It is not possible to write a modifyIORef that *doesn't* leak memory!
Why? Or can one read about it somewhere?
Possibly, Don meant that 'modifyIORef' is defined in a way that does not allow to enforce evaluation of the result of the modification function (a typical problem with fmap-style library functions): modifyIORef ref f = readIORef ref >>= writeIORef ref . f No matter whether 'f' is strict or not, the 'writeIORef r' doesn't evaluate its result, just stores the unevaluated application: > r<-newIORef 0 > modifyIORef r (\x->trace "done" $ x+1) > modifyIORef r (\x->trace "done" $ x+1) > readIORef r done done 2 If it had been defined like this instead mRef r ($) f = readIORef r >>= (writeIORef r $) . f it would be possible to transform the strictness of 'writeIORef r' to match that of 'f': > r<-newIORef 0 > mRef r ($) (\x->trace "done" $ x+1) > mRef r ($) (\x->trace "done" $ x+1) > readIORef r done done 2 > r<-newIORef 0 > mRef r ($!) (\x->trace "done" $ x+1) done > mRef r ($!) (\x->trace "done" $ x+1) done > readIORef r 2 Claus