1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2010-2025, VU University Amsterdam
    7                              CWI, Amsterdam
    8                              SWI-Prolog Solutions b.v.
    9    All rights reserved.
   10
   11    Redistribution and use in source and binary forms, with or without
   12    modification, are permitted provided that the following conditions
   13    are met:
   14
   15    1. Redistributions of source code must retain the above copyright
   16       notice, this list of conditions and the following disclaimer.
   17
   18    2. Redistributions in binary form must reproduce the above copyright
   19       notice, this list of conditions and the following disclaimer in
   20       the documentation and/or other materials provided with the
   21       distribution.
   22
   23    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   24    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   25    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   26    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   27    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   28    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   29    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   30    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   31    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   32    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   33    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   34    POSSIBILITY OF SUCH DAMAGE.
   35*/
   36
   37:- module(ansi_term,
   38          [ ansi_format/3,              % +Attr, +Format, +Args
   39            ansi_get_color/2,           % +Which, -rgb(R,G,B)
   40            ansi_hyperlink/2,           % +Stream,+Location
   41            ansi_hyperlink/3            % +Stream,+URL,+Label
   42          ]).   43:- autoload(library(error), [domain_error/2, must_be/2, instantiation_error/1]).   44:- autoload(library(lists), [append/3]).   45:- autoload(library(utf8), [utf8_codes/3]).   46
   47/** <module> Print decorated text to ANSI consoles
   48
   49This library allows for exploiting the color and attribute facilities of
   50most modern terminals using ANSI escape sequences. This library provides
   51the following:
   52
   53  - ansi_format/3 allows writing messages to the terminal with ansi
   54    attributes.
   55  - It defines the hook prolog:message_line_element/2, which provides
   56    ansi attributes and hyperlinks for print_message/2.
   57
   58The behavior of this library is controlled by two Prolog flags:
   59
   60  - `color_term`
   61    When `true`, activate the color output for this library.  Otherwise
   62    simply call format/3.
   63  - `hyperlink_term`
   64    Emit terminal hyperlinks for url(Location) and url(URL, Label)
   65    elements of Prolog messages.
   66
   67@see    http://en.wikipedia.org/wiki/ANSI_escape_code
   68*/
   69
   70:- multifile
   71    prolog:console_color/2,                     % +Term, -AnsiAttrs
   72    supports_get_color/0,
   73    hyperlink/2,                                % +Stream, +Spec
   74    tty_url_hook/2.                             % +For, -URL
   75
   76color_term_flag_default(true) :-
   77    stream_property(user_input, tty(true)),
   78    stream_property(user_error, tty(true)),
   79    stream_property(user_output, tty(true)),
   80    \+ getenv('TERM', dumb),
   81    !.
   82color_term_flag_default(false).
   83
   84init_color_term_flag :-
   85    color_term_flag_default(Default),
   86    create_prolog_flag(color_term, Default,
   87                       [ type(boolean),
   88                         keep(true)
   89                       ]),
   90    create_prolog_flag(hyperlink_term, false,
   91                       [ type(boolean),
   92                         keep(true)
   93                       ]).
   94
   95:- init_color_term_flag.   96
   97
   98:- meta_predicate
   99    keep_line_pos(+, 0).  100
  101:- multifile
  102    user:message_property/2.  103
  104%!  ansi_format(+ClassOrAttributes, +Format, +Args) is det.
  105%
  106%   Format text with ANSI  attributes.   This  predicate  behaves as
  107%   format/2 using Format and Args, but if the =current_output= is a
  108%   terminal, it adds ANSI escape sequences according to Attributes.
  109%   For example, to print a text in bold cyan, do
  110%
  111%     ==
  112%     ?- ansi_format([bold,fg(cyan)], 'Hello ~w', [world]).
  113%     ==
  114%
  115%   Attributes is either a single attribute, a   list  thereof or a term
  116%   that is mapped to concrete  attributes   based  on the current theme
  117%   (see prolog:console_color/2). The attribute names   are derived from
  118%   the ANSI specification. See the source   for sgr_code/2 for details.
