BiobaseFasta (hackage): Stream-based handling of FASTA files. The user selects a window size, the library then handles the window. For each window, the previous (past) window is available, in case some data sits on the boundary between windows.  Greg Schwartz' http://hackage.haskell.org/package/fasta package is a lot more complete. This one is mostly tailored to my usage requirements (and may at some point use his library).
Keywords: bioinformatics|bsd3|library|program