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Malte UblandClaude Opus 5.5
86299300f6
`Array.isArray(v)` is a compile-time constant when `v` has a static type: an `T[]` or a tuple is an array, every other kind is not. The fold only accepted a bare variable, a record field or a field read, because only those could be dropped without losing effects, so `Array.isArray(make())` and `Array.isArray([1, 2])` reached an SC1090 refusal. A computed argument now folds the same way after being evaluated for its effects: the value goes into a sequence whose discarded statement is the argument. Unions keep answering by their runtime tag, and `unknown` values keep their real runtime test. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
29 lines
952 B
TypeScript
29 lines
952 B
TypeScript
// `Array.isArray` over a computed argument: a call, an array literal, an
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// object literal. The answer comes from the argument's static type, so the
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// argument is evaluated for its effects and its value discarded. A union
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// argument keeps answering by its runtime tag, and side effects of the
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// argument survive the fold.
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function rows(): number[] {
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return [1, 2];
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}
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function label(): string {
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return "rows";
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}
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type Pair = [number, string];
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function pair(): Pair {
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return [3, "three"];
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}
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let evals = 0;
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function counted(): number[] {
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evals++;
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return [];
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}
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console.log(Array.isArray(rows()), Array.isArray([1, 2]), Array.isArray({ a: 1 }));
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console.log(Array.isArray(label()), Array.isArray(pair()));
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console.log(Array.isArray(counted()), evals);
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const maybe: string | string[] = ["a"];
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const narrow: string | string[] = "a";
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console.log(Array.isArray(maybe), Array.isArray(narrow));
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console.log(Array.isArray(rows().map((n) => n * 2)));
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