TES3111 October 2001 Artificial Intelligence LISP

TES3111 October 2001 Lisp resources We will use an implementation of LISP called Allegro Common LISP. Different LISP interpreters include - Eg interpreter: Allegro LISP, Harlequin LISP, Corman Lisp.  Text books:  ANSI Common LISP, P.Graham, Prentice Hall, 1995 (recommended)  Common LISP:the language, G.L. Steele, Digital Press, 1990 (2nd Edition)

TES3111 October 2001 Lisp resources-cont  Common LISP: A gentle introduction to Symbolic Computing, David Touretzky, Addison Wesley,  Paradigms of Artificial Intelligence Programming: Case Studies in Common LISP, Peter Norvig, Academic Press/Morgan Kaufmann, 1992.

TES3111 October 2001 Useful websites for LISP Allegro Lisp download – Association of LISP users – Common LISP Open Code Collection – AI special interest group of the ACM - –

TES3111 October Lisp is interactive There is an interpreter that evaluates inputs. An input is processes in 3 steps: 1. Reads input and construct expression from the input. 2. Evaluates the expression for meaning. 3. Prints the results of the evaluation, including signaling of erros if necessary. These 3 steps can be customized by the programmer.

TES3111 October Lisp is a dynamic language Programs are developed incrementally, by making small changes to the source code. Interpreter evaluates the changed definitions and then immediately run the results. New definitions and data structures can be added at any time. This features are ideal for prototyping.

TES3111 October Lisp has symbols Symbols are the basic type of data in use. Symbols are used to build bigger, more complex expressions. Example of symbols: –HELLO –23-worldofsports

TES3111 October LISP has lists Lists are delimited using parenthesis (…). Anything can be placed in a list, including other lists (nested lists). For example: (1 orange 2 3) (once (upon a) time) Empty list is represented as () Caution: elements within a list are separated with a white space, and NOT a comma,

TES3111 October Lisp classifies data It does not classify variables. A variable is just a symbol. It can hold any type of value. A variable do not have to be declared before it is used. Lisp defines different types of data (rather than defining different types of variables)

TES3111 October Lisp classifies data - contd Lisp Expression numbersymbolsequence integer float ratio keyword listvector string

TES3111 October Lisp classifies data - contd Integer - a counting number like 1, 2,3 …100, -23 float - real number. Example 1.59, ratio - a fraction, example 99/23, 4/5 symbol - a sequence of alphanumeric characters, eg: BOO, ID4… keyword - a symbol prefixed with a colon. Eg- :BOO, :ID4 list - a collection of zero or m ore expressions inside (..) vector - 1 dimensional collection of expressions in sequential memory string - a vector of 0 or more characters inside “ ”

TES3111 October Lisp uses prefix notation Operators appear in front of their operands. The infix (10 + 3) is written as (+ 10 3)

TES3111 October Lisp is functional Lisp functions take data, operates on it, and return the results. The returned results are called function values. Functions can return any number of values. To call a function, place it as the first element of an input list, followed by its operands. Example: ( ) (setf x (+ 2 3)) All operations are done through functions

TES3111 October Programs and data Lisp makes no distinction between programs and data. A program can be treated as a set of instruction or as a list of symbols. This makes it possible to write programs that generate another program, or programs that analyze other programs.

TES3111 October Lisp evaluation is easy to understand Rule 1 : If an expression is a constant, the interpreter will return the value of the constant. No more rules apply. Examples: ‘socrates, 4.5 Rule 2: If Rule 1 is not applicable, and the expression is a symbol, then Lisp treats the symbol as a variable and no more rules are considered. If the variable has a value, Lisp will return that value. Otherwise it will report an error.

TES3111 October Lisp evaluation is easy to understand Rule 3: If Rule 1 and Rule 2 do not apply and the expression is a LIST, then Lisp treats it as a function call and no more rules are considered. –You should at this point remember that the first element in the list is assumed to be a defined a function. The remaining elements are data. Each expression in the data is evaluated left to right. Rule 4: If Rules 1 to 3 do not apply, then there is an error!

TES3111 October Lisp is easy to learn??? How to study and program in Lisp? –Read the first few chapters of a good introductory Lisp book. –Read up the definition of each Lisp function that you encounter. –Start developing a good programming style from the very start!

TES3111 October 2001 Data Structures S-Expression - Symbolic expression. It can be an Atom, a List or a collection of S- Expression enclosed by (…) Atom - String of characters beginning with a letter, digit. Eg: Artificial, intelligence, etc.

TES3111 October 2001 Data Structures List - a collection of S-Expression enclosed by ( …. ). Eg: (One of these days) (one ((two) three) four)

TES3111 October 2001 Basic Operations üA LISP program contains functions applied to its arguments. üFunctions return LISP objects. ü2 types of functions - PREDICATE and COMMANDS

TES3111 October 2001 Predicate A function that tests for some condition involving its arguments and returns –Nil if the condition is FALSE (Nil stands for FALSE) –True for any other case. In LISP TRUE is represented using a special LISP variable T.

TES3111 October 2001 Command A command performs operations on its arguments and returns an S-expression. Format of a command is: ( Arg1 Arg2 … Argn) Note: All commands and predicates must be enclosed in (…). To differentiate from lists… a list is preceded with a single quote ‘ Eg : (CAR a b c) versus ‘(CAR a b c)

TES3111 October 2001 Function - CAR CAR - returns the first element of a list. Examples: (CAR ‘(a b c d e)) (CAR ‘((an orange) a day)) (CAR '(1 (2 3 (4 5)) 6)) (CAR ‘()) Note that in the last example, the argument to CAR is a null- list.