Imported From: https://issues.scala-lang.org/browse/SI-797?orig=1
Reporter: @DRMacIver

@DRMacIver said:
The problem is worse than I thought actually. A single instance of Either can use up unbounded space.

The problem is that x holds onto x.swap which holds onto x.swap.swap which...

You get the idea.

I'm more and more convinced that making any of these lazy vals is useless and actively dangerous.

@mcdirmid said:
Another case for stable defs.

Geoffrey Alan Washburn (washburn) said:
Stable defs would be a useful thing to have, but it is not clear whether we could come up with a set of restrictions and a way of testing them that works well. I've made a note to think about it more later.

In trunk, and in the 2.7.1 branch, I've converted most of the uses of lazy val to def. I left isLeft and isRight as think that they are unlikely to hold onto an unbounded amount of memory. If someone can prove me wrong, let me know and I'll change them as well.

@DRMacIver said:
isLeft and isRight are unlikely to hold onto an unbounded amount of memory, but they don't serve a useful purpose either and continue to bloat the memory footprint of Either instances. It's not like it's an expensive computation. You could make it even less so by making them abstract and override them in Left and Right.

While I'm making suggestions, wouldn't it be better if Either were a sealed abstract class so you could get pattern match warnings?

@ijuma said:
I noticed the same thing as DRMacIver but actually tested this to have some numbers to back me up. Making isLeft and isRight a def is actually faster, even if by a very small amount (JDK6u4), so there's no reason to make them lazy vals.

This is not unexpected because lazy vals involve a volatile read for thread-safety reasons (if I am not mistaken) and instanceof checks are intrinsic operations in recent JVMs.

Geoffrey Alan Washburn (washburn) said:
I've made all these changes. Given that Left and Right are final, I cannot see any sensible way someone could be using Either as a mixin.

Further suggestions are welcome, but put them in new tickets to make them easier to keep track of separately.

@ijuma said:
Thanks!

@odersky said:
Milestone next_bugfix deleted