I spent several days last week trying to track a cause of a 100% slowdown after some trivial changes I made. The profiler didn't show any slowdown, presumably because it was dependent on optimizations, so I had to revert to tweak-run-measure cycle. It turned out the slowdown was caused by some unevaluated thunks that were kept around in long-lived IORefs. This is not the first time I was bitten by too laziness, either. What made things worse this time is that there is no way do declare the following: data Label = LabelRef {labelId:: !Unique, reference:: (IORef !LabelState), -- illegal origin:: Maybe !Label} -- illegal No container data type can be annotated as strict. That means I have to pepper my code with explicit evaluations to HNF before every writeIORef (reference label): newState `seq` writeIORef (reference label) newState What is the reason for this restriction on where strictness annotations can appear? Is it purely an implementation problem or is there a reason emanating from Haskell design? If former, how hard would it be to fix?