services.xserver.windowManager.xmonad.haskellPackages' is not of type package'

[wbadart]

Describe the bug
NixOS config fails to build with services.xserver.windowManager.xmonad enabled, seemingly due to a breaking change either in its haskellPackages option or in pkgs.haskellPackages (on unstable):

error: A definition for option `services.xserver.windowManager.xmonad.haskellPackages' is not of type `package'. Definition values:
- In `/nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/services/x11/window-managers/xmonad.nix'

I have a strong feeling this is related to 37f575c.

To Reproduce
Steps to reproduce the behavior:

1. echo "{vm0={pkgs,...}: {services.xserver.windowManager.xmonad.enable = true;};}" > test.nix
2. nixos-build-vms test.nix

Expected behavior
I expected the default value to type-check. I tried overriding it with xmonad.haskellPackages = pkgs.haskellPackages and xmonad.haskellPackages = pkgs.haskellPackages.ghc_8_10_2 but got the same error.

Notify maintainers

• @Scriptkiddi: do you have any suggestions on a new value here that would type-check?

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.92, NixOS, 21.03pre265961.891f607d530 (Okapi)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.10`
 - channels(will): `""`
 - channels(root): `"nixos-21.03pre267299.5ff6700bb82, home-manager"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
- haskellPackages
# a list of nixos modules affected by the problem
module:
- services.xserver.windowManager.xmonad

Thanks all!

[wbadart]

For the sake of bookkeeping: this may also be tangentially related to #107539, in the sense of many attributes of pkgs not being literal packages.

[Scriptkiddi]

#111534

[Scriptkiddi]

#111581 reverted the changes

[wbadart]

Thanks!