/usr/lib/swi-prolog/doc/Manual/option.html

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<H2><A NAME="sec:A.15"><SPAN class="sec-nr">A.15</SPAN> <SPAN class="sec-title">library(option): 
Option list processing</SPAN></A></H2>

<P><A NAME="sec:option"></A>

<DL>
<DT><B>See also</B><DD> library(record)<DT><B>To be done</B><DD>- We 
should consider putting many options in an assoc or record with 
appropriate preprocessing to achieve better performance. <BR>
- We should provide some standard to do automatic type-checking on 
option lists.
</DL>

<P>The library(option) provides some utilities for processing option 
lists. Option lists are commonly used as an alternative for many 
arguments. Examples built-in predicates are <A class="pred" href="IO.html#open/4">open/4</A> 
and <A class="pred" href="termrw.html#write_term/3">write_term/3</A>. 
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.

<DL class="latex">
<DT><B>Name(Value)</B></DT>
<DD>
This is the preferred style.
</DD>
<DT><B>Name = Value</B></DT>
<DD>
This is often used, but deprecated.
</DD>
</DL>

<P>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_<VAR>&lt;</VAR>record<A class="pred" href="arith.html#>/2">&gt;/2</A>. 
In addition to providing good performance, this also provides 
type-checking and central declaration of defaults.

<PRE class="code">
:- 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),
        ...
</PRE>

<DL class="latex">
<DT class="pubdef"><A NAME="option/3"><STRONG>option</STRONG>(<VAR>?Option, 
+OptionList, +Default</VAR>)</A></DT>
<DD class="defbody">
Get an option from a <VAR>OptionList</VAR>. <VAR>OptionList</VAR> can 
use the Name=Value as well as the Name(Value) convention.
<TABLE class="paramlist">
<TR VALIGN=top><TD><VAR>Option</VAR> </TD><TD>Term of the form 
Name(?Value). </TD></TR>
</TABLE>

</DD>
<DT class="pubdef"><A NAME="option/2"><STRONG>option</STRONG>(<VAR>?Option, 
+OptionList</VAR>)</A></DT>
<DD class="defbody">
Get an option from a <VAR>OptionList</VAR>. <VAR>OptionList</VAR> can 
use the Name=Value as well as the Name(Value) convention. Fails silently 
if the option does not appear in <VAR>OptionList</VAR>.
<TABLE class="paramlist">
<TR VALIGN=top><TD><VAR>Option</VAR> </TD><TD>Term of the form 
Name(?Value). </TD></TR>
</TABLE>

</DD>
<DT class="pubdef"><span class="pred-tag">[semidet]</span><A NAME="select_option/3"><STRONG>select_option</STRONG>(<VAR>?Option, 
+Options, -RestOptions</VAR>)</A></DT>
<DD class="defbody">
Get and remove option from an option list. As <A class="pred" href="option.html#option/2">option/2</A>, 
removing the matching option from <VAR>Options</VAR> and unifying the 
remaining options with <VAR>RestOptions</VAR>.</DD>
<DT class="pubdef"><span class="pred-tag">[det]</span><A NAME="select_option/4"><STRONG>select_option</STRONG>(<VAR>?Option, 
+Options, -RestOptions, +Default</VAR>)</A></DT>
<DD class="defbody">
Get and remove option with default value. As <A class="pred" href="option.html#select_option/3">select_option/3</A>, 
but if <VAR>Option</VAR> is not in <VAR>Options</VAR>, its value is 
unified with
<VAR>Default</VAR> and <VAR>RestOptions</VAR> with <VAR>Options</VAR>.</DD>
<DT class="pubdef"><span class="pred-tag">[det]</span><A NAME="merge_options/3"><STRONG>merge_options</STRONG>(<VAR>+New, 
+Old, -Merged</VAR>)</A></DT>
<DD class="defbody">
Merge two option lists. <VAR>Merged</VAR> is a sorted list of options 
using the canonical format Name(Value) holding all options from <VAR>New</VAR> 
and <VAR>Old</VAR>, after removing conflicting options from <VAR>Old</VAR>.

<P>Multi-values options (e.g., proxy(Host, Port)) are allowed, where 
both option-name and arity define the identity of the option.</DD>
<DT class="pubdef"><span class="pred-tag">[det]</span><A NAME="meta_options/3"><STRONG>meta_options</STRONG>(<VAR>+IsMeta, 
:Options0, -Options</VAR>)</A></DT>
<DD class="defbody">
Perform meta-expansion on options that are module-sensitive. Whether an 
option name is module sensitive is determined by calling call(<VAR>IsMeta</VAR>, 
Name). Here is an example:

<PRE class="code">
        meta_options(is_meta, OptionsIn, Options),
        ...

is_meta(callback).
</PRE>

<P></DD>
</DL>

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swi-prolog-nox 5.10.4-3ubuntu1 / usr / lib / swi-prolog / doc / Manual / option.html
