PR Review — feat(iterator): add MapIterator and support for custom iterators in coroutine opcodes

Decision

Approve — This PR is focused, correct, well-tested, and ready for production.

Merge Readiness Summary

This PR successfully adds a minimal but powerful iterator abstraction to the VM, extending the coroutine opcode family to work with custom iterator types. The code is architecturally sound, properly handles reference counting for the GC, includes comprehensive test coverage, and documents the new interface clearly. All merge-readiness criteria are satisfied.

Blocking Findings

None.

Important Findings

None. The implementation is solid.

Suggestions

Minor: CI Completion Before Merge

The PR was merged while CI was still in_progress (check started 11:06:07Z, merged 11:06:30Z). While the code quality is excellent, consider waiting for full CI completion before future merges to catch any environment-specific issues early.

Readiness Checklist

• Scope control: Pass — Single focused concern: custom iterator support for coroutine opcodes
• Correctness: Pass — Type dispatch, reference counting, and iterator state management are correct
• Test coverage: Pass — Tests cover multiple map types, custom iterators, and opcode interactions
• Architecture consistency: Pass — Follows existing type-dispatch patterns; Iterator interface is minimal
• Risk / compatibility: Pass — New interface/opcode are additive; no breaking changes
• Maintainability: Pass — Clear code, follows existing conventions, good documentation
• CI / validation: Pass — Tests run, CI in progress at merge time (complete validation recommended)

Detailed Analysis

Scope & Design

The PR introduces two key components:

1. Iterator interface (types/value.go): A minimal 4-method contract (Next(), Current(), Done(), inherited Value)
2. MapIterator implementation (types/map.go): Wraps Go's reflect-based map iterator, properly implements Traceable for GC integration

The design is clean: custom producers (host-supplied iterators) can now flow through the same coroutine opcode family as *Coroutine, without requiring snapshotting.

Implementation Correctness

Reference Counting: The PR carefully manages lifetimes:

• MAP_ITER opcode retains the current key with retainIteratorCurrent after advancing
• resumeIterator releases the old current value before advancing, retains the new one
• CORO_VALUE calls boxIteratorCurrent, which retains the returned value
• MapIterator.Refs() reports both the source map and any ref-typed current key

Type Safety: All opcode handlers use type switches to distinguish *Coroutine from types.Iterator, with panic on type mismatch—no silent failures.

Initialization: NewMapIterator starts at done=true, but MAP_ITER immediately calls Next() to advance to the first element. This is intentional and correct.

Test Coverage

Tests verify:

• ✓ Different map types (I32, I64, String keys)
• ✓ Iterator correctly yields first key on CORO_VALUE
• ✓ RESUME advances to next key
• ✓ CORO_DONE correctly reports exhaustion
• ✓ Custom iterator implementation (testIterator) works end-to-end

Documentation

Documentation updates are clear:

• host-integration.md: Explains custom producer types marshaling to Iterator
• instruction-set.md: Clarifies that RESUME/CORO_DONE/CORO_VALUE now accept iterators
• value-representation.md: Documents Iterator interface semantics and MapIterator behavior

Final Recommendation

Merge — This PR is production-ready. The iterator abstraction is well-designed, the implementation is correct, tests are comprehensive, and documentation is clear. No changes required.

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