Comment by lmm

Things like this exist. They're not that useful in practice, because it's like live editing PHP directly on the server, only more so. Most of the value of edits to a piece of code doesn't come from running it once, it comes from having that edit in a durable, managed place. And however much effort you put into your code management database, it's hard to beat the amount of tooling that exists for the worse-is-better "mostly-ascii files on a unix filesystem" model.