XMonad is now compiled using Cabal "new-build" mechanism. This means that config
file should now be compiled using "build" script approach. When EXAMPLES option
is enabled, the port installs example build script. To migrate your config:

% pkg install hs-cabal-install
% cp /usr/local/share/examples/xmonad/build ~/.xmonad/build
% cp /usr/local/share/examples/xmonad/xmonad-config.cabal ~/.xmonad/xmonad-config.cabal
% cabal new-update

Make sure your config is named ~/.xmonad/config.hs, or change corresponding line
in the .cabal file. Building the config for the first time requires decent amount
of time, so it is recommended to manually run the build script before the first
startup:

% ~/.xmonad/build

It is possible to use devel/stack to build the config, as well as other means.
See https://github.com/xmonad/xmonad-testing/ for all available options.