Taking an action

There's more to rules than passive accumulation of results sets. Let's rewrite an earlier example as follows

engine.rule "self-printer" do
  forall {
    has :A, :friend, :B
  make {
    action { |token|
      puts "%s and %s are friends" % [ token[:A], token[:B] ]

The make section (aliased as do!) contains actions to be taken whenever the bottom of the matcher list if reached (we say "the production node is activated"). The simplest one is action, which simply runs the block with the resulting token.

Avoid modifying the engine inside action blocks (i.e., asserting and retracting facts and installing new rules). There are safer ways of doing this. Action blocks should only be used for side effects.