crazy idea: weaken the effectively final restriction

mark.yagnatinsky at barclays.com mark.yagnatinsky at barclays.com
Wed Oct 6 17:39:38 UTC 2021 

Oh yeah, if you swap the lines, then one compiles and the other doesn't.
Not nice I admit.  Not nice at all.

-----Original Message-----
From: forax at univ-mlv.fr <forax at univ-mlv.fr> 
Sent: Wednesday, October 6, 2021 1:22 PM
To: Yagnatinsky, Mark : Markets Pre Trade <mark.yagnatinsky at barclays.com>
Cc: Brian Goetz <brian.goetz at oracle.com>; raffaello giulietti <raffaello.giulietti at gmail.com>; jdk-dev <jdk-dev at openjdk.java.net>
Subject: Re: crazy idea: weaken the effectively final restriction


CAUTION: This email originated from outside our organisation - forax at univ-mlv.fr Do not click on links, open attachments, or respond unless you recognize the sender and can validate the content is safe.
----- Original Message -----
> From: "mark yagnatinsky" <mark.yagnatinsky at barclays.com>
> To: "Remi Forax" <forax at univ-mlv.fr>, "Brian Goetz" 
> <brian.goetz at oracle.com>
> Cc: "raffaello giulietti" <raffaello.giulietti at gmail.com>, "jdk-dev" 
> <jdk-dev at openjdk.java.net>
> Sent: Mercredi 6 Octobre 2021 19:05:57
> Subject: RE: crazy idea: weaken the effectively final restriction

> Re: guarded switch: I confess I have not been watching the latest 
> developments in pattern matching.  (Enhanced instanceof is great 
> though!) If it turns out that effectively final makes sense there too, 
> then I guess that's another strike against my idea; might as well 
> re-use the same concept with lambdas too.
> 
> Re: reading code too fast could result in overlooking an assignment: 
> this has recently become a "solved problem" for me, because IntelliJ 
> by default underlines local variables that are NOT effectively final.

IDEs are marvelous but if an IDE is required to understand the semantics of the languages then something is very wrong.

> 
> I do have one quibble with what you said though: my proposed semantics 
> do NOT behave differently in the two examples you listed.
> In particular, neither example compiles, since there is an assignment 
> to the variable AFTER it was already captured.
> Of course, your misunderstanding is also a strike against my idea: if 
> I can't even explain it to experts, what hope is there that mere 
> mortals can keep this straight?

yes, the two lines should be swapped, for my defense, i typed that message at a swimming pool cheering for my daughter at the same time :)

