I’ve written a Bitcoin payment channel server/client in Haskell. I know it’s not exactly a well-known problem, but it felt very practical to me. So there’s a lot of logic, but also quite a lot of input/output to handle (e.g. saving payments to the database)

https://github.com/runeksvendsen/rbpcp-handler

https://github.com/runeksvendsen/bitcoin-payment-channel/blo...