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https://github.com/vercel-labs/scriptc.git
synced 2026-11-02 07:43:42 +08:00
Chris Tate
e9977107fe
A const that kept its initializer's layout records the initializer's checker type by declaration symbol. A function body lowers once per instance, and only an instance for a wider argument keeps the layout, so the record from that instance made the function's own lowering bind the wider layout for `grow(alias)`: the argument converted into a copy and the callee's writes did not reach the caller. The recorded type now applies only while it maps to the layout of the binding the current lowering resolves.
69 lines
2.4 KiB
TypeScript
69 lines
2.4 KiB
TypeScript
// An array or record passed to a parameter, returned, or bound to a const
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// under a type that lacks some of its optional fields stays the same value:
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// identity comparisons and writes through any of the names see one object.
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interface Entry {
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key: string;
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hits: number;
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note?: string;
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}
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type Counted = { key: string; hits: number };
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function same(list: Counted[], other: Entry[]): boolean {
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return (list as unknown) === other;
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}
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function keep(list: Counted[]): Counted[] {
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return list;
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}
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function first(list: Counted[]): Counted {
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return list[0]!;
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}
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function grow(list: Counted[], key: string): Counted[] {
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list.push({ key, hits: 0 });
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return list;
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}
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const entries: Entry[] = [{ key: "a", hits: 1, note: "first" }];
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console.log(same(entries, entries), keep(entries) === entries, first(entries) === entries[0]);
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const grown = grow(entries, "b");
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grown.push({ key: "c", hits: 2 });
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console.log(grown === entries, entries.length, entries.map((e) => e.key).join(","));
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function local(list: Entry[]): boolean {
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const view: Counted[] = list;
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const kept = keep(list);
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view[0]!.hits += 10;
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return view === list && kept === list;
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}
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console.log(local(entries), entries[0]!.hits, entries[0]!.note);
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// An annotated const that kept its initializer's layout is that value when it
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// is passed on: the callee's writes reach both names.
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function track(list: Counted[], key: string): void {
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list.push({ key, hits: 0 });
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}
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function viaAlias(): string {
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const source: Entry[] = [{ key: "s", hits: 1, note: "kept" }];
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const view: Counted[] = source;
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track(view, "t");
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return `${view.length} ${source.length} ${view === source}`;
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}
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console.log(viaAlias());
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const moduleSource: Entry[] = [{ key: "m", hits: 1 }];
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const moduleView: Counted[] = moduleSource;
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track(moduleView, "n");
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console.log(moduleView.length, moduleSource.length, moduleView === moduleSource);
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// The same function body lowered for a wider argument and for its own type:
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// each lowering passes its own binding on.
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function addTwice(list: Counted[], key: string): number {
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const alias: Counted[] = list;
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track(alias, key);
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track(alias, `${key}2`);
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return list.length;
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}
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const wideList: Entry[] = [{ key: "w", hits: 1, note: "n" }];
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const plainList: Counted[] = [{ key: "p", hits: 1 }];
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console.log(addTwice(wideList, "a"), addTwice(plainList, "b"), addTwice(wideList, "c"));
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console.log(wideList.length, plainList.length);
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