PureYAML/docs/USAGE.md

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

The first release target is 0.1.0. After the tag is published, depend on the tagged package version:

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

Until that tag exists, 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]
Emit YAML from a value tree	PureYAML.dump(_:options:)	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
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
""")

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.

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.

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.

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, 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.