Slow compilation time is actually a great reason for having a REPL. e.g. Clojure and Julia are both REPL-based languages, though both secretly need to be compil

Slow compilation time is actually a great reason for having a REPL. e.g. Clojure and Julia are both REPL-based languages, though both secretly need to be compiled (and do so relatively slowly). In practice you don't notice the compile step at all, since after compiling and running once you can just compile 1-2 lines at a time in the REPL and it patches the running program for you – it's all instant. A more interesting issue for REPLs is how expression- or statement-oriented the language is (c.f. Martin Odersky's concerns about the upcoming Java REPL). In Java/CXX it's much more likely for a function to want to change an external object, in which case you'll need to set up the environment, run the code, check the environment for the change, for every line you write. That's much more likely to be the nail in the coffin for this.