35.4.2 Declarations

Declarations in files containing CHR affect the compilation and thus the behavior of the rules at runtime.

The mandatory handler declaration precedes any other CHR specific code. Example:

     handler minmax.

A handler name must be a valid Prolog atom. Per module, only one constraint handler can be defined.

The constraints must be declared before they are used by rules. With this mandatory declaration one lists the constraints the rules will later talk about. The declaration can be used more than once per handler. Example:

     constraints  leq/2, minimum/3, maximum/3.

The following optional declaration allows for conditional rule compilation. Only the rules mentioned get compiled. Rules are referred to by their names (see CHR Syntax). The latest occurrence takes precedence if used more than once per handler. Although it can be put anywhere in the handler file, it makes sense, as with other declarations, to use it early. Example:

     rules antisymmetry, transitivity.

To simplify the handling of operator declarations, in particular during fcompile/1, operator/3 declarations with the same denotation as op/3, but taking effect during compilation and loading, are helpful. Example:

     operator(700, xfx, ::).
     operator(600, xfx, :).