Command Filters

A command filter intercepts < > type commands from throttles or the serial monitor and can be used to override, block or modify existing commands and implement completely new commands.

The following code, added to myAutomation.h like any other EXRAIL code, gives a basic example.

STEALTH_GLOBAL(
  void myFilter(Print * stream, byte & opcode, byte & paramCount, int16_t p[]) {
    (void)stream;
    // use command <U locoid> to display name from roster
    if (opcode == 'U' && paramCount == 1) {
      auto locoId=p[0];
      auto name=RMFT2::getRosterName(locoId);
      if (!name) return; // caller will <X> this
      opcode=0; // caller can now ignore this
      StringFormatter::lcd(0, F("Loco %d name %S"), locoId, name);
    }
  }
)

All <> commands will be broken into an opcode and numeric parameters before being passed to myFilter.
You can only have one myFilter function but it can filter multiple commands by inspecting the opcode.
The filter function can inspect or modify the opcode and parameters, make changes and return. If the opcode is set to 0, the caller will assume that the filter has completely handled the command and will not check it further.
Output directed to the stream parameter will be transmitted back to the originating throttle. If no output is expected then casting to void is a good way of eliminating the “unused parameter” compiler warning.
the <U> and <u> opcodes are reserved for use by users. They will not clash with any future DCC-EX commands.
Although this is a conventional c++ function, the use of STEALTH_GLOBAL in EXRAIL avoids the need to #include a vast array of weird and wonderful API and memory management definitions.
To recognize keyword parameters (e.g <1 MAIN>) you would test using the _hk suffix like this

if (opcode == '1' && paramCount == 1 && p[0] == "MAIN"_hk)