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https://github.com/vercel-labs/scriptc.git
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Malte Ubl
bf17c23af9
TypeScript types `read ?? "text"` as number when the read is typed number, and `() => xs[i]` as returning number. Declared functions already widened their returns for both (the primitive result union and the optional read), but arrow functions and function expressions kept the checker's return, so the closure narrowed its result and threw "a 'string' value is not representable" or "undefined is not representable" (the scanbox `[string, () => unknown][]` probe table). Unannotated TypeScript lambdas now get the same widened return as declared functions. A const or global binding of such a lambda keeps the checker's parameters and takes the widened result, and calls through the binding count as optional or primitive-union results for the enclosing function's return.
75 lines
2.1 KiB
TypeScript
75 lines
2.1 KiB
TypeScript
// A missing read (an out-of-range typed-array or array read the checker
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// types as `number`) defaulted with `??` or `||` to a string, or returned
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// as is, gives a lambda a result its inferred return type does not spell:
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// TypeScript types `read ?? "text"` and `() => xs[i]` as number. The
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// closure returns the runtime value, through bindings, callbacks taking
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// `unknown`, calls of the binding, and the enclosing function's return.
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const base: number = process.argv.length;
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const chars = new Uint16Array(4);
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chars[0] = 108;
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const nums: number[] = [7];
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const far = base + 100;
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const a = chars[far];
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const c0 = chars[0];
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// The scanbox probe table: every arrow is typed `() => number` by
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// inference and stored in a `() => unknown` slot.
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const probes: [string, () => unknown][] = [
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["plus", () => a + 1],
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["or", () => a | 0],
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["max", () => Math.max(a, 1)],
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["fcc", () => JSON.stringify(String.fromCharCode(c0, a))],
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["and", () => c0 && 5],
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["or2", () => chars[far] || 7],
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["nullish", () => a ?? "dflt"],
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["nullish present", () => c0 ?? "dflt"],
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["tern", () => (a ? "yes" : "no")],
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["bang", () => !a],
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["concat", () => a + "s"],
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["missing", () => nums[far]],
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];
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for (const [name, f] of probes) {
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try {
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console.log(name, f());
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} catch (e) {
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console.log(name, "threw", (e as Error).message);
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}
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}
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function take(f: () => unknown): unknown {
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return f();
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}
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console.log("take nullish", take(() => a ?? "d1"));
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console.log("take missing", take(() => nums[far]));
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const bound = () => a ?? "d2";
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console.log("bound", bound());
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const block = () => {
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return a ?? "d3";
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};
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console.log("block", block());
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const expression = function () {
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return a || "d4";
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};
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console.log("expression", expression());
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const read = () => nums[far];
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console.log("read", read(), read() === undefined);
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function viaBinding(i: number): number {
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const get = () => nums[i];
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return get();
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}
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console.log("via binding", viaBinding(far), viaBinding(0));
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function declared() {
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return a ?? "d5";
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}
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console.log("declared", declared());
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class Lexer {
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peek(i: number) {
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return chars[i] ?? "eof";
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}
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}
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console.log("method", new Lexer().peek(far), new Lexer().peek(0));
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