Module gen_pnet_struct

Description

Callback function definitions for Petri net structure definition.

Version: 0.1.4

This module defines the gen_pnet_struct behaviour.
Required callback functions: place_lst/0, trsn_lst/0, init_marking/2, preset/1, is_enabled/3, fire/3.

Authors: Jörgen Brandt (joergen.brandt@onlinehome.de).

Description

Callback function definitions for Petri net structure definition.

There are six callbacks that define the Petri net structure and its initial marking:

place_lst/0 returns the names of the places in the net
trsn_lst/0 returns the names of the transitions in the net
init_marking/2 returns the initial marking for a given place
preset/1 returns the preset places of a given transition
is_enabled/3 determines whether a given transition is enabled in a given mode
fire/3 returns which tokens are produced on what places if a given transition is fired in a given mode that enables this transition

We have a look at each of them in turn.

place_lst/0

The place_lst/0 function lets us define the names of all places in the net.

Example:

  place_lst() ->
    [coin_slot, cash_box, signal, storage, compartment].

Here, we define the net to have the five places in the cookie vending machine.

trsn_lst/0

The trsn_lst/0 function lets us define the names of all transitions in the net.

Example:

  trsn_lst() ->
    [a, b].

Here, we define the net to have the two places a and b in the cookie vending machine.

preset/1

The preset/1 lets us define the preset places of a given transition.

Example:

  preset( a ) -> [coin_slot];
  preset( b ) -> [signal, storage].

Here, we define the preset of the transition a to be just the place coin_slot while the transition b has the places signal and storage in its preset.

init_marking/2

The init_marking/2 function lets us define the initial marking for a given place in the form of a token list. The argument UsrInfo is the user info field that has been generated in the actor interface callback init/1.

Example:

  init_marking( storage, _UsrInfo ) -> [cookie_box, cookie_box, cookie_box];
  init_marking( _Place, _UsrInfo )  -> [].

Here, we initialize the storage place with three cookie_box tokens. All other places are left empty.

is_enabled/3

The is_enabled/3 function is a predicate determining whether a given transition is enabled in a given mode. The UsrInfo argument is the user info field that has been created with init/1.

Example:

  is_enabled( a, #{ coin_slot := [coin] }, _UsrInfo )                      -> true;
  is_enabled( b, #{ signal := [sig], storage := [cookie_box] }, _UsrInfo ) -> true;
  is_enabled( _Trsn, _Mode, _UsrInfo )                                     -> false.

Here, we state that the transition a is enabled if it can consume a single coin from the coin_slot place. Similarly, the transition b is enabled if it can consume a sig token from the signal place and a cookie_box token from the storage` place. No other configuration can enable a transition. E.g., managing to get a `button token on the coin_slot place will not enable any transition.

fire/3

The fire/3 function defines what tokens are produced when a given transition fires in a given mode. As arguments it takes the name of the transition, and a firing mode in the form of a hash map mapping place names to token lists. The fire/3 function is called only on modes for which is_enabled/3 returns true. The fire/3 function is expected to return either a {produce, ProduceMap} tuple or the term abort. If abort is returned, the firing is aborted. Nothing is produced or consumed.

Example:

  fire( a, _Mode, _UsrInfo ) ->
    {produce, #{ cash_box => [coin], signal => [sig] }};
  fire( b, _Mode, _UsrInfo ) ->
    {produce, #{ compartment => [cookie_box] }}.

Here, the firing of the transition a produces a coin token on the cash_box place and a sig token on the signal place. Similarly, the firing of the transition b produces a cookie_box token on the compartment place. We do not need to state the tokens to be consumed because the firing mode already uniquely identifies the tokens to be consumed.

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