create_logtalk_flag/3 — The Logtalk Handbook v3.47.0 documentation

# `create_logtalk_flag/3`

## Description

    create_logtalk_flag(Flag, Value, Options)

Creates a new Logtalk flag and sets its default value. User-defined flags can be queried and set in the same way as predefined flags by using, respectively, the current_logtalk_flag/2 and set_logtalk_flag/2 built-in predicates. For a description of the predefined compiler flags, please see the Compiler flags section in the User Manual.

This predicate is based on the specification of the SWI-Prolog `create_prolog_flag/3` built-in predicate and supports the same options: `access(Access)`, where `Access` can be either `read_write` (the default) or `read_only`; `keep(Keep)`, where `Keep` can be either `false` (the default) or `true`, for deciding if an existing definition of the flag should be kept or replaced by the new one; and `type(Type)` for specifying the type of the flag, which can be `boolean`, `atom`, `integer`, `float`, or `term` (which only restricts the flag value to ground terms). When the `type/1` option is not specified, the type of the flag is inferred from its initial value.

## Modes and number of proofs

    create_logtalk_flag(+atom, +ground, +list(ground)) - one

## Errors

`Flag` is a variable:
`instantiation_error`
`Value` is not a ground term:
`instantiation_error`
`Options` is not a ground term:
`instantiation_error`
`Flag` is neither a variable nor an atom:
`type_error(atom, Flag)`
`Options` is neither a variable nor a list:
`type_error(atom, Flag)`
`Value` is not a valid value for flag Flag:
`domain_error(flag_value, Flag + Value)`
`Flag` is a system-defined flag:
`permission_error(modify, flag, Flag)`
An element `Option` of the list `Options` is not a valid option
domain_error(flag_option,Option)
The list `Options` contains a `type(Type)` option and `Value` is not of type `Type`
type_error(Type, Value)

## Examples

    % create a new boolean flag with default value set to false:
    | ?- create_logtalk_flag(pretty_print_blobs, false, []).

See also

current_logtalk_flag/2, set_logtalk_flag/2