Passing private functions


I have a simple helper function to clock stuff.
I want to put it in a my X.Helper module (assuming my project is X) to use it wherever I need.

  def clock(mark, fun, args) do
    {time, result} =, args)
    IO.puts(mark <> ": #{Kernel.inspect(result)} Time: #{time} μs")

But when I pass a function from the module Y I get an error module Y is not available

Reluctantly (as it would break the decoupling) I tried to alias the Y function but that does not seem to work either.

What is the common approach to this kind of stuff?

Could you share the code where you’re calling the function?

Module Y may need to be aliased or specified fully. E.g. MyApp.FooContext.Y.some_fun(arg1, arg2).

If it’s being passed as a parameter then you’d want something like &MyApp.FooContext.Y.some_fun/2.

The actual module:

defmodule Trail.Store do
    alias Trail.Store
    alias Trail.Helpers

    def append(key, value) do
        Helpers.clock("Append", &Trail.Store.append_as_binary/2, [data_path(key), value])

  defp append_as_binary(path, value) ..

And the Helper

defmodule Trail.Helpers do
  alias Trail.Store
  def clock(mark, fun, args) do
    {time, result} =, args)
    IO.puts(mark <> ": #{Kernel.inspect(result)} Time: #{time} μs")
    append_as_utf meta_path(mark), time

  defp append_as_utf ...
  defp meta_path ...

Where the append function is the entry point and the append_as_binary gives the module not available.

Could you please tell us the exact error you get?

From a first glance over what we have seen so far, it should work.

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Thank you for inspecting it.

     ** (UndefinedFunctionError) function Trail.Store.append_as_binary/2 is undefined or private
     code: assert, m2) == {:ok}
       (trail) Trail.Store.append_as_binary("/xxx", yyy)
       (stdlib) timer.erl:181:
       (trail) lib/trail/helpers.ex:18: Trail.Helpers.clock/3
       (trail) lib/trail/store.ex:10: Trail.Store.append/2
       test/trail_test.exs:38: (test)

where xxx and yyy are the key and value passed into the function.

I really dislike the idea to alias Trail.Store (or any specific module) in the Helper module.

Ah, now I remember…

&M.f/a exposes the remote call to the receiving module, which then isn’t allowed to do that, you need to omit the module name and do Helpers.clock("Append", &append_as_binary/2, [data_path(key), value]) instead…


Total turnaround :slight_smile:

It does work and I can remove all of the aliases from the helper.

1 Like