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webtype.org#227 includes-tuple · par 3

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.

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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.