Manipulate the AST before compilation

Questions / Help
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1

Hi there,

Is there any way to change/edit the code/ast of a module before it compiles? I am trying to use the Interceptor library to add logging functionality to a module. In order to work, this library requires all functions to be traced to either be wrapped in a special do block or to be annotated.

Now, I would like for the user to be able to just write his regular code without having to worry about this wrapping/annotation at all. As such, my question is if it would be possible to leverage some metaprogramming, module callbacks or something else to automatically make the necessary changes to the code at compile time.

For example, let’s say I want to trace all functions, could I make something such that the code below:

defmodule Stack do
  use GenServer
  require Interceptor, as: I

      def start_link(default) when is_list(default) do
        GenServer.start_link(__MODULE__, default)
      end
  
      @impl true
      def init(stack) do
        {:ok, stack}
      end
  
      @impl true
      def handle_call(:pop, _from, [head | tail]) do
        {:reply, head, tail}
      end
  
      @impl true
      def handle_cast({:push, element}, state) do
        {:noreply, [element | state]}
      end

end

Either gets manipulated to look like this:

defmodule Stack do
  use GenServer
  require Interceptor, as: I

    I.Intercept do 
       def start_link(default) when is_list(default) do
         GenServer.start_link(__MODULE__, default)
       end

       @impl true
       def init(stack) do
         {:ok, stack}
       end

       @impl true
       def handle_call(:pop, _from, [head | tail]) do
         {:reply, head, tail}
       end

       @impl true
       def handle_cast({:push, element}, state) do
         {:noreply, [element | state]}
       end
    end
end

Or gets manipulated to look like this:

defmodule Stack do
  use GenServer
  require Interceptor, as: I

      @intercept true
      def start_link(default) when is_list(default) do
        GenServer.start_link(__MODULE__, default)
      end
  
      @impl true
      @intercept true
      def init(stack) do
        {:ok, stack}
      end
  
      @impl true
      @intercept true
      def handle_call(:pop, _from, [head | tail]) do
        {:reply, head, tail}
      end
  
      @impl true
      @intercept true
      def handle_cast({:push, element}, state) do
        {:noreply, [element | state]}
      end

end

Any help would be greatly appreciated!

2

By design we don’t allow so. Any transformation in the code has to be explicit. So you have to wrap it with the block yourself. :slight_smile:

3

That is unfortunate, at least in my case. Thanks for your reply!