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webtype.orgwebtype.org#232 zip-tuples · par 3

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    No. 232 · August 9, 2026 · Moderate

    Zip

    Implement `Zip<A, B>` so it pairs elements of two tuples positionally. When the tuples differ in length, stop at the shorter one.

    01

    Try the puzzle yourself

    Par 3

    Puzzle

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

    Checks

    4
    • Zip<[1, 2], ['a', 'b']>—
      → [[1, 'a'], [2, 'b']]
    • Zip<[], []>—
      → []
    • Zip<[1, 2, 3], ['a']>—
      → [[1, 'a']]
    • Zip<[1], ['a', 'b', 'c']>—
      → [[1, 'a']]

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

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    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 10, 2026

    The solution

    type Zip<A extends readonly unknown[], B extends readonly unknown[]> =
  A extends readonly [infer AH, ...infer AR]
    ? B extends readonly [infer BH, ...infer BR]
      ? [[AH, BH], ...Zip<AR, BR>]
      : []
    : []

    The common wrong answer

    type Zip<A extends readonly unknown[], B extends readonly unknown[]> =
  A extends readonly [infer AH, ...infer AR]
    ? B extends readonly [infer BH, ...infer BR]
      ? [AH, BH, ...Zip<AR, BR>]
      : []
    : []

    Without the inner brackets the pairs are spread flat into the result: `[1, "a", 2, "b"]` instead of `[[1, "a"], [2, "b"]]`. Zipping is about structure, and the structure lives in those brackets.

    Line by line

    1. [[AH, BH], ...Zip<AR, BR>]

      The inner `[AH, BH]` is one pair; the spread appends the pairs from the rest. Nesting a tuple inside a tuple is how you keep the pairing visible in the type.

    2. : []

      Both fall-through branches end the recursion, so whichever tuple runs out first stops the zip. That gives the shorter-of-the-two behaviour without measuring either length.

    Takeaway

    Walking two structures at once is just two destructuring conditionals nested. The interesting design decision is what happens when they disagree in length — and here, doing nothing special is the answer.

    Uses

    Variadic tuplesParametersVariadic tuplesTS2493TS2554