Ben Franksen
An array of TVars is certainly *much* too inefficient for what I have in mind w.r.t. both memory and cpu time.
You must be a lot more confident than I if you say this without benchmarking first. :-) IME, there are (at least) two possible problems here, 1) transactions scale (quadratically, I think) with the number of TVars touched, so if any transaction touch a large part of the array, it's going to cost you, and 2) every element of your array will have a pointer to it, making GC potentially expensive. Perhaps you can get around the latter by tuning GC, e.g. +RTS -A100M might help.
Or should I use a high-level approach, something like a Data.Sequence.Seq of medium sized chunks (TVar (IOVector e))?
I'm not sure exactly what you mean here, but if you're going to touch contigous segments of the array, why not TArray (Vector e) or similar? -k -- If I haven't seen further, it is by standing in the footprints of giants