[Proposal] Upgrading the @value decorator

owenhilyard · May 1, 2025, 7:16pm

mojo/proposals/upgrading-value-decorator.md

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# Upgrading the `@value` decorator

April 2025. Status: Proposed, not implemented.

The `@value` decorator on struct declarations were added really early in the
evolution of Mojo as a way to bring up the language and get the lifecycle
methods ergonomic enough to use. That said, it has a number of problems, and we
have traits now, which provide a better way to solve this.

This whitepaper explores the problem with them and recommends a solution that
involves traits (which didn’t exist when these were added).

Here’s a simple example of this decorator:

```mojo
@value
struct Simple:
  var x: Int
  var y: String
```

This file has been truncated. show original

This is a discussion thread for the linked proposal.

Author: @clattner

owenhilyard · May 1, 2025, 7:26pm

I agree with the problems that @value is currently causing. As much as I would prefer the general solution, that’s a ways off, and I like this more than what we have right now. However, I think that a RecordLike or SimpleValue which is at least Copyable & Movable & ExplicitlyCopyable would be desirable for the reasons mentioned in the proposal, and I’d like to see that brought in scope for the proposal.

bgreni · May 1, 2025, 9:07pm

I generally support this new behaviour, but I’m not sure why we need to change the name @value. @memberwise_init is a bit of a mouthful, so I’d rather we keep @value and just change how it works to match this proposal.

owenhilyard · May 1, 2025, 9:10pm

This is a major behavioral change, and we will need a migration period, so I don’t think we can re-use @value.

Nick · May 2, 2025, 12:18am

I think this proposal is a step in the right direction. +1.

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[Proposal] Upgrading the @value decorator

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