Avoid reconstructing dictionaries in recursive instance methods
Ganesh writes: I have a problem in GHC 6.10 with functions in a class instance calling other functions in the same class instance. It seems that the dictionary is freshly constructed for this call, despite being already available.
The reason I care is that I want to memoise some expensive computations inside the dictionary for each instance. [Obviously I also have to make sure that the dictionary isn't constructed multiple times by external callers, but I can make other arrangements to ensure that.]
To see the problem in action, run main from the attached code. In GHC 6.8 and before, this only executes the trace statement once. In GHC 6.10, the trace statement executes twice, at all optimisation levels.
This seems related to #2902 (closed), but I'm a little unclear on whether it's the same problem or not.