RFD: predicate caching#262
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Add design for caching predicate results using file-based watch paths. Two mechanisms: syntactic (watch field on skill groups) and runtime (stdout JSON from shell predicates). Coordinates with PM interface for depends-on() caching via Cargo.lock.
nikomatsakis
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Much better! Left a few more comments.
nikomatsakis
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Jul 22, 2026
switch to granular events on a non_exhaustive enum, and replace Volatile with WatchTime.
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| ## SDK helper | ||
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| The `symposium-sdk` crate provides a helper that reads an environment variable and emits its watch event: |
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(we should probably also have a helper to e.g. read and deserialize a JSON/TOML file or just to read a file to a string)
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What does this PR do?
Updated V4
RFD for predicate caching. Custom predicates currently fork a process on every sync. This adds a small JSONL protocol that lets a predicate report the inputs its result depends on, so Symposium can cache the result and skip reevaluation until one of those inputs changes.
Protocol
We add three granular events on a #[non_exhaustive] enum:
WatchFile(PathBuf)- result depends on this fileWatchEnv(String)- result depends on this environment variableWatchTime(usize)- result becomes stale after this many millisecondsA predicate can emit any number of events during one run. Its exit status still determines true or false. No events means cached indefinitely.
WatchTime(0)means never cached.shell(cmd)emits WatchTime(0).Storage
Cache lives at
~/.symposium/cache/predicates.jsonand is discarded on Symposium version upgrade.Disclosure questions
AI disclosure.I used an AI tool for research, autocomplete, or in other minimal ways.
AI tool authored most of this RFD