2. Handling Messages

Now let's make the bot respond to messages.

Register a Message Handler

Update index.ts to add a message handler:

import { Effect } from "effect"
import { runHazelBot } from "@hazel/bot-sdk"

runHazelBot({
	setup: (bot) =>
		Effect.gen(function* () {
			yield* Effect.log("Task Bot is starting...")

			// Register a handler for new messages
			yield* bot.onMessage((message) =>
				Effect.gen(function* () {
					yield* Effect.log(`New message: ${message.content}`)
				}),
			)
		}),
})

Run the bot and send a message - you'll see it logged in the console!

Access Message Properties

The message object contains useful information:

yield *
	bot.onMessage((message) =>
		Effect.gen(function* () {
			// Message content
			yield* Effect.log(`Content: ${message.content}`)

			// Who sent it
			yield* Effect.log(`Author ID: ${message.authorId}`)

			// Where it was sent
			yield* Effect.log(`Channel ID: ${message.channelId}`)

			// When it was sent
			yield* Effect.log(`Created at: ${message.createdAt}`)
		}),
	)

Reply to Messages

Use bot.message.reply to respond:

yield *
	bot.onMessage((message) =>
		Effect.gen(function* () {
			// Reply to messages containing "hello"
			if (message.content.toLowerCase().includes("hello")) {
				yield* bot.message.reply(message, "Hello! How can I help?")
			}
		}),
	)

Prevent Infinite Loops

When your bot sends a message, it will trigger the onMessage handler again. Always check to avoid responding to your own messages:

yield *
	bot.onMessage((message) =>
		Effect.gen(function* () {
			// Skip messages that start with our response prefix
			if (message.content.startsWith("Task Bot:")) {
				return
			}

			if (message.content.toLowerCase().includes("help")) {
				yield* bot.message.reply(message, "Task Bot: Use /task-add and /task-list!")
			}
		}),
	)

React to Task Mentions

For our Task Bot, let's react to any message mentioning "task":

import { Effect } from "effect"
import { runHazelBot } from "@hazel/bot-sdk"

runHazelBot({
	setup: (bot) =>
		Effect.gen(function* () {
			yield* Effect.log("Task Bot is starting...")

			yield* bot.onMessage((message) =>
				Effect.gen(function* () {
					// React to messages mentioning "task"
					if (message.content.toLowerCase().includes("task")) {
						yield* bot.message.react(message, "📝")
					}
				}),
			)
		}),
})

Multiple Handlers

You can register multiple handlers for the same event:

// Handler 1: React to task mentions
yield *
	bot.onMessage((message) =>
		Effect.gen(function* () {
			if (message.content.toLowerCase().includes("task")) {
				yield* bot.message.react(message, "📝")
			}
		}),
	)

// Handler 2: Log all messages
yield *
	bot.onMessage((message) =>
		Effect.gen(function* () {
			yield* Effect.log(`[${message.channelId}] ${message.content}`)
		}),
	)

All handlers run in parallel when a message arrives.

What's Next?

Message handlers are great for reactions, but for structured input, slash commands are better.

Next: Slash Commands

Add type-safe slash commands