Capitalize a given string using Proper Case (dipping a toe into the F# water)

ProperCase.fs

// capitalizes the first letter of every word in the given string
let properCase (s:string) =
    // recursive helper function
    let rec helper (s:string) (i:int) (capitalize:bool) (result:string) =
        // if we're at the end of the string, return our result
        if s.Length - 1 < i then
            result
        else
            // recurse to the next character in the input string
            (helper s (i + 1) (s.[i] = ' ') (result +
                // we need a string object (not a char) for ToUpper and concatenation       
                if capitalize then s.[i].ToString().ToUpper()
                              else s.[i].ToString()))

    // start the recursion
    helper s 0 true ""

[<EntryPoint>]
let main argv =
    match argv.Length with
    // no values provided; display help message; dirty exit
    | 0 -> printfn "Syntax: FSharpConsole.exe <string value>"; 1
    // 1 value provided; process input directly; clean exit
    | 1 -> printfn "Proper Case using direct call: %s" (properCase argv.[0]); 0
    // >1 value provided; concatenate before processing; clean exit
    | _ -> printfn "Proper Case using concatenation: %s" (properCase (String.concat " " argv)); 0

Tail-call optimized!