Hacker News Reader

Comment by kaoD | Hacker News Reader

For those unfamiliar with TS, the above is just...

    function flat([head, ...tail]) {
      return Array.isArray(head)
        ? [...flat(head), ...flat(tail)]
        : [head, ...flat(tail)]
    }

...in TS syntax.

View on HN

Well, it is the type of that, in TS syntax. Few are the statically-typed languages that can even express that type.

Java: List<Object>

Python: list[Any]

...what am I missing?

You're missing the input type, essentially. Those are just array types. The TypeScript type signature more of a function type, it expresses flattening a n-dimensional array (input type) into a flat array (output type).

You're missing the specialisation of Object/Any. For example Array.flat called with [int, [bool, string]] returns a type [int, bool, string]. Admittedly this is somewhat niche, but most other languages can't express this - the type information gets erased.

Reply on news.ycombinator.com