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PureYAML/docs/USAGE.md

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PureYAML Usage

PureYAML is a dependency-free Swift package for parsing, emitting, validating, and typed-converting the YAML subset currently covered by the test corpus.

Installation

Depend on the tagged package version:

.package(url: "https://github.com/mihaelamj/PureYAML.git", .upToNextMinor(from: "0.1.0"))

For unreleased development snapshots, use the main branch:

.package(url: "https://github.com/mihaelamj/PureYAML.git", branch: "main")

Add the product to the target that reads or writes YAML:

.product(name: "PureYAML", package: "PureYAML")

Then import the library:

import PureYAML

Package.swift for PureYAML itself keeps dependencies: []. Consumers do not need C libraries, generated parser files, JavaScript tooling, or Foundation-only support code to use the library target.

Public Entry Points

The README examples are mirrored in Tests/DocumentationExampleTests.swift. Those tests are the executable contract for the short-form examples.

Need API Result
Parse YAML into an ordered value tree PureYAML.parse(_:) PureYAML.Model.Value
Parse a YAML stream into indexed documents PureYAML.parseStream(_:) [PureYAML.Stream.Document]
Parse and validate with a report-carrying failure PureYAML.parseValidated(_:) or PureYAML.parseValidatedStream(_:) Parsed artifacts or PureYAML.Validation.ReportError
Parse YAML while preserving explicit tags PureYAML.parseTagged(_:) PureYAML.Tagged.Node
Parse a YAML stream while preserving explicit tags PureYAML.parseTaggedStream(_:) [PureYAML.Tagged.Document]
Construct typed values from tagged nodes PureYAML.Tagged.Constructor Caller-owned output type
Emit YAML from a value tree PureYAML.dump(_:options:) String
Emit YAML from stream documents PureYAML.dump(_:options:) with [PureYAML.Stream.Document] String
Validate a value tree PureYAML.validate(_:using:strict:) [PureYAML.Validation.Issue] or a thrown issue collection
Validate stream documents PureYAML.validate(_:using:strict:) with [PureYAML.Stream.Document] [PureYAML.Stream.Issue] or a thrown stream issue collection
Produce non-throwing validation diagnostics PureYAML.validationReport(_:) or PureYAML.validationReports(_:) PureYAML.Validation.Report or PureYAML.Validation.BatchReport
Produce diagnostic-first reports for damaged YAML PureYAML.diagnosticValidationReport(_:) or PureYAML.diagnosticValidationReports(_:) Reports with preflight, parse, and validation diagnostics
Decode a typed value from YAML PureYAML.decode(_:from:) Decodable value
Encode a typed value to a value tree PureYAML.encode(_:) PureYAML.Model.Value
Encode a typed value to YAML PureYAML.encodeToYAML(_:options:) String

PureYAML intentionally does not expose arbitrary Any dictionary-and-array load or dump APIs. Use Model.Value for unknown schemas and typed Codable for known schemas.

Model Usage

Use PureYAML.Model.Value when the caller needs to inspect YAML structure, preserve mapping order, validate duplicate keys, or handle YAML that does not map cleanly to a Swift Codable type.

The model currently represents:

  • null
  • booleans
  • integers
  • doubles
  • strings
  • ordered mappings
  • sequences

Mappings preserve insertion order and retain duplicate keys. That is deliberate: validation can report duplicate-key diagnostics instead of losing information by collapsing a mapping into a Swift dictionary too early.

Use PureYAML.parseStream(_:) when an input can contain multiple YAML documents. The result is an array of PureYAML.Stream.Document values:

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

Use PureYAML.dump(_:) with [PureYAML.Stream.Document] to emit a multi-document stream. The dumper emits explicit --- before every document and preserves the array order rather than sorting by document.index:

let yaml = PureYAML.dump(documents)

The same emitter options apply to each document body. Flow collections, literal-block scalar selection, and complex mapping key emission are delegated to the single-document dumper.

PureYAML.parse(_:) remains the single-document API. It accepts one implicit or explicit document, including an explicit empty document, and throws unsupportedMultiDocumentStream when a second document is present.

Merge keys are expanded during parsing. Plain << and explicit !!merge keys inherit mapping entries from a mapping or sequence of mappings; local keys override inherited keys, and duplicate local keys remain visible to validation. Quoted "<<": and explicitly string-tagged !!str << keys remain ordinary string keys.

If a caller needs a JSON-like dictionary after validation, build that projection in application code after deciding how duplicate keys, unsupported tags, and complex keys should behave. Keeping that step outside PureYAML makes the lossy boundary explicit.

Tagged Usage

Use PureYAML.parseTagged(_:) when the caller needs to preserve explicit YAML tags for compatibility analysis or project-specific conversion. The tagged model is separate from Model.Value so normal parsing remains small and dependency-free. Tagged parsing preserves source shape for diagnostics and compatibility analysis. Use parse(_:) or parseStream(_:) when the caller needs semantic merge-key expansion into Model.Value.

let tagged = try PureYAML.parseTagged("""
payload: !!binary |
  YWJj
custom: !<tag:example.com,2026:thing> {name: Example}
""")

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

The default tagged validator reports unsupported built-in tags such as !!binary, !!timestamp, !!set, !!omap, and !!pairs. It also reports built-in tags applied to the wrong node kind, such as !!seq {name: Example}. Project-specific tags are preserved and allowed by default.

Tagged validation supports the same strict and non-strict issue behavior as ordinary validation. Start from PureYAML.Tagged.Validator.blank when a caller wants only project-specific tag rules.

