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repro_newt_length.js
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repro_newt_length.js
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// Reproduces a bug where Prelude_length$27 (while-loop length) returns 0
// for a 4-element linked list under ant after GC pressure.
//
// In the full newt bootstrap, lvl2ix calls length(ctx.env) and gets 0 back
// instead of 4, producing a negative de Bruijn index (-3 instead of 1),
// which cascades into "Missing case for Prelude.MkEq".
//
// The bug does not reproduce in isolation without GC pressure.
const Nil = () => ({ tag: 0 });
const Cons = (h1, h2) => ({ tag: 1, h1: h1, h2: h2 });
// While-loop length (Prelude_length$27 from newt bootstrap)
const length_go = (_, xs, acc) => {
while (1) {
let xs1 = xs;
let acc1 = acc;
if ((xs1.tag) == (1)) {
xs = xs1.h2;
acc = (acc1) + (1);
continue;
} else {
return acc1;
}
}
};
const length_prime = (xs) => length_go(xs, xs, 0);
// lvl2ix from newt: l - k - 1
const lvl2ix = (l, k) => l - k - 1;
// Build a VVar-like object (matches newt's object shape)
const VVar = (h0, h1, h2) => ({ tag: 0, h0: h0, h1: h1, h2: h2 });
const emptyFC = { tag: 0, h0: "", h1: { tag: 0, h0: 0, h1: 0, h2: 0, h3: 0 } };
const Lin = () => ({ tag: 0 });
// Simulates what quoteAlt_mkenv does: builds an env list with VVar nodes
function mkenv(lvl, env, names) {
while (1) {
if ((names.tag) == (1)) {
let newVar = VVar(emptyFC, lvl, Lin());
lvl = lvl + 1;
env = Cons(newVar, env);
names = names.h2;
continue;
} else {
return env;
}
}
}
// Simulates the eval/quote cycle that builds up env lists
function simulateEval(env, depth) {
if (depth <= 0) return env;
// Build up environment like newt's eval does
let fresh = VVar(emptyFC, length_prime(env), Lin());
let newEnv = Cons(fresh, env);
// Allocate throwaway objects to create GC pressure
for (let i = 0; i < 100; i++) {
let junk = { tag: 5, h0: emptyFC, h1: i, h2: { tag: 0 }, h3: "x", h4: { tag: 0, h0: env, h1: { tag: 0 } } };
}
return simulateEval(newEnv, depth - 1);
}
let fail = false;
// Run many iterations to trigger GC
for (let round = 0; round < 500; round++) {
// Start with a 4-element env (matching the failing case)
let env = Cons(VVar(emptyFC, 3, Lin()),
Cons(VVar(emptyFC, 2, Lin()),
Cons(VVar(emptyFC, 1, Lin()),
Cons(VVar(emptyFC, 0, Lin()),
Nil()))));
let len = length_prime(env);
if (len !== 4) {
console.log("FAIL round " + round + ": length(env) = " + len + ", expected 4");
console.log(" lvl2ix would give: " + lvl2ix(len, 1) + " (expected 2, got " + lvl2ix(len, 1) + ")");
fail = true;
break;
}
// Simulate eval growing the environment
let bigEnv = simulateEval(env, 20);
let bigLen = length_prime(bigEnv);
if (bigLen !== 24) {
console.log("FAIL round " + round + ": length(bigEnv) = " + bigLen + ", expected 24");
fail = true;
break;
}
// mkenv path (like quoteAlt_mkenv)
let names = Cons("a", Cons("b", Cons("c", Nil())));
let extEnv = mkenv(length_prime(env), env, names);
let extLen = length_prime(extEnv);
if (extLen !== 7) {
console.log("FAIL round " + round + ": length(extEnv) = " + extLen + ", expected 7");
fail = true;
break;
}
}
if (!fail) {
console.log("PASS");
} else {
process.exit(1);
}

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