  119%   Some commonly used attributes are:
  120%
  121%     - bold
  122%     - underline
  123%     - fg(Color), bg(Color), hfg(Color), hbg(Color)
  124%       For fg(Color) and bg(Color), the colour name can be '#RGB' or
  125%       '#RRGGBB'
  126%     - fg8(Spec), bg8(Spec)
  127%       8-bit color specification.  Spec is a colour name, h(Color)
  128%       or an integer 0..255.
  129%     - fg(R,G,B), bg(R,G,B)
  130%       24-bit (direct color) specification.  The components are
  131%       integers in the range 0..255.
  132%
  133%   Defined color constants are below.  =default=   can  be  used to
  134%   access the default color of the terminal.
  135%
  136%     - black, red, green, yellow, blue, magenta, cyan, white
  137%
  138%   ANSI sequences are sent if and only if
  139%
  140%     - The =current_output= has the property tty(true) (see
  141%       stream_property/2).
  142%     - The Prolog flag =color_term= is =true=.
  143
  144ansi_format(Attr, Format, Args) :-
  145    ansi_format(current_output, Attr, Format, Args).
  146
  147ansi_format(Stream, Class, Format, Args) :-
  148    stream_property(Stream, tty(true)),
  149    current_prolog_flag(color_term, true),
  150    !,
  151    class_attrs(Class, Attr),
  152    phrase(sgr_codes_ex(Attr), Codes),
  153    atomic_list_concat(Codes, ;, Code),
  154    with_output_to(
  155        Stream,
  156        (   keep_line_pos(current_output, format('\e[~wm', [Code])),
  157            format(Format, Args),
  158            keep_line_pos(current_output, format('\e[0m'))
  159        )
  160    ),
  161    flush_output.
  162ansi_format(Stream, _Attr, Format, Args) :-
  163    format(Stream, Format, Args).
  164
  165sgr_codes_ex(X) -->
  166    { var(X),
  167      !,
  168      instantiation_error(X)
  169    }.
  170sgr_codes_ex([]) -->
  171    !.
  172sgr_codes_ex([H|T]) -->
  173    !,
  174    sgr_codes_ex(H),
  175    sgr_codes_ex(T).
  176sgr_codes_ex(Attr) -->
  177    (   { sgr_code(Attr, Code) }
  178    ->  (   { is_list(Code) }
  179        ->  list(Code)
  180        ;   [Code]
  181        )
  182    ;   { domain_error(sgr_code, Attr) }
  183    ).
  184
  185list([]) --> [].
  186list([H|T]) --> [H], list(T).
  187
  188
  189%!  sgr_code(+Name, -Code)
  190%
  191%   True when code is the Select   Graphic  Rendition code for Name.
  192%   The defined names are given below. Note that most terminals only
  193%   implement this partially.
  194%
  195%     | reset                       | all attributes off    |
  196%     | bold                        |                       |
  197%     | faint                       |       |
  198%     | italic                      |       |
  199%     | underline                   |       |
  200%     | blink(slow)                 |       |
  201%     | blink(rapid)                |       |
  202%     | negative                    |       |
  203%     | conceal                     |       |
  204%     | crossed_out                 |       |
  205%     | font(primary)               |       |
  206%     | font(N)                     | Alternate font (1..8) |
  207%     | fraktur                     |       |
  208%     | underline(double)           |       |
  209%     | intensity(normal)           |       |
  210%     | fg(Name)                    | Color name    |
  211%     | bg(Name)                    | Color name    |
  212%     | framed                      |       |
  213%     | encircled                   |       |
  214%     | overlined                   |       |
  215%     | ideogram(underline)         |       |
  216%     | right_side_line             |       |
  217%     | ideogram(underline(double)) |       |
  218%     | right_side_line(double)     |       |
  219%     | ideogram(overlined)         |       |
  220%     | left_side_line              |       |
  221%     | ideogram(stress_marking)    |       |
  222%     | -Off                        | Switch attributes off |
  223%     | hfg(Name)                   | Color name    |
  224%     | hbg(Name)                   | Color name    |
  225%
  226%   @see http://en.wikipedia.org/wiki/ANSI_escape_code
  227
  228sgr_code(reset, 0).
  229sgr_code(bold,  1).
  230sgr_code(faint, 2).
  231sgr_code(italic, 3).
  232sgr_code(underline, 4).
