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, Huet (1997) introduced a simple data structure, the Zipper, that
addresses this problem – we will explain the Zipper briefly in section 2. A drawback
of the Zipper is that the type of cursor locations depends on the structure of the
term type, i.e. each term type gives rise to a different type of location (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. Sections 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.