ADR 0084: Class-Side Runtime Method Installation and Fun Dispatch

Status

Accepted (in progress, 2026-05-24) — implementation epic BT-2259

Context

Problem

Beamtalk can install and dispatch instance methods at runtime — this is how >> live patching (ADR 0066), Behaviour compile:source: (ADR 0082), and the programmatic ClassBuilder (methods:, ADR 0038) all work. It cannot do the same for class-side methods. There is no way to:

• give a programmatic ClassBuilder a callable class method (Object classBuilder name: #C; classMethods: #{ #zero => [...] }; register),
• live-patch a class method (Counter class >> reset => …), or
• compile:source: a class-side selector (ADR 0082's class-side path).

This blocks the "first-class ClassBuilder" epic (BT-2259) and the class-side half of the live-editing story that ADR 0066 / ADR 0082 already sanctioned at the syntax level (ClassName class >> sel => body parses today with an is_class_method flag, but has no runtime dispatch path).

Current state

Instance side (works). Instance dispatch checks a runtime method source before the compiled class chain and invokes it as a fun that threads state:

• beamtalk_dispatch:check_extension/2 → invoke_extension(Fun, Args, Self, State) runs fun(Args, Self, State) -> {Result, NewState} and commits NewState back to the gen_server (beamtalk_dispatch.erl:459-466).
• The fun is compiler-generated: super-sends and self-sends are lowered to explicit dispatch calls, and instance-variable writes are threaded as {Result, NewState}. It is never a naive user closure.

Class side (does not work). Class-method dispatch only ever calls a compiled function:

• beamtalk_class_dispatch:apply_class_method_in_context/6 (beamtalk_class_dispatch.erl:457-493) does erlang:apply(DefiningModule, class_<sel>, [ClassSelf, ClassVars | Args]) and nothing else. There is no runtime-method lookup and no fun path.
• beamtalk_class_builder:build_compiled_class_info/8 stores class_methods => ClassMethods verbatim — unlike instance methods, class methods are not run through build_method_map/1, so builder-supplied funs are never wrapped into a dispatchable shape (beamtalk_class_builder.erl:316-318).
• There is no put_class_method/4 to mirror the instance-side beamtalk_object_class:put_method/4 (beamtalk_object_class.erl:642).

The BT-873 precedent (the reason for caution)

BT-873 removed "Path 2" — closure-based dynamic class dispatch — because it was fundamentally broken: state mutation was silently dropped, super calls did not work, and self-sends were broken (ADR 0038, "Single Path to Class Creation"). Any proposal to install class methods as runtime funs must explain why it will not reproduce those failures.

Constraints

1. No BT-873 repeat. Runtime class-method funs must thread class-variable state, resolve super up the metaclass chain, and make self sends dispatch as class messages — correctly, not "mostly".
2. Class variables, not instance variables. Class methods mutate class variables via a distinct protocol: a compiled class method returns {class_var_result, Result, NewClassVars} (or a plain value), and the class gen_server commits NewClassVars (beamtalk_class_dispatch.erl:99,116,421). This is not the instance {Result, NewState} protocol.
3. self is the class. Inside a class method, self new / self otherCM must dispatch as class messages against ClassSelf = #beamtalk_object{class = Tag, class_mod = DefiningModule, pid = self()} (beamtalk_class_dispatch.erl:465-469).
4. One registry entry. An edited or builder-defined class stays a single beamtalk_class_registry entry; instances keep working; the class stays navigable (method_source / SystemNavigation).
5. Document/codegen rule. All Core Erlang stays in the Document / docvec! API — no format! for codegen (CLAUDE.md, BT-875).

Decision

Add a class-side runtime method path that mirrors the compiled class-method calling convention exactly, fed only by compiler-generated funs.

1. Calling convention — identical to compiled class methods

A runtime-installed class-method fun has the same signature and return protocol as a compiled class_<Selector> function:

%% Compiled (today):   DefiningModule:class_<sel>(ClassSelf, ClassVars, A1, ..., An)
%% Runtime fun (new):  fun(ClassSelf, ClassVars, A1, ..., An) -> Result
%%                                                            |  {class_var_result, Result, NewClassVars}

arity = n + 2. Class-variable mutation is threaded through the existing {class_var_result, Result, NewClassVars} return that every caller already handles. No new state protocol is introduced.

