Change new alias syntax to alias this : identifier

[9rnsr]

A new alias this syntax is introduced by #177, but after that, I thought that was not good.
Then, I'd like to replace it to the syntax alias this : identifier;.

Reasons:

1. alias this is a completely different concept with normal alias declaration.
2. That is more consistent with the base class list class D : C, I1, I2 {}.
3. That is fairly easy to enhanced for the multiple alias this in the future, like alias this : ident1, ident2;.

How about?

Blocked by dlang/dmd#1341

[9rnsr]

If this is acceptable, we should merge fixes before 2.061 release.

[alexrp]

I think this change makes a lot of sense. @WalterBright @andralex opinions?

[ghost]

But alias newSym = oldSym stays the same?

[9rnsr]

@AndrejMitrovic Yes, this change is just for alias this syntax only.

[ghost]

That does mean that people will have to get used to two syntaxes. The change in this pull is logical though (especially with multiple alias this), so I support it.

[dnadlinger]

@AndrejMitrovic: Three syntaxes (alias xyz this, alias this = xyz, alias this : xyz).

[9rnsr]

Three syntaxes (alias xyz this, alias this = xyz, alias this : xyz).

Might you misunderstand? The proposal in here is: we should have two syntaxes (alias xyz this and alias this : xyz) instead of (alias xyz this and alias this = xyz).

[dnadlinger]

@9rnsr: Ah, okay, I didn't look at the diff and got it wrong. In this case, I'm rather indifferent towards this change: It is a new syntax to remember, but then AliasThis is sufficiently different in behavior to a normal AliasDeclaration so that conflating them syntax-wise is questionable anyway.

[braddr]

I thinking having 'alias a = b' be different when 'a' is 'this' is a horrible idea. Either both change to use ':' or neither do. Also, a syntax change really ought to go through the newsgroup, not stealth through a pull.

[ghost]

so I support it

Thinking about it some more, the feature doesn't really pull its own weight, so I've changed my mind.

1 alias this is a completely different concept with normal alias declaration.

Agreed.

2 That is more consistent with the base class list class D : C, I1, I2 {}.

Maybe, but it wouldn't make much sense in structs. And we don't want people to assume that using alias this : A, B in a struct means the struct can also inherit via struct S : A, B. So there can be potential confusion here.

Also, a similar argument like your own point 1): alias this is a different feature from inheritance, so using the same syntax might not be a good idea.

3 That is fairly easy to enhanced for the multiple alias this in the future, like alias this : ident1, ident2;.

Using multiple alias this should be very rare, and you can still use two alias declarations instead via alias this = A; alias this = B;, so the new syntax doesn't fix a blocking issue.

[ntrel]

1. alias this is a completely different concept with normal alias declaration.

If we want to allow aliasing of superclass constructors, the current alias this syntax appears to be in conflict with normal aliases:
alias this = super.this;

So making alias this have different syntax might be helpful (if it's not too late to change).

See discussion (from 2009):
http://forum.dlang.org/post/op.u36pc3y4o7cclz@korden-pc

[9rnsr]

I opened a pull dlang/dmd#1413 to remove the syntax from the 2.061 release. We should leave room for debate about the syntax.

[andralex]

What's the status of this?

[tgehr]

Debate still pending.

[JinShil]

A quick summary to aid the discussion:

1. [enh] Issue 3011 - alias should have assignment syntax dmd#1187 introduces a new assignment syntax: alias newname = existingname and alias this = existingname syntax.
2. After the fact, it is discovered that the alias this = existingname conflicts with the ability to alias base (i.e. super class) constructors: alias this = super.this
3. Kill the syntax alias this = sym; dmd#1413 reverts the "=" syntax for alias this only.
4. This pull request and the corresponding DMD pull request, Change new alias syntax to alias this : identifier dmd#1341, suggests a colon syntax, alias this : existingname, as a workaround resulting 4 aliasing syntaxes...

// normal aliasing
alias existingname newname;
alias newname = existingname;

// this aliasing
alias existingname this;
alias this : existingname;

... with the justification that "this aliasing" and "normal aliasing" are different concepts

An alternative opThis proposal was posted by @tgehr on the forum (http://forum.dlang.org/post/ld5c9r$18lt$1@digitalmars.com)

More debate is ensuing on the DMD pull request, dlang/dmd#1341, and should probably continue there instead of here.

[CyberShadow]

Closing, seeing that the DMD pull request has been closed in January.