SWI-Prolog 7.7.1 Reference Manual: Section A.19

## A.19 library(option): Option list processing

See also
  - `library(record)`
  - Option processing capabilities may be declared using the directive predicate_options/3.
To be done
  We should consider putting many options in an assoc or record with appropriate preprocessing to achieve better performance.

The `library(option)` provides some utilities for processing option lists. Option lists are commonly used as an alternative for many arguments. Examples of built-in predicates are open/4 and write_term/3. Naming the arguments results in more readable code, and the list nature makes it easy to extend the list of options accepted by a predicate. Option lists come in two styles, both of which are handled by this library.

Name(Value)
  This is the preferred style.
Name = Value
  This is often used, but deprecated.

Processing options inside time-critical code (loops) can cause serious overhead. One possibility is to define a record using `library(record)` and initialise this using make_<record>/2. In addition to providing good performance, this also provides type-checking and central declaration of defaults.

    :- record atts(width:integer=100, shape:oneof([box,circle])=box).

    process(Data, Options) :-
            make_atts(Options, Attributes),
            action(Data, Attributes).

    action(Data, Attributes) :-
            atts_shape(Attributes, Shape),
            ...

Options typically have exactly one argument. The library does support options with 0 or more than one argument with the following restrictions:

- The predicate option/3 and select_option/4, involving default are meaningless. They perform an `arg(1, Option, Default)`, causing failure without arguments and filling only the first option-argument otherwise.
- meta_options/3 can only qualify options with exactly one argument.

[semidet]option(?Option, +OptionList, +Default)
  Get an Option from OptionList. OptionList can use the Name=Value as well as the Name(Value) convention.

  Option | Term of the form Name(?Value).

[semidet]option(?Option, +OptionList)
  Get an Option from OptionList. OptionList can use the Name=Value as well as the Name(Value) convention. Fails silently if the option does not appear in OptionList.

  Option | Term of the form Name(?Value).

[semidet]select_option(?Option, +Options, -RestOptions)
  Get and remove Option from an option list. As option/2, removing the matching option from Options and unifying the remaining options with RestOptions.
[det]select_option(?Option, +Options, -RestOptions, +Default)
  Get and remove Option with default value. As select_option/3, but if Option is not in Options, its value is unified with Default and RestOptions with Options.
[det]merge_options(+New, +Old, -Merged)
  Merge two option lists. Merged is a sorted list of options using the canonical format Name(Value) holding all options from New and Old, after removing conflicting options from Old.

  Multi-values options (e.g., `proxy(Host, Port)`) are allowed, where both option-name and arity define the identity of the option.

[det]meta_options(+IsMeta, :Options0, -Options)
  Perform meta-expansion on options that are module-sensitive. Whether an option name is module-sensitive is determined by calling `call(IsMeta, Name)`. Here is an example:

              meta_options(is_meta, OptionsIn, Options),
              ...

      is_meta(callback).

  Meta-options must have exactly one argument. This argument will be qualified.

  To be done
    Should be integrated with declarations from predicate_options/3.
[det]dict_options(?Dict, ?Options)
  Convert between an option list and a dictionary. One of the arguments must be instantiated. If the option list is created, it is created in canonical form, i.e., using Option(Value) with the Options sorted in the standard order of terms. Note that the conversion is not always possible due to different constraints and convertion may thus lead to (type) errors.

  - Dict keys can be integers. This is not allowed in canonical option lists.
  - Options can hold multiple options with the same key. This is not allowed in dicts.
  - Options can have more than one value (`name(V1,V2)`). This is not allowed in dicts.

  Also note that most system predicates and predicates using this library for processing the option argument can both work with classical Prolog options and dicts objects.