Guard

A Guard will either coerce an input to a value that matches some condition, or it will fail.

A Guard is used in the E language to guard the definition of a variable or the return value of a method. Implementors of Guard should always override __printOn/1 to print a guard expression reflecting the guard's value.

It is planned but not yet implemented that all objects used as guards must be DeepFrozen.

Protocol
If specimen coerces to a reference that matches the condition represented by this guard, return that reference; otherwise fail (according to optEjector ) with a problem explaining why not.

If optEjector is null, then throw the problem. Otherwise, call optEjector with the problem. optEjector should perform a non-local exit, and so should not return. If optEjector returns anyway, then throw the problem after all.

(See also: the non-optional ejectors proposal.)

Experimental. See guard-based auditing.