About 7 years ago such a tool existed:
http://www.cs.kent.ac.uk/people/staff/cr3/toolbox/haskell/GHood/
GHood was never intended to visualize graph reduction directly (*). instead, it visualized observations - ie, you could see if and when which parts of an observed data structure was inspected during a program run, but not the reductions themselves, and definitely not sharing as in the cyclic programming example earlier in this thread. GHood was very helpful in visualizing some aspects of non-strict evaluation, and its main advantage over Hood was in observing relative strictness, dynamically: you could not only see which parts of an observed data structure were used at all, but by observing both the input and the output of a function, you could see the demands for the output drive the demands for the input. very useful for spotting strictness bugs, such as demanding too much of the input too early, instead of just as much as needed to produce just as much as used for producing the output. see the example applets on that GHood page. claus (*) it was, however, the lack of reduction animation in haskell implementations that kept me looking for such opportunities (i had grown up, functionally, with the kiel reduction systems, and their built-in support for displaying and editing intermediate reduction results, with higher-order functions, free variables, avoiding name clashes, and all, and was missing all that support in the supposedly more modern world of haskell..) an earlier, simpler, example was the use of overloading to visualize intermediate expressions, so you could see the difference between foldr and foldl, etc: http://www.cs.kent.ac.uk/people/staff/cr3/toolbox/haskell/R.hs btw, in one of the previous incarnations of this thread, Wolfram Kahl used the HOP System to generate graphical traces of term-graph reductions: http://www.cas.mcmaster.ca/~kahl/HOPS/