Reduce<T> returns T instead of Option<T>

Consider this reduce method:

public static T reduce<T>(IEnumerable<T> list, Func<T, T, T> reducer) =>
    match(headOrNone(list),
        Some: x => fold(tail(list), x, reducer),
        None: () => failwith<T>("Input list was empty")
    );

I was surprised to see that it throws an exception when list is empty.

Wouldn’t the functional approach be to have the return type be Option<T> and return None when list is empty?

[louthy]

I have added reduceOrNone and reduceBackOrNone

[TysonMN]

Here is the equivalent in Haskell:

http://hackage.haskell.org/package/base-4.12.0.0/docs/src/Data.Foldable.html#foldl1

Here is the equivalent in F#:

https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/list.reduce['t]-function-[fsharp]

Both throw exceptions.

I find it interesting that Java is more functional (in my opinion) in this regard than language-ext, F#, or even Haskell.

public interface Stream<T> {
  Optional<T> reduce(BinaryOperator<T> accumulator);
}

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#reduce-java.util.function.BinaryOperator-