Hi Yotam,

The way I usually do this is by making a minimal required subset module
type and then a default module that includes the overrides:

  module DefaultMonad(M :
    sig
      type 'a m
      val return : 'a -> 'a m
      val (>>=) : 'a m -> ('a -> 'b m) -> 'b m
    end) : Monad with type 'a m = 'a M.m =
  struct
    open M (* for >>= *)
    let (>>) m f = m >>= λ _ → f
    include M (* fill in the signature and possibly override above
                 definition *)
  end

Then you can pass your incomplete module (without >>) to DefaultMonad,
which implements a default for you, which you can still override in a
complete module.

Regards,
Pippijn