yesod/yesod-eventsource/README.md

yesod-eventsource

It's easy to send an event from an HTTP client to a server: just send an HTTP request. However, sending events from the server to the client requires more sophisticated approaches. Server-sent events are a standardized way of pushing events from the server to the client.

This package allows your Yesod application to easily send server-sent events. On the client side, you may use the EventSource JavaScript object on browsers that support it (https://developer.mozilla.org/en-US/docs/Server-sent_events/EventSource) or a polyfill for browsers that don't (we support Remy's polyfill out-of-the-box, although that requires you to explicitly support it).