Tracking issue for ControlFlow enum, for use with try_fold and in Try · Issue #75744 · rust-lang/rust (original) (raw)

(edited to turn this into a tracking issue, as it's referenced by the unstable attributes)

This is a tracking issue for the std::ops::ControlFlow type.
The feature gate for the issue is #![feature(control_flow_enum)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also uses as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement
• Block interconversion with Results
  • Done with Implement the new desugaring from try_trait_v2 #84767
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)
  • As part of stabilizing, mention in the docs for Iterator::try_fold and Iterator::try_for_each

Unresolved Questions

• Should we change the generic parameter order? https://github.com/rust-lang/rust/pull/76204/files#r481357223 done in change the order of type arguments on ControlFlow #76614
• What methods make sense
  • (probably not the into_try ones, https://github.com/rust-lang/rust/pull/76204/files#r481515347) Removed those ones in Demote ControlFlow::{from|into}_try to pub(crate) #85645
• Are the CONTINUE/BREAK constants valuable? See Stabilize ControlFlow::{BREAK, CONTINUE} #102697
  • Removed in Remove ControlFlow::{BREAK, CONTINUE} #107398
• Should it have B = () too? (Might be nice for try_for_each uses.)

Implementation history

Initial PR that added LoopState as an implementation detail: #45595
PR that exposed as unstable and renamed to ControlFlow: #76204
Added BREAK/CONTINUE associated constants: #76318
Changed type parameter order and defaulted C = (): #76614
Add is_break and is_continue methods: #78200

I work with an organization that has a large amount of Rust graph traversal code as part of its core business. We used to use itertools's fold_while for short-circuiting functional-style loops over slices of our graphs, which is now deprecated in favor of try_fold.

try_fold is great, but the lack of a standard library provided Try implementation that makes the loop semantics clear is confusing. We created our own, which is fine, but I think it would make a lot of sense to expose LoopState and provide an example in the docs.

Originally related to: rust-itertools/itertools#469