2. Storage — the class gen_server class_methods map

Runtime class methods are stored as #{block => Fun, arity => Arity} in the class gen_server's class_methods map, mirroring how instance methods are stored in instance_methods (beamtalk_object_class.erl:642-644). Two changes:

• New beamtalk_object_class:put_class_method/4 (mirrors put_method/4): installs/replaces one class method, clears stale class-side signature and return-type entries (as put_method does for the instance side).

• beamtalk_class_builder:build_compiled_class_info/8 runs classMethods: funs through build_method_map/1 (the wrapper instance methods already use), so builder-supplied class-method funs become #{block, arity} entries.

• put_class_method/4 keeps two ETS-backed stores in sync, because class-method resolution is a two-step lookup that must work for inherited methods without a per-send gen_server hop:

  1. Discoverability — which ancestor defines the selector. Inherited resolution walks the chain via beamtalk_class_metadata:lookup_methods/1 (ETS: selectors + module per class, beamtalk_class_dispatch.erl:589). init/1 and apply_class_info/2 already insert maps:keys(ClassMethods) here (beamtalk_object_class.erl:419,1078), so register-time builder class methods are discoverable by subclasses automatically; a live put_class_method must add its selector here too.
  2. Retrieval — get the installed fun from that ancestor. The metadata table holds only selectors + module, so it cannot return the fun (Copilot review, PR #2297). Runtime class-method funs are therefore also recorded in a dedicated ETS store keyed by {DefiningClass, Selector} -> #{block, arity} (an extension of beamtalk_class_metadata or a sibling table), written by both build_compiled_class_info/8 (register-time) and put_class_method/4 (live). After the chain walk resolves DefiningClass, dispatch does a cheap ETS read on {DefiningClass, Selector} — no gen_server hop, the same cost model as the existing metadata lookups.

  The gen_server class_methods map stays the source of truth (reflection, update_class rebuild, local_class_methods); the ETS store is the dispatch-time read cache, kept in sync on every install / replace / remove. Compiled methods are absent from the retrieval store — they are still erlang:apply'd by module — so the compiled hot path is untouched. The instance side has no equivalent obligation because instance dispatch consults the extension registry / gen_server map directly; the class side is genuinely different and the implementation must not assume symmetry.

3. Dispatch — check the map first, then fall back to compiled

beamtalk_class_dispatch:apply_class_method_in_context/6 runs after the chain walk has resolved the DefiningClass/DefiningModule for the selector. It does a cheap ETS read on the retrieval store (§2) for a runtime fun before the compiled apply — both keyed by the defining class, so this works identically for own and inherited methods:

%% DefiningClass already resolved by find_class_method_in_chain/2.
case beamtalk_class_metadata:lookup_class_method_fun(DefiningClass, Selector) of
    {ok, #{block := Fun}} ->
        apply(Fun, [ClassSelf, ClassVars | Args]);     %% runtime fun (ETS read, no hop)
    error ->
        FunName = class_method_fun_name(Selector),
        erlang:apply(DefiningModule, FunName, [ClassSelf, ClassVars | Args])  %% compiled (today)
end

Same arguments, same {class_var_result, …} handling, same error classification. A runtime override of a compiled selector shadows it (last writer wins), matching instance-side live-patch semantics.

This adds one ETS branch before the compiled apply; it does not violate ADR 0006 (unified dispatch) — it is the same "runtime source shadows compiled" pattern the instance side already uses for extensions, with a single resolution path. To honour BT-2008 (no per-send gen_server hops on the hot path), the retrieval is an ETS read keyed by the already-resolved DefiningClass — never a gen_server call — and is gated so compile-time-only classes (no runtime class methods) skip it entirely, e.g. a per-class "has runtime class methods" flag in the metadata row, checked before the ETS read.

