- See also
- -
library(record)
- Option processing capabilities may be declared using the directive predicate_options/3.
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.
SWI-Prolog dicts provide a convenient and efficient
alternative to option lists. For this reason, both built-in predicates
and predicates that use this library support dicts transparantly.
Processing option lists inside time-critical code (loops) can cause
serious overhead. The above mentioned dicts is the preferred
mitigation. A more portable alternative 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.
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,
+Options)
- Get an Option from Options. Fails silently if the
option does not appear in Options. If Option
appears multiple times in Options, the first value is used.
| Option | Term of the form Name(?Value). |
| Options | is a list of Name(Value) or Name=Value
or a dict. |
- [det]option(?Option,
+Options, +Default)
- Get an Option from Options. If Option
does not appear in Options, unify the value with Default.
If Option appears multiple times in
Options, the first value is used. For example
?- option(max_depth(D), [x(a), max_depth(20)], 10).
D = 20.
?- option(max_depth(D), [x(a)], 10).
D = 10.
| Option | Term of the form Name(?Value). |
| Options | is a list of Name(Value) or Name=Value
or a dict. |
- [semidet]select_option(?Option,
+Options, -RestOptions)
- Get and remove Option from Options. As option/2,
removing the matching option from Options and unifying the
remaining options with
RestOptions. If Option appears multiple times in Options,
the first value is used. Note that if Options contains
multiple terms that are compatible to Option, the first is
used to set the value of Option and the duplicate appear in 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 sets. If Old is a dict, Merged is
a dict. Otherwise
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 conversion 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. This predicate removes all but the first option
on the same key.
- 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.