A general sequence representation with arbitrary annotations, for use as a base for implementations of various collection types, as described in section 4 of

• Ralf Hinze and Ross Paterson, "Finger trees: a simple general-purpose data structure", Journal of Functional Programming 16:2 (2006) pp 197-217. http://www.soi.city.ac.uk/~ross/papers/FingerTree.html

For a directly usable sequence type, see Data.Sequence in the base package, which is a specialization of this structure.