PureXML

PureXML is a dependency-free XML package written entirely in Swift.

The goal is a Linux-, Windows-, and WebAssembly-compatible XML reader/writer that does not pull in libxml2, expat, or Foundation's XMLParser. 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

It is a sibling project to PureYAML and follows the same structure, rules, and verification gates.

Roadmap

The roadmap uses the TileDown Mermaid palette: green for shipped work, yellow for review, purple for epic grouping, and gray for open work.

flowchart TB
  classDef done fill:#ddf9e4,stroke:#34c759,color:#111827
  classDef review fill:#fff7d6,stroke:#ffcc00,color:#111827
  classDef epic fill:#f2e5ff,stroke:#af52de,color:#111827
  classDef todo fill:#f2f4f7,stroke:#8e8e93,color:#111827
  LDone["In main now"]:::done
  LReview["PR in review"]:::review
  LEpic["Epic grouping"]:::epic
  LTodo["Open issue, no PR"]:::todo
  LDone --> LReview
  LReview --> LEpic
  LEpic --> LTodo

The goal is full libxml2 feature parity in pure Swift with zero dependencies, by clean-room reimplementation (behavior reproduced from the private PureXML-research analysis, no upstream source copied). The streaming core plus DTD validation (content models, attributes, ID/IDREF) and an XPath 1.0 subset is shipped; the remaining libxml2 surface is tracked as epics. Deliberate non-goals: network fetching (nanohttp/nanoftp) and the threading/memory infrastructure stay out, and external resolution is opt-in through an injected resolver so XXE stays closed.

Parsing and I/O:

flowchart TB
  classDef done fill:#ddf9e4,stroke:#34c759,color:#111827
  classDef review fill:#fff7d6,stroke:#ffcc00,color:#111827
  classDef epic fill:#f2e5ff,stroke:#af52de,color:#111827
  classDef todo fill:#f2f4f7,stroke:#8e8e93,color:#111827
  Core["Streaming core + DTD validation (shipped)"]:::done
  Push["#1 Push / feed streaming API"]:::epic
  Sax["#19 SAX2 push callbacks"]:::done
  Html["#20 HTML parser and serializer"]:::epic
  Enc["#28 Full encoding support"]:::done
  Ent["#29 External and parameter entities"]:::done
  TreeApi["#31 Tree manipulation API"]:::done
  Writer["#32 xmlTextWriter"]:::done
  Reader["#33 xmlTextReader with validation"]:::epic
  Save["#34 Serialization option parity"]:::done
  Chars["#35 Character and name validity"]:::done
  Core --> Push
  Core --> Sax
  Core --> Html
  Core --> Enc
  Core --> Ent
  Core --> TreeApi
  Core --> Writer
  Core --> Reader
  Core --> Save
  Core --> Chars

Querying:

flowchart TB
  classDef done fill:#ddf9e4,stroke:#34c759,color:#111827
  classDef review fill:#fff7d6,stroke:#ffcc00,color:#111827
  classDef epic fill:#f2e5ff,stroke:#af52de,color:#111827
  classDef todo fill:#f2f4f7,stroke:#8e8e93,color:#111827
  XPathFull["#21 Full XPath 1.0"]:::epic
  Pattern["#22 xmlPattern streaming match"]:::epic
  XPointer["#23 XPointer"]:::epic
  XPathFull --> Pattern
  XPathFull --> XPointer

Validation:

flowchart TB
  classDef done fill:#ddf9e4,stroke:#34c759,color:#111827
  classDef review fill:#fff7d6,stroke:#ffcc00,color:#111827
  classDef epic fill:#f2e5ff,stroke:#af52de,color:#111827
  classDef todo fill:#f2f4f7,stroke:#8e8e93,color:#111827
  Regexp["#30 Regular expression engine"]:::epic
  Xsd["#2 XSD / RELAX NG validation"]:::epic
  Schematron["#25 Schematron validation"]:::epic
  Regexp --> Xsd
  Xsd --> Schematron

Transformation and integrity:

flowchart TB
  classDef done fill:#ddf9e4,stroke:#34c759,color:#111827
  classDef review fill:#fff7d6,stroke:#ffcc00,color:#111827
  classDef epic fill:#f2e5ff,stroke:#af52de,color:#111827
  classDef todo fill:#f2f4f7,stroke:#8e8e93,color:#111827
  Xslt["#3 XSLT 1.0"]:::epic
  XInclude["#24 XInclude, URI, xml:base"]:::epic
  C14n["#26 Canonical XML"]:::epic
  Catalog["#27 XML Catalog"]:::epic
  Xslt --> C14n
  XInclude --> Catalog

Status

The node model, the emitter, and a streaming parser are implemented and usable today.

• Model (PureXML.Model): Node, Element, Attribute, QualifiedName. Preserves document order and the distinction between text, CDATA, comments, and processing instructions.
• Emitting (PureXML.Emitting): Serializer turns a node tree into well-formed XML, with pretty-printed and compact options and correct text and attribute escaping.
• Validation (PureXML.Validation): structural checks such as duplicate attribute names. Schema validation (DTD/XSD/RELAX NG) is out of scope for the library target.
• Parsing (PureXML.Parsing): a streaming, iterative parser. EventReader is a pull-based event core that consumes input through a character-source closure and emits one Event at a time, never holding the whole document in memory (it drives chunked input). Parser builds a Model.Node tree iteratively over the event core. Handles elements, attributes, text, the five predefined entities and numeric character references, comments, CDATA, and processing instructions. Safe by default: <!DOCTYPE> is rejected, which removes the DTD-based threat classes (XXE, entity-expansion DoS). DTD support, namespaces-as-resolved-URIs, and validation are deliberate future layers.

Usage

import PureXML

// Build a tree and emit it (works today).
let element = PureXML.Model.Element(
    "book",
    attributes: [.init("id", "bk101")],
    children: [.element(.init("title", children: [.text("XML Developer's Guide")]))],
)

let xml = PureXML.serialize(.element(element))
try PureXML.validate(.element(element))

// Parse it back into a tree.
let node = try PureXML.parse(xml)

// Or stream events without building a tree (drives chunked input too).
var reader = PureXML.events(xml)
while let event = try reader.next() {
    // handle .startElement / .characters / .endElement / ...
}

Attribution

PureXML is informed by the behavior of established XML parsers (libxml2, expat, Foundation's XMLParser) and by the W3C XML 1.0 specification, but it does not copy their implementation into Sources/. See ATTRIBUTION.md.

Development Contract

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

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

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
swiftformat . --config .swiftformat --lint
swiftlint --config .swiftlint.yml --strict
swift build
swift test

License

MIT.