Functional pearl : weaving a web

Abstract

Suppose, you want to implement a structured editor for some term type, so that the user can navigate through a given term and perform edit actions on subterms. In this case you are immediately faced with the problem of how to keep track of the cursor movements and the user's edits in a reasonably efficient manner. In a previous pearl G. Huet (1997) introduced a simple data structure, the Zipper, that addresses this proble - we will explain the Zipper briefly in Sec. 2. A drawback of the Zipper is that the type of cursor locations depends on the structure of the term type, that is, each term type gives rise to a different type of locations (unless you are working in an untyped environment). In this pearl we present an alternative data structure, the Web, that serves the same purpose but that is parametric in the underlying term type. Sec. 3-6 are devoted to the new data structure. Before we unravel the Zipper and explore the Web, let us first give a taste of their use.