prometeu-studio/docs/agendas/Dynamic Semantics - Execution Model Agenda.md

Dynamic Semantics - Execution Model Agenda

Status: Working agenda (pre-spec)
Purpose: close the observable execution model for PBS before drafting the dynamic semantics spec

1. Context

Static semantics already fixes a large part of the source model, but several runtime-visible questions remain open:

• exact evaluation order,
• what counts as a trap versus an ordinary control-flow outcome,
• how FRAME_SYNC and deterministic safepoints constrain execution,
• what host calls are allowed to observe or perturb execution order.

These decisions should be closed before effect surfaces and memory/lifetime rules are written as normative dynamic semantics.

2. Decisions to Produce

This agenda must end with decisions for:

1. The observable sequencing model for expressions, statements, and blocks.
2. The normative evaluation order for arguments, receivers, selectors, and chained apply.
3. The abrupt-completion model: return, break, continue, result propagation, and traps.
4. The interaction between host calls, safepoints, and deterministic execution.

3. Core Questions

Q1. Global execution model

• Is PBS execution defined as single-threaded per program instance in v1?
• Which execution steps are observable to user code, and which remain VM-internal?

Q2. Expression evaluation order

• Is expression evaluation strictly left-to-right unless a construct says otherwise?
• For f(a, b) / f apply (a, b), what is the order of evaluating callee, receiver, and argument expressions?
• For f1 apply f2 apply x, what is the runtime order implied by the right-associative parse?

Q3. Single-evaluation guarantees

• Which constructs normatively evaluate subexpressions exactly once?
• Does that guarantee apply to switch selectors, if conditions, opt else fallback, handle expr, and bind(context, fn_name) context expressions?

Q4. Blocks and control transfer

• How are statement sequencing and tail-expression evaluation defined inside blocks?
• At what point does return, break, or continue abort further evaluation in the current construct?

Q5. Trap model

• Which runtime failures are traps rather than status values or compile-time rejection?
• Must trap categories be stable and named in the dynamic semantics spec?
• Are traps deterministic for the same program input and runtime capability set?

Q6. Host interaction

• Can host calls introduce extra traps beyond ordinary PBS runtime failures?
• Where are safepoints guaranteed around host calls, allocation-heavy operations, and loop back-edges?

Q7. Determinism boundary

• Which behaviors must be identical across conforming implementations?
• Which details may vary as implementation strategy without changing observable behavior?

4. Expected Spec Material

After discussion, this agenda should directly feed sections of Dynamic Semantics Specification.md covering:

1. execution state and step ordering,
2. evaluation order by construct,
3. abrupt completion and trap propagation,
4. host-call interaction constraints,
5. determinism requirements for conforming runtimes.

5. Non-Goals

• Revisiting grammar or typing unless the current surface is impossible to specify dynamically.
• Defining diagnostics wording in detail.
• Designing allocator APIs or collection libraries.

6. Inputs

• 1. Language Charter.md
• 3. Core Syntax Specification.md
• 4. Static Semantics Specification.md
• Heap Model - Agenda.md