PureYAML.parseTaggedStream(_:) returns indexed tagged documents, and PureYAML.Tagged.Validator can validate those documents with PureYAML.Stream.Issue diagnostics.

PureYAML deliberately does not construct Foundation values for tags such as !!timestamp or !!binary. Those tags stay visible in the tagged tree and stay ordinary strings in Model.Value; callers own any Date, Data, or domain-specific conversion.

Use PureYAML.Tagged.Constructor when the application owns that conversion. A constructor is empty by default, so unsupported tags fail with an exact ConstructionError instead of silently becoming Any dictionaries:

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

Handlers can be registered for scalar, sequence, and mapping nodes. The context passed to each handler includes the root node, current node, path, and a recursive construct(_:at:) method that keeps nested construction diagnostics path-aware.

When a caller intentionally wants to drop tag metadata, use the explicit fallback:

let model = try PureYAML.Tagged.Constructor<PureYAML.Model.Value>
    .modelValueErasingTags
    .construct(tagged)

That fallback preserves mapping order and duplicate keys, then erases scalar, sequence, mapping, and key tags. It does not run tagged validation implicitly; call PureYAML.Tagged.Validator first when unsupported built-in tags should block construction.

Typed Conversion

Use typed conversion when the YAML shape is already known. Current coverage includes:

  • scalar Decodable and Encodable values;
  • keyed mapping-backed structs;
  • optional scalar fields;
  • nested keyed structs;
  • unkeyed sequences;
  • nested sequences;
  • keyed structs with sequence properties;
  • dictionary-like string-keyed mappings where YAML semantics allow them;
  • keyed super coders and default super coders.

Typed decoding runs the default validator first. Duplicate mapping keys are reported before Swift Decodable construction starts.

Complex mapping keys are supported in the model layer and can be parsed, validated, and dumped:

let value = try PureYAML.parse("""
? [Detroit Tigers, Chicago Cubs]
:
  - 2001-07-23
""")

if case let .mapping(mapping) = value {
    let firstKey = mapping.pairs.first?.keyNode
}

Keyed Codable remains string-keyed. Sequence and mapping keys stay visible in PureYAML.Model.Value, but are skipped by KeyedDecodingContainer.allKeys and cannot be read as Swift coding keys. Inspect Model.Value directly when a caller needs those keys.

Emitting Options

The default dumper emits deterministic block-style YAML with quoted strings. Callers can opt into:

  • conservative plain strings with .plainWhenSafe;
  • safe literal block scalars for multiline strings with .literalBlockWhenMultiline;
  • compact flow collections with collectionStyle: .flow.

Literal block emission is conservative. If a multiline string would not round-trip through the current parser, the dumper keeps it as a quoted scalar.

Validation

The default validator rejects duplicate mapping keys anywhere in the value tree. Validation issues include:

  • severity;
  • reason;
  • path.

Strict validation treats warnings as failures. Non-strict validation returns warnings but still throws when errors are present.

Use PureYAML.Validation.Validator.blank when the caller wants only project-specific validation rules.

Stream validation preserves document indexes without changing document-local paths:

let documents = try PureYAML.parseStream("""
---
title: First
title: Second
---
routes:
  - name: Users
    name: People
""")

do {
    try PureYAML.validate(documents)
} catch let collection as PureYAML.Stream.Issue.Collection {
    print(collection.description)
}

The issue descriptions include document[0], document[1], and so on, while the nested validation path still describes the location inside that document.

Use validation reports when a caller needs to process many faulty files without throwing away the rest of the batch. Parse failures become diagnostics, ordinary validation issues keep document indexes and paths, and warnings can optionally mark a source invalid:

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

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

PureYAML does not write files from the library target. Applications that need a validation document should write markdown, yaml, or json through their own file IO layer.

Use diagnostic-first reports when the source may be too damaged for the parser to build a value tree. PureYAML.Validation.PreflightScanner scans the full raw string first and reports common line-level failures such as tab indentation, missing spaces after mapping or sequence indicators, trailing whitespace, and invalid control characters. Parsing still runs afterward, but if parsing fails the report contains the preflight diagnostics plus the parser diagnostic instead of pretending a parsed artifact exists.

let report = PureYAML.diagnosticValidationReport(
    """
    title: "open
    summary:Missing
    \tmetadata:
    """,
    file: "openapi.yaml",
)

let json = report.jsonDescription(title: "YAML Validation")

Production tools that prefer throwing control flow can use parseValidated(_:) or parseValidatedStream(_:). On failure, catch PureYAML.Validation.ReportError and serialize its embedded report into the tool-owned error body:

do {
    let document = try PureYAML.parseValidated(yaml, file: "openapi.yaml")
    // Continue with application-owned semantic validation.
} 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.
}

Cross-Platform Verification

Before relying on a migration, run the same gates used by the repository:

bash scripts/check-all.sh
bash scripts/check-linux.sh
bash scripts/check-wasm.sh

scripts/check-all.sh includes style, namespacing, forbidden-pattern, changelog, roadmap, SwiftFormat, SwiftLint, host build, host tests, Linux build and test, Windows build and test in hosted CI, and WASM build checks. The explicit Linux and WASM scripts are useful when recording release or migration evidence.

Support Boundaries

PureYAML is not yet a complete YAML 1.2 implementation. Before replacing another YAML stack, read MIGRATION.md and compare your input corpus with the supported and unsupported behavior listed there.