QMK Alt Layer Code

/* remove ALT keys,
 * press the keycode by itself,
 * release the keycode on key up,
 * return false to avoid sending the original keypress [;'/
 */
#define NO_ALT(kc)              \
  if (record->event.pressed) {  \
    del_mods(MOD_MASK_ALT);     \
    register_code16(kc);        \
  } else {                      \
    unregister_code16(kc);      \
  }                             \
  return false;


bool alt_down = false;

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
  //if an ALT key is held down
  if (alt_down) {
    switch (keycode) {
      case KC_LBRC:         //if ALT + [
        NO_ALT(KC_UP);      //  press UP
      case KC_SCLN:         //if ALT + ;
        NO_ALT(KC_LEFT);    //  press LEFT
      case KC_QUOT:         //if ALT + '
        NO_ALT(KC_DOWN);    //  press DOWN
      case KC_SLSH:         //if ALT + /
        NO_ALT(KC_RIGHT);   //  press RIGHT
      default:
        return true;
    }
  }
  switch (keycode) {
      case KC_LALT:  //for left or right ALT,
      case KC_RALT:  //set alt_down to true if pressed
        if (record->event.pressed) {
            alt_down = true;
        } else {  //on ALT keyup
            alt_down = false;
        }
        break;
  return true;
}