swi-prolog-nox 6.6.6-5 / usr / lib / swi-prolog / library / option.pl

/*  $Id$

    Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (C): 1985-2011, University of Amsterdam
			      VU University Amsterdam

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA

    As a special exception, if you link this library with other files,
    compiled with a Free Software compiler, to produce an executable, this
    library does not by itself cause the resulting executable to be covered
    by the GNU General Public License. This exception does not however
    invalidate any other reasons why the executable file might be covered by
    the GNU General Public License.
*/

:- module(swi_option,
	  [ option/2,			% +Term, +List
	    option/3,			% +Term, +List, +Default
	    select_option/3,		% +Term, +Options, -RestOpts
	    select_option/4,		% +Term, +Options, -RestOpts, +Default
	    merge_options/3,		% +New, +Old, -Merged
	    meta_options/3		% :IsMeta, :OptionsIn, -OptionsOut
	  ]).
:- use_module(library(lists)).
:- use_module(library(error)).
:- set_prolog_flag(generate_debug_info, false).

:- meta_predicate
	meta_options(1, :, -).

/** <module> Option list processing

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.

@tbd	We should consider putting many options in an assoc or record
	with appropriate preprocessing to achieve better performance.
@see	library(record)
@see	Option processing capabilities may be declared using the
	directive predicate_options/3.
*/

%%	option(?Option, +OptionList, +Default) is semidet.
%
%	Get  an  Option  Qfrom  OptionList.  OptionList  can  use  the
%	Name=Value as well as the Name(Value) convention.
%
%	@param Option	Term of the form Name(?Value).

option(Opt, Options, Default) :-	% make option processing stead-fast
	functor(Opt, Name, Arity),
	functor(GenOpt, Name, Arity),
	(   get_option(GenOpt, Options)
	->  Opt = GenOpt
	;   arg(1, Opt, Default)
	).


%%	option(?Option, +OptionList) is semidet.
%
%	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.
%
%	@param Option	Term of the form Name(?Value).

option(Opt, Options) :-			% make option processing stead-fast
	functor(Opt, Name, Arity),
	functor(GenOpt, Name, Arity),
	get_option(GenOpt, Options), !,
	Opt = GenOpt.

get_option(Opt, Options) :-
	memberchk(Opt, Options), !.
get_option(Opt, Options) :-
	functor(Opt, OptName, 1),
	arg(1, Opt, OptVal),
	memberchk(OptName=OptVal, Options), !.


%%	select_option(?Option, +Options, -RestOptions) is semidet.
%
%	Get and remove Option from an option list. As option/2, removing
%	the matching option from  Options   and  unifying  the remaining
%	options with RestOptions.

select_option(Opt, Options0, Options) :-	% stead-fast
	functor(Opt, Name, Arity),
	functor(GenOpt, Name, Arity),
	get_option(GenOpt, Options0, Options),
	Opt = GenOpt.

get_option(Opt, Options0, Options) :-
	selectchk(Opt, Options0, Options), !.
get_option(Opt, Options0, Options) :-
	functor(Opt, OptName, 1),
	arg(1, Opt, OptVal),
	selectchk(OptName=OptVal, Options0, Options).

%%	select_option(?Option, +Options, -RestOptions, +Default) is det.
%
%	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.

select_option(Option, Options, RestOptions, Default) :-
	functor(Option, Name, Arity),
	functor(GenOpt, Name, Arity),
	(   get_option(GenOpt, Options, RestOptions)
	->  Option = GenOpt
	;   RestOptions = Options,
	    arg(1, Option, Default)
	).


%%	merge_options(+New, +Old, -Merged) is det.
%
%	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.

merge_options([], Old, Merged) :- !,
	canonicalise_options(Old, Merged).
merge_options(New, [], Merged) :- !,
	canonicalise_options(New, Merged).
merge_options(New, Old, Merged) :-
	canonicalise_options(New, NCanonical),
	canonicalise_options(Old, OCanonical),
	sort(NCanonical, NSorted),
	sort(OCanonical, OSorted),
	ord_merge(NSorted, OSorted, Merged).

ord_merge([], L, L) :- !.
ord_merge(L, [], L) :- !.
ord_merge([NO|TN], [OO|TO], Merged) :-
	sort_key(NO, NKey),
	sort_key(OO, OKey),
	compare(Diff, NKey, OKey),
	ord_merge(Diff, NO, NKey, OO, OKey, TN, TO, Merged).

ord_merge(=, NO, _, _, _, TN, TO, [NO|T]) :-
	ord_merge(TN, TO, T).
ord_merge(<, NO, _, OO, OKey, TN, TO, [NO|T]) :-
	(   TN = [H|TN2]
	->  sort_key(H, NKey),
	    compare(Diff, NKey, OKey),
	    ord_merge(Diff, H, NKey, OO, OKey, TN2, TO, T)
	;   T = [OO|TO]
	).
ord_merge(>, NO, NKey, OO, _, TN, TO, [OO|T]) :-
	(   TO = [H|TO2]
	->  sort_key(H, OKey),
	    compare(Diff, NKey, OKey),
	    ord_merge(Diff, NO, NKey, H, OKey, TN, TO2, T)
	;   T = [NO|TN]
	).

sort_key(Option, Name-Arity) :-
	functor(Option, Name, Arity).

%%	canonicalise_options(+OptionsIn, -OptionsOut) is det.
%
%	Rewrite option list from possible Name=Value to Name(Value)

canonicalise_options(In, Out) :-
	memberchk(_=_, In), !,		% speedup a bit if already ok.
	canonicalise_options2(In, Out).
canonicalise_options(Options, Options).

canonicalise_options2([], []).
canonicalise_options2([Name=Value|T0], [H|T]) :- !,
	H =.. [Name,Value],
	canonicalise_options2(T0, T).
canonicalise_options2([H|T0], [H|T]) :- !,
	canonicalise_options2(T0, T).


%%	meta_options(+IsMeta, :Options0, -Options) is det.
%
%	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.
%
%	@tbd	Should be integrated with declarations from
%		predicate_options/3.

meta_options(IsMeta, Context:Options0, Options) :-
	must_be(list, Options0),
	meta_options(Options0, IsMeta, Context, Options).

meta_options([], _, _, []).
meta_options([H0|T0], IM, Context, [H|T]) :-
	meta_option(H0, IM, Context, H),
	meta_options(T0, IM, Context, T).

meta_option(Name=V0, IM, Context, Name=M:V) :-
	call(IM, Name), !,
	strip_module(Context:V0, M, V).
meta_option(O0, IM, Context, O) :-
	compound(O0),
	O0 =.. [Name,V0],
	call(IM, Name), !,
	strip_module(Context:V0, M, V),
	O =.. [Name,M:V].
meta_option(O, _, _, O).