cat.fs

open System
open System.IO
open System.Text

let tryReadFile path = async {
    if File.Exists path then
        use file = File.OpenRead path
        let! bytes = file.AsyncRead (int file.Length)
        return Some (Encoding.UTF8.GetString bytes)
    else return None }

[<EntryPoint>]
let main args =
    Async.RunSynchronously <| async {
        match Array.tryItem 0 args with
        | Some path ->
            // old syntax
            //let! text = tryReadFile path
            //match text with ...
            //| Some text -> Console.Write text
            //| None -> Console.Error.WriteLine (sprintf "%s: No such file or directory" path)
            
            // using match!
            match! tryReadFile path with
            | Some text -> Console.Write text
            | None -> Console.Error.WriteLine (sprintf "%s: No such file or directory" path)
        | None -> () }
    0