Get function signature at runtime


I’m trying to create a macro that takes in input a module name and automatically creates a defdelegate for each function defined in that module.


defmodule MyApp.TargetModule1 do
  def foo(name, opts \\ []) do
    name <> to_string(opts)

  def bar(name) do

defmodule MyApp.TargetModule2 do
  def baz(name) do
    "Ciao" <> name

defmodule MyApp.DelegateModule do
  delegate_all MyApp.TargetModule1
  # the line above will generate the following:
  # defdelegate foo(name, opts \\ []), to: MyApp.TargetModule1
  # defdelegate bar(name), to: MyApp.TargetModule1

  delegate_all MyApp.TargetModule2
  # the line above will generate the following:
  # defdelegate baz(name), to: MyApp.TargetModule2

What I’ve done so far

I achieved this using Code.fetch_docs/1 and Code.Typespec.fetch_specs/1 but unfortunately these functions won’t work unless the .beam file isn’t already written on disk so when I modify any target module I get the :module_not_found error.

I know I can get the function list with MyApp.TargetModule1.__info__(:functions) but this will return a simple list of functions and arities without arguments names and possibly default arguments.


I already looked into ExUnit.DocTest but it also uses Code.fetch_docs/1 and I don’t know how does it work! Tests are compiled after the main lib files?

How can I get function signature (possibly also specs and documentations) from a module at runtime?

The macro does include a require Module into the testcase, which makes the compiler enforce that Module is compiled before the file/module having the require Module.

I also include a require Module and indeed when I call Code.ensure_loaded/1 I get an {:ok, module} response but nevertheless if I call Code.fetch_docs/1 right after the requrie and ``Code.ensure_loaded/1 I get amodule_not_found` error.

This is my prototype

quote do
  require unquote(target)
  {:module, target} = Code.ensure_loaded(unquote(target)) # -> {:module, target}
  {_, _, _, _, _, _, functions} = Code.fetch_docs(target) # -> {:error, :module_not_found}

That doesn’t matter, as Erlang do not have notion of “default arguments” so what Elixir does to support them is to simply create multiple functions:

def foo(a, b \\ 1), do: …

Is the same as:

def foo(a), do: foo(a, 1)

def foo(a, b), do: …

Yes, exactly that.

AFAIK there is no way to fetch docs for in-memory module unless you store generated BEAM data in variable.

The only feasible approach is to use mod.__info__(:functions) (or mod.module_info(:functions) if you are willing to do filtering on your own).

1 Like

With mod.__info__(:functions) and mod.module_info(:functions) I can’t know parameters name and which parameter ha a default.

My current work around, since I’m working on an umbrella project, is to put every delegate module in a separate app. In my specific case this workaround should work, but I was hoping in a more reliable solution.