PureYAML

PureYAML is a dependency-free YAML package written entirely in Swift.

The goal is a Linux-, Windows-, and WebAssembly-compatible replacement for the YAML pieces that currently force packages such as OpenAPIDoctor through C-backed parsers. The package is intentionally strict about portability:

• no external SwiftPM dependencies
• no bundled C sources
• no Foundation requirement in the library target
• root Swift package layout
• macOS, Linux, Windows, and WASI build gates

Status

The current release is 0.2.0. It includes block mappings, block sequences, ordered mappings, common scalars, quoted strings, comments, flow collections, literal and folded block scalars, anchors, aliases, YAML directives, document markers, explicit built-in scalar tags, merge-key expansion, complex mapping keys, multi-document stream parsing, and a matching dumper with block and flow output policies, including multi-document stream dumping with explicit document starts. It also includes path-aware validation for structural YAML checks such as duplicate mapping keys, stream validation that preserves document indexes, and diagnostic-first validation reports for damaged YAML input. Callers that need explicit tag metadata can use PureYAML.parseTagged(_:) or PureYAML.parseTaggedStream(_:) and run tagged validation over the preserved source-shaped node tree.

It is not yet a full YAML 1.2 implementation. The internal event parser now recognizes anchors, aliases, tags, flow collections, and block scalar styles from scanner tokens, and the public value parser composes those events into PureYAML.Model.Value. The tagged parser preserves explicit scalar and collection tags without adding Foundation-backed constructors; callers can opt into PureYAML.Tagged.Constructor when they want a typed, project-owned construction policy. Use normal parsing when semantic merge-key expansion is required. Directives beyond the selected compatibility subset and automatic Data/Date construction remain out of scope for the library target. Unsupported gaps are pinned by executable tests so they produce exact errors, exact validation issues, or exact fallback value trees instead of silent parser drift.

PureYAML now has checked-corpus Yams parse-success parity, structured diagnostics and validation, JSON compatibility proof gates, internal token/event streaming, real-corpus property tests, and repeatable performance comparison artifacts. Remaining work is tracked under the production hardening and validation readiness epic: true chunked input streaming, throughput toward libyaml parity, a broader bad-YAML mutation suite, and an OpenAPIDoctor integration dry run.

Attribution

PureYAML is informed by Yams and its bundled CYaml / libyaml-derived parser, but it does not copy their implementation into Sources/. See ATTRIBUTION.md.

Documentation

• Usage: public APIs, validation, emitter options, and cross-platform gates.
• Corpus gates: real-world YAML seed sources, generated mutations, artifact format, and hardening acceptance criteria.
• Migration and support boundaries: what PureYAML can replace today and which YAML features are deliberately unsupported.
• Release process: verification commands, tag commands, and release-publishing steps.

Usage

The examples in this section are covered by Tests/DocumentationExampleTests.swift.

import PureYAML

let document = try PureYAML.parse("""
openapi: 3.1.0
info:
  title: Example API
servers:
  - url: /
""")

let yaml = PureYAML.dump(document)

try PureYAML.validate(document)

Use parseTagged(_:) when the caller needs to inspect explicit tags before project-specific construction:

let tagged = try PureYAML.parseTagged("""
value: !!timestamp 2001-01-23
""")

let tagIssues = PureYAML.Tagged.Validator().collect(tagged)

Use PureYAML.Tagged.Constructor when a project owns a custom tag policy:

let env = try PureYAML.Tagged.Constructor<String>()
    .constructingScalar(tag: .init("!Env")) { scalar, _ in
        scalar.rawValue
    }
    .construct(try PureYAML.parseTagged("!Env DATABASE_URL"))

Use parseStream(_:) when the input may contain more than one YAML document. Each returned document carries its stream index, and validation preserves that index when reporting issues:

let documents = try PureYAML.parseStream("""
---
title: First
---
- Swift
- YAML
""")

try PureYAML.validate(documents)
let streamYAML = PureYAML.dump(documents)

Typed conversion is available for scalar values, keyed mapping-backed structs, dictionary-like string mappings, unkeyed sequences, nested containers, and keyed super coders:

struct Info: Codable {
    var title: String
    var version: Int
}

let title = try PureYAML.decode(String.self, from: "Example")
let info = try PureYAML.decode(Info.self, from: """
title: Example API
version: 1
""")
let tags = try PureYAML.decode([String].self, from: """
- Swift
- YAML
""")
let value = try PureYAML.encode(42)
let yaml = try PureYAML.encodeToYAML(info)

Typed decoding runs the default validator before constructing Swift values, so duplicate mapping keys are reported as validation issues instead of silently choosing one value.

