Associative List

letter.ml

let insert l a = 
    if List.mem_assoc a l
    then 
      let n = List.assoc a l in (a, n+1)::(List.remove_assoc a l)
    else (a, 1)::l

let letters (word : string) : char MultiSet.t  = 
  let rec insert (l :  char MultiSet.t) (c : string) (i : int) : char MultiSet.t =
    if ( String.length c  > 1 ) then
      insert (  MultiSet.insert l (String.get c i) )  ( String.sub c 1  ((String.length c) - 1)  )  0
    else   
      MultiSet.insert l (String.get c 0)
  in insert MultiSet.empty word 0
;;