japgolly’s signature for reduce above is slightly wrong, it should be `[A] -> ((A,A) -> A) -> Maybe A`.

I.e. there is no initial value to pass in, but the result is an Optional to handle the empty iterator case. That’s how rust does it, for example:

https://doc.rust-lang.org/std/iter/trait.Iterator.html#metho...

No, the sum of the first ten natural numbers is always 55. It is not "initialised" to some other number beforehand.