A.15 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 Qfrom 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.