Complex mapping keys are available through PureYAML.Model.Pair.keyNode. String-keyed lookup and keyed Codable remain string-only, while model inspection, validation, and dumping preserve sequence and mapping keys.

Emitter options are explicit. The default is deterministic block-style output with quoted strings. Callers can opt into conservative plain strings, safe literal block scalars for multiline strings, and compact flow collections:

let readable = PureYAML.Emitting.Options(
    scalarStyle: .literalBlockWhenMultiline
)

let compact = PureYAML.Emitting.Options(collectionStyle: .flow)

let yaml = PureYAML.dump(document, options: readable)
let compactYAML = PureYAML.dump(document, options: compact)

Literal block emission is intentionally conservative: multiline strings whose lines would not round-trip through the current parser are emitted as quoted strings instead. The policy allows more plain-safe block content, such as hashes without comment boundaries and indicator-like text that the parser keeps as scalar content. Flow collections always use inline scalar output.

Validation

Validation is path-aware and explicit. The default validator rejects duplicate mapping keys anywhere in a parsed document. Callers can use strict mode, where warnings fail validation, or non-strict mode, where warnings are returned while errors still throw.

let issues = try PureYAML.validate(document, strict: false)

For production-style validation over many YAML inputs, use the non-throwing report API. It validates every source, turns parse failures into diagnostics, and can render Markdown, YAML, or JSON that a consuming tool writes to disk:

let report = PureYAML.validationReports([
    .init(name: "valid.yaml", yaml: "title: Ready"),
    .init(name: "broken.yaml", yaml: "title: \"open"),
])

let markdown = report.markdownDescription(title: "YAML Validation")
let yaml = report.yamlDescription(title: "YAML Validation")
let json = report.jsonDescription(title: "YAML Validation")

For production tools that need an error body for damaged YAML, use the diagnostic-first APIs. They scan the raw source for common spacing and control character problems before parsing, then include parser and model-validation diagnostics in the same report. parseValidated(_:) and parseValidatedStream(_:) throw PureYAML.Validation.ReportError when the input cannot safely produce parsed artifacts:

do {
    let document = try PureYAML.parseValidated(yaml, file: "openapi.yaml")
    try PureYAML.validate(document)
} catch let error as PureYAML.Validation.ReportError {
    let body = error.report.jsonDescription(title: "YAML Validation")
    // Return body from the application-owned CLI or HTTP error response.
}

Custom validation rules can be layered onto the default validator or attached to a blank validator when callers want only project-specific checks. Validation tests pin exact issue paths, descriptions, severity handling, rule traversal order, strict/non-strict behavior, and duplicate-key diagnostics.

Development Contract

PureYAML must stay dependency-free and portable. Before merging changes:

• Swift tools version: 6.1
• Package products: PureYAML
• SwiftPM dependencies: none
• Hosted CI matrix: macOS build and test, Linux build and test, Windows build and test, WASM build

bash scripts/check-all.sh

That command expands to:

bash scripts/check-style.sh
bash scripts/check-namespacing.sh
bash scripts/check-forbidden-patterns.sh
bash scripts/check-changelog-touched.sh
swiftformat . --config .swiftformat --lint
swiftlint --config .swiftlint.yml --strict
swift build
swift test
bash scripts/check-linux.sh
bash scripts/check-wasm.sh

The pre-push hook runs scripts/check-all.sh so local pushes exercise macOS, Claw Mini Linux, and WASM before hosted CI repeats those gates and adds Windows.

scripts/check-wasm.sh expects a Swift toolchain with a matching Swift Wasm SDK. For Swift 6.3.2, install the SDK with:

swift sdk install https://download.swift.org/swift-6.3.2-release/wasm-sdk/swift-6.3.2-RELEASE/swift-6.3.2-RELEASE_wasm.artifactbundle.tar.gz --checksum a61f0584c93283589f8b2f42db05c1f9a182b506c2957271402992655591dd7c

License

MIT.