Question

I am converting the OCaml Format module to F#; see my earlier question.

To get started I changed

type size

external size_of_int : int -> size = "%identity"

external int_of_size : size -> int = "%identity"

to

let size_of_int = sizeof<int>

and a few other adjustments which I know is not correct, but it allowed me to convert all code related to open_box and close_box expect these three lines.

Now I have to just change these three lines so that I can test the subset of the format module that I did convert.

I know that the lines with size_of_int and int_of_size while external will probably rely on some function in the F# core. I also know that %identity can probably be ignored for the conversion.

My best guess is that I only need to create a simple type named size with size_of_int and int_of_size, but how?

EDIT

Based on answer by Jeffrey Scofield I was able to create the following F# code.

type size =
  interface
    abstract size_of_int : int -> size
    abstract int_of_size : size -> int
  end

type size = int
let size_of_int i = i
let int_of_size s = s

which allowed my subset of the Format module to successfully compile.

EDIT

Jack, who answered below, has a version at FSharpx.Compatibility.OCaml.Format.Format.fs
I havent’ tested it, but it is the most compelete version I have found at present.

Answer 1

These lines:

type size
external size_of_int : int -> size = "%identity"
external int_of_size : size -> int = "%identity"

Create an abstract type that is identical to int. The conversion functions
are no-ops (the identity function). I don’t know the F# idioms, but in
OCaml you can use an interface file and avoid the cleverness with “%identity”.

(* Interface file *)
type size
val size_of_int : int -> size
val int_of_size : size -> int

(* Implementation file *)
type size = int

let size_of_int i = i
let int_of_size s = s

Hopefully this translates more readily into F#.