Make a channel using the endpoint URI from Slack (create a new app, then add a new webhook using the "Incoming Webhooks" page to get the URI). e.g. channel @@ Uri.of_string "https://hooks.slack.com/services/..."
post channel ~key message records that key is now set to message, and posts message to channel if it has changed. e.g. post to_dev ~key:"build-status" s