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    class_attrs(Class, Attr),
  151    Attr \== [],
  152    !,
  153    phrase(sgr_codes_ex(Attr), Codes),
  154    atomic_list_concat(Codes, ;, Code),
  155    with_output_to(
  156        Stream,
  157        (   keep_line_pos(current_output, format('\e[~wm', [Code])),
  158            format(Format, Args),
  159            keep_line_pos(current_output, format('\e[0m'))
  160        )
  161    ),
  162    flush_output.
  163ansi_format(Stream, _Attr, Format, Args) :-
  164    format(Stream, Format, Args).
  165
  166sgr_codes_ex(X) -->
  167    { var(X),
  168      !,
  169      instantiation_error(X)
  170    }.
  171sgr_codes_ex([]) -->
  172    !.
  173sgr_codes_ex([H|T]) -->
  174    !,
  175    sgr_codes_ex(H),
  176    sgr_codes_ex(T).
  177sgr_codes_ex(Attr) -->
  178    (   { sgr_code(Attr, Code) }
  179    ->  (   { is_list(Code) }
  180        ->  list(Code)
  181        ;   [Code]
  182        )
  183    ;   { domain_error(sgr_code, Attr) }
  184    ).
  185
  186list([]) --> [].
  187list([H|T]) --> [H], list(T).
  188
  189
  190%!  sgr_code(+Name, -Code)
  191%
  192%   True when code is the Select   Graphic  Rendition code for Name.
  193%   The defined names are given below. Note that most terminals only
  194%   implement this partially.
  195%
  196%     | reset                       | all attributes off    |
  197%     | bold                        |                       |
  198%     | faint                       |       |
  199%     | italic                      |       |
  200%     | underline                   |       |
  201%     | blink(slow)                 |       |
  202%     | blink(rapid)                |       |
  203%     | negative                    |       |
  204%     | conceal                     |       |
  205%     | crossed_out                 |       |
  206%     | font(primary)               |       |
  207%     | font(N)                     | Alternate font (1..8) |
  208%     | fraktur                     |       |
  209%     | underline(double)           |       |
  210%     | intensity(normal)           |       |
  211%     | fg(Name)                    | Color name    |
  212%     | bg(Name)                    | Color name    |
  213%     | framed                      |       |
  214%     | encircled                   |       |
  215%     | overlined                   |       |
  216%     | ideogram(underline)         |       |
  217%     | right_side_line             |       |
  218%     | ideogram(underline(double)) |       |
  219%     | right_side_line(double)     |       |
  220%     | ideogram(overlined)         |       |
  221%     | left_side_line              |       |
  222%     | ideogram(stress_marking)    |       |
  223%     | -Off                        | Switch attributes off |
  224%     | hfg(Name)                   | Color name    |
  225%     | hbg(Name)                   | Color name    |
  226%
  227%   @see http://en.wikipedia.org/wiki/ANSI_escape_code
  228
  229sgr_code(reset, 0).
  230sgr_code(bold,  1).
  231sgr_code(faint, 2).
  232sgr_code(italic, 3).
  233sgr_code(underline, 4).
  234sgr_code(blink(slow), 5).
  235sgr_code(blink(rapid), 6).
  236sgr_code(negative, 7).
  237sgr_code(conceal, 8).
  238sgr_code(crossed_out, 9).
  239sgr_code(font(primary), 10) :- !.
  240sgr_code(font(N), C) :-
  241    C is 10+N.
  242sgr_code(fraktur, 20).
  243sgr_code(underline(double), 21).
  244sgr_code(intensity(normal), 22).
  245sgr_code(fg(Name), C) :-
  246    (   ansi_color(Name, N)
  247    ->  C is N+30
  248    ;   rgb(Name, R, G, B)
  249    ->  sgr_code(fg(R,G,B), C)
  250    ).
  251sgr_code(bg(Name), C) :-
  252    !,
  253    (   ansi_color(Name, N)
  254    ->  C is N+40
  255    ;   rgb(Name, R, G, B)
  256    ->  sgr_code(bg(R,G,B), C)
  257    ).
