Foundation

Foundation is Apple’s “base layer” framework — it provides date/time handling, URL parsing, JSON encoding, data buffers, networking (URLSession / URLRequest / URLError), filesystem access, and more. tswift measures Foundation coverage through generated .swiftinterface inventories and per-type runtime registries.

Legend: ✅ Implemented · 🟡 Partial · ⬜ Missing · 🚫 Out of scope. Types marked 🛡️ elsewhere on this site were hardened by ground-truthing against the real Swift 6.3 Foundation (probe programs run on the Apple toolchain, outputs matched byte-for-byte in golden fixtures).

Member counts below are generated by python3 tools/framework-inventory/coverage.py --framework foundation --emit-json — nothing on this page is hand-typed.

How it renders

    
flowchart LR
  src["Swift source"]
  interp["Interpreter\n+ tswift-foundation\nbuiltins"]
  host["Host services\n(defaults / fs / network)"]

  src --> interp --> host

  style src   fill:#1a0d2e,stroke:#a855f7,color:#eeeef5
  style host  fill:#0d2e1a,stroke:#34c759,color:#eeeef5

  

Out of scope

Platform- and device-specific APIs are intentionally excluded (see frameworks/foundation/scope.toml): FileManager, FileHandle, NSObject, Bundle, and friends. Remaining unimplemented members are largely locale-dependent format styles, rangeOf* string-range APIs, and resource/bookmark filesystem members.

A 2026-07 coverage pass (59.8% → 70.3% implemented) closed the mechanical gaps and re-confirmed, member by member, why the rest are intentionally out of scope rather than simply missing:

• customMirror / explicit hash(into:) / encode(to:) across every type — no Mirror reflection subsystem in this runtime; Hashable/ Encodable conformance is already exercised indirectly via == and the JSONEncoder/PropertyListEncoder machinery.
• Date/DateComponents FormatStyle/ISO8601FormatStyle/ParseStrategy symbol-builder DSL (attributedStyle, conversational*, dateSeparator, era, genericLocation, chained .day()/.month()/.year() symbol builders, parse/parseStrategy, …) — a locale-aware, ICU-shaped formatting DSL; consistent with the existing Gregorian/UTC/en_US-only stance for Calendar/DateFormatter.
• URL filesystem members (documentsDirectory/cachesDirectory/… well-known directories, bookmarkData, security-scoped resource access, resourceValues/setResourceValues) — need real filesystem/sandbox semantics this runtime doesn’t model.
• Data raw-memory APIs (span/mutableSpan, withUnsafeBytes, withUnsafeMutableBytes, copyBytes, enumerateBytes) — no raw memory model.
• IndexSet.Index Collection conformance (startIndex/endIndex/ index(after:)/formIndex/opaque-Index subscript/comparison) — for x in indexSet element iteration already works, which covers real usage; the opaque-Index API surface only calls itself. Tripwire: if a real need for indexSet[someIndex]-style access appears, model Index as a (range_ordinal, offset) pair.
• JSONDecoder.allowsJSON5 — honoring the flag means writing a real JSON5 parser (comments, unquoted keys, single quotes, trailing commas); a flag that silently still parses strict JSON would be a semantic lie, so it’s tracked separately rather than stubbed.
• Decimal.infinity / .signalingNaN / formTruncatingRemainder — the first two contradict this runtime’s documented invariant that Decimal has no infinity representation; formTruncatingRemainder was confirmed against real Apple swiftc 6.3.2 to be declared @available(*, unavailable, message: "Decimal does not yet fully adopt FloatingPoint.") — it isn’t callable from any real Swift program either, so implementing it here would diverge from Darwin, not converge with it.
• Calendar.enumerateDates / .dates — deferred (not skipped): they need inout stop-flag write-back through closure invocation and a lazy Sequence<Date> return type respectively, neither of which exist in this runtime yet.

Hardening

Types marked 🛡️ were validated in dedicated hardening passes: small probe programs were run on the real Apple Swift toolchain to capture ground-truth outputs, then encoded as golden fixtures (harden_foundation_*.swift, harden_json.swift). This process found and fixed real divergences — ISO8601 leap-year/fractional-second validation, standardized path edge cases, percent-encoding charset rules, Decimal precision boundaries, and JSON strategy-enum API surface.

Roadmap

Next coverage pushes (daily-use weighted):

1. Calendar.dates / enumerateDates sequence APIs (needs closure stop-flag write-back + a lazy Sequence<Date> primitive)
2. URLComponents.rangeOf* members (needs Range<String.Index> bridging)
3. URLSession bytes(for:) async streaming and downloadTask support
4. Remaining Data / URL member gaps (byte-buffer and filesystem APIs, scoped down per the Out-of-scope notes above)

Locale-aware FormatStyle DSLs, filesystem/bookmark APIs, and reflection (customMirror) are tracked as permanent out-of-scope items (see above), not roadmap items.