On 5/13/25 11:04, Basile B. wrote:
> On Monday, 12 May 2025 at 23:29:36 UTC, Meta wrote:
>> On Monday, 12 May 2025 at 23:23:27 UTC, Monkyyy wrote:
>>> On Monday, 12 May 2025 at 23:13:23 UTC, Meta wrote:
>>>> Hello, I've been working on a couple different DIPs that propose various enhancements to D.
>>>>
>>>> [...]
>>>
>>> I think dip 2 is far more important, dip 1 has work arounds and I dislike the complexity around constructors as is
>>
>> Please leave your feedback in the respective threads I linked. Thank you!
> 
> I'll drop a comment here anyway because it doesn't fit to the DIP section I think.
> 
> D really needs "variable declaration" as expressions. My opinion is that here you are again faced to a special case that the general case, i.e "variable declaration" as expression, would have solved.
> 
> ```d
> (auto a, auto b) = call(); // two VarDeclExp in the LHS
> ```
> ...

No, this syntax would not work just because there would be variable declaration as expression. You still need a) tuple literals b) dedicated unpacking logic.


> Let me enumerate all the cases we have now. You can exceptionally declare variable as expression in
> 
> - the IfStatement
> - the WithStatement
> - the WhileStatement
> - the SwitchStatement
> 
> and now what is proposed is another case: as tuple element.
> ...

Actually that is not true. The DIP does not propose dedicated tuple syntax to be added. It's just about unpacking. Anyway, it's not like that blocks work on your vision at all.

> Cant just people open their eyes ? That language construct tends to become a well defined expression.
> 
> A little joke to finish, I occasionally use that thing to count if something new is worth:
> 
> ```d
> printf("for the %dnth time\n", static int count++);
> ```
> 
> Yeah that kind of things work when you have "variable declaration" as expression.

Go ahead and implement it. It's harder, not a simplification. Suddenly, whenever you are parsing any expression, you will have to take into account the possibility that it is actually a variable declaration. The way DMD deals with lvalues is also ill-equipped to allow this to be added easily. Furthermore, you now have to do proper scope handling for short-circuiting operations.

On 5/16/25 13:24, Basile B. wrote:
> On Wednesday, 14 May 2025 at 16:56:23 UTC, Timon Gehr wrote:
>> On 5/13/25 11:04, Basile B. wrote:
>>> [...]
>>
>> No, this syntax would not work just because there would be variable declaration as expression. You still need a) tuple literals b) dedicated unpacking logic.
>>
>>
>>> [...]
>>
>> Actually that is not true. The DIP does not propose dedicated tuple syntax to be added. It's just about unpacking. Anyway, it's not like that blocks work on your vision at all.
>>
>>> [...]
>>
>> Go ahead and implement it. It's harder, not a simplification. Suddenly, whenever you are parsing any expression, you will have to take into account the possibility that it is actually a variable declaration. The way DMD deals with lvalues is also ill-equipped to allow this to be added easily. Furthermore, you now have to do proper scope handling for short-circuiting operations.
> 
> It **is** implemented but not in **D**.
> ...

I don't know why you seem to think I am not aware of this.

Clearly I meant go ahead and implement it in DMD, and deal with the politics and technical debt, in addition to refactoring an evolving code base that was not originally developed with this feature in mind.

I don't think it is a bad idea, it's just not what I had chosen to invest my increasingly scarce spare time in. IIRC Andrei also wanted this, so I don't think it is impossible for this to make it into the language.

It's just that my analysis of the cost/benefit ratio makes this an unappealing prospect to me at the moment.

> I see what would be the problem with D or rather I see what "you guys think" the problem would be. I think that this is wrong, it's not like if the parser does not already have to perform an arbitrary count of lookups in certain situations.

There is no fundamental problem, given infinite resources. I just went with a less controversial incremental language addition that was also simple enough to implement and improves language ergonomics meaningfully.

Anyway, I don't understand why you are approaching this in such a belligerent manner. (My eyes are open, thank you very much, it seems that did not magically implement your feature in DMD though.) Just understand we have to start somewhere and I am not willing to maintain my own fork of the language indefinitely until it reaches your arbitrary standard of orthogonality of features.

Again, tuple unpacking does not conflict at all with variable declarations within expressions, and variable declarations within expressions, even if you add tuple literals with dedicated unpacking assignment logic, actually do not directly cover key use cases such as unpacking a foreach loop variable, unpacking at global scope, or unpacking in a function argument list.