  258sgr_code(framed, 51).
  259sgr_code(encircled, 52).
  260sgr_code(overlined, 53).
  261sgr_code(ideogram(underline), 60).
  262sgr_code(right_side_line, 60).
  263sgr_code(ideogram(underline(double)), 61).
  264sgr_code(right_side_line(double), 61).
  265sgr_code(ideogram(overlined), 62).
  266sgr_code(left_side_line, 62).
  267sgr_code(ideogram(stress_marking), 64).
  268sgr_code(-X, Code) :-
  269    off_code(X, Code).
  270sgr_code(hfg(Name), C) :-
  271    ansi_color(Name, N),
  272    C is N+90.
  273sgr_code(hbg(Name), C) :-
  274    !,
  275    ansi_color(Name, N),
  276    C is N+100.
  277sgr_code(fg8(Name), [38,5,N]) :-
  278    ansi_color8(Name, N).
  279sgr_code(bg8(Name), [48,5,N]) :-
  280    ansi_color8(Name, N).
  281sgr_code(fg(R,G,B), [38,2,R,G,B]) :-
  282    between(0, 255, R),
  283    between(0, 255, G),
  284    between(0, 255, B).
  285sgr_code(bg(R,G,B), [48,2,R,G,B]) :-
  286    between(0, 255, R),
  287    between(0, 255, G),
  288    between(0, 255, B).
  289
  290off_code(italic_and_franktur, 23).
  291off_code(underline, 24).
  292off_code(blink, 25).
  293off_code(negative, 27).
  294off_code(conceal, 28).
  295off_code(crossed_out, 29).
  296off_code(framed, 54).
  297off_code(overlined, 55).
  298
  299ansi_color8(h(Name), N) :-
  300    !,
  301    ansi_color(Name, N0),
  302    N is N0+8.
  303ansi_color8(Name, N) :-
  304    atom(Name),
  305    !,
  306    ansi_color(Name, N).
  307ansi_color8(N, N) :-
  308    between(0, 255, N).
  309
  310ansi_color(black,   0).
  311ansi_color(red,     1).
  312ansi_color(green,   2).
  313ansi_color(yellow,  3).
  314ansi_color(blue,    4).
  315ansi_color(magenta, 5).
  316ansi_color(cyan,    6).
  317ansi_color(white,   7).
  318ansi_color(default, 9).
  319
  320rgb(Name, R, G, B) :-
  321    atom_codes(Name, [0'#,R1,R2,G1,G2,B1,B2]),
  322    hex_color(R1,R2,R),
  323    hex_color(G1,G2,G),
  324    hex_color(B1,B2,B).
  325rgb(Name, R, G, B) :-
  326    atom_codes(Name, [0'#,R1,G1,B1]),
  327    hex_color(R1,R),
  328    hex_color(G1,G),
  329    hex_color(B1,B).
  330
  331hex_color(D1,D2,V) :-
  332    code_type(D1, xdigit(V1)),
  333    code_type(D2, xdigit(V2)),
  334    V is 16*V1+V2.
  335
  336hex_color(D1,V) :-
  337    code_type(D1, xdigit(V1)),
  338    V is 16*V1+V1.
  339
  340%!  prolog:console_color(+Term, -AnsiAttributes) is semidet.
  341%
  342%   Hook that allows  for  mapping  abstract   terms  to  concrete  ANSI
  343%   attributes. This hook  is  used  by   _theme_  files  to  adjust the
  344%   rendering based on  user  preferences   and  context.  Defaults  are
  345%   defined in the file `boot/messages.pl`, default_theme/2.
  346%
  347%   @see library(theme/dark) for an example  implementation and the Term
  348%   values used by the system messages.
  349
  350
  351                 /*******************************
  352                 *             HOOK             *
  353                 *******************************/
  354
  355%!  prolog:message_line_element(+Stream, +Term) is semidet.
