[PARROT] Add "a ?= 2" support (Elvis assignment): should be expanded to "a = a == null ? 2 : a"

I've suggested last week the creation of the "?:=" constructor in Groovy:

https://jira.codehaus.org/browse/GROOVY-5291

It was rejected and a new JIRA was requested to be created with a new operator.

Then, I've talked to the Grails users in their mailing list to get some feedback, which can be read here:

http://grails.1312388.n4.nabble.com/Help-improving-Groovy-syntax-tt4384137.html

Then, Phil DeJarnett has suggested using "?=" instead of "?:=". Not only I preferred this suggestion, but I was changed my mind about its meaning too.

It would be used as a caching/memoization operator mostly. It would be similar to Ruby's "||=", except for this specific situation:

Ruby:

a = nil
a ||= false # a will be false
a ||= true # a will be true

That is why "a ||= value" is expanded to "a = a || value"

But for caching/memoization, I'd prefer "a ?= value" to be expanded to "a = a == null ? value : a". This way we would have:

Proposed Groovy syntax:

def a = null
a ?= false; assert a == false
a ?= true; assert a == true

a = null; a ?= new Object(); assert a instanceof Object

I'll actually fill a new ticket on Ruby Redmine too for proposing the same syntax and semanthics