Added some ones in Ruby

Would be useful to add a condition (and test) to raise error if self doesn't respond to the method passed

Needs random tests

You say that we should raise an exception in certain cases, but you never test that.

Had to graph out what was going on in this one. Super cool!

Great separation of concerns

This one is pretty clever, I have to say.

This is really cool and concise - you used all the same methods as my solution with 1/3rd the verbosity. I love kata tackles like this that make the most of Ruby's functional enumerable methods.

Props on writing the full method out. This is very easy to follow, too.

Now THIS is best practices. Beautiful!

Eloquent, simple, ideomatic.

Now that is concise!