven’t been able to find any answer in this forum yet.

iex(33)> fn x, y -> x + y end
#Function<13.126501267/2 in :erl_eval.expr/5>

  • What is the meaning of the double 13.126501267 ?

What do you mean by “double”? Anyway, there is no meaning in that number. It is opaque., though the /2 indicates arity of the anonymous function.

1 Like

Sorry, I meant a floating point number when I said “double”. Thanks for the reply!

It’s not double. The dot is just a separator. Similar to as in a PID individual segments have their own meaning, but which is not documented nor necessary to know.


If you look in the Elixir code, in the “inspect” module, you can see that the output is derived from calling Function.info. Calling Function.info on an anonymous function on my machine gives me:

iex(5)> some_function = fn x -> x * x end
#Function<7.126501267/1 in :erl_eval.expr/5>
iex(6)> Function.info(some_function)
  pid: #PID<0.109.0>,
  module: :erl_eval,
  new_index: 7,
  new_uniq: <<241, 72, 50, 109, 70, 84, 198, 45, 20, 94, 42, 25, 184, 243, 5,
  index: 7,
  uniq: 126501267,
  name: :"-expr/5-fun-4-",
  arity: 1,
  env: [
    {[], :none, :none,
       {:clause, 5, [{:var, 5, :_x@1}], [],
        [{:op, 5, :*, {:var, 5, :_x@1}, {:var, 5, :_x@1}}]}
  type: :local

So you can see that the inspect value for the function #Function<7.126501267/1 in :erl_eval.expr/5> consists of the function’s index, uniq and arity pieces.

According to the documentation on Function.info:

:index - (integer) an index into the module function table.

:uniq - (integer) a unique value for this function. This integer is
    calculated from the compiled code for the entire module.

:arity - (integer) the number of arguments the function is to be called

Now… having figured all that out, it’s still the case that there’s not much the average user can do with that information (well… the arity is useful metadata) - most folks can treat it as a blob of stuff that uniquely identifies the function.