agent: builder model: opus max_turns: 100 budget_usd: 20 —
match over error that names a struct error type by its name (#673)Found while reviewing #673 step 1 (commits std::regex: run-time patterns… and keeps: a parameter of a generic type…, local on develop, not pushed). Part I, Telling failures apart: a match over error “is complete when it names every type that can arrive: each variant of it, or the type by a bare name”. For an error type that is a struct this does not hold.
target/fast/nikaia, built from this tree)std struct error type by its path. regex::dynamic has throws = ["regex::Error"] in std.contracts.
use std::regex
fn main() throws {
let re = regex::dynamic("(") catch {
match error {
regex::Error => { println(f"bad pattern: {error}"); return }
}
}
println(f"compiled {re.is_match("(")}")
}is refused with NK1151 (“doesn’t handle else”) though regex::Error is the only type that can arrive. With an added else => { … } arm it passes the checker and rustc fails: Expected unit struct, unit variant or constant, found struct regex::Error - a rustc message reaching the user.
struct Bad { why: String } with impl Error for Bad, thrown by risky, caught by match error { Bad => { println(f"bad: {error}"); 0 } }: accepted and runs (bad: negative), but rustc warns Convert the identifier to snake case: the lowering writes Bad as a Rust binding pattern, which matches anything. It works only because one type arrives; with two types the second arm would be unreachable in Rust.ConfigError::NotFound(p), Part I’s example) and else are lowered correctly today; follow how the error set reaches the match there (ADR-280’s envelope) and lower a type named by its bare name or its path as the arm for that member of the set.Bad, regex::Error, io::IoError) as an arm’s pattern in a match over error covers that member of the error set; the match is complete when every member that can arrive is covered (Part I). A name that is not a type in the set keeps its existing refusal.tests/compiler/tests/, .nika + .stdout): a program struct error caught by its name; two program struct error types arriving at one catch, each named, the right arm taken for each (no else); regex::Error caught by its path without else (probe A); one with a type that cannot arrive, keeping today’s refusal (.stderr). Run each once against the unfixed compiler (stash the fix) and say in Tests: that it failed there.tests/compiler/tests/regex_a_bad_pattern_is_a_regex_error.nika without its wrapper function: regex::dynamic("(") catch { match error { regex::Error => … } } directly in main, the .stdout unchanged.target/fast/nikaia test --both-settings --project tests/compiler (all, it is a check//emit/ change); the guard set of .claude/agents/builder.md; clippy clean; lower-std at its fixpoint. One commit per step that stands alone (the fix with its tests; the regex test rewrite), Part of #673. Commit, do not push: the supervisor pushes the step-1 commits and these together. Changelog: one line under Changelog:.
The error set’s representation cannot tell two struct types apart without a design choice (report it under Owner:); a .stderr outside your tests that would change; an error in another agent’s file.
Read docs/builder-facts.md.
docs/builder-facts.md first. develop in the main checkout; git status at start: the two untracked docs/briefs/b673-regex-dynamic.md*.report.md files are not yours, never touch or stage them.git add <files> only; commit after each step; no push (this brief’s exception).cargo fmt, cargo clippy --all-targets -- -D warnings, scripts/pre-push-check.sh, and the gate of .claude/agents/builder.md (#852).CHANGELOG.md, badges; no new NK code. New tests in Nikaia only; never the full suite beyond tests/compiler. No foreground sleep, no polling; scripts in files.NIKAIA_CACHE_LIMIT=4G; delete target/release/nikaia-scratch at the end; report GB freed./home/user/Nikaia/docs/briefs/b673b-catch-by-struct-type.md.report.md)Fields of docs/architect.md, Briefs and reports: Result: done|noop|partial|blocked (the bare word alone), Files, Commit, Body (optional), Fixes, Pushed (no, by the brief), Tests, Gate (green|red), Deviation, Found, Needs, Changelog, Owner, Disk freed, Rs2nika: none. none where nothing, as the bare word. Final answer, one line: done <sha>, report <path> (or BLOCKED: ...).