Chris Stringer (1) [Avatar] Offline
It looks like there's an error on the last line of listing 2.18.

In the ternary conditional operator expression - specialMix isn't force-unwrapped.
Craig Grummitt (48) [Avatar] Online
You're right! That exclamation mark seems to have been lost in the editing process, thanks for pointing it out! It sounds as though you've worked out what it should be, but for anyone else reading this before it gets fixed, the line of code should read:

let prepareMartini = specialMix != nil ? specialMix! : defaultMix