Xylem

A fast, memory-safe XML parser written in pure Swift. Xylem targets UTF-8 XML 1.0 well-formedness with namespace-aware SAX/DOM parsing and XPath 1.0. It does not implement DTD validation, external entity resolution, XSLT, or schema validation — by design.

Why Xylem

• Fast. The repository includes reproducible SAX, DOM, and XPath benchmarks over representative fixtures. Use them for change-to-change regression tracking on your hardware.
• Safe. No external entity resolution and no user-defined entity expansion from DTD declarations. This keeps XXE-style resolution paths out of scope. Memory safety is enforced by the language, not by discipline.
• Modular. Use only what you need. SAX, DOM, and XPath are separate modules built on a shared XMLCore. An embedded target that only needs streaming parsing pulls in SAXParser alone — no DOM allocator, no XPath evaluator.
• Zero dependencies. Pure Swift. No Foundation, no C, no iconv, no system libraries.
• Cross-platform. Runs anywhere the Swift compiler targets — macOS, Linux, Windows, WASM, embedded.

Modules

What Xylem Supports

XML 1.0 (UTF-8 well-formedness profile): Elements, attributes, namespace declaration/resolution (default + prefixed bindings), strict QName checks (single :, valid prefix/local forms), processing instructions, CDATA sections, comments, DOCTYPE tokenization (name + public/system IDs), character references, and predefined entities (&, <, >, ', ").

XML declaration: version is required and must be 1.0; optional encoding and standalone pseudo-attributes are parsed (standalone must be yes or no).

XPath 1.0: 12 of 13 axes (namespace axis is parsed but returns no nodes), all node tests, predicates, union, arithmetic/comparison operators, all 27 core functions. Variables are parsed but evaluate to the empty set (relevant only in XSLT contexts).

Parsing APIs: Event-driven SAX-style streaming, document tree construction, XPath evaluation.

What Xylem Does Not Support

These omissions are deliberate — they reduce attack surface, binary size, and complexity.

• DTD validation semantics (<!ELEMENT>, <!ATTLIST>, <!ENTITY>, <!NOTATION>, conditional sections)
• User-defined entity expansion from DTD declarations
• External entity resolution
• Full XML 1.1 semantics
• Encoding declarations / non-UTF-8 input
• Full XML Namespaces conformance-suite matrix coverage (current automated XMLTS profile is not namespace-specific groups)
• XInclude
• Canonical XML (C14N)
• Streaming / pull reader
• Push parser
• XSLT
• XML Schema (XSD) / RelaxNG
• XQuery
• XPath 2.0+

Xylem assumes well-formed, UTF-8-encoded XML input.

Implementation Snapshot

Current architecture is split into four modules:

• XMLCore: UTF-8-first lexer/token model, scalar/name validation, entity expansion, and shared byte-span utilities.
• SAXParser: streaming parser with namespace resolution, strict well-formedness state tracking, and callback-based processing.
• DOMParser: flat-arena document builder on top of SAX events (single storage arena, stable references, no per-node heap allocation).
• XPath: XPath 1.0 parser/evaluator over DOM documents with strict type/error behavior.

Current conformance/testing posture:

• UTF-8 XML 1.0 well-formedness is the primary target.
• Namespace behavior (scope, QName validity, reserved bindings, duplicate expanded-name checks) is covered in focused SAX/DOM unit tests.
• DTD internal subset/entity-declaration semantics are tracked by disabled tests and explicit conformance allowlists.
• Conformance coverage includes focused DOM/SAX suites plus XMLTS-driven ConformanceTests (valid/sa + not-wf/sa, scoped for current feature set and not namespace-specific XMLTS groups).

Requirements

• Swift 6.2+

Running Tests

swift test

Run a specific suite:

swift test --filter SAXParser
swift test --filter DOMParser
swift test --filter XPath
swift test --filter ConformanceTests

Conformance Tests (XMLTS)

ConformanceTests use the W3C XML Test Suite artifact (xmlts20130923) from the canonical source.

Fetch via SwiftPM command plugin:

swift package plugin --allow-writing-to-package-directory xmlts -- --fetch

Verify local artifact:

swift package plugin --allow-writing-to-package-directory xmlts -- --verify

The plugin verifies SHA-256 before extraction.

Run conformance-only:

swift test --filter ConformanceTests

Benchmarks

For benchmark setup, fixtures, and baseline workflows, see Benchmarks/BENCHMARKING.md.

License

BSD-3-Clause. See LICENSE for details.