[throwaway2037]

Re: Properties: What is the gain? What is the "pain point"? More typing? Can you share a language where you feel properties is a significant gain? (C#?)

Re: Streams API that allows checked exceptions. Brian Goetz has written about this issue. (Search StackOverflow.com) They did not allow because of the optionally parallel (threaded) nature of streams. If multiple parallel streams throw an exception, when/when/where/how are the exceptions re-thrown? Moving exceptions across thread boundaries is a tricky thing in any language. If you don't care about parallel streams, there are many open source projects that effective create a nearly identical Streams API but allow throws Exception everywhere. Are these open source solutions insufficient for your needs?

Re: adding functional methods like... Is there an advantage other than typing nine less characters?

[int_19h]

> Moving exceptions across thread boundaries is a tricky thing in any language.

Not really. C++ has std::exception_ptr for that exact reason, and C# has ExceptionDispatchInfo. Both are very straightforward - you catch the exception, get a handle for it, and then re-throw that handle in another context, with original stack trace preserved.

Multiple concurrent exceptions is also a solved problem. In C#, if you do something like Task.WhenAll(), and one or more task throws, you get back an AggregateException, which can be inspected to see which task threw what. The same can be applied to async streams.