Yaron Minsky wrote:
  

module type Abs_int : sig
type t
val to_int : t -> int
val of_int : int <- t
end

And then you write concrete module Int that implements this signature.  You
can then write:

module Row : Abs_int = Int
module Col : Abs_int = Int

    

This approximates my idea of "optimal" well enough that I'll rewrite
some of my old code to use it.  It loses some opportunities for compiler
optimization, but otherwise seems perfect.  No repetitive boilerplate to
write over and over, no unnecessary boxing, efficient conversion to the
base type (well, it still requires a function call to find out that
nothing needs to change, and maybe there's a shallow copy created in
this process), easy on the eyes/fingers syntax for conversion and
declaration and a readable type name for ocamlc to report when things go
wrong.