An opaque handle describing how to connect an external event source to a Flow pipeline. Build one with connect or join, then pass it to Flow.await or Flow.subscribe.

Build a channel from a subscription source and a command that opens or attaches it. The command is fired once when the channel is first awaited.

-- Port declarations:
--   port onMessage : (String -> msg) -> Sub msg
--   port subscribe : String -> Cmd msg
myChannel : Channel Model String
myChannel =
    Flow.Channel.connect
        (\callback -> Ports.onMessage callback)
        (\_ -> Ports.subscribe "myTopic")

Build a channel from a subscription source when no startup command is needed.

-- Port declaration:
--   port onKeyDown : (String -> msg) -> Sub msg
keyChannel : Channel Model String
keyChannel =
    Flow.Channel.join (\callback -> Ports.onKeyDown callback)

Filter events for a channel. Only events for which the predicate returns True are forwarded; others are silently dropped.

The predicate receives the internal channel key and the event payload. In most cases you only need the payload:

Flow.Channel.filter (\_ msg -> msg /= "") myChannel

Fire the channel's setup command and suspend until the first value arrives. The callback decides what to do next: Just flow to continue, Nothing to close the subscription.

This is the low-level primitive that Flow.await and Flow.subscribe are built on. Prefer those unless you need custom continuation logic.

Re-register on an already-open channel without firing the setup command again. Used to loop inside a subscription after the channel has been opened with open.