[MLton] Memory layout debugging

[Wesley W. Terpstra]

Something I would find very useful is to be able to get a graphical
picture of the memory used by my application from a specified root.
MLton.size is nice, but I was thinking of a similar function that
instead created a graphviz dot format file. Nodes in the graph are
addresses in the heap, and arcs correspond to objptrs. I bet this
would be a very easy picture for 'dot' to render as cycles are not
that common in SML programs' memory. This part I am sure I'd have no
trouble implementing.

To make this picture easier to understand, it would be helpful if
objects had their point of allocation or type marked. I know that
-profile alloc is able to record the # of allocations at a point in
the program. However, I also understand that there may be several
points in the program which allocate a given objectType. What sort of
sensible information could I provide based on the object type?

The other question is how to get this information back to ML. I could
just spew the graphviz data to stderr, but this seems crude. I could
also allocate a C-side string that I keep growing as the dotfile
grows, then convert this string to an ML-string inside the basis
library. What's best?

I think that a tool like this would not only help the working
programmer, but would also help MLton development pick the best places
to make representation optimizations.

Thoughts?