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