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# register_option(++Name, ?InitialValue, +Options)

Register an option

Name
  Name of the option (any ground term)
InitialValue
  Initial value to be given to option
Options
  List of options related to this option

## Description

This predicate sets up an option with name Name and initial value InitialValue. Name must be ground; InitialValue can be any term but should normally be ground, since it will be copied when stored and retrieved, which means that any retrieved non-ground value will contain fresh copies of variables rather than the original ones.

Options must be a list. The following terms may appear in this list:

validation_pred(Pred/Arity)
  Pred is used to validate any value to be assigned to the option. This can be used to ensure that only legal values are assigned to the option. Arity must be either 1 or 2, corresponding to the arity of the predicate to be called: if Arity is 1 then Pred(NewValue) is called, if Arity is 2 then Pred(Name, NewValue) is called; Name is the name of the option and NewValue is the value to be validated. If the call to Pred succeeds then the value is assigned to the option; if it fails the value is not assigned. Any choice points left by the call to Pred are pruned upon success.

  Only one validation predicate may be defined for any option; if more are specified then each replaces the one before it.

notify_pred(Pred)
  Specifies that Pred(Name, OldValue, NewValue) should be called whenever the value of the option Name is changed. OldValue is the old value of the option that has just been replaced and NewValue is the new value that has just been assigned. Pred should not modify OldValue or NewValue in any way, as this may interfere with other notification predicates. Note that any choice points left by the call to Pred are pruned upon success.

  See also request_notification/2.

### Modules

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

## See Also

set_option / 2, get_option / 2, get_all_options / 1, request_notification / 2