rose-ash/plans/smalltalk-on-sx.md

Smalltalk-on-SX: blocks with non-local return on delimited continuations

The headline showcase is blocks — Smalltalk's closures with non-local return (^expr aborts the enclosing method, not the block). Every other Smalltalk on top of a host VM (RSqueak on PyPy, GemStone on C, Maxine on Java) reinvents non-local return on whatever stack discipline the host gives them. On SX it's a one-liner: a block holds a captured continuation; ^ just invokes it. Message-passing OO falls out cheaply on top of the existing component / dispatch machinery.

End-state goal: ANSI-ish Smalltalk-80 subset, SUnit working, ~200 hand-written tests + a vendored slice of the Pharo kernel tests, classic corpus (eight queens, quicksort, mandelbrot, Conway's Life).

Scope decisions (defaults — override by editing before we spawn)

• Syntax: Pharo / Squeak chunk format (! separators, Object subclass: #Foo …). No fileIn/fileOut images — text source only.
• Conformance: ANSI X3J20 as a target, not bug-for-bug Squeak. "Reads like Smalltalk, runs like Smalltalk."
• Test corpus: SUnit ported to SX-Smalltalk + custom programs + a curated slice of Pharo Kernel-Tests / Collections-Tests.
• Image: out of scope. Source-only. No become: between sessions, no snapshotting.
• Reflection: class, respondsTo:, perform:, doesNotUnderstand: in. become: (object-identity swap) in — it's a good CEK exercise. Method modification at runtime in.
• GUI / Morphic / threads: out entirely.

Ground rules

• Scope: only touch lib/smalltalk/** and plans/smalltalk-on-sx.md. Don't edit spec/, hosts/, shared/, or any other lib/<lang>/**. Smalltalk primitives go in lib/smalltalk/runtime.sx.
• SX files: use sx-tree MCP tools only.
• Commits: one feature per commit. Keep ## Progress log updated and tick roadmap boxes.

Architecture sketch

Smalltalk source
    │
    ▼
lib/smalltalk/tokenizer.sx  — selectors, keywords, literals, $c, #sym, #(…), $'…'
    │
    ▼
lib/smalltalk/parser.sx     — AST: classes, methods, blocks, cascades, sends
    │
    ▼
lib/smalltalk/transpile.sx  — AST → SX AST (entry: smalltalk-eval-ast)
    │
    ▼
lib/smalltalk/runtime.sx    — class table, MOP, dispatch, primitives

Core mapping:

• Class = SX dict {:name :superclass :ivars :methods :class-methods :metaclass}. Class table is a flat dict keyed by class name.
• Object = SX dict {:class :ivars} — ivars keyed by symbol. Tagged ints / floats / strings / symbols are not boxed; their class is looked up by SX type.
• Method = SX lambda closing over a self binding + temps. Body wrapped in a delimited continuation so ^ can escape.
• Message send = (st-send receiver selector args) — does class-table lookup, walks superclass chain, falls back to doesNotUnderstand: with a Message object.
• Block [:x | … ^v … ] = lambda + captured ^k (the method-return continuation). Invoking ^ calls k; outer block invocation past method return raises BlockContext>>cannotReturn:.
• Cascade r m1; m2; m3 = (let ((tmp r)) (st-send tmp 'm1 ()) (st-send tmp 'm2 ()) (st-send tmp 'm3 ())).
• ifTrue:ifFalse: / whileTrue: = ordinary block sends; the runtime intrinsifies them in the JIT path so they compile to native branches (Tier 1 of bytecode expansion already covers this pattern).
• become: = swap two object identities everywhere — in SX this is a heap walk, but we restrict to oneWayBecome: (cheap: rewrite class field) by default.

Roadmap

Phase 1 — tokenizer + parser

• Tokenizer: identifiers, keywords (foo:), binary selectors (+, ==, ,, ->, ~= etc.), numbers (radix 16r1F, scaled 1.5s2), strings '…''…', characters $c, symbols #foo #'foo bar' #+, byte arrays #[1 2 3], literal arrays #(1 #foo 'x'), comments "…"
• Parser: chunk format (! ! separators), class definitions (Object subclass: #X instanceVariableNames: '…' classVariableNames: '…' …), method definitions (extend: #Foo with: 'bar ^self'), pragmas <primitive: 1>, blocks [:a :b | | t1 t2 | …], cascades, message precedence (unary > binary > keyword)
• Unit tests in lib/smalltalk/tests/parse.sx

Phase 2 — object model + sequential eval

• Class table + bootstrap: Object, Behavior, Class, Metaclass, UndefinedObject, Boolean/True/False, Number/Integer/Float, String, Symbol, Array, Block
• smalltalk-eval-ast: literals, variable reference, assignment, message send, cascade, sequence, return
• Method lookup: walk class → superclass; cache hit-class on (class, selector)
• doesNotUnderstand: fallback constructing Message object
• super send (lookup starts at superclass of defining class, not receiver class)
• 30+ tests in lib/smalltalk/tests/eval.sx

Phase 3 — blocks + non-local return (THE SHOWCASE)

• Method invocation captures a ^k (the return continuation) and binds it as the block's escape
• ^expr from inside a block invokes that captured ^k
• BlockContext>>value, value:, value:value:, …, valueWithArguments:
• whileTrue: / whileTrue / whileFalse: / whileFalse as ordinary block sends — runtime intrinsifies the loop in the bytecode JIT
• ifTrue: / ifFalse: / ifTrue:ifFalse: as block sends, similarly intrinsified
• Escape past returned-from method raises BlockContext>>cannotReturn:
• Classic programs in lib/smalltalk/tests/programs/:
  • eight-queens.st
  • quicksort.st
  • mandelbrot.st
  • life.st (Conway's Life, glider gun)
  • fibonacci.st (recursive + memoised)
• lib/smalltalk/conformance.sh + runner, scoreboard.json + scoreboard.md

Phase 4 — reflection + MOP

• Object>>class, class>>name, class>>superclass, class>>methodDict, class>>selectors
• Object>>perform: / perform:with: / perform:withArguments:
• Object>>respondsTo:, Object>>isKindOf:, Object>>isMemberOf:
• Behavior>>compile: — runtime method addition
• Object>>becomeForward: (one-way become; rewrites the class field of aReceiver)
• Exceptions: Exception, Error, signal, signal:, on:do:, ensure:, ifCurtailed: — built on top of SX handler-bind/raise

Phase 5 — collections + numeric tower

• SequenceableCollection/OrderedCollection/Array/String/Symbol
• HashedCollection/Set/Dictionary/IdentityDictionary
• Stream hierarchy: ReadStream/WriteStream/ReadWriteStream
• Number tower: SmallInteger/LargePositiveInteger/Float/Fraction
• String>>format:, printOn: for everything

Phase 6 — SUnit + corpus to 200+

• Port SUnit (TestCase, TestSuite, TestResult) — written in SX-Smalltalk, runs in itself
• Vendor a slice of Pharo Kernel-Tests and Collections-Tests
• Drive the scoreboard up: aim for 200+ green tests
• Stretch: ANSI Smalltalk validator subset

Phase 7 — speed (optional)

• Method-dictionary inline caching (already in CEK as a primitive; just wire selector cache)
• Block intrinsification beyond whileTrue: / ifTrue:
• Compare against GNU Smalltalk on the corpus

Progress log

Newest first. Agent appends on every commit.

• (none yet)

Blockers

Shared-file issues that need someone else to fix. Minimal repro only.

• (none yet)