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agent: builder model: opus max_turns: 120 budget_usd: 30 —

Brief: #859 steps 2 and 8, a type as an argument (schema(User)) and f[T]() with NK1233 narrowed

Score: owner’s direction (2026-10-31): #859 first; #1019 (Json::read[T], ADR-grammar-2) waits on exactly these two steps. Steps 1, 3, 4 are built (78897e3, 3feae8b, 528c72a; 0.0.1317): T::kind, T::name, field.ty::…, match T::kind. Steps 5-7 are not yours.

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

struct User { name: String, age: i64 }
fn schema[T](T) -> String {
    match T::kind {
        Text => "string",
        I64 => "integer",
        Struct(s) => {
            var out = "{"
            for field in s::fields { out += f"{field.name}:{schema(field.ty)} " }
            out + "}"
        }
        else => "?"
    }
}
fn pick[T](n: i64) -> String { T::name }
fn main() {
    println(schema(User))          // step 2: a type the call names
    println(pick[User](1))         // step 8: no NK1233, nothing else says what T is
}

fn schema[T](T) is expected to be a parse error today, and pick[User](1) too (or NK1233-less refused); confirm and say what you found.

Decision (accepted records, no owner decision needed)

Locations

To build

  1. Step 2. fn f[T](T) parses; a call fills T from the type written there (a named type, a type parameter, field.ty, a match-arm binding), recursion included (schema(field.ty), schema(it)). Tests in tests/compiler/tests/: type_as_argument.nika + .stdout (the schema probe above over a struct holding a String, an i64 and a nested struct); type_as_argument_value_refused.nika + .stderr (schema(5) and, in a normal fn g(x: i64), g(User): both NK1230). Commit parser, check, emit: a type as an argument where the parameter list holds one (ADR-330 D1, #859 step 2), Part of #859.
  2. Step 8. f[T](…) at a call fills the type parameters in order. Tests: type_arguments_at_a_call.nika + .stdout (pick[User](1), no warning) and type_arguments_warned.nika + .stderr (fn id[T](x: T) -> T, id[i64](3): warning[NK1233] naming #514). Commit parser, check: f[T]() fills type parameters; NK1233 only where a value could say the type (ADR-330 D9, ADR-grammar-2 D11, #859 step 8), Part of #859.

If step 8 is too large for this run, commit step 2 and as much of step 8 as is verified, and say what is left under Needs:.

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:); lower-std fixpoint clean; cargo build --profile fast -p nikaia; the gate of .claude/agents/builder.md; pushed. Changelog: one line per step. Run the existing reflection tests (reflected_field_ty, match_on_a_kind*, type_kind_*) too.

Stop and report if

An edit is needed outside crates/nikaia/src/parser/mod.rs, crates/nikaia-std/src/tools/check_*.nika, emit_*.nika, crates/nikaia/src/build_time.rs, their lowered files and the new tests; a form beyond ADR-330 / ADR-grammar-2 is needed (a method’s x.f[T]() is not decided: refuse it as today); 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/b859c-type-argument.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, 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: ... / 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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