Hi Cafe, We are lucky to have a plethora of data structures out there. But it does make choosing one off hackage difficult at times. In this case I'm *not* looking for a O(1) access bit vector (Data.Vector.Unboxed seems to be the choice there), but an efficient representation for a list of bits (cons,head,tail). Let's say that you want to represent tree indices as you walk down a binary tree. [Bool] is a simple choice, you only add to the front of the list (0/1 = Left/Right), sharing the tails. But [Bool] is quite space inefficient. Something like [Int] would allow packing the bits more efficiently. A Lazy ByteString could amortize the space overhead even more... but in both cases there's a tiny bit of work to do in wrapping those structures for per-bit access. That's probably the right thing but I wanted to check to see if there's something else recommended, perhaps more off-the-shelf. What about just using the Data.Bits instance of Integer? Well, presently, the setBit instance for very large integers creates a whole new integer, shifts, and xors: http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-Bits.html#se... (I don't know if it's possible to do better. From quick googling GMP seems to use an array of "limbs" rather than a chunked list, so maybe there's no way to treat large Integers as a list and update only the front...) Advice appreciated! Thanks, -Ryan