  233sgr_code(blink(slow), 5).
  234sgr_code(blink(rapid), 6).
  235sgr_code(negative, 7).
  236sgr_code(conceal, 8).
  237sgr_code(crossed_out, 9).
  238sgr_code(font(primary), 10) :- !.
  239sgr_code(font(N), C) :-
  240    C is 10+N.
  241sgr_code(fraktur, 20).
  242sgr_code(underline(double), 21).
  243sgr_code(intensity(normal), 22).
  244sgr_code(fg(Name), C) :-
  245    (   ansi_color(Name, N)
  246    ->  C is N+30
  247    ;   rgb(Name, R, G, B)
  248    ->  sgr_code(fg(R,G,B), C)
  249    ).
  250sgr_code(bg(Name), C) :-
  251    !,
  252    (   ansi_color(Name, N)
  253    ->  C is N+40
  254    ;   rgb(Name, R, G, B)
  255    ->  sgr_code(bg(R,G,B), C)
  256    ).
  257sgr_code(framed, 51).
  258sgr_code(encircled, 52).
  259sgr_code(overlined, 53).
  260sgr_code(ideogram(underline), 60).
  261sgr_code(right_side_line, 60).
  262sgr_code(ideogram(underline(double)), 61).
  263sgr_code(right_side_line(double), 61).
  264sgr_code(ideogram(overlined), 62).
  265sgr_code(left_side_line, 62).
  266sgr_code(ideogram(stress_marking), 64).
  267sgr_code(-X, Code) :-
  268    off_code(X, Code).
  269sgr_code(hfg(Name), C) :-
  270    ansi_color(Name, N),
  271    C is N+90.
  272sgr_code(hbg(Name), C) :-
  273    !,
  274    ansi_color(Name, N),
  275    C is N+100.
  276sgr_code(fg8(Name), [38,5,N]) :-
  277    ansi_color8(Name, N).
  278sgr_code(bg8(Name), [48,5,N]) :-
  279    ansi_color8(Name, N).
  280sgr_code(fg(R,G,B), [38,2,R,G,B]) :-
  281    between(0, 255, R),
  282    between(0, 255, G),
  283    between(0, 255, B).
  284sgr_code(bg(R,G,B), [48,2,R,G,B]) :-
  285    between(0, 255, R),
  286    between(0, 255, G),
  287    between(0, 255, B).
  288
  289off_code(italic_and_franktur, 23).
  290off_code(underline, 24).
  291off_code(blink, 25).
  292off_code(negative, 27).
  293off_code(conceal, 28).
  294off_code(crossed_out, 29).
  295off_code(framed, 54).
  296off_code(overlined, 55).
  297
  298ansi_color8(h(Name), N) :-
  299    !,
  300    ansi_color(Name, N0),
  301    N is N0+8.
  302ansi_color8(Name, N) :-
  303    atom(Name),
  304    !,
  305    ansi_color(Name, N).
  306ansi_color8(N, N) :-
  307    between(0, 255, N).
  308
  309ansi_color(black,   0).
  310ansi_color(red,     1).
  311ansi_color(green,   2).
  312ansi_color(yellow,  3).
  313ansi_color(blue,    4).
  314ansi_color(magenta, 5).
  315ansi_color(cyan,    6).
  316ansi_color(white,   7).
  317ansi_color(default, 9).
  318
  319rgb(Name, R, G, B) :-
  320    atom_codes(Name, [0'#,R1,R2,G1,G2,B1,B2]),
  321    hex_color(R1,R2,R),
  322    hex_color(G1,G2,G),
  323    hex_color(B1,B2,B).
  324rgb(Name, R, G, B) :-
  325    atom_codes(Name, [0'#,R1,G1,B1]),
  326    hex_color(R1,R),
  327    hex_color(G1,G),
  328    hex_color(B1,B).
  329
  330hex_color(D1,D2,V) :-
  331    code_type(D1, xdigit(V1)),
  332    code_type(D2, xdigit(V2)),
  333    V is 16*V1+V2.
  334
  335hex_color(D1,V) :-
  336    code_type(D1, xdigit(V1)),
  337    V is 16*V1+V1.
  338
  339%!  prolog:console_color(+Term, -AnsiAttributes) is semidet.