ClassSelf.class_mod for funs. For a compiled method, class_mod is the defining module, and super/self-helpers are co-resident named exports in that module. An anonymous fun has no class_<sel> export and may not live in the defining BEAM module at all, so class_mod = DefiningModule cannot be relied on to locate the fun's helpers. The fun must be self-contained: all of its super/self/helper calls are captured in the closure at compile time (see §4), so dispatch only needs ClassSelf to carry the correct class identity (tag + defining class name) for further self-sends — not a module it can erlang:apply into.

4. Funs are compiler-generated — the BT-873 guard

The runtime funs are produced by the compiler, reusing the lowering it already applies to named class_<sel> functions, emitting it into an anonymous fun instead of a module export. The safety of this ADR rests on this lowering being concretely specified — "the compiler will do it right" is a plan, not an argument — so the implementation (Phase 2) must establish the following contract, each independently tested for the fun path before Phase 2 is considered done:

1. Class-variable threading. Writes to class variables lower to the existing {class_var_result, Result, NewClassVars} return; the dispatch wrapper commits NewClassVars. Test: a builder/patched class method that mutates a class variable, called twice, accumulates (the BT-873 "dropped state" regression).
2. super resolution without module identity. super sel inside a class-method fun lowers to an explicit, compile-time class-name-keyed superclass dispatch (e.g. beamtalk_class_dispatch:class_self_dispatch( SuperclassName, Sel, ClassVars, Args)), with SuperclassName resolved from the defining class at compile time and captured in the closure. It must not rely on erlang:apply(DefiningModule, …) or on ClassSelf.class_mod, because an anonymous fun has no such export (see §3). Test: a builder class method whose body calls super, resolving to the superclass's class method.
3. self-sends. self new / self otherCM lower to explicit class-message dispatch against the ClassSelf the wrapper passes in. Test: a class method that calls another class method and self new.

They are never naive user closures. This is precisely the property BT-873's Path 2 lacked — Path 2 wrapped raw blocks with no lowering, so it dropped state and broke super/self. With the contract above, the runtime fun is behaviourally equivalent to a compiled class method — verified by parity tests, not assumed — differing only in location (an anonymous fun in the gen_server map vs a named export). The one thing a named export can do that an anonymous fun cannot is call module-local helper functions by name; the lowering must therefore either inline such helpers into the closure or route them through a stable module (the stdlib runtime), never through the (possibly absent) defining module.

5. Scope boundary — own methods vs cross-class extensions

This ADR covers a class installing/redefining its own class methods (builder creation + live edit of a class you own). Cross-class class-side extensions (SomeForeignClass class >> sel from another package) belong to ADR 0066's extension-registry model and would use a symmetric class-side extension registry; that is explicitly out of scope here and noted as future work.

REPL session

>> c := Object classBuilder
     name: #Tally;
     superclass: Object;
     classVars: #{ #total => 0 };
     classMethods: #{ #bump => [:self | self.total := self.total + 1. self.total] };
     register
=> Tally
>> Tally bump
=> 1
>> Tally bump
=> 2                                  // class variable threaded — not dropped

>> Counter class >> reset => self.count := 0   // live class-method patch
=> a CompiledMethod (#reset in Counter class)
>> Counter reset
=> 0

Error example

>> Object classBuilder name: #Bad; superclass: Object;
     classMethods: #{ #oops => [:self | super nope] }; register
>> Bad oops
=> error: Bad class does not understand #nope
         (super resolved up the metaclass chain — no silent drop)

Prior Art

Pharo / Squeak Smalltalk

A class method is just a method in the metaclass's method dictionary; the System Browser installs a CompiledMethod into Counter class exactly as it installs one into Counter. There is no second mechanism — instance side and class side are symmetric. Adopted: the symmetry — class methods get the same runtime-install path as instance methods. Adapted: Pharo compiles to bytecode in the image; we compile block bodies to BEAM funs and store them in the class gen_server, but the calling convention matches the compiled class method so there is one behavioural contract, not two.

