Debugging feature that links shapes back to source code

[cdsmith]

It would be great if you could run a drawing in a debug mode, which would let you select shapes, and point you to the source code that drew them. What is this black magic? Actually, all that would be required to is to add a HasCallStack constraint to all of the drawing primitives, and have them store source code references alongside constructors for the Picture type! Then it's just a matter of building the UI.

Performance is definitely a concern. We'd want to only store the source locations in a Picture when the debugging mode is enabled, and carefully design the code to minimize performance impact when this function isn't used. For example, perhaps the source locs could be stored in an external hash map that's built up as the picture is evaluated, rather than in the ADT itself.

[cdsmith]

Proposed UI: Add a hover button to drawingOf for debugging, akin to the ones that exist now for animationOf and simulationOf. The button would enable the debug mode, and redraw the picture, saving source locations. We can also automatically enable the feature when you pause an animation. I'd propose just omitting this feature from simulation and interaction modes, since those are less picture-focused, and I already advise students to be willing to change main to a drawingOf to debug their drawings in isolation, anyway.

Another thing that would be great in conjunction with this is to adopt gloss's features that automatically let you scale and translate a drawing or animation using mouse controls. Then you can zoom in, and choose tiny little shapes or visual artifacts to see where they came from.

[cdsmith]

Just recording my thoughts here. Turns out that the idea of recording source locations as a benign side effect only when debugging is enabled is not workable, because once the runtime evaluates a thunk, there is no way to mark it as unevaluated again and turn it back into a thunk. So we'd actually need to store source locations in the Picture all the time.

Probably a premature optimization, anyway. Maybe this is cheap enough to not matter.

[cdsmith]

This was done last summer by @three, so I'm closing.