It's one of the questions I ask when I interview people.

I could care less whether you remember some algorithm you could look up in Knuth. I want to know if you can put yourself in your customer's place and design an API that meets their needs.

Language design is one of the hardest skills to master, and as the qualities of a good language include maintaining backward compatibility and longevity, most people who do master it never get to apply what they learned. Instead we only get to see their practice pieces. Their almost but not quite masterpieces.

It’s why I could immediately see that XML was going to fail in the long run. Everyone is a language designer? Sure, that’s going to work out.

APIs are tiny languages, so learning to do it well is a microcosm of language design. You don’t have to be good at all parts, but if the API you need isn’t in your wheelhouse, everyone will know. And if you build enough of them you’ll discover the limits of your skill.