/usr/share/doc/swi-prolog-doc/UserGuide/mlingual.html

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<H2><A NAME="sec:11.7"><SPAN class="sec-nr">11.7</SPAN> <SPAN class="sec-title">Multi-lingual 
applications</SPAN></A></H2>

<A NAME="sec:mlingual"></A>

<P><font size=-1>XPCE</font> provides some support for building 
multi-lingual applications without explicitly mapping terms all the 
time. This section provides an overview of how multi-lingual operation 
was realised in a simulator for optics.

<P>When writing a multi-lingual application, several different types of 
information needs to be translated. We will discuss each of them below.

<P>
<UL>
<LI><I>Labels</I><BR>
Labels as they are used by the subclasses of <A class="" href="summary.html#class:dialog_item">dialog_item</A>, 
menu items, etc. These can be mapped by redefining a number of methods 
that realise the default mapping between internal names and externally 
visible names:

<DL>
<DT><STRONG>dialog_item &lt;-label_name:</STRONG> <VAR>Id</VAR></DT>
<DD class="defbody">
Label This method performs the mapping that ensures that the code
<CODE>text_item(user_name, '')</CODE> renders as:

<P>
<CENTER>
</CENTER>

<P>This method may be redefined to return another name or image object, 
depending on the current language mapping.</DD>
<DT><STRONG>menu_item &lt;-label_name:</STRONG> <VAR>Id</VAR></DT>
<DD class="defbody">
Label Similar to <A NAME="idx:dialogitemgetlabelname:513"></A>`<B>dialog_item<CODE>&lt;-</CODE>label_name</B>'.</DD>
<DT><STRONG>dialog_group &lt;-label_name:</STRONG> <VAR>Id</VAR></DT>
<DD class="defbody">
Label Similar to <A NAME="idx:dialogitemgetlabelname:514"></A>`<B>dialog_item<CODE>&lt;-</CODE>label_name</B>', 
but is, in the current implementation, not allowed to return an <A class="" href="summary.html#class:image">image</A>. 
This method needs to be redefined separately as <A class="" href="summary.html#class:dialog_group">dialog_group</A> 
(a super-class of <A class="" href="summary.html#class:tab">tab</A>) is 
not in the same branch of the inheritance hierarchy as <A class="" href="summary.html#class:dialog_item">dialog_item</A>.

<P></DD>
</DL>

<P>In the current implementation, window and frame labels are not 
covered by this schema.

<P>
<LI><I>Error messages</I><BR>
Although it is convenient to present error messages directly using the 
report mechanism described in <A class="sec" href="report.html">section 
10.7</A>, this approach is not very well suited for multi-lingual 
applications. A better approach is to use <A class="" href="summary.html#class:error">error</A> 
objects, as described in
<A class="sec" href="errors.html">section 10.8</A>.

<P>Using error objects is slightly more cumbersome as errors need to be 
declared separately, but they improve the embedding possibilities using 
error handling, and the mapping from an error identifier to a format 
string provides the indirection necessary in multi-lingual applications.

<P>
<LI><I>Other messages and help text</I><BR>
There is no special support for other textual information, help-texts, 
etc.
</UL>

<P>Below is a summary of the file <CODE>language.pl</CODE> as using in 
the optics simulator to reach at an English/Dutch application.

<P>
<HR>

<PRE class="code">
:- module(language,
          [ message/2,                  % Id, Message
            current_language/1,         % -Language
            set_language/1              % +Language
          ]).
:- use_module(pce).
:- use_module(configdb).
:- require([ concat_atom/2
           , is_list/1
           , memberchk/2
           ]).

:- dynamic
        current_language/1.

current_language(english).              % the default
%current_language(dutch).

set_language(Lan) :-
        retractall(current_language(_)),
        assert(current_language(Lan)),
        make_errors.

%       message(+Term, -Translation)
%       The heart of the translator. Map a term
%       (normally an atom, but if can be an arbitrary
%       Prolog term, into an image or atom. If no
%       translation is found, the default case and
%       underscore translation is performed.

message(Term, Translation) :-
        current_language(Lan),
        term(Term, Translations),
        (   is_list(Translations)
        -&gt;  T =.. [Lan, Translation0],
            memberchk(T, Translations),
            (   is_list(Translation0)
            -&gt;  concat_atom(Translation0, Translation)
            ;   Translation = Translation0
            )
        ;   Translation = Translations
        ), !.
message(Term, Translation) :-
        get(Term, label_name, Translation).


                 /*******************************
                 *    MAP DIALOG IDENTIFIERS    *
                 *******************************/

:- pce_extend_class(dialog_item).

label_name(DI, Id:name, Label:'name|image') :&lt;-
        "Multi-lingual label service"::
        message(Id, Label0),
        (   atomic(Label0)
        -&gt;  get(DI, label_suffix, Suffix),
            get(Label0, ensure_suffix, Suffix, Label)
        ;   Label = Label0
        ).

:- pce_end_class.

:- pce_extend_class(dialog_group).

label_name(_DI, Id:name, Label:name) :&lt;-
        "Multi-lingual label service"::
        (   message(Id, Label),
            atomic(Label)
        -&gt;  true
        ;   get(Id, label_name, Label)
        ).

:- pce_end_class.

:- pce_extend_class(menu_item).

default_label(_MI, Id:name, Label:'name|image') :&lt;-
        "Multilingual label service"::
        message(Id, Label).

:- pce_end_class.

                 /*******************************
                 *        GENERIC LABELS        *
                 *******************************/

%       term(+Term, -Translated)
%
%       Term translates a term. There are three examples
%       here. The first only contains the translation
%       for an English label name into a Dutch one. The
%       second replaces all labels named 
%       image. The last is for generating a more
%       elaborate message from an identifier.

term(settings,
     [ dutch('Instellingen')
     ]).
term(label,
     image('label.lbl')).
term(start_named_test(Name),
     [ english(['Click "OK" to start test "', Name, '"']),
       dutch(['Klik op "OK" om aan de toets "', Name,
              '" te beginnen'])
     ]).
     

                 /*******************************
                 *             ERRORS           *
                 *******************************/

%       error(Id, Kind, Translations)
%
%       Specify and create the required error messages.
%       An object that detects there are too many
%       instruments directs this information to the user
%       by
%
%               ...
%               send(MySelf, error, max_instruments, 5),
%               ...

error(max_instruments, error,
      [ dutch('%IU kunt niet meer dan %d van deze \
               instrumenten gebruiken'),
        english('%IYou can not use more than %d of \
                 these instruments')
      ]).

make_errors :-
        current_language(Lan),
        T =.. [Lan, Message],
        error(Id, Kind, Messages),
        (   memberchk(T, Messages)
        -&gt;  true
        ;   Message = Id
        ),
        new(_E, error(Id, Message, Kind, report)),
        fail.
make_errors.

:- initialization make_errors.</PRE>

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