SWI-Prolog -- create_prolog

Availability:built-in

[YAP]create_prolog_flag(+Key, +Value, +Options)

Create a new Prolog flag. The ISO standard does not foresee creation of new flags, but many libraries introduce new flags. Prolog flags are particularly useful for managing mutating global settings in a threaded environment. Were predicates are either local to a thread or shared between all threads, a thread inherits its flags from the thread that creates it (see thread_create/3) while modifications are local to the calling thread.

SWI-Prolog flags are typed. If the type is not explicitly defined using the type(Type) option (see below), the type is determined from the initial value. Defined types are boolean (if the initial value is one of false, true, on or off), atom if the initial value is any other atom, integer if the value is an integer that can be expressed as a 64-bit signed value. Any other initial value results in an untyped flag that can represent any valid Prolog term.

By default, the new flag is added to the global flag table, making the value available to all threads that have not set the flag. If the flag was already defined locally in the calling thread, the value is updated both for the calling thread and in the global flag table. See the local(+Boolean) option.

Options is a list of the options below. See also the Prolog flag user_flags.

access(+Access): Define access rights for the flag. Values are read_write and read_only. The default is read_write.
type(+Atom): Define a type restriction. Possible values are boolean, atom, oneof(ListOfAtoms), integer, float and term. The default is determined from the initial value. Note that term restricts the term to be ground.
keep(+Boolean): If true, do not modify the flag if it already exists. Otherwise (default), this predicate behaves as set_prolog_flag/2 if the flag already exists.
If the flag has a value, but this value is incompatible with the specified type, a warning is printed and the flag gets the value and type specified by this call to create_prolog_flag/3.
local(+Boolean): If true (default false), if the flag does not exist, create it only in the calling thread. The flag is only visible in the calling thread and threads that inherit from the calling thread.
warn_not_accessed(+Boolean): If true and the flag is never read using current_prolog_flag/2, print a warning. This option is used for options set using the commandline option -D<flag>[=<value>].