Ramblings on Lisp


In the world of programming languages Lisp can be considered truly ancient along with its companion FORTRAN. However it has survived till this day. Lisp had its origins as early as 1958 when John McCarthy of MIT published the design of the language in an ACM paper titled “Recursive Functions of Symbolic Expressions and Their Computation by Machine”. Lisp was invented by McCarthy as a mathematical notation of computer programs and is based on Lambda Calculus described by Alonzo Church in 1930.

Lisp has not had the popularity of other more recent languages like C, C++ and Java partly because it has an unfamiliar syntax and also has a very steep learning curve. The Lisp syntax can be particularly intimidating to beginners with its series of parentheses. However it is one of predominant language that is used in AI domain.

Some of the key characteristics of Lisp are

Lisp derives its name from LISt Processing. Hence while most programming languages try to compute on data, Lisp computes on a data structure namely the list. A list can be visualized as a collection of elements which can be either data or functions or lists themselves. Its power comes from the fact that the language includes in its syntax some of the operations that can be performed on a lists and lists of lists. In fact many key features of the Lisp Language have found their way into more current languages like Python, Ruby and Perl.

Second Lisp is a symbolic processing language. This ability to manipulate symbols gives Lisp a powerful edge over other Programming Languages in AI domains like theorem proving or natural language processing.

Thirdly Lisp uses a recursive style of programming. This makes the code much shorter than other languages. Recursion enables the expression of the problem as a combination of a terminating condition and a self describing sub problem.  However the singular advantage that Lisp has over other programming languages is that it uses a technique called “tail recursion” . The beauty of tail recursion is that computing  space is of the order of O(1) and not O(n) that is common in languages like C,C++,Java where the size of the stack grows with each subsequent recursive call.

Lisp blurs the distinction between functions and data. Functions use other functions as arguments during computations. Lisp lists are functions that operate on other functions and data in a self repeating fashion.

The closest analogy to this is to think of machine code which is sequence of 32 bit binary words. Both the logic and the data on which they operate are 32 bit binary words and cannot be distinguished unless one knows where the program is supposed to start executing. If one were to take the snapshot of consecutive memory locations we will encounter 32 bit binary words which represent either a logical or arithmetic operation on data which are also 32 bit binary words.

Lisp is a malleable language and allows the programmer to tailor the language for his own convenience. It allows the programmer to manipulate the language so that it suits the programming style of the programmer. Lisp programs evolve from the bottom-up rather from the top-down style adopted in other languages. The design methodology of Lisp programs takes an inside-out approach rather than an outside-in method.

Lisp has many eminent die-hard adherents who swear by the elegance and beauty of being able to solve difficult problems concisely. On the other hand there are those to whom Lisp represents an ancient Pharaoh’s curse that is difficult to get rid of.

However with Lisp, “Once smitten, you remain smitten”.

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