BFE.devでのAI解答例 - TypeScript タイプパズル
33. implement TupleToString<T>

解答例:

type TupleToString<T extends string[]> = T extends [infer Head, ...infer Tail]  ? Head extends string    ? `${Head}${TupleToString<Tail>}`    : never  : '';

Explanation:

• TupleToString<T extends string[]> defines a generic type T that extends an array of strings.
• T extends [infer Head, ...infer Tail] deconstructs the tuple by getting its first element as Head and the rest as Tail.
• Head extends string ? ... : never is a conditional type that checks whether Head is a string.
• ${Head}${TupleToString<Tail>} concatenates Head with the result of the recursive call of TupleToString on the rest of the tuple elements, Tail.
• : ''; specifies the base case of an empty tuple, which returns an empty string.

Note that the use of infer allows us to capture the type of Tail in each iteration of the recursive call, which ensures that the returned type is a string.