I have a potentially very large number of values to feed into a bloom filter. They're coming from IO: getValues :: (v -> k -> k) -> k -> IO k getValues update initial = go initial =<< gen where go v [] = return v go v (f:fs) = do x <- val f go (maybe v (`update` v) x) fs gen = undefined val f = undefined This streams lazily, but if I build a list (getValues (:) []), the laziness is lost; the whole list is constructed and returned before any of it can be used. Which uses too much memory of course. So I can't use Data.BloomFilter.fromListB. Seems I need to do something like this: let filter = newMB (cheapHashes 13) 33554432 --- sized for 1 million items filter' <- unsafeFreezeMB <$> getValues insertMB filter Except this won't work, the mutable bloom filter uses the ST monad. And I cannot see a way to combine the three "MB" functions into a single computation in the same ST monad. Perhaps stToIO to is what I need, but I can't work out how to use it. Help? -- see shy jo