  340%
  341%   Hook that allows  for  mapping  abstract   terms  to  concrete  ANSI
  342%   attributes. This hook  is  used  by   _theme_  files  to  adjust the
  343%   rendering based on  user  preferences   and  context.  Defaults  are
  344%   defined in the file `boot/messages.pl`, default_theme/2.
  345%
  346%   @see library(theme/dark) for an example  implementation and the Term
  347%   values used by the system messages.
  348
  349
  350                 /*******************************
  351                 *             HOOK             *
  352                 *******************************/
  353
  354%!  prolog:message_line_element(+Stream, +Term) is semidet.
  355%
  356%   Hook implementation that deals with  ansi(+Attr, +Fmt, +Args) in
  357%   message specifications.
  358
  359prolog:message_line_element(S, ansi(Class, Fmt, Args)) :-
  360    class_attrs(Class, Attr),
  361    ansi_format(S, Attr, Fmt, Args).
  362prolog:message_line_element(S, ansi(Class, Fmt, Args, Ctx)) :-
  363    class_attrs(Class, Attr),
  364    ansi_format(S, Attr, Fmt, Args),
  365    (   nonvar(Ctx),
  366        Ctx = ansi(_, RI-RA)
  367    ->  keep_line_pos(S, format(S, RI, RA))
  368    ;   true
  369    ).
  370prolog:message_line_element(S, url(Location)) :-
  371    ansi_hyperlink(S, Location).
  372prolog:message_line_element(S, url(URL, Label)) :-
  373    ansi_hyperlink(S, URL, Label).
  374prolog:message_line_element(S, begin(Level, Ctx)) :-
  375    level_attrs(Level, Attr),
  376    stream_property(S, tty(true)),
  377    current_prolog_flag(color_term, true),
  378    !,
  379    (   is_list(Attr)
  380    ->  sgr_codes(Attr, Codes),
  381        atomic_list_concat(Codes, ;, Code)
  382    ;   sgr_code(Attr, Code)
  383    ),
  384    keep_line_pos(S, format(S, '\e[~wm', [Code])),
  385    Ctx = ansi('\e[0m', '\e[0m\e[~wm'-[Code]).
  386prolog:message_line_element(S, end(Ctx)) :-
  387    nonvar(Ctx),
  388    Ctx = ansi(Reset, _),
  389    keep_line_pos(S, write(S, Reset)).
  390
  391sgr_codes([], []).
  392sgr_codes([H0|T0], [H|T]) :-
  393    sgr_code(H0, H),
  394    sgr_codes(T0, T).
  395
  396level_attrs(Level,         Attrs) :-
  397    user:message_property(Level, color(Attrs)),
  398    !.
  399level_attrs(Level,         Attrs) :-
  400    class_attrs(message(Level), Attrs).
  401
  402class_attrs(Class, Attrs) :-
  403    user:message_property(Class, color(Attrs)),
  404    !.
  405class_attrs(Class, Attrs) :-
  406    prolog:console_color(Class, Attrs),
  407    !.
  408class_attrs(Class, Attrs) :-
  409    '$messages':default_theme(Class, Attrs),
  410    !.
  411class_attrs(Attrs, Attrs).
  412
  413%!  ansi_hyperlink(+Stream, +Location) is det.
  414%!  ansi_hyperlink(+Stream, +URL, +Label) is det.
  415%
  416%   Create a hyperlink for a terminal emulator. The file is fairly easy,
  417%   but getting the line and column across is   not as there seems to be
  418%   no established standard. The  current   implementation  emits, i.e.,
  419%   inserting a capital ``L`` before the line.
  420%
  421%       ``file://AbsFileName[#LLine[:Column]]``
  422%
  423%   @see https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda
  424
  425ansi_hyperlink(Stream, Location) :-
  426    hyperlink(Stream, url(Location)),
  427    !.
  428ansi_hyperlink(Stream, Location) :-
  429    location_label(Location, Label),
  430    ansi_hyperlink(Stream, Location, Label).
  431
  432location_label(File:Line:Column, Label) =>
  433    format(string(Label), '~w:~w:~w', [File,Line,Column]).
  434location_label(File:Line, Label) =>
  435    format(string(Label), '~w:~w', [File,Line]).
