[zengid]

> The term “value identity” is not defined anywhere in this post, nor can I find it elsewhere in Mojo’s documentation, so I’m not clear on how Modular claims that Mojo solves the problem that Pin is meant to solve

I don't claim to know the answer either, but it reminds me of a great talk from Dave Abrahams, who worked on the value semantics for Swift together with Chris Lattner (who started Mojo). The talk is "Value Semantics: Safety, Independence, Projection, & Future of Programming" [0]

[0] https://www.youtube.com/watch?v=QthAU-t3PQ4

[withoutboats3]

It's clear that Mojo is in some sense inheriting Swift's notion of "value semantics," but Rust also has "value semantics" in the same sense. Rust just also has references as first class types, whereas Swift (and as far as I can tell, Mojo) only allows references as a parameter passing mode; Mojo expands on Swift's inout parameters by having an immutable reference passing mode as well.

Not being able to store references in objects does solve the problem of "self-referential structs" in that you just can't implement code like the code Rust compiles to, but that isn't at all what the quoted paragraph says about Mojo so I am quite lost as to what they mean.

[demurgos]

My understanding of _value identity_ refers to the `StableDeref`/yoke approach to self-regerential structs. The value is constructed at a stable address (usually some heap allocation) and you always access it through some pointer. The address is the value's identity. The pointer can move, but the value doesn't move.

[withoutboats3]

Could you link to a source for this in Mojo's documentation? This would be a logical interpretation, but it would mean Mojo is planning to adopt a much worse implementation of async than Rust and the post is claiming that Mojo is both faster and easier than Rust.

[demurgos]

I'm not familiar with Mojo, so my understanding above was based on their blog post that you linked, and assumptions based on context. Checking their website, I find a sentence equating "identity" with "having an address" though:

> So far, we've talked about values that live in memory, which means they have an identity (an address) that can be passed around among functions (passed "by reference").

Source: https://docs.modular.com/mojo/manual/lifecycle/life#trivial-...

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If their self referential structs require indirection, I agree that they're weaker then what's available in Rust. Hopefully they provide more details at some point. The "No pin requirement" section in particular focused on Mojo's async ergonomics, not Mojo's async perfs.