My wish is a language that lets you instantly run changes but build enforcing static types. For example, if I change the signature of a function, I can update one url that uses it and instantly see the changes, and the language/tooling tells me there are 4 places where the code broke. I guess Python or JS will be the first to reach this goal.
It sounds like a dynamic language + optional external static analysis tool which exists in a lot of languages as third party softwares, like typescript / flow.
Actually it already exists. You can do that with any bytecode-targeting JVM language.
For a compiler that can compile even with errors in the code, look at ECJ.
If you take HotSpot and attach a debugger in your IDE, then you have some hotswap capabilities [1]. You can redefine the contents of methods, add methods and add classes (e.g. lambdas). This is OK as far as it goes. But this is no inherent limitation of Java. In fact the debugger protocol lets you request arbitrary changes, it's just not all JVMs support them.
One that does is called DCEVM [2]. It fleshes out the hotswap capabilities far more extensively and lets you do things like delete methods, change prototypes, alter the inheritance hierarchies etc, all on the fly whilst the app is loaded and running.
DCEVM is an open source patchset onto HotSpot, but it's not really a pro level tool. JRebel [3] is a company that makes hotswap tools as a commercial product. Many companies use it.
These are a neat capabilities and I've used them a few times, but even with that level of power, it's not always complete for every use case. The problem is that most apps have initialized state. Web servers often do not, because they throw away state at the end of every request, and there it works better, but if you're working on e.g. a GUI or a CLI tool with a shell, or really anything where the program has to maintain state for longer than a split second, changing code becomes of more limited use because you are going to end up changing initialization code that won't re-run. You can add various ad-hoc hooks to your app to let you force re-init whilst running, but it's not totally smooth. Note: this isn't a limitation of the language or runtime. It's inherent to the concept of hotswapping code.
There is work to resolve even this problem. The Espresso JVM has full hotswap support, and a callback interface that lets the app learn when its own code is being redefined. It can use those callbacks to trigger re-initialization of any state that's required. [4] Espresso is still new but this sort of thing lights the way to apps that perhaps never need to be shut down at all, not even for upgrades.
