crazy idea: weaken the effectively final restriction

Raffaello Giulietti raffaello.giulietti at
Wed Oct 6 12:51:19 UTC 2021


what if the variable is reassigned after the capture?

Runnable f(String s) {
     Runnable r = () -> println(s);
     s = normalize(s); // perfectly useless assignment
     return r;

It would not be "effectively final from point of capture", so would this 
lead to a compilation error?


On 2021-10-05 23:55, mark.yagnatinsky at wrote:
> I'm not sure if this is the right list for this; if not I hope someone can redirect me.
> Java requires that local variables that are captured by a lambda must be effectively final.
> This restriction has the benefit that no one needs to worry about annoying questions such as "what are the semantics of a data race on a local variable" and other such horrors.
> But this benefit can still be obtained by a weaker restriction.  For instance, consider this code, which currently does not compile:
> Runnable f(String s) {
>                  s = normalize(s);
>                  return () -> println(s); // no can do: s is not effectively final
> }
> However, this seems a bit silly because although s is not final throughout the entire method, it's final where it actually matters.
> Namely, it is final from the point of capture to the end of the method, and that's the only condition we need to avoid those annoying questions.
> What do we gain by allowing the code above?  Well, if we don't allow it, how can the code above be fixed?  We would have to introduce a new local variable.
> That would actually be fine, except for another rule Java has, also for a good reason: one local variable name can NOT "shadow" another: all local variables must have distinct names.
> That is, we can do something like this: String s = "hello"; String s = s.trim();
> So we need to come up with a new name, and this has two costs:
> First, coming up with good names is hard, and coming up with two good names for what is basically the same thing is harder.
> Second, suppose that when we first wrote the method, the variable really was effectively final, and we needed to change it many months/years/decades later.
> We now need to update the appropriate usages to the new name.  This tends to clutter line-based diffs, creating more work for reviewers during pull requests, and also for code archeologists.
> Third (did I say two?): we must spend mental bandwidth deciding how to accomplish renaming things.  In the example above we have at least two options:
> Runnable f(String t) {// option 1: rename original
>                  String s = normalize(t);
>                  return () -> println(s);
> }
> Runnable f(String s) {// option 2: keep original
>                  String t = normalize(s);
>                  return () -> println(t);
> }
> Deciding which option is better might actually involve non-trivial tradeoffs.  Perhaps option 1 leads to a smaller diff, but option 2 leads to a better parameter name for the method signature.
> How important are good parameter names?  What if it's a private method with only one caller?  Etc.
> It seems that all this nuisance would go away almost for free if we just weaken the restriction to "effectively final from point of capture".
> The only possible downside I can see is that this would be a bit more annoying to properly specify in the language spec.
> So... opinions?  Good idea?  Bad idea?
> Mark.
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