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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.

Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

Examples

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

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