In OCaml

let explode str =
    let rec aux pos acc =
        if pos < 0 then acc
        else aux (pred pos) (str.[pos] :: acc)
    in aux ((String.length str)-1) []

let d2a = function
    | 'M' -> 1000
    | 'D' -> 500
    | 'C' -> 100
    | 'L' -> 50
    | 'X' -> 10
    | 'V' -> 5
    | 'I' -> 1
    | _ -> failwith "Incorrect character"

let r2d str =
    List.fold_left (fun acc x -> let x' = d2a x in acc + x' - acc mod 
+x' * 2) 0 (explode (String.uppercase str));;
[download]

• explode just converts a string into a list of its characters. The future standard library for OCaml, named Batteries included will include such a function by default.
• The functional nature of OCaml allows a natural implementation of the algorithm

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