gossamer/message_port

One end of a MessageChannel. Obtain a pair of ports via message_channel.new, send messages with post_message, and react to incoming messages via set_on_message.

Sending Gleam values

Every value passed to post_message is serialized by the structured-clone algorithm and arrives on the receiving side as Dynamic. Decode it with gleam/dynamic/decode. Primitives, tuples, and the fields of records cross intact. A record’s constructor type does not — decode the data field by field rather than expect to pattern-match the original variant. Gleam lists and dicts don’t round-trip with their normal decoders; send a tuple or a gleam/javascript/array.Array for sequences. Functions and symbols can’t be serialized; sending one returns Error(Nil).

Transferring ports

A MessagePort inside the data passed to post_message is automatically detached on this side and re-attached on the receiver — the message and any ports reachable from it ride a single atomic call. Once a port has been sent it is no longer usable on the sender; calling post_message, close, or set_on_message on the transferred port is a silent no-op. To extract a transferred port from received Dynamic data, use from_dynamic.

pub type MessagePort

pub fn close(port: MessagePort) -> Nil

pub fn from_dynamic(
  value: dynamic.Dynamic,
) -> Result(MessagePort, Nil)

pub fn post_message(
  port: MessagePort,
  data: a,
) -> Result(Nil, Nil)

pub fn set_on_message(
  port: MessagePort,
  handler: fn(dynamic.Dynamic, MessagePort) -> a,
) -> Nil