It seems like there is an inconsistency introduced in ruby 2.0 regarding the use of lambda syntax. The issue is detailed here
In ruby 2.0 the following is valid (notice the space between -> and (message))
hello_world = -> (message) { puts message }
However in ruby 1.9 it gives an error.
The issue is that in jruby-parser this doesn't parse even with CompatVersion set to 2.0. And in the older versions it won't parse, so the files using this cannot be handled with the jruby-parser.
Removing the space between the arrow and the parentheses would allow jruby-parser to parse these files. But there are uses of this syntax (with spaces) in the wild already, e.g. in slop gem. So, there are many files using this syntax that cannot be handled with jruby-parser.