Your example does not make much sense, because you can return-from foo in lambda, without enclosing it in a block. The solution proposed by OP (distinguishing

Your example does not make much sense, because you can return-from foo in lambda, without enclosing it in a block. The solution proposed by OP (distinguishing "lambda" callables from "block" callables) is in my opinion misguided, because it only solves a problem partially, in a confusing way and creates new, unnecessary entities. The return-from mechanism is simpler and more intuitive.