Feature: Member of Operator

[rikkimax]

This is a new feature, which I'm calling member of operator.
It is based upon Neat's Identifier Type.

In certain statements and expressions, there is a context, when seen with the operator it gets the member and rewrites the AST to use that instead.

Supports function calls, variable declarations, case statements for switch statements, and return expressions.
Dot expressions after it has not been implemented, although that should be done.

Why did I implement this? To prove that even I with limited knowledge can do it. It isn't a complex feature. It was less than 10 hours work with a bit of help from @dkorpel to point in the right direction!

My motivation is to make it be the mechanism to describe sum-type tags without a value type. It also applies to value type exceptions aka zero cost, to replace zero size struct support.

Test case which has all the examples in it:
master...rikkimax:dmd:member-of-property#diff-476a4c5f24136a55eb521b2a39faad10ce01dc3075af4f4ceceb82acd22242c3

But the difficult one that is worth copying Enum varF2 = identity(:Middle);.

[dlang-bot]

Thanks for your pull request and interest in making D better, @rikkimax! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#16161"

[thewilsonator]

How much of the test case needs to actually be runnable? I suspect that most of it could be made compilable.

[thewilsonator]

silly question: why does this fail? this breaks my metal model of : as shorthand for typeof(lhs) (or typeof(return) as appropriate).

[ntrel]

Sounds like it might be more useful to have a token which represents the expected type, so you can call its constructor:
fun(_(5), _.some_member).

[thewilsonator]

should also check for auto, immutable, static, shared, __gshared (any other type redundant modifier I'm forgetting?).
Also should check for initialisation at

• module scope,
• class/struct scope
• (other scopes I may be forgetting?)

[RazvanN7]

@rikkimax What are your plans with regards to this feature? This looks like something that would require a DIP.

[rikkimax]

The plan is to basically let this bitrot.

It exists to prove that it can be done without introducing a lot of complexity in the compiler.

[RazvanN7]

Ok to close? There is no practical difference in keeping this open, apart from constantly appearing in the PR queue.

[WalterBright]

This feature can be used to “type-tag” entries in sumtypes, to differentiate identically typed entries, such as (:centimeters, int | :meters, int).

An alias can do that, too:

alias centimeters = int;
alias meters = int;

[schveiguy]

Is there a drawback syntactically for using :Member? Like for ternary operations, named parameters, or array initializers?

Otherwise, I love this idea, and have loved it since I used it in Swift.

[rikkimax]

Changelog added.

[rikkimax]

Is there a drawback syntactically for using :Member? Like for ternary operations, named parameters, or array initializers?

Otherwise, I love this idea, and have loved it since I used it in Swift.

https://dlang.org/spec/arrays.html#ArrayInitializer

You need the identifier to proceed the :.

It only applies to the first term of an expression, so as far as I know there are no downsides.