You're right that "expect" combined with the error message here could be better. Specifically, I've been wanting a better name for the method, but haven't really done anything about it yet. The name itself is taken from Rust.

"unwrap_or" could perhaps be called "unwrap_or_use_default", but that's a tad too much to type. What it basically does is this:

    match the_value {
      case Some(val) -> val
      case _ -> 0
    }

i.e if you have a Some, you "unwrap" its value and return it, otherwise you return a default value.