/// A raw message.
type Message = byte[]

/// A domain-specific input type.
type Input =
  | MerchantSkuImported
  | MerchantSkuRemoved

/// A domain-specific output type.
type Output =
  | SkuUpdated

/// Decodes an input or returns an error.
let decodeInput : Message -> Result<Input, string> = undef

/// Encodes the output.
let encodeOutput : Output -> Message = undef

/// Runs the core logic of the service.
let handle : Input -> Async<Output> = undef

/// Given a host name, returns a stream of input messages.
let inputStream : string -> AsyncSeq<Message> = undef

/// Given a host name, publishes an output message.
let publish : string -> Message -> Async<unit> = undef

/// Creates a message handler given a decoder, business logic handler and a decoder.
let handler :
  (Message -> Result<Input, string>) ->
  (Input -> Async<Output>) ->
  (Output -> Async<unit>) ->
  Message -> Async<unit> = undef

/// Configuration for the service.
type Args = {
  input_host : string
  output_host : string
}

/// Runs the service.
let run (args:Args) = async {
  let inputStream = inputStream args.input_host
  let publish = publish args.output_host
  let handle = handler decodeInput handle (encodeOutput >> publish)
  do! inputStream |> AsyncSeq.iterAsync handle }