CHR Constraint Store

Module to use Prolog as an incremental constraint store for Constraint Handling Rules (CHR). Since version 7.3.28 of SWI-Prolog this behaviour can also achieved using the flag set_prolog_flag(toplevel_mode, recursive) as described here.

While basic Prolog implementations support only one-time-only queries, i.e. you can't access the resulted constraint store of a previous query, this adds the functionality of an online constraint store. You can find some background information in the Wiki.

Usage

Here is an example on how to use it:

:- use_module(examples/gcd).
:- use_module(examples/min).
:- use_module(examples/directions).
:- use_module(chr_store).

main :- chr_store([gcd, min, directions]).

Use the chr_store predicate to provide a list of modules which make use of CHR.

Commands

Currently only two methods are implemented:

• s_list: Lists all constraints of the current constraint store.
• s_stop: Stops the prompt loop.

Example

?- main.
[CHRstore] ?- gcd(18), north, min(3), west, east, min(1), gcd(12).
[CHRstore] ?- s_list.
[CHRstore] Stored constraints:
gcd(6)
min(1)
north
[CHRstore] ?- gcd(16), south, min(1).
[CHRstore] ?- s_list.
[CHRstore] Stored constraints:
gcd(2)
min(1)
[CHRstore] ?- s_stop.
[CHRstore] Stopped.
true .

ToDo

• Delete single constraints