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

Brief: members[S] builds a struct by the mapping rule (ADR-grammar-2 D3, D5, #1019 step 3a)

Facts: Traps already met

Score: #1019 step 3, the element that makes Json::read[T] a typed read; S=A, O=~3M tokens, C=~2.5M tokens, parser, grammar_types.nika, grammar checker and emitter. Follows j2 (match on a kind, 1ee4cca) and j3 (meta::Field); this step does not need j3.

Problem (probe it with target/fast/nikaia run at your start, in your scratchpad)

struct Point {
	x: i64,
	y: i64 = 0,
	label: String?,
}

struct Line {
	from: Point,
	to: Point,
}

grammar J {
	rule WS = "" -> { }
	entry rule read[T] -> T = v:Value[T] eof -> { v }
	rule Ch -> i64 = x:any if(x != '"') -> { 1 }
	rule Key -> String = "\"" s:text(Ch*) "\"" ":" -> { s }
	rule Null = "null" -> { }
	rule Value[T] -> T = match T::kind {
		I64 => d:dec[i64](digit+) -> { d }
		Text => "\"" s:text(Ch*) "\"" -> { s }
		Maybe(A) => v:Value[A] -> { v }
		Struct(_) => v:Object[T] -> { v }
	}
	rule Object[S: Struct] -> S =
		"{" => m:members[S](Key, Value[_], ","; null: Null) "}" -> { m }
}

fn main() {
	let l: Line = J::read[Line](Bytes("{\"to\":{\"x\":3,\"label\":\"b\"},\"from\":{\"y\":2,\"x\":1,\"label\":null}}".bytes().collect())) catch {
		println("error")
		return
	}
	let a = l.from.label ?? "-"
	let b = l.to.label ?? "-"
	println(f"{l.from.x} {l.from.y} {a} {l.to.x} {l.to.y} {b}")
}

Today (architect’s probe, 2026-10-31): Expected ')' here, but found ',' at members[S](Key, …. Wanted: 1 2 - 3 0 b (members in any order; to.y missing takes its default 0; from.label given null goes into the String?; to.label given).

Decision (accepted records, no owner decision needed)

Locations

To build

  1. members[S](key, value, sep; null:, unknown:) with the mapping rule. Tests in tests/compiler/tests/:
    • grammar_members_builds_a_struct.nika + .stdout: the probe above (nested struct, any order, default, null into T?), plus a missing T? member that becomes null;
    • grammar_members_a_missing_field_fails.nika + .stdout (a field without default and not T? absent: the catch prints the message), grammar_members_a_member_twice_fails.nika + .stdout, grammar_members_null_into_a_field_that_takes_none_fails.nika + .stdout (null into an i64 without default; and into one with a default takes it);
    • grammar_members_an_unknown_member.nika + .stdout: fails without unknown:, skipped with unknown: Skip (a rule that reads a value and builds nothing). Commit grammar: members[S] builds a struct by the mapping rule (ADR-grammar-2 D3, D5), Part: #1019.

Done when

The new tests pass through nikaia test (one program alone: docs/builder-facts.md, Traps already met), each failed once against the compiler without your change (say so under Tests:); j1’s, j2’s and the other grammar_* tests still pass; lower-std fixpoint clean; the cheap checks of Rules. Changelog: one line.

Stop and report if

j2 is not on develop; the backend cannot choose a value rule by a key read just before it (say what the generated Rust needs); building S needs a construction form the language does not have (D3: the compiler builds it in the lowering; no field.set); an edit outside the parser, the AST, node_numbers.nika, check_*.nika, emit_*.nika, grammar_*.nika, grammar_run.rs, crates/nikaia-std/src/grammar.rs, their lowered files and the new tests; a form beyond ADR-grammar-2; an error in another agent’s file. Stop about 15 turns before max_turns, commit what is finished and report partial.

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/j4-1019-members.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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