Rémi

> 
> -----Original Message-----
> From: Remi Forax <forax at univ-mlv.fr>
> Sent: Wednesday, October 6, 2021 12:24 PM
> To: Brian Goetz <brian.goetz at oracle.com>
> Cc: raffaello giulietti <raffaello.giulietti at gmail.com>; Yagnatinsky, Mark :
> Markets Pre Trade <mark.yagnatinsky at barclays.com>; jdk-dev 
> <jdk-dev at openjdk.java.net>
> Subject: Re: crazy idea: weaken the effectively final restriction
> 
> 
> CAUTION: This email originated from outside our organisation - 
> forax at univ-mlv.fr Do not click on links, open attachments, or respond 
> unless you recognize the sender and can validate the content is safe.
> Hi Mark,
> to complete Brian answer,
> 
> - Reassigning a local variable without being inside a loop is usually 
> seen as a code smell, because when you read the code a little to fast 
> you can miss the second assignment thus believe that the code behave 
> differently than what is written.
> 
> - The semantics you propose behave differently if the captured 
> variable is available outside the loop or not, so  Runnable r = () -> 
> println(s);  s = normalize(s);
> 
> will behave differently from
>  var s =
>  for(...) {
>    Runnable r = () -> println(s);
>    s = normalize(s);
>  }
>  ... s ...
> 
> If you see the local variable in SSA form, the semantics depend if 
> there is a phi or not.
> 
> - We also did not want to have a semantics depending on what you call 
> "the point of capture", i.e. we wanted to have a semantics that does 
> not explicitly specify when the lambda is created, this allows 
> implementations/translation strategies to create reusable lambda 
> instance either lazily or eagerly. This is something important because 
> we can support both invokedynamic which does the allocation as late as 
> possible and GraalVM native image / AppCDS that may store the lambda instance directly in the image.
> 
> You may think that we are talking about capture here so a lambda 
> instance can not be reused but a captured value can be proved to be a 
> constant by a static/runtime analysis.
> 
> - Effectively final is a semantics that is used not only for lambdas 
> but also for anonymous classes and recently for guarded patterns, for 
> a pattern the "the point of capture" is not clearly defined. Is it 
> when you want to evaluate the switch ? or when you evaluate the guard of the pattern ?
> 
> 
> So yes, we can try to tweak that rule but as Brian said, it just 
> doesn’t seem worth the complexity.
> 
> regards,
> Rémi
> 
> 
> ----- Original Message -----
>> From: "Brian Goetz" <brian.goetz at oracle.com>
>> To: "raffaello giulietti" <raffaello.giulietti at gmail.com>
>> Cc: "mark yagnatinsky" <mark.yagnatinsky at barclays.com>, "jdk-dev"
>> <jdk-dev at openjdk.java.net>
>> Sent: Mercredi 6 Octobre 2021 17:12:18
>> Subject: Re: crazy idea: weaken the effectively final restriction
> 
>> A good way to think about this is to zoom out and realize that there 
>> is a spectrum, with trade-offs.  At one end of the spectrum is “no 
>> capture at all”, which is simple and easy to reason about, but … 
>> unsatisfying.  At the other end is that we close over variables 
>> rather than values, which some languages do, which is defensible but 
>> leads to surprising bugs.  In the middle are options like “capture 
>> final variables only”, “capture effectively final variables only”, 
>> etc.  Your option is somewhat to the “right” of “effectively final.”
>> 
>> What you’re saying is that we could capture things that are not 
>> effectively final, if we can prove that all writes happen before the 
>> capture point in the execution order.  Java does flow analysis to 
>> determine definite assignment, so such an enhanced flow analysis is imaginable.
>> 
>> The way to think about this is whether the incremental expressiveness 
>> warrants the incremental complexity.  Yes, there are a small number 
>> of cases where the more refined analysis would make a difference, but 
>> it’s really not that many.
>> On the other side, “effectively final” is an easier concept to 
>> explain to developers; there is definitely incremental complexity in 
>> the user model here.
>> Is the trade worth it?  My sense: meh.  I don’t see a huge degree of 
>> leverage here.
>> 
>> A similar example where we chose a simpler rule than necessary was in `var`.
>> The intent is that local variable type inference is for 
>> implementation, not for API.  So you can use it for locals, but not 
>> for, say, method returns, because that’s API, not just implementation.
>> Invariably, some slightly-too-clever person asks “but, then why can’t 
>> I use it for the return of a *private* method?  That’s not API.  AHA!  Gotcha!”
>> 
>> The answer is of course simple; we could make the boundary of the 
>> feature arbitrarily complex, but if the incremental complexity makes 
>> it harder for people to reason about when they can use var and when 
>> not, then we’re not necessarily helping by making it incrementally 
>> more expressive.  A simple rule that is easy to reason about is often 
>> better than a fractally complex one that is slightly more powerful.
>> 
>> So, to summarize, the idea isn’t crazy, it just doesn’t seem worth it in the
>> balance between complexity and expressiveness.   I’d rather spend that
>> complexity budget on something with more leverage.
>> 
>> Cheers,
>> -Brian
>> 
>>> On Oct 6, 2021, at 8:51 AM, Raffaello Giulietti 
>>> <raffaello.giulietti at gmail.com>
>>> wrote:
>>> 
>>> Hi,
>>> 
>>> 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?
>>> 
>>> 
>>> Greetings
>>> Raffaello
>>> 
>>> 
>>> 
>>> On 2021-10-05 23:55, mark.yagnatinsky at barclays.com 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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