[mredigonda]

This video [0] is from February 2012, but as far as I know this tool is not yet implemented (or at least it is not widely used).

Basically as you are writing a function, you can write sample input for it, and it tells you the intermediate value of each variable inside this function, which lets you reason about the code, makes it trivial to know you've done something wrong, and would make developing a much more interactive experience overall.

In my experience, this is done by hand when writing a function, but a tool like this would make it automatic.

[0] https://www.youtube.com/watch?v=PUv66718DII

[ricklamers]

This gets pretty close for web applications: https://www.replay.io/

Another relevant talk: https://www.youtube.com/watch?v=Ml6Dp3F4Inc