Caesar cipher in f#

I have a project where I have to code a Caesar cipher that takes in a string and a shift amount and then encrypts the string into cipher text. I easily did this in JavaScript but now I have to do it in F#. Also no loops allowed only recursion. I'm completely stressed an开发者_开发百科d confused and running out of time so im posting here as a last resort. This is all I have so far and I feel like I'm heading in a completely wrong direction...

 let rec encrypt str shiftAmount =
if str.length > 0 then 
     strChar = str.ToUpper().Chars(0)
     strUni = int strChar
     strCoded = (((strUni + shiftAmount - 65) %26) +65)
else 

In this case it makes more sense to use a mapping function (in this case Array.Map()) and pipelining instead of recursion since you have to apply a function (character shifting) to each character in your string. Below should work for uppercase characters:

let shift(c, shiftAmount) = 
     let num = int(c) - int('A')
     let offsetNum = (num+shiftAmount)%26
     let result = offsetNum + int('A')
     if offsetNum < 0 then
        char(int('Z') + offsetNum + 1)
     else
        char(offsetNum + int('A'))

let encrypt(str:string, shiftAmount) =
    str.ToCharArray() 
    |> Array.map (fun c -> shift( int(c), shiftAmount)) 
    |> String.Concat

There might be a much more elegant solution (especially to cover both clock-wise and counter-clock wise shifting), still learning myself.

Here's a naive Caesar encoding:

let encode (str:string) shift =
    let inline flipzip a b = b, a
    str
    |> String.map (fun ch ->
        ch
        |> int
        |> flipzip (int 'A')
        ||> (-)
        |> (+) shift
        |> flipzip 26
        ||> (%)
        |> (+) (int 'A')
        |> char)

Beyond that, it's not clear to me what you're asking for other than for someone else to do all the work for you...