getAssociations.nix

     let # invert is O(n^2); it's not obvious to me that we can do much better without a lot of work
          # invert :: Map a [b] -> Map b [a]
          invert = m:
            let allBs = builtins.concatLists (builtins.attrValues m);
                exploded = builtins.concatMap (a: map (b: { ${b} = a; }) m.${a}) (builtins.attrNames m);
            in builtins.listToAttrs (map (b: { name = b; value = builtins.catAttrs b exploded; }) allBs);
          # Given a package that installs .desktop files in the usual location,
          # return a mapping from mime types to lists of desktop file names.
          # This is suitable for use in home-manager's
          # xdg.mimeApps.defaultApplications.
          getAssociations = p: invert (builtins.fromJSON (builtins.readFile (pkgs.runCommand "getAssociations" {
            inherit p;
          } ''
            for x in $(ls "$p/share/applications/") ; do
              cat "$p/share/applications/$x" | sed -n 's/MimeType=//p' | sed -e 's/;$//' -e 's/;/", "/g' -e 's/^\(.*\)$/"'"$x"'": ["\1"],/'
            done | sed -e '1 i {' -e '$ s/,$//' -e '$ a }' >$out
          '')));