Chained assignment

[avishnyak]

Summary

A statement like w = x = y = z is called a chained assignment in which the value of z is assigned to multiple variables w, x, and y. Chained assignments are often used to initialize multiple variables, as in a = b = c = d = f = 0 .

In some programming languages (C for example), chained assignments are supported because assignments are expressions, and have values. In this case chain assignment can be implemented by having a right-associative assignment, and assignments happen right-to-left. For example, i = arr[i] = f() is equivalent to arr[i] = f(); i = arr[i].

Motivation

Why are we doing this? What use cases does it support? What is the expected outcome?

Guide-level explanation

Explain the proposal as if it was already included in the language and you were teaching it to another programmer. That generally means:

• Introducing new named concepts.
• Explaining the feature largely in terms of examples.
• Explaining how programmers should think about the feature, and how it should impact the way they use EV2. It should explain the impact as concretely as possible.
• If applicable, provide sample error messages, deprecation warnings, or migration guidance.
• If applicable, describe the differences between teaching this to existing EV2 programmers and new EV2 programmers.

For implementation-oriented RFCs (e.g. for compiler internals), this section should focus on how compiler contributors should think about the change, and give examples of its concrete impact. For policy RFCs, this section should provide an example-driven introduction to the policy, and explain its impact in concrete terms.

Reference-level explanation

This is the technical portion of the RFC. Explain the design in sufficient detail that:

• Its interaction with other features is clear.
• It is reasonably clear how the feature would be implemented.
• Corner cases are dissected by example.

The section should return to the examples given in the previous section, and explain more fully how the detailed proposal makes those examples work.

Drawbacks

Why should we not do this?

This adds complexity to the language and there are other languages that treat assignment as statements versus expressions. A common trap is using an assignment in an if statement when an equality expression was intended (e.g. if (x = true)). This style of error would not be possible if assignments were statements.

Rationale and alternatives

• Why is this design the best in the space of possible designs?

• What other designs have been considered and what is the rationale for not choosing them?

• What is the impact of not doing this?

• Does it fit the stated goals EV2?
  [x] Low friction
  [x] Sparks joy in the developer and development team
  [x] Designed for the advanced use-case (i.e. not primarily intended to address scripting needs)
  [x] Encourages performant and data-oriented programming as a default

Prior art

Discuss prior art, both the good and the bad, in relation to this proposal.
A few examples of what this can include are:

This section is intended to encourage you as an author to think about the lessons from other languages, provide readers of your RFC with a fuller picture.
If there is no prior art, that is fine - your ideas are interesting to us whether they are brand new or if it is an adaptation from other languages.

Languages to consider:
[x] Go - No chained assignment
[x] Zig - No chained assignment
[x] C - c-style chained assignment
[x] C# (especially .NET Core) - c-style chained assignment
[ ] Kotlin
[x] Swift - No chained assignment
[x] Typescript/Javascript - c-style chained assignment

Note that while precedent set by other languages is some motivation, it does not on its own motivate an RFC.
Please also take into consideration that EV2 sometimes intentionally diverges from common language features.

Unresolved questions

• What parts of the design do you expect to resolve through the RFC process before this gets merged?
• What parts of the design do you expect to resolve through the implementation of this feature before stabilization?
• What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?

Future possibilities

Think about what the natural extension and evolution of your proposal would
be and how it would affect the language and project as a whole in a holistic
way. Try to use this section as a tool to more fully consider all possible
interactions with the project and language in your proposal.
Also consider how the this all fits into the roadmap for the project
and of the relevant sub-team.

This is also a good place to "dump ideas", if they are out of scope for the
RFC you are writing but otherwise related.

If you have tried and cannot think of any future possibilities,
you may simply state that you cannot think of anything.

Note that having something written down in the future-possibilities section
is not a reason to accept the current or a future RFC; such notes should be
in the section on motivation or rationale in this or subsequent RFCs.
The section merely provides additional information.