kc3-lang/libxkbcommon/doc/rules-format.md

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• Author : Peter Hutterer
  Date : 2020-06-02 16:11:33
  Hash : a2657874
  Message : doc: add the rules-format file (as markdown) Useful to have this as part of the documentation. The rendering isn't great but at least not any worse than pure text. Markdown escapes % so explaining our use of %S and %H would require a double % - not idea. Let's just wrap it as a code block and done. Includes two typo fixes too, yay. Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

• doc/rules-format.md

• The rules file
  ==============
  
  The purpose of the rules file is to map between configuration values
  that are easy for a user to specify and understand, and the
  configuration values xkbcomp uses and understands.
  
  xkbcomp uses the xkb_component_names struct, which maps directly to
  include statements of the appropriate sections, called for short
  KcCGST (see doc/keymap-format-text-v1.txt; 'G' stands for "geometry",
  which is not supported). These are not really intuitive or straight-
  forward for the uninitiated.
  
  Instead, the user passes in a xkb_rule_names struct, which consists
  of the name of a rules file (in Linux this is usually "evdev"), a
  keyboard model (e.g. "pc105"), a set of layouts (which will end up
  in different groups, e.g. "us,fr"), variants (used to alter/augment
  the respective layout, e.g. "intl,dvorak"), and a set of options
  (used to tweak some general behavior of the keyboard, e.g.
  "ctrl:nocaps,compose:menu" to make the Caps Lock key act like Ctrl
  and the Menu key like Compose). We call these RMLVO.
  
  Format of the file
  ------------------
  The file consists of rule sets, each consisting of rules (one per
  line), which match the MLVO values on the left hand side, and, if
  the values match to the values the user passed in, results in the
  values on the right hand side being added to the resulting KcCGST.
  Since some values are related and repeated often, it is possible
  to group them together and refer to them by a group name in the
  rules.
  
  Along with matching values by simple string equality, and for
  membership in a group defined previously, rules may also contain
  "wildcard" values - "*" - which always match. These usually appear
  near the end.
  
  Grammar
  -------
  (It might be helpful to look at a file like rules/evdev along with
  this grammar. Comments, whitespace, etc. are not shown.)
  
  ```
  File         ::= { "!" (Include | Group | RuleSet) }
  
  Include      ::= "include" <ident>
  
  Group        ::= GroupName "=" { GroupElement } "\n"
  GroupName    ::= "$"<ident>
  GroupElement ::= <ident>
  
  RuleSet      ::= Mapping { Rule }
  
  Mapping      ::= { Mlvo } "=" { Kccgst } "\n"
  Mlvo         ::= "model" | "option" | ("layout" | "variant") [ Index ]
  Index        ::= "[" 1..XKB_NUM_GROUPS "]"
  Kccgst       ::= "keycodes" | "symbols" | "types" | "compat" | "geometry"
  
  Rule         ::= { MlvoValue } "=" { KccgstValue } "\n"
  MlvoValue    ::= "*" | GroupName | <ident>
  KccgstValue  ::= <ident>
  ```
  
  Notes:
  
  - Include processes the rules in the file path specified in the ident,
    in order. %-expansion is performed, as follows:
  
  ```
    %%:
      A literal %.
  
    %H:
      The value of the HOME environment variable.
  
    %S:
      The system-installed rules directory (usually /usr/share/X11/xkb/rules).
  ```
  
  - The order of values in a Rule must be the same as the Mapping it
    follows. The mapping line determines the meaning of the values in
    the rules which follow in the RuleSet.
  
  - If a Rule is matched, %-expansion is performed on the KccgstValue,
    as follows:
  
  ```
    %m, %l, %v:
       The model, layout or variant, if only one was given (e.g.
       %l for "us,il" is invalid).
  
    %l[1], %v[1]:
       Layout or variant for the specified group Index, if more than
       one was given (e.g. %l[1] for "us" is invalid).
  
    %+m, %+l, %+v, %+l[1], %+v[1]
       As above, but prefixed with '+'. Similarly, '|', '-', '_' may be
       used instead of '+'.
  
    %(m), %(l), %(l[1]), %(v), %(v[1]):
       As above, but prefixed by '(' and suffixed by ')'.
  ```
  
    In case the expansion is invalid, as described above, it is
    skipped (the rest of the string is still processed); this includes
    the prefix and suffix (that's why you shouldn't use e.g. "(%v[1])").