Representing Monads with Capabilities

Programming with monads can be advantageous even in imperative languages with builtin support for side effects. However, in these languages composing monadic programs is different from composing side effecting imperative programs. This does not need to be the case, as already noticed by Filinski [1994]. We revive the well-known technique of monadic reflection in the context of modern programming languages with support for fibers, generators, or coroutines. In particular, we show how (layered) monadic reflection can be implemented in a stack safe manner and how effect safety can conveniently be approximated by capability passing.