running ghci with -prof turned on makes it die immediately (with an appropriate message about this combination being illegal). however, i find that i would almost always *rather* run ghci with -prof turned on than without it. i don't particularly want to create profiles, but i'd really like stack backtraces in ghci.

how much effort would be required to allow ghci to (at least) *use* profiled objects, if not build them itself?

Not trivial, I suspect. When -prof is turned on, lots of conventions change, including the layout of heap objects and info tables. This means the whole binary has to be compiled with -prof: that includes GHCi itself. The byte-code back-end would need some work to make it generate and interpret byte codes in a profiled world. Cheers, Simon