asahi: fn keycode in defsrc results in file parse error

[montchr]

I copied the template from https://github.com/kmonad/kmonad/blob/master/keymap/template/apple.kbd with some slight alterations in an attempt to get KMonad working with the internal Apple keyboard on an M2 MacBook Air running Asahi-NixOS.

I am configuring the defcfg block via the provided NixOS module. Then, I try this configuration:

(defsrc
 esc  f1   f2   f3   f4   f5   f6   f7   f8   f9   f10  f11  f12
 grv  1    2    3    4    5    6    7    8    9    0    -    =    bspc
 tab  q    w    e    r    t    y    u    i    o    p    [    ]    \
 caps a    s    d    f    g    h    j    k    l    ;    '    ret
 lsft z    x    c    v    b    n    m    ,    .    /    rsft up
 fn   lctl lmet lalt           spc            ralt rmet left down rght
 )

(defalias fn (around (layer-toggle apple) fn))

(deflayer default
          _    f1   f2   f3   f4   f5   f6   f7   f8   f9   f10  f11  f12
          _    _    _    _    _    _    _    _    _    _    _    _    _    _
          _    _    _    _    _    _    _    _    _    _    _    _    _    _
          _    _    _    _    _    _    _    _    _    _    _    _    _
          _    _    _    _    _    _    _    _    _    _    _    _    _
          @fn  _    _    _              _              _    _    _    _    _
          )

(defalias
  dsh dashboard
  rec record
  )

(deflayer apple
          _    brdn brup @dsh find @rec XX   prev pp   next mute vold volu
          _    _    _    _    _    _    _    _    _    _    _    _    _    _
          _    _    _    _    _    _    _    _    _    _    _    _    _    _
          _    _    _    _    _    _    _    _    _    _    _    _    _
          _    _    _    _    _    _    _    _    _    _    _    _    _
          _    _    _    _              _              _    _    _    _    _
          )

And the build fails every time when it gets to fn in the defsrc:

kmonad: Parse error at 16:2:
   |
16 |  fn   lctl lmet lalt           spc            ralt rmet left down rght
   |  ^
unexpected 'f'
expecting ')'

Based on what I see in the keycodes file, the fn keycode is only allowed on Darwin, so I imagine that's why there's a parse error.

If it helps, here's the output of wev when I press the Fn key:

[14:     wl_keyboard] key: serial: 24579; time: 13972728; key: 472; state: 1 (pressed)
                      sym: XF86Fn       (268964304), utf8: ''

Edit:

If I omit fn from the defsrc, an error occurs in the defalias where I also tried to use fn:

kmonad: Parse error at 29:44:
   |
29 | (defalias fn (around (layer-toggle apple) fn))
   |                                            ^
Unrecognized compose-char

Once I remove all traces of fn, the file parses and the build succeeds.

[slotThe]

Seems like Keycode.hs has a conditional that prevents this key from being available on non-darwin hosts:

data Keycode
  …
#ifdef darwin_HOST_OS
  | KeyFn
  | KeyLaunchpad
  | KeyMissionCtrl
  | KeySpotlight
  | KeyDictation
#endif