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Unveiling the secrets of S-expressions: Why are they so important in Lisp?
In the programming world, S-expressions (or symbolic expressions) are a representation of nested list (tree structure) data. This representation was first proposed and popularized by the Lisp programming language and has been widely used in both the source code and data of the language. S-expressions are revolutionary in programming and data structure representation. Let's take a closer look at its key features and uses.
Characteristics of S-expressions
In Lisp's traditional bracket syntax, the definition of an S-expression is quite simple: it can be an atom or an expression of the form (x . y), where x and y are S-expressions. This definition reflects Lisp's concept of representing a list as a series of "cells", each of which is an ordered pair.
This recursive definition means that both S-expressions and lists can represent any binary tree.
In addition, modern Lisp dialects such as Common Lisp and Scheme provide a syntax using data tags to indicate the shared structure of objects, which allows expressions to carry reference cycles without causing infinite recursion.
S-Expression Syntax and Data Types
The grammatical form of S-expressions has multiple variations to support the representation of different data types. The most common ones include:
- Lists and pairs: (1 () (2 . 3) (4))
- Symbols: use dashes, special symbols |a symbol with spaces|
- String: "Hello, world!"
- Integer: -98760
- Floating point number: -0.0 6.28318 6.022e23
Thus, the character # is often used to prefix syntax extensions, for example, #x10 for a hexadecimal integer or #\C for a character.
Application in Lisp
In Lisp, when expressing source code, the first element of an S-expression is usually an operator or function name, and the following elements are treated as parameters. This is called "prefix notation" or "Polish notation". For example, the C Boolean expression 4 == (2 + 2) is represented in Lisp's S-expression as (= 4 (+ 2 2)).
The precise definition of an "atom" varies among Lisp's cousins; quoted strings can usually contain any characters, but unquoted identifiers cannot contain quotes, space characters, parentheses, and other special characters. character.
S-expressions are read through the READ function, and the PRINT function is used to output S-expressions. This mutual reading and writing capability makes Lisp programs not only representations of source code, but also processable data structures. Lisp programs can be formatted as beautiful S-expressions and output in various formats through the PPRINT function.
Parsing of S-expressions
In the use of S-expressions, an important comparison is its difference with XML: S-expressions have only one inclusion form, that is, dot pairs, while XML tags can contain simple attributes, other tags, or CDATA . S-expressions are simpler than XML in simple use cases, but in advanced applications, XML provides query languages such as XPath, which makes tools and libraries for processing XML data more advantageous.
The development of standardization
Various Lisp-derived programming languages have their own S-expression syntax specifications, including Common Lisp (ANSI INCITS 226-1994 (R2004)), Scheme (R5RS and R6RS), and ISLISP. Although the Internet Draft proposed by Ron Rivest in 1997 failed to become an RFC, the general data storage and exchange specifications it defined based on Lisp S-expressions are still cited and used in other documents.
Rivest defines the format as an octet string or a finite list of other S-expressions, and describes the transmission formats of three expression structures.
These developments have undoubtedly promoted the application of S-expressions in data exchange and analysis, as well as the universality and flexibility of programming. Through these standardization developments, we see the important role of S-expressions in the next generation of programming.
In short, the status of S-expressions in Lisp is not only a grammatical requirement, but also an integrated presentation of data structure and source code. It challenges our inherent view of programming languages. So, can S-expressions find new applications and meanings in future programming?
