As I've been finding myself constantly desiring to alias expressions and identifiers for many years, I'd like to ask again whether it would be possible to extend either the alias-statement or the with-statement. It would be great to relieve the constant tension between descriptiveness, (often outside your control), and brevity in a straightforward manner.

There is something missing when your fields and parameters start out with proper names but end up as cryptic abbreviations when you start using them. Sometimes you want to look at an identifier and understand its purpose, other times you want to look at a bunch of expressions and statements and understand their pattern. I think an expressive language should support both needs without tension. It would also isolate names and reduce editing when names change.

Some thoughts about extending the with-statement were brought up here earlier:
http://forum.dlang.org/post/txpifmwpmmhsvcpbcijw@forum.dlang.org
I don't care much whether it would be with, alias or possibly something clever already existing, but a solution should be easy to use, easy to read and shouldn't introduce any possible overhead at runtime.

1) Are there any technical blockers for extending the alias- or with-statement?
2) Does it carry its own weight?

On Monday, 23 May 2016 at 14:05:43 UTC, deed wrote:
> Some thoughts about extending the with-statement were brought up here earlier:
> http://forum.dlang.org/post/txpifmwpmmhsvcpbcijw@forum.dlang.org
> I don't care much whether it would be with, alias or possibly something clever already existing, but a solution should be easy to use, easy to read and shouldn't introduce any possible overhead at runtime.

From the linked thread:

>        ref M()   { return matrix.rawArr; }
>        ref Ex1() { return e1.someProperties.someModulusX; }

If we had local refs, we could use this instead:

ref m = matrix.rawArr;

The difference is m is already computed, it is not recomputed each time m is read, unlike M(). I think the reason D doesn't support local refs is because it would make it harder to design @safe, particularly with the planned @rc ref-counting. Because M() above is only a return reference, it can't live longer than the data it references. My ref m could persist longer than matrix.rawArr using (naive) reference counting.

BTW local refs are possible in @system code:
http://forum.dlang.org/post/lmuokynffgljzvrpvkvd@forum.dlang.org

On Monday, 23 May 2016 at 15:18:51 UTC, Nick Treleaven wrote:
> On Monday, 23 May 2016 at 14:05:43 UTC, deed wrote:
>> Some thoughts about extending the with-statement were brought up here earlier:
>> http://forum.dlang.org/post/txpifmwpmmhsvcpbcijw@forum.dlang.org
>> I don't care much whether it would be with, alias or possibly something clever already existing, but a solution should be easy to use, easy to read and shouldn't introduce any possible overhead at runtime.
>
> From the linked thread:
>
>>        ref M()   { return matrix.rawArr; }
>>        ref Ex1() { return e1.someProperties.someModulusX; }
>
> If we had local refs, we could use this instead:
>
> ref m = matrix.rawArr;
>

Note that this wouldn't work with rvalues, which `with` supports.

> The difference is m is already computed, it is not recomputed each time m is read, unlike M(). I think the reason D doesn't support local refs is because it would make it harder to design @safe, particularly with the planned @rc ref-counting. Because M() above is only a return reference, it can't live longer than the data it references. My ref m could persist longer than matrix.rawArr using (naive) reference counting.

At some point during the `scope` discussion, Walter wanted to allow local `ref`s, so I guess he's not opposed to the idea. As far as I understand, the compiler already supports them internally, as they can result from lowering certain constructs, there's just no syntax for them. They wouldn't pose a problem for lifetime tracking, because they can never be assigned to, only initialized once.

On Monday, 23 May 2016 at 15:18:51 UTC, Nick Treleaven wrote:
> If we had local refs, we could use this instead:
>
> ref m = matrix.rawArr;
>
> The difference is m is already computed, it is not recomputed each time m is read, unlike M(). I think the reason D doesn't support local refs is because it would make it harder to design @safe, particularly with the planned @rc ref-counting. Because M() above is only a return reference, it can't live longer than the data it references. My ref m could persist longer than matrix.rawArr using (naive) reference counting.
>
> BTW local refs are possible in @system code:
> http://forum.dlang.org/post/lmuokynffgljzvrpvkvd@forum.dlang.org

Just to be clear: I'm not looking for anything touching the semantics, just a simple replacement mechanism/macro expansion. Recomputation or not, @safe or @system, should be no different from the expanded code. So within a scope, after aliasing 'm', 'm' should be replaced by 'matrix.rawArr'. Everything stays the same as is, even error messages.

