Is this the blessed way to write an overlay that adds a package foo to all Haskell package sets, without undoing previous overlays? If so, should we add it to the manual?

final: prev: {
  haskell = prev.haskell // {
    packageOverrides = final.lib.composeExtensions prev.haskell.packageOverrides
      (hfinal: hprev: {
        foo = ...;
      });
  };
}