Convert decimal to hexadecimal

Given a Unicode codepoint in integer form, I want to convert it to the standard U+ hexadecimal notation. The simple code snippet below achieves this, but perhaps there’s a better/simpler way to do it in OCaml ?
The closest I could find was Printf.prinf “%x”, but this only prints to stdout, and does not return a string.

let decompose_in_hex_base n=
   let rec tail_rec_decomposer=(
     fun (results,walker)->
       if walker=0
       then String.concat "" results
         let remainder=(walker mod 16)
         and new_walker=(walker / 16) in
         let char_index=(
            if remainder<=9
            then 48+remainder
            else 55+remainder
         ) in
        let result=String.make 1 (char_of_int char_index) in
   ) in
   tail_rec_decomposer ([],n);;

let unicode_point i=
   let temp=decompose_in_hex_base i in
   let offset=4-String.length(temp) in
   if offset>0
   then "U+"^(String.make offset '0')^temp
   else "U+"^temp;;

Something like Printf.sprintf "U+%04x" 123 gives you a hexadecimal string representation with 4 digits and leading zeros. I’m not familiar with the exact unicode representation that you need but it should give you an idea.


Looks like exactly what I need, thank you very much