The CHR constraints defined in a particulary .chr file
are associated with a module. The default module is user.
One should never load different .chr files with the same
CHR module name.
Every constraint used in CHR rules has to be declared. There are two ways to do this. The old style is as follows:
option(type_definition,type(list(T),[ [] , [T|list(T)] ]). option(mode,foo(+,?)). option(type_declaration,foo(list(int),float)). :- constraints foo/2, bar/0. |
The new style is as follows:
:- chr_type list(T) ---> [] ; [T|list(T)]. :- constraints foo(+list(int),?float), bar. |
The SWI-Prolog CHR compiler exploits term_expansion/2
rules to translate the constraint handling rules to plain Prolog. These
rules are loaded from the library library(chr). They are
activated if the compiled file has the .chr extension or
after finding a declaration of the format below.
:- constraints ... |
It is adviced to define CHR rules in a module file, where the module
declaration is immediately followed by including the library(chr)
library as examplified below:
:- module(zebra, [ zebra/0 ]). :- use_module(library(chr)). :- constraints ... |
Using this style CHR rules can be defined in ordinary Prolog
.pl files and the operator definitions required by CHR do
not leak into modules where they might cause conflicts.