prometeu-studio/docs/pbs/agendas/13.2. IR and Lowering Workshop 2 - Control Flow, Evaluation Order, and Propagation.md

PBS Frontend IR and Lowering Workshop 2

Status: Active

Purpose

Run the second focused discussion for 13. Lowering IRBackend Specification.md on frontend-lowering obligations for:

• expression evaluation order,
• branching behavior,
• and propagation-related constructs at frontend IR boundary.

Why This Slice Second

After Workshop 1 closes contract/preconditions, this workshop closes construct-level obligations that must be testable in frontend scope.

Proposed Meeting Order

1. Reconfirm preserved obligations from Workshop 1.
2. Close once-only evaluation obligations in frontend IR terms.
3. Close branch and abrupt-completion representation obligations.
4. Close propagation construct obligations (optional, result, !, handle) at frontend boundary.
5. Record callable-category leftovers for Workshop 3.

Decisions To Produce

1. Which evaluation-order guarantees must remain explicit in frontend IR obligations.
2. Which branch/abrupt-completion facts are mandatory in v1 frontend lowering.
3. Which propagation constructs are in-scope versus deterministic rejection in v1 slice.
4. Which diagnostics are required when unsupported propagation constructs are used.

Candidate Decisions

1. Frontend Lowering Must Preserve Once-Only Evaluation Facts

Candidate direction:

• source-observable single-evaluation points are preserved as explicit lowering obligations,
• independent of one exact IR node taxonomy.

2. Branching Obligations Are Normative At Frontend Boundary

Candidate direction:

• selected/non-selected branch semantics remain explicit in frontend lowering contract,
• with no implicit permissive fallbacks.

3. Propagation Constructs Have Explicit V1 Status

Candidate direction:

• each construct is marked supported in v1 frontend lowering or deterministic reject,
• and this status is mirrored in Gate U fixtures.

4. Diagnostics Identity Must Be Stable For Unsupported Forms

Candidate direction:

• unsupported forms emit stable code/severity/primary attribution,
• so regression tests can lock behavior.

Questions To Resolve In The Room

1. Which propagation constructs are fully represented now versus deferred?
2. What minimum frontend IR evidence is enough to prove evaluation-order obligations?
3. Which failures should be classified as parse/static errors versus lowering errors?
4. Which fixture shapes are mandatory for Gate U here?

Expected Outputs

1. a decision note on expression/control-flow lowering obligations,
2. a decision note on propagation status map (support/reject),
3. and fixture targets for frontend conformance tests.

Explicit Deferrals For Workshop 3

• callable-category representation details,
• and service/contract/callback-specific obligations.

Inputs

• docs/pbs/specs/9. Dynamic Semantics Specification.md
• docs/pbs/specs/12. Diagnostics Specification.md
• docs/pbs/specs/13. Lowering IRBackend Specification.md
• docs/general/specs/13. Conformance Test Specification.md
• docs/pbs/agendas/archive/13.1. IR and Lowering Workshop 1 - Lowering Contract and IR Status.md