Escaping runtime values in a macro with `^`

I’ve been building a tool that compiles elixir expressions into a “workflow” a directed acyclic graph of steps, conditions, and other nodes. Some examples in a livebook here. The advantage of doing this is it allows for composition of functionality at runtime - with varying levels of potential user defined logic. Should be handy for data pipelines, rule based expert systems or other runtime dependent workflows.

A big part of expert systems is to build rules at runtime, but to do this I need to be able to inject runtime values into a macro (at least as implemented - if there’s a better way to extract atomic conditionals in a function I’m all ears).

So the idea is supporting ^ as an escape value like Ecto and Explorer do, but I barely understand Macros and have been struggling to figure out how to access the runtime values in this situation.

test "escapes runtime values with '^'" do
    some_values = [:potato, :ham, :tomato]

    escaped_rule = Dagger.rule(
      name: "escaped_rule",
      condition: fn val when val in ^some_values -> true end,
      reaction: "food"

    assert match?(%Rule{}, escaped_rule)
    assert Rule.check(escaped_rule, :potato)

I want the left hand side of this rule to build a function such as:

fn val when val in [:potato, :ham, :tomato] -> true end

I noticed Jose implemented something similar in Explorer recently: explorer/query.ex at v0.5.5 · elixir-nx/explorer · GitHub but I don’t quite understand how it works yet.

I’ve tried a few variations such as:

check =
  Macro.prewalk(lhs, fn
    {:^, _meta, [{bind, _, _} = expr]} = ast ->
      Macro.Env.has_var?(context, {bind, nil}) |> IO.inspect(label: "has var?") # true
      runtime_value = Macro.Env.vars(context) |> IO.inspect() # [some_values: nil]

      if runtime_value do
        var = Macro.unique_var(bind, __MODULE__)
        quote do: unquote(var)

    ast ->

Can anyone more familiar with macros and/or escaping runtime values help me understand how this works or how you’d approach this problem? Any feedback is appreciated.


You can’t access the runtime value at compile time, because, well, it doesn’t have a value yet. :slight_smile: what you can do is strip the ^ and inject the variable into the quoted expression such that the runtime value will be used when things finally execute.

The ^ isn’t special in any way, it’s just a conventional signal to the macro that what follows should be left untouched and not further transformed.