{
  // Some scope

  alias i = instance.targetIdx;
  alias m = matrix.rawArr;

  m[i] = m[j] + m[k];
  // detected and conceptually replaced in one of the earlier compiler passes by
  // matrix.rawArr[instance.targetIdx] = matrix.rawArr[j] + matrix.rawArr[k]
}

On Monday, 23 May 2016 at 17:03:32 UTC, Marc Schütz wrote:
> On Monday, 23 May 2016 at 15:18:51 UTC, Nick Treleaven wrote:
>> If we had local refs, we could use this instead:
>>
>> ref m = matrix.rawArr;
>>
>
> Note that this wouldn't work with rvalues, which `with` supports.

OK. I suppose supporting local refs is a good reason not to allow rvalues to be passed as const ref arguments, if the function also returns by ref.

>> I think the reason D doesn't support local refs is because it would make it harder to design @safe, particularly with the planned @rc ref-counting. Because M() above is only a return reference, it can't live longer than the data it references. My ref m could persist longer than matrix.rawArr using (naive) reference counting.
>
> At some point during the `scope` discussion, Walter wanted to allow local `ref`s, so I guess he's not opposed to the idea. As

Great :-)

> far as I understand, the compiler already supports them internally, as they can result from lowering certain constructs, there's just no syntax for them. They wouldn't pose a problem for lifetime tracking, because they can never be assigned to, only initialized once.

What about:

@safe unittest
{
	RCArray!int arr;
	ref r = arr[0];
	arr.destroy; // refcount drops to zero, arr.impl memory freed
	r++; // writes to unallocated memory
}

On Monday, 23 May 2016 at 17:43:49 UTC, deed wrote:
> On Monday, 23 May 2016 at 15:18:51 UTC, Nick Treleaven wrote:
> Recomputation or not, @safe or @system, should be no different from the expanded code. So within a scope, after aliasing 'm', 'm' should be replaced by 'matrix.rawArr'. Everything stays the same as is, even error messages.

I think it's more useful to support local refs than alias expressions. In fact apart from the rvalue support, I would rather use a local ref than the existing with statement (assuming the ref was memory safe). The only exception is when using `with (EnumType)`.

Also, compile-time expressions use enum, not alias. It could be changed, but I think it's useful when reading meta-programming code to clearly see the difference.

Currently aliased symbols are expanded by the compiler in error messages, but that's probably not ideal.

On 24/05/2016 14:48, Nick Treleaven wrote:
> On Monday, 23 May 2016 at 17:03:32 UTC, Marc Schütz wrote:
>> On Monday, 23 May 2016 at 15:18:51 UTC, Nick Treleaven wrote:
>>> I think the reason D doesn't support local refs is because it would
>>> make it harder to design @safe, particularly with the planned @rc
>>> ref-counting.
>> They wouldn't pose a problem for lifetime tracking,
>> because they can never be assigned to, only initialized once.
>
> What about:
>
> @safe unittest
> {
>      RCArray!int arr;
+      arr.length = 1;
>      ref r = arr[0];
>      arr.destroy; // refcount drops to zero, arr.impl memory freed
>      r++; // writes to unallocated memory
> }

Here I think local refs must be prevented from initialization by return ref (-dip25). The @rc DIP and RCArray would use return ref. It would be OK to initialize a local ref with a ref function result that is not return ref. Naturally, this would be @safe:

auto slice = [7];
ref r = slice[0];
slice.destroy;
r++; // slice memory still allocated

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On Tuesday, 24 May 2016 at 13:48:41 UTC, Nick Treleaven wrote:
> What about:
>
> @safe unittest
> {
> 	RCArray!int arr;
> 	ref r = arr[0];
> 	arr.destroy; // refcount drops to zero, arr.impl memory freed
> 	r++; // writes to unallocated memory
> }

You're calling `destroy` explicitly, what else would you expect to happen?

