Requiring a value to be Some

I have a function that returns an `Option`. I run this on several values and then process them further. Since I’m using `map` et al I still have an `Option` at the end of it. Now I want to require that some of the values be defined. I want to be able to say something like `option.orElse(() => throw new Error("this value must be defined")`, but `orElse` directly takes an `Option`. `flatMap` does the opposite of what I need, as it will only run the function when the value is `Some`.

It would be helpful to have something like [Rust’s `unwrap_or_else`](https://doc.rust-lang.org/std/option/enum.Option.html#method.unwrap_or_else) and [Scala’s `orElse`](https://www.scala-lang.org/api/current/scala/Option.html#orElse[B%3E:A](alternative:=%3EOption[B]):Option[B]) in the general case. In this specific case, I’ve been wondering if it might be better to define `get` on `Option` instead of on `Some` and have it throw an exception on `None` (cf. [Rust](https://doc.rust-lang.org/std/option/enum.Option.html#method.unwrap), [Scala](https://www.scala-lang.org/api/current/scala/Option.html#head:A), [F#](https://msdn.microsoft.com/visualfsharpdocs/conceptual/option.get%5b%27t%5d-function-%5bfsharp%5d)).

Start by reading the Option, Some, and None implementations, including the existing orElse and get entry points. Compare the requested unwrap-or-throw behavior with the current API and related Rust, Scala, and F# semantics. Done means an agreed API behavior is implemented and its edge cases are covered by the project’s tests.

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