157 lines
5.8 KiB
Markdown
157 lines
5.8 KiB
Markdown
# Output Format
|
||
|
||
`plasp` 3 translates SAS and PDDL files into a uniform ASP fact format.
|
||
|
||
## Overview
|
||
|
||
Essentially, `plasp`’s output format consists of [state variables](#variables) that are modified by [actions](#actions) if their preconditions are fulfilled.
|
||
Variables reference [entities](#constants-objects) that are affected by the actions.
|
||
As with PDDL, the objective is to achieve a specific [goal](#goal) starting from an [initial state](#initial-state) by executing a sequence of actions.
|
||
|
||
`plasp`’s variables correspond to the multivalued variables in SAS.
|
||
PDDL predicates are turned into Boolean variables to make the output format consistent.
|
||
|
||
Actions are modeled exactly as PDDL actions and SAS operators.
|
||
|
||
## In a Nutshell
|
||
|
||
The following illustrates `plasp`’s output format for the problem of turning switches on and off.
|
||
|
||
```prolog
|
||
% declares the type "type(switch)"
|
||
type(type(switch)).
|
||
|
||
% introduces a switch "constant(a)"
|
||
constant(constant(a)).
|
||
has(constant(a), type(switch)).
|
||
|
||
% declares a variable "variable(on(X))" for switches X
|
||
variable(variable(on(X))) :- has(X, type(switch)).
|
||
|
||
% the variable may be true or false
|
||
contains(variable(on(X)), value(on(X)), true)) :- has(X, type(switch)).
|
||
contains(variable(on(X)), value(on(X)), false)) :- has(X, type(switch)).
|
||
|
||
% declares the action "action(turnOn(X))", which requires switch X to be off and then turns it on
|
||
action(action(turnOn(X))) :- has(X, type(switch)).
|
||
precondition(action(turnOn(X)), variable(on(X)), value(on(X), false)) :- has(X, type(switch)).
|
||
postcondition(action(turnOn(X)), effect(0), variable(on(X)), value(on(X), true)) :- has(X, type(switch)).
|
||
|
||
% initially, the switch is off
|
||
initialState(variable(on(constant(a))), value(on(constant(a)), false)).
|
||
|
||
% in the end, the switch should be on
|
||
goal(variable(on(constant(a))), value(on(constant(a)), true)).
|
||
```
|
||
|
||
## Syntax and Semantics
|
||
|
||
`plasp` structures the translated ASP facts into multiple sections, which are explained in the following.
|
||
|
||
### Feature Requirements
|
||
|
||
```prolog
|
||
% declares a required feature
|
||
requires(feature(<name>)).
|
||
```
|
||
|
||
`plasp` recognizes and declares advanced features used by the input problem, such as conditional effects and [axiom rules](#axiom-rules) (currently only SAS).
|
||
See the [full list of supported features](feature-requirements.md) for more information.
|
||
|
||
The feature requirement predicates may be used in meta encodings to warn about unsupported features.
|
||
|
||
### Types
|
||
|
||
```prolog
|
||
% declares a <type>
|
||
type(type(<name>)).
|
||
|
||
% specifies <constant> to be of type type(<name>)
|
||
has(<constant>, type(<name>)).
|
||
```
|
||
|
||
[Variables](#variables), [constants](#constants-objects), and [objects](#constants-objects) may be typed. Types are only available with PDDL and if typing is enabled.
|
||
|
||
### Variables
|
||
|
||
```prolog
|
||
% declares a <variable>
|
||
variable(variable(<name>)).
|
||
|
||
% adds a <value> to the domain of a <variable>
|
||
contains(<variable>, <value>).
|
||
```
|
||
|
||
`plasp`’s variables represent the current state of the planning problem.
|
||
Variables are linked to the problem's [objects](#constants-objects) and [constants](#constants-objects).
|
||
|
||
With SAS, variable names are numbers starting at 0, `variable(<number>)`.
|
||
SAS variables are inherently multivalued, which results in two or more values of the form `value(<SAS predicate>, <bool>)` for each variable.
|
||
|
||
With PDDL, Boolean variables are created from the PDDL predicates.
|
||
Variables ared named after the PDDL predicates, `variable(<PDDL predicate>).`
|
||
Each variable contains exactly two values (one `true`, one `false`) of the form `value(<PDDL predicate>, <bool>)`.
|
||
Note that with PDDL, variables and values are named identically.
|
||
|
||
### Actions
|
||
|
||
```prolog
|
||
% declares an <action>
|
||
action(action(<name>)).
|
||
|
||
% defines that as a precondition to <action>, <variable> must have value <value>
|
||
precondition(<action>, <variable>, <value>).
|
||
|
||
% defines that after executing <action>, <variable> is assigned <value>
|
||
postcondition(<action>, effect(<number>), <variable>, <value>).
|
||
|
||
% defines the condition of a conditional effect
|
||
precondition(effect(<number>), <variable>, <value>).
|
||
```
|
||
|
||
Actions may require certain variables to have specific values in order to be executed.
|
||
After applying an action, variables get new values according to the action's postconditions.
|
||
|
||
Actions may have *conditional effects*, that is, certain postconditions are only applied if additional conditions are satisfied.
|
||
For this reason, each conditional effect is uniquely identified with a predicate `effect(<number>)` as the second argument of the `postcondition` facts.
|
||
The conditions of conditional effects are given by additional `precondition` facts that take the respective `effect(<number>)` predicates as the first argument.
|
||
|
||
Unconditional effects are identified with `effect(unconditional)`.
|
||
|
||
Conditional effects are currently only supported with SAS input problems.
|
||
|
||
### Constants/Objects
|
||
|
||
```prolog
|
||
% declares a <constant> or object
|
||
constant(constant(<name>)).
|
||
|
||
% specifies <constant> to be of type type(<name>)
|
||
has(<constant>, <type>).
|
||
```
|
||
|
||
Constants and objects are the entities that are affected by [actions](#actions), for instance, the blocks in a Blocks World problem.
|
||
Constants are global for a domain, while objects are problem-specific.
|
||
|
||
`plasp` does not distinguish between the two (modeling both as constants), as both are identically used static identifiers.
|
||
|
||
### Initial State
|
||
|
||
```prolog
|
||
% initializes <variable> with a specific <value>
|
||
initialState(<variable>, <value>).
|
||
```
|
||
|
||
The initial state contains all variable assignments that hold before executing any actions.
|
||
|
||
Note that with PDDL, the initial state might not assign values to all variables. Instead, unassigned values have to be assigned `false` manually.
|
||
|
||
### Goal
|
||
|
||
```prolog
|
||
% specifies that <variable> shall obtain <value> in the end
|
||
goal(<variable>, <value>).
|
||
```
|
||
|
||
The goal specifies all variable assignments that have to be fulfilled after executing the plan.
|