Pretty Printing with Delimited Continuations

Pretty printing is the task of nicely formatting tree structured data within a given line width limit. In 1980 Oppen published a pretty printing algorithm that takes time linear in the size of the input, independent of the line width, and uses only limited look-ahead. This work inspired the development of a number of purely functional pretty printing libraries in Haskell. Here I present a new functional pretty printing algorithm that has all the nice properties of Oppen's and is surprisingly simple. A double-ended queue of delimited continuations is the key to addressing all aspects of the problem explicitly.