[ library(ech) | The ECLiPSe Libraries | Reference Manual | Alphabetic Index ]

constraints(+SpecList)

Directive for declaring SpecList to be CHR constraints

SpecList: Sequence of the form Atom/Integer, or Atom/Integer:PrioSpec

Description

Declares the predicates specified in SpecList as CHR constraints. This allows the predicate to appear in the head of a CHR rule. A constraint can be follwed by a priority specification PrioSpec, which can be one of:

1. at_lower(++N) 2. at_higher(++N) 3. at_absolute_priority(++N)

where N is an integer. This specifies the priority the CHR rules will be executed at if the specified constraint is the active constraint. at_lower and at_higher specifies that the priority is N lower or higher than the default CHR priority, and for at_absolute_priority, it is the actual priority.

Note that a predicate declared as a CHR constraint should not appear as a normal ECLiPSe predicate. Any such definition of the predicate in the user's program would be replaced by the CHR definition.

Examples

   :- constraints leq/2.
   :- op(700, xfx, leq).

   X leq Y <=> \+nonground(X), \+nonground(Y) | X @=< Y.
   X leq X <=> true.