Why not simply FnOnce, FnFew?

All in all, think of the Fn… traits as: FnOnce() is CallableThroughOwnedAccess: trait CallableThroughOwnedAccess { fn call(self); } FnMut() is CallableThroughUniqueAccess: trait CallableThroughUniqueAccess { fn call(&mut self); } Fn() is CallableThroughSharedAccess: trait CallableThrough…

FnMut and Fn both can be called multiple times, but the difference is that FnMut cannot be called concurrently - this is necessary restriction to avoid data races, in case when closure is used to modify its environment.

FnMut acts like a &mut self receiver, and Fn acts like a &self receiver, which matters not only for concurrency, but also for variance . ( Example )

Can you give an example where Rust prevents concurrent use of FnMut but allows that for Fn?

[image] maxim-ge: Can you give an example where Rust prevents concurrent use of FnMut but allows that for Fn ? Any place where function is called through shared reference. For example, any rayon::ParallelIterator method takes impl Fn and not impl FnMut, for exactly this reason - it will be sh…

For example, any rayon::ParallelIterator method takes impl Fn and not impl FnMut , for exactly this reason - it will be shared between several threads Will Rust prevent sharing FnMut or it is just a convention? Also: If we have few Fn , one FnMut and they are not synced then we do have probl…

[image] maxim-ge: Can you give an example where Rust prevents concurrent use of FnMut but allows that for Fn ? The simplest way to show concurrency is through parallelism, so you'll mainly see three kind of closures: FnOnce(), for "the callee will need to call the callback at most once" ki…

[image] maxim-ge: Will Rust prevent sharing FnMut or it is just a convention? It will not prevent sharing, but it will prevent calling. To call the FnMut, you must use unique reference to it, which is possible if you have either this (non-shared) reference or direct (also non-shared) ownershi…

[image] maxim-ge: Will Rust prevent sharing FnMut or it is just a convention? In Rust, soundness is never just a convention.

[image] Yandros: And since in Rust there are situations where both the implementors, or the callback-callers require different levels of access to the callback, it is necessary to feature that trinity / trifecta / troika / triumvirate of traits, in the same fashion that we have self/&mut self/&s…

Yeah, sure, and Future could be an alias for a HRTB-ed FnOnce, and more generally, you can express all method-based traits (≠ marker traits) as aliases of some FnOnce-based "expression" :grinning_face_with_smiling_eyes: Be it as it may, the thing is that expressing the & vs. &mut distinction is an …

Rationale behind Fn, FnMut and FnOnce design

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H2CO3 September 3, 2021, 11:40am 11

In Rust, soundness is never just a convention.

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Rationale behind Fn, FnMut and FnOnce design

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