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Malte UblandClaude Opus 5.5
3375304945
A lambda with no type parameters of its own, contextually typed by a generic signature (`send: (m) => …` for an interface's `send<T>(m: Message<T>): string`), takes that signature's type parameters: its parameter is `Message<T>`. A generic lambda written with its own type parameters already lowers once, erased, with them bound to `unknown`, but this one tried to map `Message<T>` with `T` unbound and was refused (SC2009). It now lowers the same way, erased, binding the contextual signature's type parameters to `unknown`. Parameters bound in the enclosing scope keep their bindings. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
41 lines
1.4 KiB
TypeScript
41 lines
1.4 KiB
TypeScript
// An object literal implements an interface's generic method with a plain
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// lambda (`send: (m) => …` for `send<T>(m: Message<T>): string`). The
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// lambda takes the contextual signature's type parameters as its own and
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// lowers once, erased, like a generic lambda: its parameters hold whatever
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// each call passes.
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interface Message<T = unknown> {
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content: string;
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details?: T;
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}
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interface Sender {
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name: string;
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send<T = unknown>(message: Message<T>): string;
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notify<T>(message: Message<T>): void;
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pick<K extends string>(keys: K[], fallback: K): K;
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}
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const log: string[] = [];
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function makeSender(name: string): Sender {
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return {
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name,
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send: (message) => `${name} sent ${message.content} (${message.details === undefined ? "no details" : JSON.stringify(message.details)})`,
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notify: (message) => {
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log.push(`${name} notified ${message.content}`);
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},
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pick: (keys, fallback) => keys[0] ?? fallback,
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};
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}
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const senders: Sender[] = [makeSender("first"), makeSender("second")];
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for (const sender of senders) {
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console.log(sender.send({ content: "hi", details: { n: 3 } }));
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console.log(sender.send({ content: "bare" }));
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console.log(sender.send<number[]>({ content: "list", details: [1, 2] }));
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sender.notify({ content: "ping", details: true });
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console.log(sender.pick(["a", "b"], "z"), sender.pick([], "z"));
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}
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console.log(log.join("; "));
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