We describe a design pattern that for writing programs that traverse data structures built from rich mutually-recursive data types. Such programs often have a great deal of “boilerplate” code that simply walks the structure, hiding a small amount of “real” code that constitutes the reason for the traversal.

Our technique allows most of this boilerplate to be written once and for all (perhaps even mechanically generated), leaving the programmer free to concentrate on the important part of the algorithm. These generic programs are much more robust to data structure evolution because they contain many fewer lines of type-specific code.

Our approach is simple to understand, reasonably efficient, and it handles all the data types found in conventional functional programming languages. It makes essential use of rank-2 polymorphism, an extension found in some implementations of Haskell.