Newspeak

Class-side state and methods live on the class object, mutated through ordinary message sends; the IDE edits live class objects. Adopted: class methods as first-class, runtime-editable members of the class object. Diverged: we keep files as source of truth (ADR 0004); runtime install is memory-only until flushed (ADR 0082).

Erlang / BEAM

No class concept; code:load_binary/3 hot-swaps a whole module. Our class gen_server map is the finer-grained analogue — one selector at a time — without recompiling the module. Adopted: memory-only runtime install, file remains authoritative.

Ruby

A class method is a method on the object's singleton class (metaclass); Ruby installs them at runtime with define_singleton_method / def self.x / class << self, which mutate the singleton class's method table exactly as define_method mutates the instance method table. State accessed via class instance variables is read/written through the same self (the class object). Adopted: the symmetry — runtime-defined class methods are first-class and mutate the class object's own method table, not a side registry. Diverged: Ruby has no compile step and no super-lowering concern — its dynamic dispatch resolves super at call time via the ancestor chain; Beamtalk lowers super ahead of time so the runtime fun carries explicit chain dispatch (the property that prevents BT-873's super breakage).

Beamtalk instance side (the local precedent)

The instance extension path (invoke_extension, ADR 0066) already proved that a compiler-generated fun with proper state threading dispatches correctly at runtime. This ADR is its class-side mirror, differing only where the domain genuinely differs (class-variable threading; self-is-the-class).

User Impact

• Newcomer: Object classBuilder … classMethods: …; register and Counter class >> reset => … both just work, symmetric with the instance forms they already learned. No new vocabulary.
• Smalltalk developer: matches the Pharo expectation that class-side methods are editable exactly like instance-side. Removes a surprising asymmetry.
• Erlang/BEAM developer: runtime class methods are observable in the class gen_server state (class_methods map); dispatch remains a normal apply with a predictable convention; no naive-closure surprises.
• Operator: class-method live patches are memory-only and (under ADR 0082) logged in the ChangeLog as flushable: false for dynamic classes — same audit story as instance patches.
• Tooling developer: builder/patched class methods carry class_method_source (BT-2195 channel), so SystemNavigation class-side scans see them.

Steelman Analysis

Alternative A — Class-side extension ETS registry (mirror ADR 0066 exactly)

• ⚙️ BEAM veteran: "The instance side uses an ETS extension table, not the gen_server map. Symmetry argues for the same table on the class side."
• 🎩 Smalltalk purist: "An extension registry already exists; reuse it instead of inventing a second store."
• Why not (primary): the ETS extension registry exists to add methods to foreign classes you don't own (cross-package String >> shout). Builder creation and editing your own class are naturally the class object's own state — the gen_server class_methods map, mirroring instance instance_methods. Cross-class class-side extensions will want the registry (noted as future work), but routing own-class methods through a foreign-method registry inverts ownership and complicates class new/metadata reads that already read the gen_server map. Chosen design keeps own-methods in the class, registry for foreign-methods — exactly the instance-side split.

Alternative B — A distinct closure convention fun(Args, ClassSelf, ClassVars)

• 🎨 Language designer: "Match the instance extension shape fun(Args, Self, State) for cross-side consistency."
• Why not: it would diverge from the compiled class-method convention (ClassSelf, ClassVars | Args) that the dispatch site already uses, forcing a second adapter and a second {…} return shape. Matching the compiled convention means apply_class_method_in_context changes one line (which apply to call) and the {class_var_result, …} handling is untouched. One contract, not two.

Alternative C — Status quo: class methods are compile-only

• 🏭 Operator: "Fewer runtime moving parts; class methods only come from compiled modules — maximally predictable."
• Why not: blocks BT-2259 entirely (no callable builder class methods) and leaves ADR 0066's ClassName class >> sel syntax permanently non-functional at runtime. The asymmetry (instance methods editable, class methods not) is a papercut every Smalltalker hits.

Alternative D — Generic closure dispatch (revive BT-873 Path 2)

• 🧑‍💻 Newcomer: "Just store the block and call it — simplest possible."
• Why not: this is BT-873, which was removed for dropping state and breaking super/self. The decisive difference in this ADR is that funs are compiler-lowered, not raw blocks. Rejected explicitly.

