ROT13 in F#

So, this is nothing really exciting, I wrote a simple program to do ROT13 text encoding, and in the way bumped into some minor quirks of the standard library. One of the problems with OCaml is its idiosyncratic standard library which is missing a lot of simple and useful functions. Granted, most of them you can write in less than five minutes, but it would be better if they were already done and standardized. I guess I became spoiled by Haskell. Anyway, F# seems to have inherited this as well. Below you can see the code, and a sample use case.

#light

let letters = Array.of_list(['a' .. 'z'] @ ['a' .. 'z'])

let index a c =
  let rec aux i = if a.(i) = c then i else aux (i + 1) in
  aux 0

let strmap f (s : string) =
  let sb = new System.Text.StringBuilder(s) in
  let rec aux i =
      if i = sb.Length then () else
      (sb.Chars(i) <- f (sb.Chars(i)) ; aux (i + 1)) in     
      ( aux 0; sb.ToString() )     

let rotchar c =     
    let i = index letters c in     
    letters.(i + 13)  

let rot13 s =     
    strmap rotchar s 

And now for the use case:

> rot13 "feijoada";;
val it : string = "srvwbnqn"