Extend a module from Map in OCaml

Question

I have a module StringMap built by the functor Map.Make given a type String:

module StringMap = Map.Make(String)

Besides the ordinary operations provided by Map, I would like to add more definitions in this module, for instance, my_own_function, such that I could call StringMap.my_own_function. Does anyone know where I should define this kind of functions and their signature?

Answer 1

You can use the include keyword inside a new module to add all the same functions. This was also extended to the signature in OCaml 3.12.

module StringMap =
    struct
        include Map.Make(String)
    end

If you want to access the structure of the map, you'll have to add some Obj.magic or the %identity special external function. The redefinition of the type must be exact since no type checking is happening,

module Make (Ord : Map.OrderedType) =
    struct
        include Map.Make(Ord)

        type 'a impl = Empty 
                     | Node of 'a impl * key * 'a * 'a impl * int

        external impl_of_t : 'a t -> 'a impl = "%identity"
        external t_of_impl : 'a impl -> 'a t = "%identity"

        let cardinal map =
            let rec cardinal = function
                | Empty -> 0
                | Node(l,_,_,r,_) -> cardinal l + 1 + cardinal r
            in
            cardinal (impl_of_t map)

    end