Tension points

• BEAM veteran (Alt A, ETS) vs the ownership model: resolved by scoping own-methods to the gen_server and foreign-methods to a future registry.
• Newcomer simplicity (Alt D) vs correctness (BT-873): correctness wins; the compiler does the heavy lifting so the simple-looking surface stays safe.

Alternatives Considered

See Steelman. A (class-side ETS registry) — deferred to the cross-class case. B (distinct convention) — rejected, gratuitous divergence from the compiled shape. C (compile-only) — rejected, blocks the epic. D (generic closures) — rejected, reproduces BT-873.

Consequences

Positive

• Programmatic ClassBuilder class methods become callable; ClassName class >> and class-side compile:source: (ADR 0082) get a runtime path.
• Instance/class symmetry: one runtime-install mental model.
• Minimal dispatch change — one branch added before the existing compiled apply; {class_var_result, …} handling reused verbatim.
• No new state protocol; class-variable threading reuses the compiled contract.

Negative

• Compiler must lower class-method block bodies with full class-var/self/super lowering (the work that makes it safe). This is the bulk of the effort and the BT-873-risk surface — it must be tested, not assumed.
• A class can now have a class method in two places (compiled export and gen_server map); dispatch precedence (map shadows compiled) must be documented and tested for hot-reload ordering.
• Dispatch adds a class_methods-map lookup before the compiled apply. It must read from state already resolved during the class-chain walk (DefiningModule, ClassVars) — not introduce an extra gen_server hop per class-method send (cf. BT-2008, which removed such hops). If a hop is unavoidable, the lookup must be gated on the class actually having runtime class methods.
• update_class/2 (recompile / reload) rebuilds the class_methods map from the incoming ClassInfo, which would drop runtime-installed class methods unless they are merged. This is the same memory-vs-disk reconciliation ADR 0082 governs (runtime methods are memory-only; disk wins on reload). The merge-vs-replace behaviour on reload must be decided and tested, consistent with the instance-side put_method reload semantics.

Neutral

• Cross-class class-side extensions remain unimplemented (future, ADR 0066 registry). This ADR does not address them.
• Memory-only until flushed (ADR 0082); restart reloads from disk.
• apply_class_method_in_context already special-cases runAll / run: on TestCase subclasses (test_spawn, beamtalk_class_dispatch.erl:461). A runtime class method named with a test-execution selector on a TestCase subclass is shadowed by that guard. This is pre-existing behaviour; the fun path inherits it and should be tested, not silently changed.

Implementation

Phasing: (1) runtime fun-path + put_class_method (incl. metadata update) + builder wrapping — delivers callable builder class methods, the bulk of BT-2259's value; (2) compiler lowering for classMethods: block literals with the §4 contract + parity tests; (3) class-side >> / compile:source: wiring. Phase 3 is gated on ADR 0082 being accepted (it owns the compile:source: / ChangeLog model); if 0082 stalls, Phase 3 ships as its own follow-on rather than blocking Phases 1–2. Each phase independently testable.

Migration Path

None — additive. Existing compiled class methods dispatch exactly as before (the runtime fun-path is checked first but is empty for file-defined classes that supply no classMethods: block funs). No source, codegen output, or on-disk format changes for current classes. The only behavioural change is that the previously-inert classMethods: builder key and the parsed-but-unwired ClassName class >> sel syntax start working.

Implementation Tracking

Epic: BT-2259 (Programmatic ClassBuilder parity) Status: In progress

References

• Related issues: BT-2259 (epic), BT-873 (Path 2 removal), BT-2195 (class-side method source), BT-2246 (builder methodSource:), BT-2258 (register return), BT-2008 (no per-send gen_server hops)
• Related ADRs: ADR 0038 (ClassBuilder protocol), ADR 0066 (>> extension methods), ADR 0082 (method-level edit/save), ADR 0013 (class variables / class methods), ADR 0036 (metaclass tower), ADR 0006 (unified dispatch)
• Principle: Principle 11 — Live Patching is a Message Send (docs/beamtalk-principles.md)