Unicon Book

Chapter 2
Structures
The examples in the previous chapter employed data types whose values are immutable.
For example, all operations that manipulate numbers and strings compute new values,
rather than modify existing values. This chapter presents structured types that organize
and store collections of arbitrary (and possibly mixed) types of values. When you complete
this chapter, you will understand how to use these types.
• Tables associate their elements with key values for rapid lookup.
• Lists offer efficient access by position as well as by stack or queue operations.
• Records store values using a fixed number of named fields.
• Sets support operations such as union and intersection on groups of elements.
• Using structures to represent trees, graphs, and matrices.
There are several structure types that describe different basic relationships between values.
The philosophy of structures in Unicon is to provide built-in operators and functions for
common organization and access patterns - the flexible ”super glue”that is needed by nearly
all applications. Their functionality is similar to the C++ Standard Template Library or
generic classes in other languages, but Unicon’s structure types are much simpler to learn
and use, and are well supported by the expression evaluation mechanism described in the
previous chapter.
All structure types in Icon share many aspects in common, such as the fact that structures
are mutable. The values inside them may change. In that respect, structures are
similar to a collection of variables that are bundled together. In many cases, Unicon’s
structure types are almost interchangeable! Operators like subscripts and built-in functions
such as insert() are defined consistently for many types. Code that relies on such operators
is polymorphic: it may be used with multiple structure types in an interchangeable way.
For both the structures described in this chapter and the strings described in the next
chapter, be aware that Unicon performs automatic storage management, also known as
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