  436location_label(File, Label) =>
  437    format(string(Label), '~w', [File]).
  438
  439ansi_hyperlink(Stream, Location, Label),
  440    hyperlink(Stream, url(Location, Label)) =>
  441    true.
  442ansi_hyperlink(Stream, Location, Label) =>
  443    (   location_url(Location, URL)
  444    ->  keep_line_pos(Stream,
  445                      format(Stream, '\e]8;;~w\e\\', [URL])),
  446        format(Stream, '~w', [Label]),
  447        keep_line_pos(Stream,
  448                      format(Stream, '\e]8;;\e\\', []))
  449    ;   format(Stream, '~w', [Label])
  450    ).
  451
  452is_url(URL) :-
  453    (   atom(URL)
  454    ->  true
  455    ;   string(URL)
  456    ),
  457    url_prefix(Prefix),
  458    sub_string(URL, 0, _, _, Prefix).
  459
  460url_prefix('http://').
  461url_prefix('https://').
  462url_prefix('file://').
  463
  464%!  location_url(+Location, -URL) is det.
  465%
  466%   Translate Location into a (file) URL.   This  predicate is hooked by
  467%   tty_url_hook/2  with  the  same  signature  to  allow  for  actions,
  468%   location specifiers or URL schemes.
  469
  470location_url(Location, URL),
  471    tty_url_hook(Location, URL0) =>
  472    URL = URL0.
  473location_url(File:Line:Column, URL) =>
  474    url_file_name(FileURL, File),
  475    format(string(URL), '~w#~d:~d', [FileURL, Line, Column]).
  476location_url(File:Line, URL) =>
  477    url_file_name(FileURL, File),
  478    format(string(URL), '~w#~w', [FileURL, Line]).
  479location_url(File, URL) =>
  480    url_file_name(URL, File).
  481
  482%!  tty_url_hook(+Location, -URL)
  483%
  484%   Hook for location_url/2.
  485
  486
  487%!  url_file_name(-URL, +File) is semidet.
  488%
  489%   Same as uri_file_name/2 in mode (-,+), but   as a core library we do
  490%   not wish to depend on the `clib` package and its foreign support.
  491
  492url_file_name(URL, File) :-
  493    is_url(File), !,
  494    current_prolog_flag(hyperlink_term, true),
  495    URL = File.
  496url_file_name(URL, File) :-
  497    current_prolog_flag(hyperlink_term, true),
  498    absolute_file_name(File, AbsFile),
  499    ensure_leading_slash(AbsFile, AbsFile1),
  500    url_encode_path(AbsFile1, Encoded),
  501    format(string(URL), 'file://~s', [Encoded]).
  502
  503ensure_leading_slash(Path, SlashPath) :-
  504    (   sub_atom(Path, 0, _, _, /)
  505    ->  SlashPath = Path
  506    ;   atom_concat(/, Path, SlashPath)
  507    ).
  508
  509url_encode_path(Name, Encoded) :-
  510    atom_codes(Name, Codes),
  511    phrase(utf8_codes(Codes), UTF8),
  512    phrase(encode(UTF8), Encoded).
  513
  514encode([]) --> [].
  515encode([H|T]) --> encode1(H), encode(T).
  516
  517encode1(C) -->
  518    { reserved(C),
  519      !,
  520      format(codes([C1,C2]), '~`0t~16r~2|', [C])
  521    },
  522    "%", [C1,C2].
  523encode1(C) -->
  524    [C].
  525
  526reserved(C) :- C =< 0'\s.
  527reserved(C) :- C >= 127.
  528reserved(0'#).
  529
  530%!  keep_line_pos(+Stream, :Goal)
  531%
  532%   Run goal without changing the position   information on Stream. This
  533%   is used to avoid that the exchange   of  ANSI sequences modifies the
  534%   notion of, notably, the `line_pos` notion.
  535
  536keep_line_pos(S, G) :-
  537    stream_property(S, position(Pos)),
  538    !,
  539    setup_call_cleanup(
  540        stream_position_data(line_position, Pos, LPos),
  541        G,
  542        set_stream(S, line_position(LPos))).
  543keep_line_pos(_, G) :-
  544    call(G).
