Abstract Although there is a great deal of academic interest in functional languages, there are very few large-scale functional applications. The poor interface to the file system seems to be a major factor preventing functional languages being used for large-scale programming tasks. The interfaces provided by some widely-used languages are described and some problems encountered with using these interfaces to access persistent data are discussed. Three means of improving the persistent data manipulation facilities of functional languages are considered: an improved interface to the file system, including a good binary file implementation; an interface to a database; and the provision of orthogonal persistence. Concrete examples are given using the functional programming language, Haskell. 1 Introduction Modifying a file is a common operation in application programs. Frequently, a program needs to change only small parts of a large file or files. For example, to record the arrival of a shipment, a program controlling an inventory might update the information about a single item in the stock file. We term reading or writing part of a file without needing to read or write the entire file incremental read/write.