A few weeks ago I noticed that I no longer could use
haskell-hoogle-lookup-from-website in Emacs. After a bit of experimentation I
found that the reason was that I couldn't use
xdg-open in a Nix shell.
Yesterday I finally got around to look into further.
It's caused by
XDG_DATA_DIRS rather than appending to it.
Of course someone already reported a bug already.
The workaround is to use
use nix --keep XDG_DATA_DIRS