In my current pondering of the compose-able objects them, I was thinking it would be useful to have the follow abstractions: Monoids, which were themselves tuples of Monoids. The idea was something like so: code: -------- import Data.Monoid instance Monoid (Socket2 a b) where mempty = Socket2 (mempty, mempty) Socket2 (a, b) `mappend` Socket2 (w, x) = Socket2 (a `mappend` w, b `mappend` x) data Socket2 a b = Socket2 (a, b) -------- However, this does not compile because of errors like so: code: -------- Sockets.hs:9:21: No instance for (Monoid a) arising from a use of `mempty' In the expression: mempty In the first argument of `Socket2', namely `(mempty, mempty)' In the expression: Socket2 (mempty, mempty) -------- This makes sense, but I haven't figured out a way to rewrite this to make it work. One approach I tried was to encode Monoid constraints into the data declaration (which I heard was a bad idea) but this didn't work, even using forall. Also I tried to encode it into the instance declaration, but the compiler kept complaining about errant or illegal syntax. -- frigidcode.com