In computer science, the expression code as data refers to the idea that
source code
In computing, source code, or simply code or source, is a plain text computer program written in a programming language. A programmer writes the human readable source code to control the behavior of a computer.
Since a computer, at base, only ...
written in a
programming language
A programming language is a system of notation for writing computer programs.
Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...
can be manipulated as data, such as a sequence of characters or an
abstract syntax tree
An abstract syntax tree (AST) is a data structure used in computer science to represent the structure of a program or code snippet. It is a tree representation of the abstract syntactic structure of text (often source code) written in a formal ...
(AST), and it has an
execution
Capital punishment, also known as the death penalty and formerly called judicial homicide, is the state-sanctioned killing of a person as punishment for actual or supposed misconduct. The sentence ordering that an offender be punished in ...
semantics only in the context of a given
compiler
In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...
or
interpreter.
The notion is often used in the context of
Lisp
Lisp (historically LISP, an abbreviation of "list processing") is a family of programming languages with a long history and a distinctive, fully parenthesized Polish notation#Explanation, prefix notation.
Originally specified in the late 1950s, ...
-like languages that use
S-expressions as their main syntax, as writing programs using nested lists of symbols makes the interpretation of the program as an AST quite transparent (a property known as
homoiconicity
In computer programming, homoiconicity (from the Greek words ''homo-'' meaning "the same" and ''icon'' meaning "representation") is an informal property of some programming languages. A language is homoiconic if a program written in it can be mani ...
).
These ideas are generally used in the context of what is called
metaprogramming
Metaprogramming is a computer programming technique in which computer programs have the ability to treat other programs as their data. It means that a program can be designed to read, generate, analyse, or transform other programs, and even modi ...
, writing programs that treat other programs as their data.
For example, code-as-data allows the
serialization
In computing, serialization (or serialisation, also referred to as pickling in Python (programming language), Python) is the process of translating a data structure or object (computer science), object state into a format that can be stored (e. ...
of
first-class functions
In computer science, a programming language is said to have first-class functions if it treats function (programming), functions as first-class citizens. This means the language supports passing functions as arguments to other functions, returning ...
in a portable manner. Another use case is storing a program in a string, which is then processed by a compiler to produce an executable.
[ More often there is a reflection API that exposes the structure of a program as an object within the language, reducing the possibility of creating a malformed program.
In computational theory, Kleene's second recursion theorem provides a form of code-is-data, by proving that a program can have access to its own source code.
Code-as-data is also a principle of the ]Von Neumann architecture
The von Neumann architecture—also known as the von Neumann model or Princeton architecture—is a computer architecture based on the '' First Draft of a Report on the EDVAC'', written by John von Neumann in 1945, describing designs discus ...
, since stored programs and data are both represented as bits in the same memory device.[ This architecture offers the ability to write ]self-modifying code
In computer science, self-modifying code (SMC or SMoC) is source code, code that alters its own instruction (computer science), instructions while it is execution (computing), executing – usually to reduce the instruction path length and imp ...
. It also opens the security risk of disguising a malicious program as user data and then using an exploit to direct execution to the malicious program.
Data as Code
In declarative programming
In computer science, declarative programming is a programming paradigm—a style of building the structure and elements of computer programs—that expresses the logic of a computation without describing its control flow.
Many languages that ap ...
, the Data as Code (DaC) principle refers to the idea that an arbitrary data structure can be exposed using a specialized language semantics or API. For example, a list of integers or a string is data, but in languages such as Lisp and Perl, they can be directly entered and evaluated as code.domain-specific language
A domain-specific language (DSL) is a computer language specialized to a particular application domain. This is in contrast to a general-purpose language (GPL), which is broadly applicable across domains. There are a wide variety of DSLs, ranging ...
s and markup language
A markup language is a Encoding, text-encoding system which specifies the structure and formatting of a document and potentially the relationships among its parts. Markup can control the display of a document or enrich its content to facilitate au ...
s are cases where program execution is controlled by data elements that are not clearly sequences of commands.
References
Programming language topics
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