How to call apply/3 dynamically (with Strings)

Questions / Help
1

I am setting up a function that I want to call using strings from a database.

Once I get the data from the database, it looks like this:

q = %Question{ choices_module: "Survey", choices_function: "get_choices" }

I have a function in Survey that looks something like this:

  def get_choices(question) do
   apply(q.choices_module, q.choices_function, [q])
  end

This does not work, and I get an error that I need to send an Atom as the module name. I tried something like:

String.to_atom(q.choices_module) # => :Survey

Which is not what it expects.

Working it out in the console, I see that it wants this:

apply(Survey, :get_choices_for_question, [q])

and this call works in the console.

I think I am fundamentally not understanding how to turn "Survey" to Survey.

Any ideas? Thanks in advance.

2

In elixir module names are prefixed with Elixir. This means when you see ModuleName in iex they are really Elixir.ModuleName

iex> Atom.to_string(String)
"Elixir.String"

So if you want to create a “module” from a string you need to prefix it with “Elixir.” first.

Try:

apply(String.to_existing_atom("Elixir." <> q.mod), String.to_existing_atom(q.fun), [Atom])
"Elixir.Atom"

You might want to check that the module is not prefixed with “Elixir.” already.

2 Likes
3

It’s better and safer to use Module.concat/1 in such a case rather than manually get into the implementation details.

["String"] |> Module.concat() |> apply(:length, ["foo"])
#⇒ 3
9 Likes
4

Thanks! This is exactly what I needed… With a slight twist. I am writing this function inside the Elixir.Survey module, so I was getting (module Survey is not available).

I changed the whole thing to __MODULE__ and everything works as expected.

BUT! I will need to do something similar across modules, so this knowledge will come in handy.

Thanks!