  545
  546%!  ansi_get_color(+Which, -RGB) is semidet.
  547%
  548%   Obtain the RGB color for an ANSI  color parameter. Which is either a
  549%   color alias or  an  integer  ANSI   color  id.  Defined  aliases are
  550%   `foreground` and `background`. This predicate sends a request to the
  551%   console (`user_output`) and reads the reply. This assumes an `xterm`
  552%   compatible terminal.
  553%
  554%   @arg RGB is a term rgb(Red,Green,Blue).  The color components are
  555%   integers in the range 0..65535.
  556
  557ansi_get_color(Which0, RGB) :-
  558    \+ current_prolog_flag(console_menu, true),
  559    stream_property(user_input, tty(true)),
  560    stream_property(user_output, tty(true)),
  561    stream_property(user_error, tty(true)),
  562    supports_get_color,
  563    (   color_alias(Which0, Which)
  564    ->  true
  565    ;   must_be(between(0,15),Which0)
  566    ->  Which = Which0
  567    ),
  568    catch(keep_line_pos(user_output,
  569                        ansi_get_color_(Which, RGB)),
  570          error(timeout_error(_,_), _),
  571          no_xterm).
  572
  573supports_get_color :-
  574    getenv('TERM', Term),
  575    sub_atom(Term, 0, _, _, xterm),
  576    \+ getenv('TERM_PROGRAM', 'Apple_Terminal').
  577
  578color_alias(foreground, 10).
  579color_alias(background, 11).
  580
  581ansi_get_color_(Which, rgb(R,G,B)) :-
  582    format(codes(Id), '~w', [Which]),
  583    hex4(RH),
  584    hex4(GH),
  585    hex4(BH),
  586    phrase(("\e]", Id, ";rgb:", RH, "/", GH, "/", BH, "\a"), Pattern),
  587    stream_property(user_input, timeout(Old)),
  588    setup_call_cleanup(
  589        set_stream(user_input, timeout(0.05)),
  590        with_tty_raw(exchange_pattern(Which, Pattern)),
  591        set_stream(user_input, timeout(Old))),
  592    !,
  593    hex_val(RH, R),
  594    hex_val(GH, G),
  595    hex_val(BH, B).
  596
  597no_xterm :-
  598    print_message(warning, ansi(no_xterm_get_colour)),
  599    fail.
  600
  601hex4([_,_,_,_]).
  602
  603hex_val([D1,D2,D3,D4], V) :-
  604    code_type(D1, xdigit(V1)),
  605    code_type(D2, xdigit(V2)),
  606    code_type(D3, xdigit(V3)),
  607    code_type(D4, xdigit(V4)),
  608    V is (V1<<12)+(V2<<8)+(V3<<4)+V4.
  609
  610exchange_pattern(Which, Pattern) :-
  611    format(user_output, '\e]~w;?\a', [Which]),
  612    flush_output(user_output),
  613    read_pattern(user_input, Pattern, []).
  614
  615read_pattern(From, Pattern, NotMatched0) :-
  616    copy_term(Pattern, TryPattern),
  617    append(Skip, Rest, NotMatched0),
  618    append(Rest, RestPattern, TryPattern),
  619    !,
  620    echo(Skip),
  621    try_read_pattern(From, RestPattern, NotMatched, Done),
  622    (   Done == true
  623    ->  Pattern = TryPattern
  624    ;   read_pattern(From, Pattern, NotMatched)
  625    ).
  626
  627%!  try_read_pattern(+From, +Pattern, -NotMatched)
  628
  629try_read_pattern(_, [], [], true) :-
  630    !.
  631try_read_pattern(From, [H|T], [C|RT], Done) :-
  632    get_code(C),
  633    (   C = H
  634    ->  try_read_pattern(From, T, RT, Done)
  635    ;   RT = [],
  636        Done = false
  637    ).
  638
  639echo([]).
  640echo([H|T]) :-
  641    put_code(user_output, H),
  642    echo(T).
  643
  644:- multifile prolog:message//1.  645
  646prolog:message(ansi(no_xterm_get_colour)) -->
  647    [ 'Terminal claims to be xterm compatible,'-[], nl,
  648      'but does not report colour info'-[]
  649    ]