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.
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