Diffusion Ant (master) Code

ant/tests/repro_newt_length.js
dce8e2e6fb90master

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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