New type: `:existing_module`

[Aleksei Matiushkin]

I find myself copy-pasting the following implementation to be used to validate a dependency injection via module:

@doc false
def existing_module?(value) do
  case Code.ensure_compiled(value) do
    {:module, ^value} ->
      {:ok, value}
    {:error, error} ->
      {:error, "Cannot find the requested module ‹" <> inspect(value) <> "› (#{error})"}
  end
end

and use it in options schema declaration as

  {:custom, MyValidators, :existing_module?, []}

I believe it’d be a great candidate for another default type. I am all-in to provide a PR if this would be desired.

[Andrea Leopardi]

I'm in favor of this 👍. Would love a PR, yes please! 💟

[José Valim]

@whatyouhide I am not sure if this makes sense actually.

first, why ensure_compiled? Would it always be a compile time dependency?

then, often you want to the module to expose some API, so wouldn’t you most likely need to check it with function_exported? and need custom code anyway?

[Aleksei Matiushkin]

@josevalim the intent was to establish a compile-time dependency, yes.

The check for exported API might be needed, but at least it narrows down errors induced by typos.

[Andrea Leopardi]

After thinking more about this, I do think that this is probably out of scope for this library. You can always write a custom validator and do these checks in there. Thanks anyways @am-kantox! 💟