How to “splat” the function arguments for dynamically created function

Question

I am generating some functions in the module:

defmodule M do
  funs = [do_it: [:arg1, :arg2]]

  Enum.each(funs, fn {name, args} ->
    args = Enum.map(args, & {&1, [], Elixir})
    def unquote(name)(unquote(args)),
      do: IO.inspect(unquote(args))
  end)
end

The issue is the generated function obviously accepts one single argument, namely a list of size 2:

▶ M.__info__(:functions) 
#⇒ [do_it: 1]

The goal is to dynamically declare the function accepting two arguments. In ruby terminology, it would be to unsplat argument list.

Is there a possibility to accomplish this without pattern matching the resulting AST for {:do_it, blah, [[ list of arguments ]]} and flattening the list manually?

Aleksei Matiushkin · Accepted Answer · 2017-12-08 06:44:53Z

6

You can use Kernel.SpecialForms.unquote_splicing/1 to "splice" in the args list:

defmodule M do
  funs = [do_it: [:arg1, :arg2], do_it: [:arg1, :arg2, :arg3]]

  Enum.each(funs, fn {name, args} ->
    def unquote(name)(unquote_splicing(args)), do: :ok
  end)
end

iex(1)> M.__info__(:functions)
[do_it: 2, do_it: 3]

edited Dec 8, 2017 at 6:44

Aleksei Matiushkin

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answered Dec 8, 2017 at 6:37

Dogbert

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