F# Ping Pong Computation Expression!

pongce.fsx

// State types
type Ping = Ping
type Pong = Pong

// Builder that requires every "ping" to be
// immediately answered by a "pong"
type PingBuilder() =
  member __.Yield(()) = Pong

  member __.Run(Pong) = ()

  [<CustomOperation("ping")>]
  member __.Ping(Pong) = Ping

  [<CustomOperation("pong")>]
  member __.Pong(Ping) = Pong

let ping = PingBuilder()

// Compiles - balanced
ping {
  ping
  pong
  ping
  pong
}

// Doesn't compile - not balanced
ping {
  ping
  pong
  ping
}

// Builder that requires every "ping" to be
// eventually answered by a "pong"
type PongBuilder() =
  member __.Yield(()) = ()

  member __.Run(()) = ()

  [<CustomOperation("ping")>]
  member __.Ping(rest) = Ping, rest

  [<CustomOperation("pong")>]
  member __.Pong((Ping, rest)) = rest

let pong = PongBuilder()

// Compiles - balanced
pong {
  ping
  ping
  pong
  ping
  pong
  pong
}

// Doesn't compile - not balanced
pong {
  ping
  ping
  pong
}