CFEngine 3 limits promise evaluation to three passes.

dependencies.cf

# Run me with:
#     cf-agent -vKf ./dependencies.cf

bundle agent main {
  vars:
    any::
      "pre_eval" string => "This is set during a pre-eval pass.";
    first_pass_complete::
      "second_pass" string => "Vars before classes, so this is set 2nd pass.";
    second_pass_class::
      "third_pass" string => "The class will be set by end of 2nd pass.
                              This var will be set during the 3rd pass.";
    end_of_third_pass::
      "oops" string => "Thanks to implementation quirks, this never gets set.";

  classes:
    any::
      "first_pass_complete" expression => isvariable("pre_eval");
      "second_pass_class" expression => isvariable("second_pass");
      "end_of_third_pass" expression => isvariable("third_pass");
      "i_am_not_set_but_logically_should_be" expression => isvariable("oops");
}

# Note: This behavior is *documented*;
# see https://docs.cfengine.com/lts/guide-language-concepts-normal-ordering.html
# The point is that documenting odd behavior doesn't make it optimal design.