The document discusses the Lisp programming language. It notes that Allegro Common Lisp will be used and lists textbooks for learning Lisp. It provides 10 points on Lisp, including that it is interactive, dynamic, uses symbols and lists as basic data types, prefix notation for operators, and classifies different data types. Evaluation follows simple rules and programs can be treated as both instructions and data.

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Artificial Intelligence
LISPLISP

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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)

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Lisp resources-cont
⇒Common LISP: A gentle introduction to Symbolic
Computing, David Touretzky, Addison Wesley, 1990.
⇒Paradigms of Artificial Intelligence Programming: Case
Studies in Common LISP, Peter Norvig, Academic
Press/Morgan Kaufmann, 1992.

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Useful websites for LISP
• Allegro Lisp download
– http://www.franz.com/downloads/
• Association of LISP users
– http://www.alu.org
• Common LISP Open Code Collection
– http://clocc.sourceforge.net
• AI special interest group of the ACM -
– http://www.sigart.acm.org

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1. 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.

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2. 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.

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3. 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

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4. 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 ,

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5. 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)

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5. Lisp classifies data - contd
Lisp Expression
number symbol sequence
integer
float
ratio
keyword
list vector
string

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5. Lisp classifies data - contd
• Integer - a counting number like 1, 2 ,3 …100, -23
• float - real number. Example 1.59, -100.3
• 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 “ ”

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

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7. 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:
(+ 100 99 88)
(setf x (+ 2 3))
• All operations are done through functions

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8. 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.

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9. 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.

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9. 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!

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10. 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!

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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, 31416..etc.

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

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Basic Operations
 A LISP program contains functions applied to its
arguments.
 Functions return LISP objects.
 2 types of functions - PREDICATE and
COMMANDS

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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.

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Command
• A command performs operations on its arguments
and returns an S-expression.
• Format of a command is:
(<Command> 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)

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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.