‘Growing a Language’ and the ‘Are We There Yet’ presentations changed the way I look at languages, so right now I would say my “perfect” high-level PL would be a Java-like language (possibly running on the JVM due to ecosystem-reuse and the killer GCs and performance) where immutable “objects”/values are the first-class citizens, but method implementations can use local, mutable semantics. I would also very much add a ‘pure’ keyword marking side-effectless functions/methods.

It sort of sounds like Scala though, but I would probably settle for a weaker type system but with constraints (like JML or Clojure’s spec) that are tried to be proven at compile time but will fall-back to be checked at runtime in debug mode (and if marked so, even in prod). Also, would probably make Clojure’s atom/ref/etc concurrency primitives the “most first-class”, so other than the possibly mutable method implementations, everything would be safe+ concurrency-wise.

It is pretty much a mix between Scala and Clojure, or a non-lisp clojure with some types, so not really original..

Oh, and Haskell’s target typing for number literals, I honestly don’t understand why don’t we use that everywhere.

+ AFAIK, deadlocks can’t be statically guaranteed