In computer programming, homoiconicity is a property of some programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s, in which the primary representation of programs is also a data structure
Data structure
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
In computer science, primitive data type is either of the following:* a basic type is a data type provided by a programming language as a basic building block...
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
words homo meaning the same and icon meaning representation. This makes metaprogramming
Metaprogramming
Metaprogramming is the writing of computer programs that write or manipulate other programs as their data, or that do part of the work at compile time that would otherwise be done at runtime...
easier than in a language without this property.
History
The original source is the paper Macro Instruction Extensions of Compiler Languages, according to the early and influential paper TRAC, A Text-Handling Language: Alan Kay
Alan Kay
Alan Curtis Kay is an American computer scientist, known for his early pioneering work on object-oriented programming and windowing graphical user interface design, and for coining the phrase, "The best way to predict the future is to invent it."He is the president of the Viewpoints Research...
used and possibly popularized the term "homoiconic" through his use of the term in his 1969 PhD thesis:
Use and advantages
One advantage of homoiconicity is that extending the language with new concepts typically becomes simpler, as data representing code can be passed between the meta
Metaprogramming
Metaprogramming is the writing of computer programs that write or manipulate other programs as their data, or that do part of the work at compile time that would otherwise be done at runtime...
In computer science, an abstract syntax tree , or just syntax tree, is a tree representation of the abstract syntactic structure of source code written in a programming language. Each node of the tree denotes a construct occurring in the source code. The syntax is 'abstract' in the sense that it...
of a function may be composed and manipulated as a data structure in the meta layer, and then eval
Eval
In some programming languages, eval is a function which evaluates a string as though it were an expression and returns a result; in others, it executes multiple lines of code as though they had been included instead of the line including the eval...
A meta-circular evaluator is a special case of a self-interpreter in which the existing facilities of the parent interpreter are directly applied to the source code being interpreted, without any need for additional implementation...
.
Examples
Languages which are considered to be homoiconic include members of the Lisp
Lisp programming language
Lisp is a family of computer programming languages with a long history and a distinctive, fully parenthesized syntax. Originally specified in 1958, Lisp is the second-oldest high-level programming language in widespread use today; only Fortran is older...
Curl is a reflective object-oriented programming language for interactive web applications whose goal is to provide a smoother transition between formatting and programming...
REBOL is a cross-platform data exchange language and a multi-paradigm dynamic programming language originally designed by Carl Sassenrath for network communications and distributed computing. The language and its official implementation, which is a proprietary freely redistributable software are...
SNOBOL is a generic name for the computer programming languages developed between 1962 and 1967 at AT&T Bell Laboratories by David J. Farber, Ralph E. Griswold and Ivan P. Polonsky, culminating in SNOBOL4...
XSLT is a declarative, XML-based language used for the transformation of XML documents. The original document is not changed; rather, a new document is created based on the content of an existing one. The new document may be serialized by the processor in standard XML syntax or in another format,...
- Features :XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents....
TRAC is a programming language developed in the early 1960s by Calvin Mooers. It was one of three "first languages" recommended by Ted Nelson in Computer Lib....
Ioke is a dynamic, strongly typed, prototype-based programming language targeting the Java Virtual Machine and the Common Language Runtime. It was designed by Ola Bini, a developer of JRuby. It has a very simple homoiconic syntax, somewhat similar to Io....
The Joy programming language in computer science is a purely functional programming language that was produced by Manfred von Thun of La Trobe University in Melbourne, Australia. Joy is based on composition of functions rather than lambda calculus. It has turned out to have many similarities to...
Factor is a stack-oriented programming language created by Slava Pestov. Factor is dynamically typed and has automatic memory management, as well as powerful metaprogramming features. The language has a single implementation featuring a self-hosted optimizing compiler and an interactive development...
Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...
R is a programming language and software environment for statistical computing and graphics. The R language is widely used among statisticians for developing statistical software, and R is widely used for statistical software development and data analysis....
The term Von Neumann architecture, aka the Von Neumann model, derives from a computer architecture proposal by the mathematician and early computer scientist John von Neumann and others, dated June 30, 1945, entitled First Draft of a Report on the EDVAC...
systems (including the vast majority of general purpose computers today), raw machine code also has this property, the data type being bytes in memory.
Homoiconicity in Lisp
Lisp uses S-expressions as an external representation for data and code. S-expressions can be read with the primitive Lisp function READ. READ returns Lisp data: lists, symbols, numbers, strings. The primitive Lisp function EVAL uses Lisp code represented as Lisp data, computes side-effects and returns a result. The result will be printed by the primitive function PRINT, which creates an external S-Expression from Lisp data.
Lisp data, a list using different data types: (sub)lists, symbols, strings and integer numbers.
Lisp code. The example uses lists, symbols and numbers.
(* (sin 1.1) (cos 2.03)) ; in Infix: sin(1.1)*cos(2.03)
Create above expression with the primitive Lisp function LIST and set the variable EXPRESSION to the result
(setf expression (list '* (list 'sin 1.1) (list 'cos 2.03)) )
-> (* (SIN 1.1) (COS 2.03)) ; Lisp returns and prints the result
(third expression) ; the third element of the expression
-> (COS 2.03)
Change the COS term to SIN
(setf (first (third expression)) 'SIN)
The expression is now (* (SIN 1.1) (SIN 2.03))
Evaluate the expression
(eval expression)
-> 0.7988834
Print the expression to a string
(princ-to-string expression)
-> "(* (SIN 1.1) (SIN 2.03))"
Read the expression from a string
(read-from-string "(* (SIN 1.1) (SIN 2.03))")
-> (* (SIN 1.1) (SIN 2.03)) ; returns a list of lists, numbers and symbols
Homoiconicity in Prolog
1 ?- X is 2*5.
X = 10.
2 ?- L = (X is 2*5), write_canonical(L).
is(_, *(2, 5))
L = (X is 2*5).
3 ?- L = (ten(X):-(X is 2*5)), write_canonical(L).
-(ten(A), is(A, *(2, 5)))
L = (ten(X):-X is 2*5).
4 ?- L = (ten(X):-(X is 2*5)), assert(L).
L = (ten(X):-X is 2*5).
5 ?- ten(X).
X = 10.
6 ?-
On line 4 we create a new clause. The operator ":-" separates the head and the body of a clause. With assert/1* we add it to the existing clauses(add it to the "database"), so we can call it later. In other languages we would call it "creating a function during runtime". We can also remove clauses from the database with abolish/1, or retract/1.
* The number after the clause's name is the number of arguments it can take (it's also called arity)
We can also query the database to get the body of a clause:
7 ?- clause(ten(X),Y).
Y = (X is 2*5).
8 ?- clause(ten(X),Y), Y = (X is Z).
Y = (X is 2*5),
Z = 2*5.
9 ?- clause(ten(X),Y), call(Y).
X = 10,
Y = (10 is 2*5).
A concatenative programming language is a point-free programming language in which all expressions denote functions and the juxtaposition of expressions denotes function composition...
Cognitive dimensions of notations, design principles for programming languages' syntax.
