Design model

Platform spec article

Design model

Back to Parser and AST contracts ◎ Open feature in platform map

Spec standingStandard

Owner

Piotr Mikstacki pmikstacki@cybernomad.it

Submitter

Piotr Mikstacki pmikstacki@cybernomad.it

Current document

ADRs (3)

1. 0001-feature-hub-authority Feature hub authority Accepted

   This feature hub owns normative MUST/SHOULD contract text for Parser and AST contracts.

   Context

   Sibling articles under this feature previously restated requirements in inconsistent forms.

   Decision

   This feature hub owns normative MUST/SHOULD contract text. Sibling articles must not redefine hub requirements and should link here for authority.

   Consequences

   Contract changes start on the hub or in linked ADRs, then propagate to articles and implementation anchors.

   Verification anchors

   • site/website/src/content/docs/platform-spec/compiler/front-end/parser-and-ast-contracts/index.mdx
   • article bundle under the same feature directory.

   Open full ADR page

2. 0002-spec-over-implementation-notes Specification over implementation notes Accepted

   Platform-spec text supersedes informal crate comments for Parser and AST contracts.

   Context

   Implementation crates accumulated informal notes that diverged from published contracts.

   Decision

   Normative platform-spec prose and ADRs under this feature supersede informal comments in implementation crates until explicitly migrated into spec text.

   Consequences

   Engineers file spec/ADR updates when behavior changes; crate comments are non-authoritative for conformance arguments.

   Verification anchors

   • compiler/crates/beskid_analysis/src/beskid.pest
   • compiler/crates/beskid_analysis/src/parsing
   • compiler/crates/beskid_analysis/src/syntax/items

   Open full ADR page

3. 0003-pest-grammar-single-surface Single pest grammar surface Accepted

   Multiple parser entrypoints caused span drift.

   Context

   Multiple parser entrypoints caused span drift.

   Decision

   beskid.pest and beskid_analysis::parsing are the authoritative parse surface; AST contracts derive spans from this pipeline only.

   Consequences

   Alternate parsers must not ship without an ADR and conformance fixtures.

   Verification anchors

   • compiler/crates/beskid_analysis/src/beskid.pest
   • compiler/crates/beskid_analysis/src/parsing.

   Open full ADR page

Articles

• Contracts and edge cases Non-negotiable parser contracts and common edge-case handling rules. article Standard
• Design model Mental model for how the front-end parser and syntax model are organized. article Standard
• Examples Concrete examples that show parser contracts in daily compiler work. article Standard
• FAQ and troubleshooting Common parser and syntax questions with practical debugging guidance. article Standard
• Flow and algorithm Step-by-step parse flow from source text to syntax structures. article Standard
• Verification and traceability How to verify parser behavior and trace docs back to source. article Standard

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OKfeature · 0 SpecSection(s) · 4 H2 heading(s)

Full tree: run pnpm verify:platform-spec-layout (writes src/generated/platform-spec-layout-report.json).

Related spec docs

• Feature
  Parser and AST contracts

  Parent feature

• Feature
  Rules and diagnostics catalog

  Semantic rules consume parser outputs

• Feature
  Compiler Mod SDK

  Compiler Mod SDK contract surfaces

• Feature
  Grammar and parser contract

  Pest productions feeding AST builders

The front-end is split into three layers:

1. Grammar layer (beskid.pest): token and production definitions.
2. Parsing layer (src/parsing): traversal helpers that map parse trees into typed syntax nodes.
3. Syntax layer (src/syntax/items, src/syntax/types): stable Rust structs/enums consumed by resolver, analysis, docs, and formatter paths.

For newcomers, the practical rule is simple: if a language construct changes, update grammar and syntax together, then confirm semantic passes still read the resulting nodes as expected.

extend type in the module AST

extend type is a top-level module item for type extension (see extend type). Parser responsibilities:

1. Discovery order — Program.items preserves source order; mod host discovery walks items deterministically.
2. Merge semantics — Generated extend type contributions merge with hand-authored extensions under host validation before semantic analysis.
3. Legacy removal — MetaDefinition AST nodes must be removed from parser output.

HIR lowering for extend type is documented in AST and HIR shape contract.