The first days in the life of Project Coin

On February 27th the Compiler Group-sponsored OpenJDK project Project Coin has finally gone live. Proposals for small language changes may be submitted to the dev mailing list until March 30th 2009 after which a subset will be included in a JSR draft. In the two days since inception of the project there have been posted quite a few proposals to the list.

Neal Gafter has been very active so far and proposed no less than three language changes:

• Block expressions allow a series of statements to be written inside a parenthesized expression. This avoids the need for helper functions that are used only once or the introduction of temporary variables.
  
• Improved exception handling includes catching multiple exception types as well as improved checking for rethrown exceptions.
• Improved wildcard syntax introduces syntactic sugar for writing ? extends T as out T (covariant) and ? super T as in T (contravariant). Both in and out would be considered as context-sensitive keywords, i.e. they still can be used as identifiers elsewhere. The syntax looks an awful lot like the upcoming variance feature of C# 4.0. However, C#'s designers apparently decided on declaration-site variance similar to Scala as opposed to Java's use-site variance.
  

Josh Bloch submitted a proposal for Automatic Resource Management. People who closely followed the discussion around closures in Java and the various proposals that were brought forward should recognize this one (however, I don't know if there are significant differences to Josh's older proposals). The posting of the proposal prompted an interesting discussion with Neal Gafter who (obviously?) seems not very much in favour of this feature or at least does not agree with Josh on its specification.

Jeremy Manson of Google proposed Improved Type Inference for Generic Instance Creation that would allow you to avoid the explicit declaration of parameterized types in class instance creation expressions even though the parameterized type of the constructor is obvious from the context. Neal Gafter also chimed in on this one and pointed out some of what he believes to be omissions or shortcomings of the specification. Apparently he and Joe Darcy, the initiator of Project Coin, are working on a similar proposal that is based on a different implementation strategy.

Additionally, Joe Darcy proposed the use of Strings in switch-statements, Ruslan Shevchenko submitted a proposal for multi-line strings and Adrian Kuhn suggested the use of the default keyword for default (i.e. package-private) visibility.

Altogether a very promising start in my opinion, though I don't necessarily want to see each and every one of those proposals in the language. For example, I've not yet made up my mind about Neal's improved wildcard syntax. On a first scan of the proposal I was not sure what to make of the in and out keywords, i.e. what they should convey to the developer. After refreshing my mind a bit by reading up on the theory behind variance as well as Scala's variance annotations and C#'s upcoming support I felt a bit more like I'd groked it. Well, that's maybe worth a blog post on its own.

Anyway, the mailing list is open to anyone for submitting a proposal and/or for chiming in on any of the previously submitted proposals. I'm not sure I will however because seeing the concentrated brain power on the list makes me afraid of looking like an idiot ;) Nevertheless, discussions so far have been very interesting and I'm confident that we'll see a lot more of that in the days and weeks to come. It's good to finally see things in motion ...