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JS runtime: Use classes for Ref and Arena instead of closures #1332

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Merged
jiribenes merged 1 commit into from
Mar 23, 2026
Merged

JS runtime: Use classes for Ref and Arena instead of closures #1332

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94 changes: 54 additions & 40 deletions libraries/js/effekt_runtime.js
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Original file line number Diff line number Diff line change
@@ -1,46 +1,60 @@
// Complexity of state:
//
// get: O(1)
// set: O(1)
// capture: O(1)
// restore: O(|write operations since capture|)

// Sentinel: a node whose .value is Mem is the current root.
const Mem = null

// reusable buffer for rerooting
const _rerootPath = []

function Arena() {
const s = {
root: { value: Mem },
generation: 0,
fresh: (v) => {
const r = {
value: v,
generation: s.generation,
store: s,
set: (v) => {
const s = r.store
const r_gen = r.generation
const s_gen = s.generation

if (r_gen == s_gen) {
r.value = v;
} else {
const root = { value: Mem }
// update store
s.root.value = { ref: r, value: r.value, generation: r_gen, root: root }
s.root = root
r.value = v
r.generation = s_gen
}
}
};
return r
},
// not implemented
newRegion: () => s
};
return s
/**
* A mutable reference inside an `Arena`.
*/
class Ref {
constructor(v, s) {
this.value = v;
this.generation = s.generation;
this.store = s;
}

set(v) {
const s = this.store;
const r_gen = this.generation;
const s_gen = s.generation;
if (r_gen === s_gen) {
this.value = v;
} else {
const root = { value: Mem };
s.root.value = { ref: this, value: this.value, generation: r_gen, root };

s.root = root;
this.value = v;
this.generation = s_gen;
}
}
}

/**
* A snapshottable arena: a bag of `Ref`s whose collective state can be
* captured in O(1) and restored in O(#writes since capture).
*/
class Arena {
constructor() {
this.root = { value: Mem };
this.generation = 0;
}

/**
* Allocate a new reference with initial value v.
*/
fresh(v) {
return new Ref(v, this);
}

/**
* Region support (not implemented!).
*/
newRegion() {
return this;
}
}

function snapshot(s) {
Expand Down Expand Up @@ -87,7 +101,7 @@ function restore(store, snap) {
let _prompt = 1;

const TOPLEVEL_K = (x, ks) => { throw { computationIsDone: true, result: x } }
const TOPLEVEL_KS = { prompt: 0, arena: Arena(), rest: null }
const TOPLEVEL_KS = { stack: null, prompt: 0, arena: new Arena(), rest: null }
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@jiribenes jiribenes Mar 21, 2026

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Explicitly setting stack: null seems to help V8 so that it knows it's always the same "type".
Same for the one created in RESET.


function THUNK(f) {
f.thunk = true
Expand All @@ -108,7 +122,7 @@ const RETURN = (x, ks) => ks.rest.stack(x, ks.rest)
function RESET(prog, ks, k) {
const prompt = _prompt++;
const rest = { stack: k, prompt: ks.prompt, arena: ks.arena, rest: ks.rest }
return prog(prompt, { prompt, arena: Arena([]), rest }, RETURN)
return prog(prompt, { stack: null, prompt, arena: new Arena(), rest }, RETURN)
}

function SHIFT(p, body, ks, k) {
Expand Down
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