Term graph

[kderme]

Related Issue #2
This PR adds the option of visualizing the heap as a graph.

• Module Closure is pure and can be used by any visual backend to visualize the graph.
• Dot is used for representing the graphs (same as Graph Visualization #11).
• For each step a new dot file is created. Dot files must be rendered to png to be used by javascript.
• Nodes of the graph include not only named heap pointers, but also every term that would be represented as a separate heap object.
• Edges of a Closure point towards its free variables and other heap objects.

A step of examples/skim.hs:

This PR also adds a new example examples/graphs.hs

This is just a first attempt. There are a lot more thing that can be improved, like

• better visualizations
• rendering dot files in js
• Adding Closures for function applications (more here) so that graphs of function application are not generic thunks
• Eliminating Indirections

[edsko]

Oooooh! 😍 I'll try to take a look at this tonight. Awesome work!

[edsko]

@kderme How is the consistency of the graphs from step to step? If you're tracing the execution of a program, how much do the nodes jump around?

[kderme]

They actually jump around much less than I expected. I have all results from examples here and a script to run all.

A function can jump from bottom to top after it`s been applied (for example before and after)
Sometimes this can also affect the order of other nodes as dot does some sort of balancing (for example this vs this)

But in general there is a lot of stability.

[edsko]

Just took a closer look, really cool stuff. Have been wanting to add this but never got around to it. I'm happy to merge this as-is, or do you want to address some of the points you mention above? If not, do you want to open some tickets about those so that we don't forget?

One very minor niggle: typo: Funtion. Also, feel free to add yourself to CONTRIBUTORS and add an entry to the ChangeLog (I can do that also if you prefer).

Nice! :)

[kderme]

Happy you liked it. For now I can open some of the issues I mention above. In the future I may find some time to address them.

[edsko]

Happy you liked it. For now I can open some of the issues I mention above. In the future I may find some time to address them.

Ok, great, merging as is :)

[erikd]

???

[kderme]

oops..