How we built an interpreter for Swift

Swift can already be used in REPL mode, not sure if it's accurate to say Swift is a strictly compiled language that a interpreter was developed for (see 'swift' command line executable). Seems misleading that this is not mentioned anywhere in the article.

++ had the same reaction as you. When I was picking up Swift, the interpreter was invaluable to check my understanding (although I nearly always needed to begin with `import Foundation` to have my code actually work).

I suppose you could argue the repl is not a true interpreter (parsing and executing from an AST)

That's a JIT. It uses the same compiler infrastructure but swaps out the AoT backend and replaces it with the JIT backend in LLVM. Notably, this blog post is targeting on-device usage which a custom JIT is not allowed. You can only interpret.