Language
Country/Area
No result found
The Wonderful World of S-Expressions: Why Lisp Will Become a Revolution in Programming?
In computer programming, S-expressions (or symbolic expressions) are a way of expressing nested list (tree structure) data. S-expressions were invented and popularized in the Lisp programming language, which uses S-expressions as both a source code and data representation format.
"S-expressions can represent both data and code, which makes Lisp very flexible in expression."
The general syntax for S-expressions is constructed using parentheses and defined as an atom like "x", or of the form "(x.y)" where x and y are both S-expressions. This definition reflects the way Lisp represents lists as a series of "cells", each of which is an ordered pair.
This structure enables S-expressions to represent arbitrary binary trees. However, S-expressions can also allow circular references, in which case the structure is no longer a tree but a cyclic graph. Many modern Lisp dialects, such as Common Lisp and Scheme, provide syntax to mark these looping constructs.
According to John McCarthy's original definition, an atom can be thought of as a group of recognizable symbols. Over time, modern S-expression notation has begun to allow for a wider range of quoted strings, and lists of more than two members can be represented with a shorter notation.
"In the Lisp family of languages, S-expressions are used to represent source code and data, making them central to a revolution in programming."
There are many languages derived from Lisp, each supporting different formats of S-expression syntax, but one feature they all have in common is the use of S-expressions and prefix notation. When representing source code in Lisp, the first element of an S-expression is usually an operator or function name, and the remaining elements are considered arguments.
The canonical prefix notation takes advantage of this, and many programmers write a lot of code in this way. For example, the Boolean expression "4 == (2 + 2)" in C is expressed as "(= 4 (+ 2 2))" in Lisp.
In terms of specific data types and syntax, the S-expression format has many variations, supporting different syntax for different data types, including lists and pairs, symbols and strings, and even floating point numbers and integers.
"Lisp's automorphism means that the primary representation of a program is a data structure of the language itself."
When parsing an S-expression, it is often compared to XML. However, S-expressions have only one container form, the dotted pair, whereas XML tags can contain simple attributes, other tags, or CDATA, and use different syntaxes to do so.
While S-expressions are simpler than XML for simple use cases, XML has query languages like XPath and numerous tools and third-party libraries to simplify data processing in more advanced use cases.
As Lisp and its derivatives grew in popularity, standardization followed. Several dialects such as Common Lisp and Scheme have specifications for their S-expression syntax. Ron Rivest proposed an Internet draft in 1997 to define a syntax for general-purpose data storage and exchange based on Lisp S-expressions.
Although this draft was not ultimately approved as an RFC, its contents have been referenced and applied by multiple RFCs and other publications. The formation of these standards shows the importance of S-expressions in computer science.
In today's programming environment, S-expressions have been widely used in different application areas, including data serialization standards. It is worth noting that trend analysis by the University of London and the University of Miami indicates that more and more developers are beginning to explore the potential of S-expressions to enhance the flexibility and expressiveness of their programming.
In summary, S-expression is not just a notation, it is the core of Lisp language and represents a highly innovative structure in computer programming. What kind of enlightenment should this kind of representation bring to our future due to the commonality between code and data?
