## A.2 library(ansi_term): Print decorated text to ANSI consoles

See also
  http://en.wikipedia.org/wiki/ANSI_escape_code

This library allows for exploiting the color and attribute facilities of most modern terminals using ANSI escape sequences. This library provides the following:

- ansi_format/3 allows writing messages to the terminal with ansi attributes.
- It defines the hook prolog:message_line_element/2, which provides ansi attributes and hyperlinks for print_message/2.

The behavior of this library is controlled by two Prolog flags:

`[|]`(99, [111,108,111,114,95,116,101,114,109])
  When `true`, activate the color output for this library. Otherwise simply call format/3.
`[|]`(104, [121,112,101,114,108,105,110,107,95,116,101,114,109])
  Emit terminal hyperlinks for `url(Location)` and `url(URL, Label)` elements of Prolog messages.

[det]ansi_format(+ClassOrAttributes, +Format, +Args)
  Format text with ANSI attributes. This predicate behaves as format/2 using Format and Args, but if the `current_output` is a terminal, it adds ANSI escape sequences according to Attributes. For example, to print a text in bold cyan, do

      ?- ansi_format([bold,fg(cyan)], 'Hello ~w', [world]).

  Attributes is either a single attribute, a list thereof or a term that is mapped to concrete attributes based on the current theme (see prolog:console_color/2). The attribute names are derived from the ANSI specification. See the source for sgr_code/2 for details. Some commonly used attributes are:

  bold
  underline
  `fg(Color)` `,` ``bg(Color)`,``hfg(Color)`,`hbg(Color)```
    For `fg(Color)` and `bg(Color)`, the colour name can be’#RGB' or’#RRGGBB'
  `fg8(Spec)` `,` `bg8(Spec)`
    8-bit color specification. Spec is a colour name, `h(Color)` or an integer 0..255.
  `fg(R, G, B)` `,` `bg(R, G, B)`
    24-bit (direct color) specification. The components are integers in the range 0..255.

  Defined color constants are below. `default` can be used to access the default color of the terminal.

  - black, red, green, yellow, blue, magenta, cyan, white

  ANSI sequences are sent if and only if

  - The `current_output` has the property `tty(true)` (see stream_property/2).
  - The Prolog flag `color_term` is `true`.
[semidet,multifile]prolog:console_color(+Term, -AnsiAttributes)
  Hook that allows for mapping abstract terms to concrete ANSI attributes. This hook is used by theme files to adjust the rendering based on user preferences and context. Defaults are defined in the file `boot/messages.pl`.
  See also
    `library(theme/dark)` for an example implementation and the Term values used by the system messages.
[semidet,multifile]prolog:message_line_element(+Stream, +Term)
  Hook implementation that deals with `ansi(+Attr, +Fmt, +Args)` in message specifications.
[det]ansi_hyperlink(+Stream, +Location)
[det]ansi_hyperlink(+Stream, +URL, +Label)
  Create a hyperlink for a terminal emulator. The file is fairly easy, but getting the line and column across is not as there seems to be no established standard. The current implementation emits, i.e., inserting a capital `L` before the line.

      ``file://AbsFileName[#LLine[:Column]]``

  See also
    https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda
[semidet]ansi_get_color(+Which, -RGB)
  Obtain the RGB color for an ANSI color parameter. Which is either a color alias or an integer ANSI color id. Defined aliases are `foreground` and `background`. This predicate sends a request to the console (`user_output`) and reads the reply. This assumes an `xterm` compatible terminal.

  RGB | is a term `rgb(Red,Green,Blue)`. The color components are integers in the range 0..65535.