♻️ Invert reducer ↔ delimiter control#1164
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In order to implement critical sections that cannot be interrupted and that are also able to implement sticky return, we need to make the behavior of the reducer more context sensitive. This drops the whole validation behavior and instead completely inverts control over iteration to the delimiter from the reducer. Now, the reducer becomes a pure iterator-stepping engine. The delimiter can tell it to use "next", "return", "throw" to tell it how to advance the iterator, or it can even tell it "drop" to ignore the dequeued item entirely.
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jbolda
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May 14, 2026
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Motivation
Issues #1153, #1154, and #1159 share a single root cause: cancellation
in v4 is one-shot.
The proposed fix is to have the co-routine, after cancellation, continually return, and return, and return with each yield, and then protect "critical" sections with a shield that makes normal execution for secments of the effect stream.
This PR is an attempt to enable these refactors by allowing the conetxt control exactly how the co-routine is advanced at every stem.
Approach
Invert control from the co-routine to the delimiter.