Hash.new with a block is a sexy beast: you can specify default hash values and when a key is first used, the value will be calculated and inserted into the Hash. Based on that behavior I took a couple of minutes to hack together a small proof of concept class for lazy lists:

As I said, this is quick and dirty, especially since you have to pass the entire Hash.new block to LazyList.new right now. Other than that I quite like it though. I know there are at least two other lazy list implementations for Ruby, but it seems like both haven’t been updated in a while and I don’t particularly like their interfaces. Dunno, if I manage to simplify list definition, I may actually try to develop this into something. Don’t hold your breath though…