Help with validating macro inputs

factoryd · May 8, 2021, 10:59pm

I’ve created a set of macros to help reduce a bunch of boilerplate when dealing with CQRS applications but am stuck on one detail.

I’m not sure how to validate that a module macro argument is what I need it to be.

Example on line 40

github.com

trbngr/elixir_cqrs_tools/blob/0.2.0/example/lib/example_api/types/user_types.ex#L40


      except: [:name]


    derive_connection ListUsers, :user,
      as: :users,
      arg_types: [status: :user_status]
  end


  derive_mutation_input CreateUser


  object :user_mutations do
    derive_mutation CreateUser, :user, input_object?: true, then: &fetch_user/1
    derive_mutation SuspendUser, :user, then: &fetch_user/1
    derive_mutation ReinstateUser, :user, then: &fetch_user/1
  end
end

If CreateUser is not Module with a __command__ function – say I typo-ed it as CreateUse – I’d love to simply bail out of compilation right away with the details on why it’s invalid.

But the exception gets lost in compilation and just fails at the level of Absinthe compilation.

This works correctly when the macro is top-level as you can see here on line 7

github.com

trbngr/elixir_cqrs_tools/blob/0.2.0/example/lib/example/users.ex#L7


defmodule Example.Users do
  use Cqrs.BoundedContext
  use Cqrs.BoundedContext.Commanded


  import_commands Example.Users.Router


  query Example.Queries.GetUser
  query Example.Queries.ListUsers
end

But in the first example I receive

== Compilation error in file lib/example_api/schema.ex ==
** (Absinthe.Schema.Error) Compilation failed:
---------------------------------------
## Locations


Could not load module `Elixir.ExampleApi.Types.UserTypes`. It returned reason: `unavailable`.

I’m guessing that it’s because I’m already in the context of an Absinthe macro by calling Module.eval_quoted(__CALLER__, quoted). But I’m at the limit of my knowledge here.

Relevant code here

Second question is how I can defer checking these modules until the last possible moment?

As of now, I’m forced to call module.__info__/1 in Guards and it feels dirty.

I hope this makes sense. If not, I’m open to discussion.

factoryd · May 10, 2021, 3:47am

In case anyone is following, the fix was super simple. I have to ensure the modules are compiled first and then check for the functions.

github.com

trbngr/elixir_cqrs_tools/blob/v0.2.9/lib/cqrs/guards.ex#L36


    end
  end


  def ensure_is_dispatcher!(module) do
    unless exports_function?(module, :dispatch, 2) do
      raise "#{module} is required to export a dispatch/2 function."
    end
  end


  defp exports_function?(module, fun, arity) do
    case Code.ensure_compiled(module) do
      {:module, _} -> function_exported?(module, fun, arity)
      _ -> false
    end
  end
end