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agent: builder model: opus max_turns: 150 budget_usd: 40 —

Brief: a match on a kind in a grammar rule, and refuse (ADR-grammar-2 D2, D6, #1019 step 2)

Score: owner’s direction (2026-10-31, “bauen ja”): #1019 step 2, after j1 (type parameters on rules). S=A, O=~3M tokens, C=~2M tokens, grammar checker/emitter.

Problem (probe it with target/fast/nikaia check at your start)

grammar Lit {
    entry rule read[T] -> T = v:Value[T] eof -> { v }
    rule Value[T] -> T = match T::kind {
        Bool      => "true" -> { true } | "false" -> { false }
        Int       => d:text([0-9]+) -> { … }      // an action the existing grammar tests accept
        Maybe(A)  => "null" -> { null } | v:Value[A] -> { v }
        Text      => refuse("no strings in this test")
    }
}

Lit::read[bool]("true"), Lit::read[i64?]("null"); Lit::read[String](…) gives NK1250 with the reason; Lit::read[f64](…) (no arm) gives NK1250 naming the rule. Expected today: refused at match in a rule body. Confirm and say what you found. Use the kind names T::kind has today (Part I 4.7, ADR-330 D4); if a kind the sketch uses does not exist, take the one that does and say so in Body:.

Decision (accepted records, no owner decision needed)

Locations

To build

  1. What j1 (06e4e63) left open, needed here: substitute a rule’s type parameters inside its actions, guards and the match scrutinee of each copy (grammar_types.nika), and check a bound ([S: Struct]) at the call. Test grammar_a_type_parameter_in_an_action.nika + .stdout, and the bound refused in grammar_a_rule_bound_is_checked.nika + .stderr (the code a generic function’s unmet bound gives today).
  2. match T::kind in a rule body with per-arm patterns, only the taken arm emitted per copy, refuse("…"), NK1250 (rule and, through nested Value[A], the path of type arguments that led there), NK1310. Tests in tests/compiler/tests/: grammar_a_match_on_a_kind_chooses_the_pattern.nika + .stdout, grammar_a_type_no_arm_takes_is_refused.nika + .stderr (NK1250), grammar_refuse_in_an_arm_says_why.nika + .stderr, grammar_a_match_on_a_runtime_value_is_refused.nika + .stderr (NK1310). Commit grammar: a match on a kind chooses a rule's pattern, refuse (ADR-grammar-2 D2, D6), Part of #1019.

Done when

The new tests pass through nikaia test --project tests/compiler; each failed once against the compiler without your change (say so under Tests:); the existing grammar_* tests and j1’s tests still pass; lower-std fixpoint clean; the cheap checks of Rules. Changelog: one line.

Stop and report if

j1 is not on develop (its report says Result: done); members[S] is needed (step 3); an edit outside the parser, the AST, check_*.nika, emit_*.nika, grammar_*.nika, grammar_run.rs, the code registry, their lowered files and the new tests; a form beyond ADR-grammar-2; an error in another agent’s file.

Not yours, never stage: every file git status shows modified or untracked at your start, and every docs/briefs/* file except your report.

Rules (CLAUDE.md; this block is pasted into every brief, also a fix-forward)

Report (at most 15 lines, to exactly /home/user/Nikaia/docs/briefs/tools/j2-1019-match-on-a-kind.md.report.md)

Fields of docs/architect.md, Briefs and reports: Result: done|noop|partial|blocked (the bare word alone on that line: Result: done, never a summary after it, which cuts the batch, b869a 2026-10-27; Owner: is a question for the owner or none, never a role), Files, Commit, Body (optional, one paragraph for the commit message), Fixes, Pushed, Tests, Gate (green|red), Deviation, Found, Needs, Changelog, Owner, Disk freed, and for a move Rs2nika: (the kept N of M lines of rs2nika --score, then the three costliest hand edits; none for a task that moves no Rust, none (redesign) for a brief with Shape: redesign and no Convert: line). none where nothing, and then the bare word: Deviation: none (a note) cuts the batch (batch.nika compares exactly); a note goes into the commit message. Found: is only for a defect outside your change (it cuts the batch); a process event such as another session resetting or rewriting your files is no Found:, it goes under Needs: (m3a 2026-10-28); a converter draft that came out wrong is no Found:, it is one of the costliest edits under Rs2nika:, which the converter lane reads (e12-4b 2026-10-27); how you solved your own step goes into the commit message. Deviation: is only a step of the brief built otherwise than written or not built (a choice the brief left to you, such as keep what X needs and say so, is no deviation: it goes into the commit message; nor is building two steps in one commit when every step is built, e12-3a 2026-10-27, nor a step another commit on develop already built, shown by your test failing with that commit’s change reverted, b889 2026-10-27: name the commit in Body:, nor a test’s expected .stdout/.stderr taken from what the program really prints where the brief’s sketch of it was short or wrong, b934 2026-10-29: say so in Body:, nor a lowering that follows the ADR governing it where the brief’s expected lowering contradicts that ADR, b933 2026-10-29: name the ADR in Body:, nor a lowering other than the brief’s where rustc cannot take the brief’s, b1029 2026-10-31: say why in Body:, nor re-keying an allow-list entry (KNOWN) whose hash changed for the same program and the same reason, b-fixforward-sweep2 2026-10-28: say so in Body:, nor an existing test’s expected .stdout/.stderr updated because your change rightly changes the message it prints, b1012b 2026-10-31: name the test in Body:); how you verified (e.g. a test not run against the unfixed compiler) goes under Tests:. A noop says why under Tests: (what was probed), not under Deviation:. Final answer, one line: done <sha>, report <path> (or BLOCKED: ... / PUSH REJECTED ...).

Attribution: Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_01UVxJDvLe1uS47UcqA9y1U2

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