The Bits Between The Lambdas: Binary Data in a Lazy Functional Language

For the programmer, storage media are usually assumed to have a minimum atomic unit of transffi- of one byte. However. sometimes it is useful to have an even finer storage granularity of one bit; for instance in order to compress data. This paper describes an API in the lazy functional language Haskell for treating storage media as arbitrary-length streams of bits, without byte-alignment constraints. So far as possible, storage media are treated uniformly. In particu lar, bit-stream memory and binary files share the same API a new and useful abstraction over memory management and filo management. This uniformity of access leads to a novel technique for lazy random-access to files in a purely functional manner. We also describe a technique for automatically deriving compressed binary representations of user-defined data structures, whose operations provide both in-heap data compression and convenient high-level binary I/O. Of many possible applications, we illustrate the processing of Huffman-encoded image data, and a bibliographic information system which uses lazy B-trees for efficient storage management.