  356%
  357%   Hook implementation that deals with  ansi(+Attr, +Fmt, +Args) in
  358%   message specifications.
  359
  360prolog:message_line_element(S, ansi(Class, Fmt, Args)) :-
  361    class_attrs(Class, Attr),
  362    ansi_format(S, Attr, Fmt, Args).
  363prolog:message_line_element(S, ansi(Class, Fmt, Args, Ctx)) :-
  364    class_attrs(Class, Attr),
  365    ansi_format(S, Attr, Fmt, Args),
  366    (   nonvar(Ctx),
  367        Ctx = ansi(_, RI-RA)
  368    ->  keep_line_pos(S, format(S, RI, RA))
  369    ;   true
  370    ).
  371prolog:message_line_element(S, url(Location)) :-
  372    ansi_hyperlink(S, Location).
  373prolog:message_line_element(S, url(URL, Label)) :-
  374    ansi_hyperlink(S, URL, Label).
  375prolog:message_line_element(S, begin(Level, Ctx)) :-
  376    level_attrs(Level, Attr),
  377    stream_property(S, tty(true)),
  378    current_prolog_flag(color_term, true),
  379    !,
  380    (   is_list(Attr)
  381    ->  sgr_codes(Attr, Codes),
  382        atomic_list_concat(Codes, ;, Code)
  383    ;   sgr_code(Attr, Code)
  384    ),
  385    keep_line_pos(S, format(S, '\e[~wm', [Code])),
  386    Ctx = ansi('\e[0m', '\e[0m\e[~wm'-[Code]).
  387prolog:message_line_element(S, end(Ctx)) :-
  388    nonvar(Ctx),
  389    Ctx = ansi(Reset, _),
  390    keep_line_pos(S, write(S, Reset)).
  391
  392sgr_codes([], []).
  393sgr_codes([H0|T0], [H|T]) :-
  394    sgr_code(H0, H),
  395    sgr_codes(T0, T).
  396
  397level_attrs(Level,         Attrs) :-
  398    user:message_property(Level, color(Attrs)),
  399    !.
  400level_attrs(Level,         Attrs) :-
  401    class_attrs(message(Level), Attrs).
  402
  403class_attrs(Class, Attrs) :-
  404    user:message_property(Class, color(Attrs)),
  405    !.
  406class_attrs(Class, Attrs) :-
  407    prolog:console_color(Class, Attrs),
  408    !.
  409class_attrs(Class, Attrs) :-
  410    '$messages':default_theme(Class, Attrs),
  411    !.
  412class_attrs(Attrs, Attrs).
  413
  414%!  ansi_hyperlink(+Stream, +Location) is det.
  415%!  ansi_hyperlink(+Stream, +URL, +Label) is det.
  416%
  417%   Create a hyperlink for a terminal emulator. The file is fairly easy,
  418%   but getting the line and column across is   not as there seems to be
  419%   no established standard. The  current   implementation  emits, i.e.,
  420%   inserting a capital ``L`` before the line.
  421%
  422%       ``file://AbsFileName[#LLine[:Column]]``
  423%
  424%   @see https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda
  425
  426ansi_hyperlink(Stream, Location) :-
  427    hyperlink(Stream, url(Location)),
  428    !.
  429ansi_hyperlink(Stream, Location) :-
  430    location_label(Location, Label),
  431    ansi_hyperlink(Stream, Location, Label).
  432
  433location_label(File:Line:Column, Label) =>
  434    format(string(Label), '~w:~w:~w', [File,Line,Column]).
  435location_label(File:Line, Label) =>
  436    format(string(Label), '~w:~w', [File,Line]).
  437location_label(File, Label) =>
  438    format(string(Label), '~w', [File]).
  439
  440ansi_hyperlink(Stream, Location, Label),
  441    hyperlink(Stream, url(Location, Label)) =>
  442    true.
