I had an idea for a way to attach small-but-necessary amounts of information to a ByteArray# or MutableByteArray# (and potentially also to Array# and ArrayArray#). The type would look something like "TaggedByteArray# 8#". This, for instance, would represent the address of a primitive array of bytes, with an alignment of 8, and with the last 3 bits having a number between 0 and 7 in them. Code to access the elements of the ByteArray# would disregard the last bits, and the garbage collector would simply ensure that the tag was retained after collection passes. "ByteArray#" itself would simply become an alias for "TaggedByteArray# 1#", and the existing functions would have their types changed to work with the new TaggedByteArray#s. The primary potential problems I see are how to have a KnownNat-style typeclass before Integer is defined, and how to store both the array and the tag in an ArrayArray#.