[isaacimagine]

By 'red' I was referring to anything effectful, whether that be concurrency, IO, state, or otherwise. Composing any one effect is a solved problem (e.g. monad); composing arbitrarily many effects is a lot harder. Recent languages like Koka describe effects over the free monad; because they are described over a single monad, and this single monad composes, effects compose.

I am by no means an expert in this area, but I hope this clarifies the intent of my original comment. What transformations and/or constraints exist for composable heterogeneous effects?

Koka: http://koka-lang.org/