Parameter Base.Monad.Make.1-X

The following identities ought to hold (for some value of =):

• return x >>= f = f x
• t >>= fun x -> return x = t
• (t >>= f) >>= g = t >>= fun x -> (f x >>= g)

Note: >>= is the infix notation for bind)

The map argument to Monad.Make says how to implement the monad's map function. `Define_using_bind means to define map t ~f = bind t ~f:(fun a -> return (f a)). `Custom overrides the default implementation, presumably with something more efficient.

Some other functions returned by Monad.Make are defined in terms of map, so passing in a more efficient map will improve their efficiency as well.