On Wednesday, 25 May 2016 at 19:47:06 UTC, Nick Treleaven wrote:
> On 24/05/2016 14:48, Nick Treleaven wrote:
>> What about:
>>
>> @safe unittest
>> {
>>      RCArray!int arr;
> +      arr.length = 1;
>>      ref r = arr[0];
>>      arr.destroy; // refcount drops to zero, arr.impl memory freed
>>      r++; // writes to unallocated memory
>> }
>
> Here I think local refs must be prevented from initialization by return ref (-dip25). The @rc DIP and RCArray would use return ref. It would be OK to initialize a local ref with a ref function result that is not return ref. Naturally, this would be @safe:
>
> auto slice = [7];
> ref r = slice[0];
> slice.destroy;
> r++; // slice memory still allocated

To elaborate: neither `scope` nor reference counting can ever protect you against explicit premature destruction of a still-referenced owner. But there is a slightly different problematic scenario:

RCArray!int arr = [7];
ref r = arr[0];
arr = [9];        // this releases the old array
r++;              // use after free

But this issue exists even without locale `ref`s:

void foo() {
    RCArray!int arr = [7];
    bar(arr, arr[0]);
}

void bar(ref RCArray!int arr, ref int r) {
    arr = [9];    // this releases the old array
    r++;          // use after free
}

It can be solved in one of two ways: Either by making the owner (`arr`) non-mutable during the existence of the references, thereby forbidding the call to `bar()` (I would prefer this one, as it's cleaner and can be used for many more things, e.g. the byLine problem), or by making the owner live longer by inserting the appropriate AddRef/Release pairs whenever such a situation arises.

On Thursday, 26 May 2016 at 08:29:41 UTC, Marc Schütz wrote:
> On Wednesday, 25 May 2016 at 19:47:06 UTC, Nick Treleaven wrote:
>> On 24/05/2016 14:48, Nick Treleaven wrote:
>>> @safe unittest
>>> {
>>>      RCArray!int arr;
>> +      arr.length = 1;
>>>      ref r = arr[0];
>>>      arr.destroy; // refcount drops to zero, arr.impl memory freed
>>>      r++; // writes to unallocated memory
>>> }
>>
>> Here I think local refs must be prevented from initialization by return ref (-dip25). The @rc DIP and RCArray would use return ref. It would be OK to initialize a local ref with a ref function result that is not return ref.
>> ...
>
> To elaborate: neither `scope` nor reference counting can ever protect you against explicit premature destruction of a still-referenced owner. But there is a slightly different problematic scenario:
>
> RCArray!int arr = [7];
> ref r = arr[0];
> arr = [9];        // this releases the old array
> r++;              // use after free

This is the same situation. There's nothing special about destroy, it just assigns arr = arr.init. destroy is @safe so in fact it must work with safe RC. You seem to have ignored my suggestion (which maybe wasn't clear enough) to statically prevent the above from compiling using:

RCArray(T) {
    ...
    ref opIndex(size_t) return;

Local refs cannot be assigned from a function returning ref if that function has any parameters marked with the return attribute. If there is no attribute, local refs + function returning ref is OK.

> But this issue exists even without locale `ref`s:
>
> void foo() {
>     RCArray!int arr = [7];
>     bar(arr, arr[0]);
> }
>
> void bar(ref RCArray!int arr, ref int r) {
>     arr = [9];    // this releases the old array
>     r++;          // use after free
> }

This is the reason for the @rc DIP.

> It can be solved in one of two ways: Either by making the owner (`arr`) non-mutable during the existence of the references, thereby forbidding the call to `bar()` (I would prefer this one, as it's cleaner and can be used for many more things, e.g. the byLine problem)

I don't see directly how this affects byLine.front, that does not return a reference.