Could you not provide MVar free calls for use inside a withMVar function, minimising locking overhead?

Are you suggesting something like withDiffArray :: DiffArray ix e -> (UnlockedDiffArray ix e -> IO a) -> IO a and then a bunch of fast operations on UnlockedDiffArray? The problem with this is that DiffArrays live in the pure world, not the IO world. This is one good reason they have to be protected by MVars: the imperative oeprations happen behind the scenes, so the programmer gets no chance to do his own locking or otherwise ensure that the DiffArray is uesd safely. Functions like withDiffArray don't work too well in the pure world, because you have to arrange that the "inner" part is fully deepSeq'd before you can release the lock. Cheers, Simon