So again, you are just asking: "Why did you not do significantly more work to also cover use cases like `if((auto a = foo()) && a.bar()){}` ?"

It's not hard to understand why I did not do it, it's basic economics. x) The unpacking DIP would not even be in the state it is now without Meta and Nick stepping up to help push it over the finish line. I did the basic implementation of UnpackDeclaration in _2018_! It's about time for this feature to land, and I am not going to entertain the idea of delaying it just because you seem to think rewriting big portions of DMD would have been so much easier.

On 5/16/25 18:32, Timon Gehr wrote:
> I am not going to entertain the idea of delaying it just because you seem to think rewriting big portions of DMD would have been so much easier.

Even just getting tuple assignment to work is annoying:
https://github.com/tgehr/dmd/pull/12#pullrequestreview-2779380671

Feel free to fix the limitations of that PR, I think it's a necessary prerequisite to supporting tuple unpacking via variable declaration as expression.

On 5/16/25 23:45, Basile B. wrote:
> On Friday, 16 May 2025 at 16:43:05 UTC, Timon Gehr wrote:
>> On 5/16/25 18:32, Timon Gehr wrote:
>>> I am not going to entertain the idea of delaying it just because you seem to think rewriting big portions of DMD would have been so much easier.
>>
>> Even just getting tuple assignment to work is annoying:
>> https://github.com/tgehr/dmd/pull/12#pullrequestreview-2779380671
>>
>> Feel free to fix the limitations of that PR, I think it's a necessary prerequisite to supporting tuple unpacking via variable declaration as expression.
> 
> You really dont seem to get the point I tried to raise.

I am capable of understanding the points you are putting out, you are just not correct about the ones that are most important in the context of this tread.

> It's not directly about tuples.

You are making some statements and implications about tuple unpacking that are not true in the context of D/DMD, this is what I was primarily responding to, but I am happy to contradict your other incorrect statements and agree with any correct ones.

> It's about the fact that variable declarations as expressions would solve the problem

"The problem" being tuple unpacking, so actually it turns out that your point is, at least in part, directly about tuples.

> directly.

- It is not true that that would solve the problem "directly", you still need to do all the unpacking logic, and more, on top. It does not suffice to be able to declare variables anywhere in an expression.

- While it can work in theory, and the unpacking DIP is forward-compatible with a design that does work this way, it is not an efficient step towards achieving the goal of unpacking given the actual current circumstances we are faced with.

> Bonus that would make the grammar much more simple for plenty of Statements.


This is also not true. Instead of "IfCondition", they would state "Expression". And the "IfCondition" rules would be moved over to "Expression". Big deal. The grammar stays basically at the same level of complexity and any simplifications that are made are incidental and unrelated to the feature. Likely, the grammar would even get a bit more blown up because actually it can be awkward to make a variable declaration inline and have to always refer to it directly in the expression, so probably you'd want something that is at least as capable as a let binding (which C++17 has introduced as a special case in some statements).

Anyway, parsing this is not even the primary issue, though I anticipated some resistance anyway due to the pervasiveness of the contexts in which the new grammar rule would apply and some (mostly misguided) concerns about parser performance.

The biggest part of the effort here is making the new expression type work sensibly in all contexts in which expressions can occur, in a big code base that already assumes that expressions will not declare named variables. One challenge with the implementation of this feature is that it is not fully self-contained, the contexts have to play ball somehow.

This is of course something that is completely invisible if you build a new language that nobody is already using, as a greenfield effort where one of the design goals is to support variable declarations anywhere.

> That's just astonishing how people refuse to see that.

This claim is a pure hallucination. Besides exaggerations of supposed benefits to abstract notions such as grammar complexity as well as their significance, I don't see any posts here that are particularly confused about how variable declarations as expressions would work or what it would do to the grammar, parser, or semantic analysis routines.

It's not some secret knowledge, it's just not something that D has done.

It does seem like you have a distorted view of the benefit/cost ratio that others should assign to this feature in D. I would happily use it now and then if it were added, and would have integrated it with tuple unpacking if it had already been available.

However, before anything like this has any chance to be added there will be weeks of forum debate. If you recall, Guido van Rossum gave up his position as Python's BDFL over a somewhat similar feature, so I will be astonished right back at you for not seeing the potential for this to create a huge amount of drama that is not conducive to the current primary goal of being able to unpack tuples. x)

Declaring variables anywhere in an expression in a D program just does not rank very high on my list of priorities.

> I mean, are you (plurally) blind ?
It is interesting how you have not implemented this in DMD yourself, yet expect people to have done it unless they have some sort of handicap.