Data.ByteString (formerly FPS) provides packed strings (byte arrays held by a
ForeignPtr), along with a list interface to these strings.  It lets you do
extremely fast IO in Haskell; in some cases, even faster than typical C
implementations, and much faster than [Char].  It uses a flexible "foreign
pointer" representation, allowing the transparent use of Haskell or C code to
manipulate the strings.

Data.ByteString is written in Haskell98 + the foreign function interface and
cpp. It has been tested successfully with GHC 6.4 and 6.5, and hugs March 2005.

WWW: http://www.cse.unsw.edu.au/~dons/fps.html