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

Capability is being grown toward libxml2 parity in pure Swift, guided by the private PureXML-research analysis. Shipped vs 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
  Model["Node model + emitter"]:::done
  Stream["Streaming pull parser"]:::done
  Escape["Iterative emitter + escaping"]:::done
  Limits["Bounded-by-default limits"]:::done
  Bytes["Byte input + encoding detection"]:::done
  Namespaces["Namespace resolution"]:::todo
  ByteStream["Streaming byte decode"]:::todo
  DTD["Optional DTD + entities"]:::todo
  Validation["Schema validation"]:::todo
  XPath["XPath 1.0"]:::todo
  Model --> Stream
  Stream --> Escape
  Escape --> Limits
  Limits --> Bytes
  Bytes --> Namespaces
  Namespaces --> ByteStream
  ByteStream --> DTD
  DTD --> Validation
  Validation --> XPath

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.