How to dynamically call an operator in Elixir

Question

I'm working my way through Dave's upcoming book on Elixir, and in one exercise I would like to dynamically construct a function reference to Kernel.+/2, Kernel.-/2 etc, based on the contents of one character of a string, '+', '-' and so on.

Based on another SO question I expected to be able to call apply/3 passing Kernel, :+ and the two numbers like this:

apply(Kernel, :+, [5, 7])

This doesn't work because (if I understand right) Kernel.+/2 is a macro, not a function. I looked up the source code, and + is defined in terms of __op__, and I can call it from iex:

__op__(:+, 5, 7)

This works until I put the :+ into a variable:

iex(17)> h = list_to_atom('+')
:+
iex(18)> __op__(h, 5, 7)
** (CompileError) iex:18: undefined function __op__/3
    src/elixir.erl:151: :elixir.quoted_to_erl/3
    src/elixir.erl:134: :elixir.eval_forms/4

And I'm guessing there's no way to call __op__ using apply/3.

Of course, the brute-force method gets the job done.

  defp _fn(?+), do: &Kernel.+/2
  defp _fn(?-), do: &Kernel.-/2
  defp _fn(?*), do: &Kernel.*/2
# defp _fn(?/), do: &Kernel.//2   # Nope, guess again
  defp _fn(?/), do: &div/2        # or &(&1 / &2) or ("#{div &1, &2} remainder #{rem &1, &2}")

But is there something more concise and dynamic?

José Valim nailed it with his answer below. Here's the code in context:

  def calculate(str) do 
    {x, op, y} = _parse(str, {0, :op, 0})
    apply :erlang, list_to_atom(op), [x, y]
  end

  defp _parse([]     , acc      )                 , do: acc
  defp _parse([h | t], {a, b, c}) when h in ?0..?9, do: _parse(t, {a, b, c * 10 + h - ?0})
  defp _parse([h | t], {_, _, c}) when h in '+-*/', do: _parse(t, {c, [h], 0})
  defp _parse([_ | t], acc      )                 , do: _parse(t, acc)

(not too relevant to your question) my solution looks so different than yours! gist.github.com/nielsbom/0d689e00d927cc057ad7c5595d7e8f06 — Niels Bom
– Niels Bom, Commented Mar 18, 2019 at 19:00

José Valim · Accepted Answer · 2015-02-23 09:23:03Z

14

You can just use the Erlang one:

apply :erlang, :+, [1,2]

We are aware this is confusing and we are studying ways to make it or more explicit or more transparent.

UPDATE: Since Elixir 1.0, you can dispatch directly to Kernel (apply Kernel, :+, [1, 2]) or even use the syntax the OP first attempted (&Kernel.+/2).

edited Feb 23, 2015 at 9:23

answered Jan 12, 2014 at 20:55

José Valim

51.5k12 gold badges133 silver badges117 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Daniel Ashton Over a year ago

By the way, what would have been the proper incantation for the / macro, since &Kernel.//2 doesn't work?

José Valim Over a year ago

This should do it: &(Kernel./)/2 (or &(&1 / &2))

Jordan Dimov Over a year ago

If you have the operator as a single-quoted string, use List.to_atom(operator) as the second argument to apply

Collectives™ on Stack Overflow

How to dynamically call an operator in Elixir

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related