irc-client 
An IRC client library.
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Built on irc-conduit.
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Handles a connection to a single IRC server.
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Manages "event handlers", calling them as appropriate on receipt of messages.
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Provides default event handlers for some common messages (e.g., server PINGs).
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Executes each event handler in its own thread, and uses a message queue to guarantee thread-safe message delivery.
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Provides a few helper functions for common operations.
The documentation of the latest developmental version is available online.
Note
This used to be a part of yukibot, so if you want the history from before this was split out into its own library, check there.
Contributing
Bug reports, pull requests, and comments are very welcome!
Feel free to contact me on GitHub, through IRC (#haskell on freenode), or email (mike@barrucadu.co.uk).