[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.