Hmm, that’s interesting - I enjoy REPL-based development, having spent a lot of time working in Python, and a little Clojure. How does the REPL experience play with the static typing (that I think) Julia has? If you change the type signature of a function in the REPL what happens?

Julia is a dynamic language, it doesn't have static typing. What happens is that it's dynamic nature is actually a superposition of all possible static implementations of a function. So if you call a * b with an integer it will JIT compile an integer based version of the product, while if you do it with matrices it will compile a matrix product version. Julia Base has 361 implementations of product, which any library or programmer can freely add at any point and the compiler will always match at compile time the most specific version defined for the combination of all arguments (multiple dispatch). You also usually don't need type hints when calling methods, the compiler will infer the types by itself and choose the optimal implementation.

If you define one method with the exact same parameters as an older one it will just redefine it. As a side-note, it has safe points where the JIT will work (usually the global namespace, in which the REPL runs each command), and otherwise it can't see newly declared methods (as it is running already compiled code) unless you use a method to force it (like invokelatest or eval). The period between safe-points is called the world age.