simple shift

gistfile1.fs

let shift (s:string) (k:int) = 
    System.String(
        s.ToCharArray()
        |> Array.map (function
            | x when 'a' <= x && x <='z' ->
                let code = int(x)-int('a')
                char(int('a') + (code + k + 26) % 26)
            | x -> x )
    )