Parameter Make.1-X

type 'a t
val bind : 'a t -> f:('a -> 'b t) -> 'b t
val return : 'a -> 'a t
val map : [ `Define_using_bind | `Custom of 'a t -> f:('a -> 'b) -> 'b t ]

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.