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kotlinlang

... where does the default `S` of the result even come from? O.o

The first element. (And it throws if you have 0 elements)

So it upcasts the first element to `S`? Ok, weird design.

how is it weird? you wouldn't be able to
```sealed class Node<out T>
data class Leaf<out T>(val data: T) : Node<T>()
data class Tree<out T>(val left: Node<T>, val right: Leaf<T>) : Node<T>()
```
```listOf(Leaf(1), Leaf(2), Leaf(3)).reduce(::Tree)
```
otherwise

... call it unexpected. In Scala, reduce/fold were similar, the distinction was over `reduce` vs. `reduceRight`. Haskell has only one anyway IIRC.

the names ~are closer to~ match most other languages such as JS and Python

Ah yup, I stand corrected: <https://hoogle.haskell.org/?hoogle=(a%20-%3E%20a%20-%3E%20a)%20-%3E%20t%20a%20-%3E%20a>

Didn't read the type signature `&lt;S, T: S&gt;` on reduce correctly.