Comment by bloovis

As others have mentioned, Crystal is close to Ruby in many ways, such that some simpler code will port straight over. I've managed to port a large Ruby application (the sup email client) to Crystal, and a lot of the code just worked, but I still had tweak just about everything else to get it to compile. The hardest bits were the places that used Ruby's dynamic nature, e.g., constructing method names at runtime and then calling them with send, or creating methods on the fly, or data structures that mixed up types freely.

Crystal's intent, as I see it, is very different from Ruby's. Because it compiles down to machine code in a single executable, it's good for making things that are fast and easy to deploy. I've used it to make small web services as well as the bigger thing I mentioned above.