module NonNegativeInt = 

  type T = private | NonNeg of int

  type MaybeT = 

    | Ok of T

    | Error of string

  let create i = 

    if (i >= 0 )

    then 

        Ok (NonNeg i)

    else Error "i is negative"

  let unwrap (t:T ) =

    match t with 

    | NonNeg i -> i

open NonNegativeInt

​

module Test =

  let show (input:MaybeT) =

     match input with

     | Ok i -> 

        printfn "%i" ( 10*NonNegativeInt.unwrap (i))

     | Error e -> printfn "wronge: %s" e

open Test

​

show (NonNegativeInt.create -42)

show (NonNegativeInt.create 42)

// union cases not accessible 

//show (Ok (NonNeg -1))

​