Common Lisp Derek Debruin Mark Larue Vinh Doan

Problem Domains There was a need in the early 1960s to define a programming language whose computational model could be based on some well characterized mathematical principles rather than on the efficiency of implementation. Lisp become one of the best examples of these language definitions.

John McCarthy Lisp was invented by John McCarthy in the late 1950's as a formalism for reasoning about the use of recursion equations as a model for computation. The name Lisp derives from "List Processing Language". Of computer languages still in widespread use today, only FORTRAN is older.

McCarthy Cont… Around 1956 McCarthy understood the central role of a programming language for his scientific goal -- artificial intelligence. McCarthy desired for an algebraic list processing language for artificial intelligence work on the IBM 704 computer (1958)

Variation in Lisp Since the inception of Lisp many variations have been created. Of each of these variations, there also has been many dialects born. The most widely-known general-purpose Lisp dialects are Common Lisp and Scheme.

Common Lisp Rapid was beneficial in early development, but demand for a standard eventually grew. Developed to standardize the divergent variants of Lisp. It is not an implementation but more of a language specification.

Basic Concepts Everything is a list. Everything. Language can be redefined at run-time. Code is data and can be generated at run- time. –This is one of the primary reasons that Lisp is so popular in artificial intelligence.

Greenspun’s Tenth Rule Any sufficiently complicated C or Fortran program contains an ad-hoc, informally- specified, bug-ridden, slow implementation of half of Common Lisp. Lisp is often considered the “Grand-Daddy of them all.” Many important features of other languages come from Lisp.

Macros Not the same as C’s preprocessor macros Lisp macros formalize the ability of Lisp to implement other languages and to change it’s defined behavior dynamically.

Examples The S-expression is the basic syntax of Lisp: a parenthesized list. (list 1 2 3) The basic definition of a list. (cons 1 2) constructs a basic list with contents (1 2)

Examples (cont.) The car function yields the “head” (first element) of a list: (car (1 2 3)) → 1 The cdr function yields the “tail” (everything following the head) of a list: (tail (1 2 3)) → (2 3)

Examples (cont.) Quoting prevents lists from being evaluated automatically. ‘(one two three) is a quoted list. Quoting prevents the list from being evaluated as three identifiers “ one,” “ two,” and “ three.”

Complete Example An unquoted form is evaluated when encountered and again when used: (setq a ‘b) (setq b ‘c) (eval a) → c

Comparing Lisp to C++ Lisp is a functional programming language; C++ is procedural. This causes Lisp and C++ to have numerous differences in syntax, function definition, typing, etc.

Prefix Syntax Lisp expressions use prefix notation. The expression: (* l 2 3 4) evaluates to 24. The equivalent expression in C++ would use infix notation: 1 * 2 * 3 * 4;

Functions Functions are created through the use of the special operator lambda. lambda takes an argument list and an expression, evaluating the arguments according to the expression. The expression: ((lambda (arg) (* arg 2)) 3) describes a function that takes one argument, 3, and multiplies it by 2, yielding a result of 6.

Functions (cont.) Contrast this with C++, where a function stores an argument which is explicitly manipulated by statements that follow: int lambda( int arg ) { return arg * 2; }

Cons and Implicit Typing Lisp uses lists as its primary data structure. Lists are constructed similarly to other functional languages, using cons, append, etc. Data is implicitly typed in list and can therefore be mixed in a given list. For example, (cons ‘a (2 3)) evaluates to (a 2 3).

Lists and Typing (cont.) In C++ data is explicitly typed. C++ also lacks a primitive list data structure—the closest equivalent is the array, which may contain only one data type. The following code is not valid in C++: int list[3] = {‘a’, 2, 3};

Impact and the Future McCarthy pioneered the if-then-else structure through Lisp’s cond structure Lisp has had a resurgence in popularity lately. The lack of variables in Lisp causes programs without side-effects. This may prove very useful in parallel computing.