Lisp (programming language)
LISP (which stands for "LISt Processing") is a programming language oriented towards functional programming. Its prominent features include prefix-notation syntax, dynamic typing (variables are type-neutral, but values have implicit type), and the ability to treat source code as first-class objects. Not counting the various machine languages and assembly languages, Lisp is the second-oldest programming language still in widespread use; only Fortran is older. Like Fortran, Lisp has changed greatly since its early days. Strictly speaking, Lisp is now not so much a single language as a class of similar languages, known as "Lisp dialects".
Lisp History
Lisp was invented by John McCarthy in 1958 while he was at MIT. McCarthy published a paper in CACM in 1960, entitled "Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I" (part II was never published.) Lisp was originally implementated on an IBM 704 computer, and two instructions on that machine became the primitive Lisp operations for decomposing lists: car (Contents of Address Register) and cdr (Contents of Decrement Register). Even today, the Lisp operations for returning the first item in a list and the rest of the list are named car and cdr respectively.
Because of its expressiveness and flexibility, Lisp became popular with the artificial intelligence community. However, Lisp had its downsides as well: Lisp program generate a large amount of intermediate output, which take up memory and have to be garbage collected. This made it difficult to run Lisp on stock hardware. In the 1970s, an increasing user community and generous government funding led to the creation of Lisp machines: dedicated hardware for running Lisp programs.
During the 1980s and 1990s, a great effort was made to unify the numerous Lisp dialects into a single language. The new language, Common Lisp, was essentially a superset of the dialects it replaced. In 1994, ANSI published the Common Lisp standard, "ANSI X3.226-1994 Information Technology Programming Language Common Lisp." Unfortunately, by this time the world market for Lisp was much smaller than in its heyday.
The language is amongst the oldest programming languages still in use as of the time of writing in 2001. Algol, Fortran and COBOL are of a similar vintage, and Fortran and COBOL are also still being used.
Syntax
Lisp is an expression-oriented language. Unlike most other languages, no distinction is made between "expressions" and "statements"; all code and data are written as expressions. When an expression is evaluated, it produces a value (or list of values), which then can be embedded into other expressions.
McCarthy's 1958 paper introduced two types of syntax: S-expressions (Symbolic Expressions), which are also called sexp's, and M-expressions (Meta Expressions), which express functions of S-expressions. M-expressions never found favour, and almost all Lisps today use S-expressions to manipulate both code and data. The heavy use of parentheses in S-expressions has been criticized - one joke acronym for Lisp is "Lots of Irritating Superfluous Parenthesis" - but the S-expression syntax is also responsible for much of Lisp's power: the syntax is extremely regular, which facilitates manipulation by computer.
The reliance on expressions gives Lisp great flexibility. Because Lisp functions are themselves written as lists, they can be processed exactly like data - Lisp programs can be written to manipulate other Lisp programs. This is known as metaprogramming. Many Lisp dialects exploit this feature using macro systems, which make it possible to extend the language almost without limit.
A Lisp list is written with its elements separated by whitespace, and delimited by parentheses. For example,
(1 2 "foo").
is a list whose elements have the values 1, 2, and "foo". These values are implicitly typed: they are respectively two integers and a string, and do not have to be declared as such. The empty list () is also represented as nil.
Expressions are written as lists, using prefix notation. The first element in the list is the name of a form, i.e. a function, operator, macro, or "special form" (see below.) The remainder of the list are the arguments. For example, the function list returns its arguments as a list, so the expression
(list 1 2 "foo")
evaluates to the list (1 2 "foo"). If any of the arguments are expression, they are recursively evaluated before the enclosing expression is evaluated. For example,
(list 1 2 (list 3 4))
evaluates to the list (1 2 (3 4)). Note that the third argument is a list; lists can be nested.
Arithmetic operators are treated similarly. The expression
(+ 1 2 3 4)
evaluates to 10. Note that this is much more compact (if less familiar) than "1+2+3+4", which is the equivalent under infix notation.
"Special forms" provide Lisp's control structure. For example, the special form if takes three arguments. If the first argument is non-nil, it evaluates to the second argument; otherwise, it evaluates to the third argument. Thus, the expression
(if nil
(list 1 2 "foo")
(list 3 4 "bar"))
evaluates to (3 4 "bar"). (Of course, this would be more useful if a non-trivial expression had been substituted in place of nil!)
Another special form, defun, is used to define functions. The arguments to defun are a list of arguments and the expression that the function evaluates to.
Example programs
Here are some examples of Lisp code. While not typical of Lisp programs used in industry, they are typical of Lisp as it is usually taught in computer science courses.
As the reader may have noticed from the above discussion, Lisp syntax lends itself naturally to recursion. Often, mathematically intricate problems are easily written in Lisp. Paradoxically, "easy" tasks are often difficult to accomplish in Lisp. For example, Lisp syntax is ill-equipped for performing consecutive tasks (a la. procedural programming), which is the starting point of most other languages.
This function evaluates to the factorial of its argument:
(defun factorial (n)
(if (<= n 1)
1
(* n (factorial (- n 1)))))
This is an alternative function, which is more efficient because it uses tail recursion:
(defun factorial (n &optional (acc 1))
(if (<= n 1)
acc
(factorial (- n 1) (* acc n))))
This function takes a list argument and evaluates to the reverse of the list. (Lisp actually has a built-in reverse function which does the same thing.)
(defun reverse (l &optional acc)
(if (atom l)
acc
(reverse (cdr l) (cons (car l) acc))))
Object systems
Various object systems and models have been built on top of, alongside, or into Lisp, including:
CLOS features multiple inheritance, multiple dispatch ("multimethods"), and a powerful system of "method combinations". In fact, Common Lisp, which includes CLOS, was the first object-oriented language to be officially standardized.
Genealogy and Variants
- LISP, McCarthy's original version, developed at MIT.
- MACLisp, developed for MIT's Project MAC (no relation to Apple's Macintosh), direct descendant of LISP.
- ZetaLisp, used on the Lisp machines, direct descendant of MACLisp.
- InterLisp, developed at MIT, later adopted as a "west coast" Lisp for the Xerox lisp machines. A small version called "InterLisp 65" was published for Atari's 6502-based computer line.
- Franz Lisp, originally a Berkeley project; later run by Franz, Inc.
- Common Lisp, descended mainly from ZetaLisp and Franz, with some InterLisp input.
- Gold Hill Common Lisp, an early PC implementation of Lisp.
- Coral Lisp, an implementation of Lisp for the Macintosh.
- Scheme, a minimalist Lisp originally designed for teaching; the first Lisp dialect to use lexical scoping rather than dynamic scoping.
- AutoLISP/Visual LISP, customization language for the AutoCAD product.
- Emacs Lisp, scripting language for the Emacs editor.
- Oak Lisp, an object-oriented Lisp based on Scheme.
- Cambridge Lisp, originally implemented on IBM mainframes; published by Metacomco for the Amiga.