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ALPprolog --- Action Logic Programs in Propositional Logic

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Website:
http://alpprolog.sourceforge.net

Status:
Stable

Developer:
conrad.drescher@web.de

ALPprolog is a Prolog implementation of an action programming language.

It requires ECLiPSe Prolog (available at http://www.eclipse-clp.org).

It implements a fragment of the theoretical framework of Action Logic Programs
as described in

http://www.computational-logic.org/content/projects/wisslogpubs/DreSchiThi-FroCoS09.pdf


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Overview
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The fragment can be characterized as follows:

* States are built from ground fluent (i.e. mutable) and non-fluent (also called 
  auxiliary) properties using the logical connectives "and", "or", and "not".
  We assume that there are only finitely many objects in the domain and make a
  domain closure assumption. Hence states are a notational variant of classical
  propositional logic.
* An ALPprolog program describes the strategy of an agent that executes the program
  online. Hence only simple substitutions are used for evaluating queries like
  e.g. ?- holds(safe(Cell)). In the offline setting reasoning by cases via
  disjunctive substitutions ensures that the Action Logic Program framework
  becomes logically complete.
* The agent can execute two kinds of actions: Physical actions that affect the
  environment, and sensing actions that only affect the agent's knowledge thereof.
  Sensing actions may result in disjunctive information --- e.g. one of the
  surrounding cells might be safe, but it might not be clear which one. Physical
  actions are assumed to be deterministic, i.e. there are no disjunctive effects.


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World descriptions with ALPprolog
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With ALPprolog you can program strategies for autonomous agents in
dynamic domains. ALPprolog programs are ordinary Prolog programs with
two additional predicates:

* holds/1 to test whether some state property holds currently

* execute/1 to execute some action

An ALP describing a breadth first naive planning strategy is the following:

strategy :- holds(goal).
strategy :- do(A), strategy.

In words if the goal description is satisfied at the current state then the
strategy was successful. Otherwise we randomly pick some action A and keep on
searching. As one can see ALP programs are read as temporally ordered sequences
from left to right.

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Required Files
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The overall structure of an ALPprolog domain is as follows:

(1) Create a module world.ecl --- describing the dynamic domain
(2) Import this module into alpprolog.ecl
(3) Create a file strategy.ecl importing world.ecl and alpprolog.ecl

For illustration purposes with this distribution comes an implementation of 
the Wumpus world with the respective files wumpus_world.ecl and 
wumpus_strategy.ecl. Below is a brief explanation of the language features and 
their use.

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Sample Domain - Wumpus world
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Let us first recall the wumpus world:
The wumpus world is a well-known challenge problem in the reasoning about 
action community. It consists of a grid-like world: cells may contain pits, one 
cell contains gold, and one the fearsome Wumpus. The agent dies if she enters a 
cell containing a pit or the Wumpus. But she carries one arrow so that she can 
shoot the Wumpus from an adjacent cell. If the agent is next to a cell contain-
ing a pit (the Wumpus), she can detect that one of the surrounding cells 
contains a pit (the Wumpus), but doesn't know which one:
She perceives a breeze if she is in a cell next to a pit, and she perceives a 
stench if she is in a cell next to the Wumpus. She knows the contents of the 
already visited cells --- in particular she can detect the gold if she is in 
the same cell. Getting the gold without getting killed is the agent's goal.

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world.ecl:
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A world description is composed as follows:

* Describe the initial state as a ground formula in prime implicate form:
  A formula in conjunctive normal form is in prime implicate form iff it is
  closed under resolution, subsumption, and tautology deletion. This should
  be included as a Prolog fact: E.g. 

           inital_state([at(agent,1),[at(wumpus,2),wumpus(3)]]). 

  This Prolog fact describes the initial state 

           at(agent,1) /\ ( at(wumpus,2) \/ at(wumpus,3) )

  We know where the agent is, but the exact location of the fearsome wumpus
  is unknown. This initial state talks about "fluents": These are the mutable 
  properties of the dynamic domain. Formulas describing states are Prolog lists.
  If a list element is a term it is a singleton clause, if it is a list it is
  a disjunctive clause. A negated fluent is e.g. neg(alive(wumpus)).

* Describe the actions, using action/3 facts. E.g. the fact

           action(shoot,
                  [carries(agent,arrow),
                   at(wumpus,Cell1),
                   at(agent,Cell2),
                   connected(Cell2,Cell1)],
                  [neg(carries(agent,arrow)),
                   neg(alive(wumpus))]).

  has the following meaning: The action "shoot" is possible if the agent is
  currently armed, and next to a cell containing the wumpus. Finally, the
  effects of the action are that the agent is no longer armed, and that the
  wumpus gets killed.

* You can define as many additional auxiliary static properties of the domain
  as you like using Prolog clauses. These have to be listed in a fact

           aux([connected]).

  so that they can be distinguished from fluent properties (the default).

* If the dynamic domain includes sensing then you have to first list the 
  respective predicates, e.g.

           sensors([glitter,stench]).

  Describing sensor axioms is a little complicated. The meaning of an observation
  depends both upon the sensing result and the agent's current state. For example,
  the meaning of sensing whether an adjacent cell contains gold depends both on the
  observation (Yes/No) and the agent's current location. This is the sensor axiom
  for the wumpus agent that is trying to sense whether there is gold:

  sensor_axiom(glitter(X),[ [X-true] -
                            [at(agent,Y)] -
                            [at(gold,Y)],
                          
                            [X-false] -
                            [at(agent,Y)]-
                            [neg(at(gold,Y))]
                          ]).

  This sensor has just two possible sensor values, true and false. The meaning
  of the sensing result depends on the agents current location at(agent,Y).
  If e.g. the sensing result was true then at(gold,Y) is adjoined to the
  current state description. If the meaning of the sensor result does not
  depend on the current state then the empty list may be used. In general, the
  meaning may be described by any formula in prime implicate form. Variables may
  be used provided that these can approriately grounded by evaluating the state
  condition against the agent's current state knowledge. A sensing action
  is performed simply by evaluating a non-ground sensed property.

  For sensing to work in simulation you also have to provide a predicate
  real_world/1. This is used by alpprolog.ecl to determine the appropriate
  sensing results.


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strategy.ecl:
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A strategy is an arbitrary Prolog program, using the special predicates
holds/1 and execute/1. See wumpus_strategy.ecl for an example.


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A Guide to the Distribution
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The directory 'ALPprolog' contains the files

* wumpus_strategy.ecl
* wumpus_world_small.ecl
* wumpus_world_big.ecl
* alpprolog.ecl

Compile 'wumpus_strategy.ecl' and call 'main'. You can choose a world size in
the files 'wumpus_world_*.ecl' (from 4x4 to 32x32).

The 'small' world contains fewer ground fluents in the agents state
representation, and the 'bigger' one contains much more (allow some time
for compilation).

If you want to use 'big' instead of 'small' adapt both 'wumpus_strategy.ecl'
and 'alpprolog.ecl'.

The subdirectory 'FluxVersion' contains two subdirectories, 'Ground' and 
'Nonground'. 'Ground' is essentially the same as 'ALPprolog', only using Flux
for the reasoning instead. Again, if you want to use 'big' instead of 'small' 
adapt both 'wumpus_strategy.ecl' and 'flux.ecl'.

'Nonground' uses quantification over variables to keep the state representation 
small, and is much more efficient. Compile 'wumpus.pl' and call 'main'. You 
can choose a world size in 'wumpus.pl'.