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What is a 'non-nominal type' in Swift?

I see this error when trying to extend unorthodox 'types' like (Int, Int), or Any:

Non-nominal type 'Any' cannot be extended

So what makes a type non-nominal? What's the difference between a non-nominal type like Any or (Int) and a regular nominal type like Int?

Solution

  • This is somewhat of a guess (edit: it's wrong, look at Brent's answer), but here goes:

    Any is a protocol, not an actual type. The word "Nominal" implies naming (based on the root of the word).

    So you can't extend Any because it's a protocol, not an actual type, and you can't extend (Int, Int) because that's just a tuple literal, again not an actual type that you could specify by name.

    Update:

    You can, of course, extend protocols. Any is not a protocol, it's (shocker) a non-nominal type which is something else. Read Brent's answer; he did a good job.