Universe / Topic Slang — Implementation Plan

RakuAST slang that makes caller topic ($_) an explicit operand for universe-based set operators (and optionally bare navigation), eliminating nqp::ctxcaller / getlexrel digs.

Related: Raku/problem-solving#524 (callsite $_ injection — we do not wait on a core will ast trait; we use a Qwiratry slang instead).

Goals

1. Universe ops work under given (and any lexical $_), including inside Test subtest blocks.

2. No runtime topic dig for those ops (universe-from-caller / navigation-topic via ctxcaller retired for this path).

3. RakuAST only — legacy grammar unsupported for Qwiratry.

4. query-ast { } still returns Qwiratry operator nodes with the universe captured as a real AST operand (not resolved only at eval time via NQP).

5. Prefer a Slangify Grammar + Actions approach (same family as Mold slang).

Non-goals for the first landing (historical; Phase 3 bare-nav also landed):

• Supporting non-RakuAST Rakudo builds.

• Lizmat’s proposed core will ast trait.

• One-use Topic activation (blocked by infix categoricals / MAIN refresh).

Problem (short)

Universe operators (parens ⩂ ⊙ ⊡ ∁) and bare nav prefixes used to call navigation-topic(nqp::ctxcaller(…)). After 6.d, $_ is not dynamic; given’s topic is often invisible to NQP lexpad walks from a callee. Explicit $_ = $universe works; given often does not. Fixed by Topic slang + explicit $universe / bare-nav helpers (no dig).

Target semantics

Universe infixes / prefix

Preferred runtime signature sketch:

multi sub infix:<⋂>(Mu $left, Mu $right, Mu $universe = $*QWIRATRY-UNIVERSE) { ... }
# Actions always pass $universe explicitly as lexical $_ from the callsite AST.
# Default optional only as a safety net during migration; remove dig entirely.

If a three-arg infix is awkward in RakuAST ApplyInfix (binary-only), Actions emit an explicit call to &infix:<⋂> (or a named helper &qwiratry-universe-nand) with three args: $left, $right, $_.

Bare navigation (Phase B)

Architecture

Landed load order (two uses — one-use activation is not viable on current Rakudo):

use Qwiratry::Query::Slang;         # re-exports Ops (operators)
use Qwiratry::Query::Slang::Topic;  # Slangify TopicGrammar/TopicActions — AFTER Ops
        │
        ▼
Importer CU parse → Actions rewrite PREFIX/INFIX / given
        │
        ▼
Rewrite matching universe and bare-nav nodes:
  inject RakuAST::Var::Lexical.new('$_') as universe / LHS
        │
        ▼
Runtime helpers: no ctxcaller; universe/LHS is a normal argument

Why two uses: (1) Slangify + many infix is export in the same CU prevents Topic activation; (2) importing operator categoricals refreshes MAIN and wipes a Topic mixin applied in the same use. Thin Query::Slang re-exports Ops via sub EXPORT { Map.new(Ops::EXPORT::DEFAULT.WHO.pairs) }.

Reuse patterns from lib/Qwiratry/Mold/Slang.rakumod:

• Topic is a file-level compunit (no unit module) so Slangify runs in the importer.

• use v6.e.PREVIEW.

• RakuAST-only actions (no legacy grammar role).

• Mold slang loads Ops then Topic so mold when bodies get the same rewrites.

Phases

Phase 0 — Spike (½–1 day)

Prove Actions can rewrite one universe form end-to-end.

1. Minimal Slangify actions that intercept ∁ (simplest: one operand today).

2. User code: given @U { say (∁ @A).List } must work inside a subtest.

3. Confirm injected node is lexical $_ (DEPARSE / AST dump).

4. Confirm query-ast({ given …; ∁ @A }) or query-ast({ $_ = …; ∁ @A }) still yields a SetDifferenceOperator (or successor) with universe operand represented somehow (literal capture vs topic sentinel — decide in spike).

Exit: spike branch or throwaway script + short note of which AST node types were rewritten (ApplyPrefix vs Call::Name vs custom).

Phase 1 — Runtime API without dig

1. Change universe multis to take explicit $universe (positional 3rd or leading, pick one and stick to it).

2. Remove universe-from-caller from those multis (keep navigation-topic for bare nav until Phase B).

3. Temporary bridge: if $universe undefined, throw a clear error pointing at slang/given/$_ = — do not silently dig.

4. Update unit tests that used $_ = @U to keep working; restore given once slang lands.

Phase 2 — Topic slang for all universe glyphs

Ops: ⋂ ⩂ ⊙ ⊡ ∁.

1. Grammar/Actions: identify applications of these operators in the importer AST.

2. Rewrite to pass lexical $_ as $universe.

3. Document: universe must be the topic (given U { … } or $_ = U; …).

4. Tests:

   • t/spec/Operators/06.1-set-relational.rakutest §6.1.2 with given @U

   • nested subtest + given

   • query-ast round-trip / planner smoke

5. Changelog + Operators.md note (RakuAST required; topic = universe).

Phase 3 — Bare navigation (optional follow-up)

1. Same slang: rewrite bare nav prefixes to supply LHS = $_.

2. Fix t/spec/Operators/05-navigation-topic.rakutest under given.

3. Retire navigation-topic / ctxcaller from those prefixes.

4. Infix $x ⪪ $y unchanged.

Phase 4 — Cleanup

1. Delete unused NQP topic helpers (navigation-topic / universe-from-caller / use nqp dig).

2. META6 provides Query::Slang, ::Ops, ::Topic; Extending / Operators / README document two-use load order; RakuAST-only.

3. Full zef test on moar-2026.07+ — Topic acceptance green; remaining FAIL is table FK/catalog / t/operator/io skip-all (pre-existing vs Topic work).

Design choices (settled)

Open points for the spike

1. Exact RakuAST node shape for custom ops under 2026.07+ (ApplyPrefix / ApplyInfix / Call).

2. Whether Actions should mixin and wrap existing prefix:sym / infix:sym methods, or post-process statement ASTs.

3. How query-ast represents “universe is topic”: store NavQueryTopic-like sentinel vs require topic to be resolvable when the block runs under $*QWIRATRY-BUILD-QUERY.

4. Interaction with Mold slang when both are used in one CU (activation order).

Test plan (acceptance)

• given @U { (@A ⩃ @B).List } inside subtest → expected NAND result

• Same for ⩂ ⊙ ⊡ ∁

• query-ast({ … }) still returns non-Seq operator AST for planners

• Clear error if universe op used with undefined topic (no silent wrong U)

• (Phase B) given %repo { (⪪ <owner>).List } works

• Full zef test on moar-2026.07+ (06.1 + 05-navigation-topic verified; suite still FAIL on pre-existing table FK/catalog, io skip-all, unrelated)

Landing notes

Ops live in Qwiratry::Query::Slang::Ops. Topic slang is a separate use Qwiratry::Query::Slang::Topic after Ops/Slang: infix categoricals from operator import overwrite MAIN if Topic is mixed in the same use. Public entry Qwiratry::Query::Slang re-exports Ops via sub EXPORT.

Suggested first PR

Phase 0 spike + Phase 1 API change + Phase 2 for ∁ and ⋂ only, with §6.1.2 tests restored to given. Remaining glyphs and bare nav in follow-ups.