# PBS AST Workshop 3

Status: Closed (2026-03-05)

## Purpose

Close AST obligations for statement and expression surfaces, including precedence/associativity representation and unsupported-form boundaries.

## Decisions To Produce

1. Mandatory statement/expression node families for v1 AST.
2. Representation obligations for precedence and associativity outcomes.
3. Deterministic rejection policy for unsupported statement/expression forms.
4. Minimum node consistency rules used by diagnostics and lowering.

## Candidate Decisions

### 1. Parsing Outcomes Must Be Visible in AST Shape

- Precedence and associativity results are reflected by explicit tree shape.
- No hidden post-parse rewrites that change source-observable meaning.

### 2. Unsupported Forms Are Deterministic Reject

- Forms outside supported syntax/lowering slice must fail with stable diagnostics.
- They must not collapse into permissive placeholder AST nodes.

### 3. AST Consistency for Downstream Use

- Statement/expression nodes needed by static semantics and lowering must always carry coherent spans and child structure.

## Questions To Resolve

1. Which expression forms are mandatory in v1 AST conformance?
2. Which recoveries are acceptable without masking parse errors?
3. What exact AST checks should Gate U fixtures assert?

## Expected Outputs

1. Decision note on statement/expression AST model.
2. Decision note on unsupported-form policy.
3. Fixture targets for precedence/associativity and rejection cases.

## Decision Outcome (2026-03-05)

Decision record: `docs/pbs/decisions/AST Statements and Expressions Decision.md`.

1. Mandatory statement/expression node families cover the supported v1 PBS syntax slice, including `block`, `let`, `return`, expression statement, `identifier`, literals, `unary`, `binary`, `call`, and `group`.
2. Precedence and associativity outcomes are normative via AST shape.
3. Non-associative/forbidden chained forms are deterministic reject with stable diagnostics.
4. AST remains structural; semantic compatibility/type rules stay in static semantics/linking.
5. Unsupported forms are deterministic reject and cannot be masked by permissive placeholder nodes.
6. Recovery is allowed, but recovered AST must remain structurally coherent with stable attribution.
7. Gate U evidence requires positive/negative fixtures with AST-shape and diagnostics assertions.

## Inputs

- `docs/pbs/specs/3. Core Syntax Specification.md`
- `docs/pbs/specs/11. AST Specification.md`
- `docs/pbs/specs/12. Diagnostics Specification.md`
- `docs/pbs/specs/13. Lowering IRBackend Specification.md`
- `docs/pbs/agendas/11.2. AST Workshop 2 - Declarations and Type Surfaces.md`