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    No. 227 · August 4, 2026 · Moderate

    Includes

    Implement Includes<T, U> so it reports whether the tuple T contains exactly the type U. boolean does not count as containing true — the match must be exact. Equal from the harness is in scope.

    01

    Try the puzzle yourself

    Par 3

    Puzzle

    includes-tuple.ts
    Stroke 1 of 3Not run yet

    Replace ??? — your solution is checked against the cases below. Tab indents; press Escape then Tab to move focus out.

    Checks

    4
    • Includes<[1, 2, 3], 2>—
      → true
    • Includes<[1, 2, 3], 4>—
      → false
    • Includes<[], 1>—
      → false
    • Includes<[boolean], true>—
      → false

    How a check is judged Exact type equality, not assignability — an intersection is not the same as the flattened object.

    How everyone did

    Fewer than 5 people have solved this one so far. The distribution appears once there is enough of a sample to mean anything.

    Short game

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    Archive
    02

    Annotated solution

    Published August 5, 2026

    The solution

    type Includes<T extends readonly unknown[], U> =
  T extends readonly [infer H, ...infer R]
    ? Equal<H, U> extends true
      ? true
      : Includes<R, U>
    : false

    The common wrong answer

    type Includes<T extends readonly unknown[], U> =
  U extends T[number] ? true : false

    Short and wrong. T[number] is the union of element types and extends asks about assignability, so Includes<[boolean], true> answers true — true is assignable to boolean even though the tuple contains no such element. Assignability is not membership.

    Line by line

    1. Equal<H, U> extends true

      Equal returns the type true or the type false, so it has to be tested with extends true rather than used directly as a condition. This is the puzzle where the harness itself becomes a tool.

    2. : false

      Reaching the empty tuple means every element was checked and none matched. Exhausting the list *is* the negative answer.

    Takeaway

    When a puzzle says "exactly", extends is the wrong tool — it tests assignability, which is one-directional. Exact identity needs Equal, and this is why every check on this site is built on it.

    Uses

    RecursionGet a value by a typed deep pathMake an object deeply readonlyTS2456TS2589TS7023