Demo Linear States

datasort state = 
| uninit
| listening
| active
| closed

absvtype socket (state)  

typedef port = int 
typedef addr = string

extern fun create  (): socket (uninit)

extern fun listen  (!socket (uninit)    >> socket (listening), port): void
extern fun accept  (!socket (listening) >> socket (active)): void

extern fun connect (!socket (uninit)    >> socket (active), addr): void

extern fun send (!socket (active), int): void
extern fun recv (!socket (active)): int 

extern fun close {s:state} (socket (s)): void


implement main0 () = let 
    val sock = create ()
    val _ = listen (sock, 8080)
    val _ = accept (sock) // block until someone connects
    val _ = send (sock, 1)
    val x = recv (sock)
in 
    close sock
end