Editing Lisp (programming language) (section)

===Atoms===
In the original '''LISP''' there were two fundamental [[data type]]s: atoms and lists. A list was a finite ordered sequence of elements, where each element is either an atom or a list, and an atom was a [[number]] or a symbol. A symbol was essentially a unique named item, written as an [[alphanumeric]] string in [[source code]], and used either as a variable name or as a data item in [[symbolic processing]]. For example, the list {{Lisp2|(FOO (BAR 1) 2)}} contains three elements: the symbol {{Lisp2|FOO}}, the list {{Lisp2|(BAR 1)}}, and the number 2.

The essential difference between atoms and lists was that atoms were immutable and unique. Two atoms that appeared in different places in source code but were written in exactly the same way represented the same object,{{Citation needed|date=November 2008}} whereas each list was a separate object that could be altered independently of other lists and could be distinguished from other lists by comparison operators.

As more data types were introduced in later Lisp dialects, and [[programming style]]s evolved, the concept of an atom lost importance.{{Citation needed|date=November 2008}} Many dialects still retained the predicate ''atom'' for [[legacy compatibility]],{{Citation needed|date=November 2008}} defining it true for any object which is not a cons.