  443ansi_hyperlink(Stream, Location, Label) =>
  444    (   location_url(Location, URL)
  445    ->  keep_line_pos(Stream,
  446                      format(Stream, '\e]8;;~w\e\\', [URL])),
  447        format(Stream, '~w', [Label]),
  448        keep_line_pos(Stream,
  449                      format(Stream, '\e]8;;\e\\', []))
  450    ;   format(Stream, '~w', [Label])
  451    ).
  452
  453is_url(URL) :-
  454    (   atom(URL)
  455    ->  true
  456    ;   string(URL)
  457    ),
  458    url_prefix(Prefix),
  459    sub_string(URL, 0, _, _, Prefix).
  460
  461url_prefix('http://').
  462url_prefix('https://').
  463url_prefix('file://').
  464
  465%!  location_url(+Location, -URL) is det.
  466%
  467%   Translate Location into a (file) URL.   This  predicate is hooked by
  468%   tty_url_hook/2  with  the  same  signature  to  allow  for  actions,
  469%   location specifiers or URL schemes.
  470
  471location_url(Location, URL),
  472    tty_url_hook(Location, URL0) =>
  473    URL = URL0.
  474location_url(File:Line:Column, URL) =>
  475    url_file_name(FileURL, File),
  476    format(string(URL), '~w#~d:~d', [FileURL, Line, Column]).
  477location_url(File:Line, URL) =>
  478    url_file_name(FileURL, File),
  479    format(string(URL), '~w#~w', [FileURL, Line]).
  480location_url(File, URL) =>
  481    url_file_name(URL, File).
  482
  483%!  tty_url_hook(+Location, -URL)
  484%
  485%   Hook for location_url/2.
  486
  487
  488%!  url_file_name(-URL, +File) is semidet.
  489%
  490%   Same as uri_file_name/2 in mode (-,+), but   as a core library we do
  491%   not wish to depend on the `clib` package and its foreign support.
  492
  493url_file_name(URL, File) :-
  494    is_url(File), !,
  495    current_prolog_flag(hyperlink_term, true),
  496    URL = File.
  497url_file_name(URL, File) :-
  498    current_prolog_flag(hyperlink_term, true),
  499    absolute_file_name(File, AbsFile),
  500    ensure_leading_slash(AbsFile, AbsFile1),
  501    url_encode_path(AbsFile1, Encoded),
  502    format(string(URL), 'file://~s', [Encoded]).
  503
  504ensure_leading_slash(Path, SlashPath) :-
  505    (   sub_atom(Path, 0, _, _, /)
  506    ->  SlashPath = Path
  507    ;   atom_concat(/, Path, SlashPath)
  508    ).
  509
  510url_encode_path(Name, Encoded) :-
  511    atom_codes(Name, Codes),
  512    phrase(utf8_codes(Codes), UTF8),
  513    phrase(encode(UTF8), Encoded).
  514
  515encode([]) --> [].
  516encode([H|T]) --> encode1(H), encode(T).
  517
  518encode1(C) -->
  519    { reserved(C),
  520      !,
  521      format(codes([C1,C2]), '~`0t~16r~2|', [C])
  522    },
  523    "%", [C1,C2].
  524encode1(C) -->
  525    [C].
  526
  527reserved(C) :- C =< 0'\s.
  528reserved(C) :- C >= 127.
  529reserved(0'#).
  530
  531%!  keep_line_pos(+Stream, :Goal)
  532%
  533%   Run goal without changing the position   information on Stream. This
  534%   is used to avoid that the exchange   of  ANSI sequences modifies the
  535%   notion of, notably, the `line_pos` notion.
  536
  537keep_line_pos(S, G) :-
  538    stream_property(S, position(Pos)),
  539    !,
  540    setup_call_cleanup(
  541        stream_position_data(line_position, Pos, LPos),
  542        G,
  543        set_stream(S, line_position(LPos))).
  544keep_line_pos(_, G) :-
  545    call(G).
  546
  547%!  ansi_get_color(+Which, -RGB) is semidet.
