Agent "action" model. [Ant][18/11/2002]

An agents observable behaviour is the action (or script) that it outputs. The
new model for writing agent scripts removes the complexities of the existing
technique.

Each agent is passed a single variable (unlike the existing scripts which could
pass any number of variables). An agent generates sub-agents (its children),
and for each child passes an input, a template and a unique variable. An agent
can perform actions before and after its children.

A new style script is specified with a new tag called "action". The template for
an agent with this new script is:

myagent = [ ...
            [ "action",
              [ "now", "somecommand" ],
              [ "later", "anothercommand" ],
              ...
            ]
          ];

The script part is a list with the head being the string "action" and the
proceeding elements are the commands. Each command is a tuple, with its first
element being either "now" or "later" indicating when the command should be
executed. The second element in the tuple is the command string. A command
string may contain the following variables to be substituted into the string:

    $i = the input string to the agent
    $t = the token/string that was matched by the agent
    $v = the variable name that was passed to the agent
    $s1 = the first substring of the input
    $s2 = the second substring of the input (and so on for 3rd, 4th, etc)
    $p1 = the variable name to be passed to the first child agent
    $p2 = the variable name to be passed to the second child agent (etc)

This is model is much simpler than the old style scripting as you no longer
need concern yourself with multiple parameters, variable names or the famous
"$$/%%"! You can still achieve everything that was possible with the old style
scripts.

An example of a very simple expression calculator using agent "actions":

  init_calc =
    [ "\n", "calc", [] ];
  calc =
    [ "read_prefix", [[], 0], "expr",
      [ "action",
        [ "later", "writeln('=',$p1);" ] ] ];
  expr =
    [ "pivot", "+", [ "expr", "expr" ],
      [ "ignore", ["bras"] ],
      [ "action",
        [ "now", "$v is $p1 + $p2;" ] ],
      [ "fail", "term" ] ];
  term =
    [ "read_all", [["0","9"]],
      [ "action",
        [ "now", "$v = $i;" ] ] ];

See example.e for a comparison to the old style scripts.

Comments/suggestions to csvcx@dcs.