It isn't, I'm afraid.

By the way, I'm well aware of the problem with effects and value restriction and such (hence my qualification above, with purity). However, I don't care if it fails from time to time (and I think I remember some check about purity being done somewhere in the module system). For my use-case, I don't care if I need to have access to the original source-code -- I mean to see the concrete definition of the functor rather than its signature -- either (though it seems a limitation in general).