  548%
  549%   Obtain the RGB color for an ANSI  color parameter. Which is either a
  550%   color alias or  an  integer  ANSI   color  id.  Defined  aliases are
  551%   `foreground` and `background`. This predicate sends a request to the
  552%   console (`user_output`) and reads the reply. This assumes an `xterm`
  553%   compatible terminal.
  554%
  555%   @arg RGB is a term rgb(Red,Green,Blue).  The color components are
  556%   integers in the range 0..65535.
  557
  558ansi_get_color(Which0, RGB) :-
  559    \+ current_prolog_flag(console_menu, true),
  560    stream_property(user_input, tty(true)),
  561    stream_property(user_output, tty(true)),
  562    stream_property(user_error, tty(true)),
  563    supports_get_color,
  564    (   color_alias(Which0, Which)
  565    ->  true
  566    ;   must_be(between(0,15),Which0)
  567    ->  Which = Which0
  568    ),
  569    catch(keep_line_pos(user_output,
  570                        ansi_get_color_(Which, RGB)),
  571          error(timeout_error(_,_), _),
  572          no_xterm).
  573
  574supports_get_color :-
  575    getenv('TERM', Term),
  576    sub_atom(Term, 0, _, _, xterm),
  577    \+ getenv('TERM_PROGRAM', 'Apple_Terminal').
  578
  579color_alias(foreground, 10).
  580color_alias(background, 11).
  581
  582ansi_get_color_(Which, rgb(R,G,B)) :-
  583    format(codes(Id), '~w', [Which]),
  584    hex4(RH),
  585    hex4(GH),
  586    hex4(BH),
  587    phrase(("\e]", Id, ";rgb:", RH, "/", GH, "/", BH, "\a"), Pattern),
  588    stream_property(user_input, timeout(Old)),
  589    setup_call_cleanup(
  590        set_stream(user_input, timeout(0.05)),
  591        with_tty_raw(exchange_pattern(Which, Pattern)),
  592        set_stream(user_input, timeout(Old))),
  593    !,
  594    hex_val(RH, R),
  595    hex_val(GH, G),
  596    hex_val(BH, B).
  597
  598no_xterm :-
  599    print_message(warning, ansi(no_xterm_get_colour)),
  600    fail.
  601
  602hex4([_,_,_,_]).
  603
  604hex_val([D1,D2,D3,D4], V) :-
  605    code_type(D1, xdigit(V1)),
  606    code_type(D2, xdigit(V2)),
  607    code_type(D3, xdigit(V3)),
  608    code_type(D4, xdigit(V4)),
  609    V is (V1<<12)+(V2<<8)+(V3<<4)+V4.
  610
  611exchange_pattern(Which, Pattern) :-
  612    format(user_output, '\e]~w;?\a', [Which]),
  613    flush_output(user_output),
  614    read_pattern(user_input, Pattern, []).
  615
  616read_pattern(From, Pattern, NotMatched0) :-
  617    copy_term(Pattern, TryPattern),
  618    append(Skip, Rest, NotMatched0),
  619    append(Rest, RestPattern, TryPattern),
  620    !,
  621    echo(Skip),
  622    try_read_pattern(From, RestPattern, NotMatched, Done),
  623    (   Done == true
  624    ->  Pattern = TryPattern
  625    ;   read_pattern(From, Pattern, NotMatched)
  626    ).
  627
  628%!  try_read_pattern(+From, +Pattern, -NotMatched)
  629
  630try_read_pattern(_, [], [], true) :-
  631    !.
  632try_read_pattern(From, [H|T], [C|RT], Done) :-
  633    get_code(C),
  634    (   C = H
  635    ->  try_read_pattern(From, T, RT, Done)
  636    ;   RT = [],
  637        Done = false
  638    ).
  639
  640echo([]).
  641echo([H|T]) :-
  642    put_code(user_output, H),
  643    echo(T).
  644
  645:- multifile prolog:message//1.  646
  647prolog:message(ansi(no_xterm_get_colour)) -->
  648    [ 'Terminal claims to be xterm compatible,'-[], nl,
  649      'but does not report colour info'-[]
  650    ]