[RyanCavanaugh]

This is how JavaScript intellisense in Visual Studio used to work, except that the program was executed "behind the scenes" using a trimmed-down VM that could execute without side effects or infinite loops. It was eventually abandoned due to poor performance, predictability, and stability.

The problem is this dilemma: If you have to wait for a "real" execution of a program, then very reasonable expectations like "I can see a local variable I just declared" doesn't work. If you try to fake-execute a program, you have problems like trying to figure out what to do with side-effecting calls, loops, and other control flow problems.

Trying to reconcile a previous type snapshot with an arbitrary set of program edits was tried by an early version of TypeScript and wholly abandoned because it's extremely difficult to get right, and any wrongness quickly propagates and corrupts the entire state. The flow team is still trying this approach and is having a very hard time with it, from what I can tell.