smalltalk: String>>format: + universal printOn: + 18 tests

plans/smalltalk-on-sx.md

@@ -91,7 +91,7 @@ Core mapping:
 - [x] `HashedCollection`/`Set`/`Dictionary`/`IdentityDictionary`. Implemented as user classes in `runtime.sx`. `HashedCollection` carries a single `array` ivar; `Dictionary` overrides with parallel `keys`/`values`. Set: `add:` (dedup), `addAll:`, `remove:`, `includes:`, `do:`, `size`, `asArray`. Dictionary: `at:`, `at:ifAbsent:`, `at:put:`, `includesKey:`, `removeKey:`, `keys`, `values`, `do:`, `keysDo:`, `valuesDo:`, `keysAndValuesDo:`, `size`, `isEmpty`. `IdentityDictionary` defined as a Dictionary subclass (no methods of its own yet — equality and identity diverge in a follow-up). Class-side `new` calls `super new init`. Added Array primitive `add:` (append). 29 tests in `lib/smalltalk/tests/hashed.sx`.
 - [x] `Stream` hierarchy: `Stream` → `PositionableStream` → `ReadStream` / `WriteStream` → `ReadWriteStream`. User classes with `collection` + 0-based `position` ivars. ReadStream: `next`, `peek`, `atEnd`, `upToEnd`, `next:`, `skip:`, `reset`, `position`/`position:`. WriteStream: `nextPut:`, `nextPutAll:`, `contents`. Class-side `on:` constructor; `WriteStream class>>with:` pre-fills + `setToEnd`. Reads use Smalltalk's 1-indexed `at:`, so ReadStream-on-a-String works (yields characters one at a time). 21 tests in `lib/smalltalk/tests/streams.sx`. Bumped `test.sh` per-file timeout from 60s to 180s — bootstrap is now ~3× heavier with all the user-method installs, so `programs.sx` runs in ~64s.
 - [x] `Number` tower: `SmallInteger`/`LargePositiveInteger`/`Float`/`Fraction`. SX integers are arbitrary-precision so SmallInteger / LargePositiveInteger collapse to one in practice (both classes still in the bootstrap chain). Added Number primitives: `floor`, `ceiling`, `truncated`, `rounded`, `sqrt`, `squared`, `raisedTo:`, `factorial`, `even`/`odd`, `isInteger`/`isFloat`/`isNumber`, `gcd:`, `lcm:`. **Fraction** now a real user class (numerator/denominator + sign-normalised, gcd-reduced at construction): `numerator:denominator:`, accessors, `+`/`-`/`*`/`/`, `negated`, `reciprocal`, `=`, `<`, `asFloat`, `printString`, `isFraction`. 47 tests in `lib/smalltalk/tests/numbers.sx`.
-- [ ] `String>>format:`, `printOn:` for everything
+- [x] `String>>format:`, `printOn:` for everything. `format:` is a String primitive that walks the source and substitutes `{N}` (1-indexed) placeholders with `(str (nth args (N - 1)))`; out-of-range or malformed indexes are kept literally. `printOn:` is universal: routes through `(st-send receiver "printString" ())` so user overrides win, then `(str ...)` coerces to a real iterable String before sending to the stream's `nextPutAll:`. `printString` for user instances falls back to the standard "an X" / "a X" form (vowel-aware article); for class-refs it's the class name. 18 tests in `lib/smalltalk/tests/printing.sx`. Phase 5 complete.
 
 ### Phase 6 — SUnit + corpus to 200+
 - [ ] Port SUnit (TestCase, TestSuite, TestResult) — written in SX-Smalltalk, runs in itself
@@ -108,6 +108,7 @@ Core mapping:
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-25: String>>format: + universal printOn: + 18 tests (`lib/smalltalk/tests/printing.sx`). `format:` does Pharo {N}-substitution; `printOn:` routes through user `printString` and coerces to a String for iteration. Phase 5 complete. 638/638 total.
 - 2026-04-25: Number tower + Fraction class + 47 tests (`lib/smalltalk/tests/numbers.sx`). 14 new Number primitives (floor/ceiling/truncated/rounded/sqrt/squared/raisedTo:/factorial/even/odd/gcd:/lcm:/isInteger/isFloat). Fraction with normalisation + arithmetic + comparisons + asFloat. 620/620 total.
 - 2026-04-25: Stream hierarchy + 21 tests (`lib/smalltalk/tests/streams.sx`). ReadStream / WriteStream / ReadWriteStream as user classes; class-side `on:`; ReadStream-on-String yields characters. Bumped `test.sh` per-file timeout 60s → 180s — heavier bootstrap pushed `programs.sx` past 60s. 573/573 total.
 - 2026-04-25: HashedCollection / Set / Dictionary / IdentityDictionary + 29 tests (`lib/smalltalk/tests/hashed.sx`). Set: dedup add:, remove:, includes:, do:, addAll:. Dictionary: parallel keys/values backing; at:put:, at:ifAbsent:, includesKey:, removeKey:, keysDo:, keysAndValuesDo:. Class-side `new` chains `super new init`. Array primitive `add:` added. 552/552 total.