I always found Hack interesting. It's PHP slowly transformed step by step into something closer to Java (Java isn't as bad as new grads claim, compared to PHP i

I always found Hack interesting. It's PHP slowly transformed step by step into something closer to Java (Java isn't as bad as new grads claim, compared to PHP it's actually great). It's pretty much exclusively used at Meta. Basically what they did is they had repeated codemods (changes to the entire codebase with automated tooling) that bit by bit moved PHP closer to Java. More and more static typing, generics, all the common Java ADTs (Vector, Map, Set, Pair, etc.), bytecode+JIT execution, etc. Essentially instead of rewriting the codebase to a better language they just changed the language itself. Which makes sense since PHP is a much smaller codebase than the Meta backend.