CSE 341 -- S. Tanimoto Introduction Scheme Scheme is a dialect of Lisp. Lisp is worth learning for the profound enlightenment experience you will have when you finally get it; that experience will make you a better programmer for the rest of your days, even if you never actually use Lisp itself a lot. -- Eric Raymond* * in How to Become a Hacker, quoted by Paul Graham (see the CSE 341 Syllabus page) CSE 341 -- S. Tanimoto Introduction

Lisp and Scheme Implementations LISP = LISt Processing (Intended for processing symbolic information) Lisp implementations from noncommercial sites: GNU Common Lisp (GCL) - U of Texas mods to Kyoto Common Lisp CLISP -- developed in Germany. Lisp implementations from www.franz.com: Allegro Common Lisp for Windows, Web edition. Allegro Common Lisp for Linux DrScheme: MzScheme with a GUI. (free) Cadence Research: Chez Scheme (commercial version) Petite Chez Scheme (free interp. w/out compiler) CSE 341 -- S. Tanimoto Introduction

Scheme: Our Objectives Motivation and History Interactive programming Functional programming List manipulation with recursive functions Polymorphism Language extensibility via macro definitions Closures (compare with objects) Scheme on the Web CSE 341 -- S. Tanimoto Introduction

History of Lisp and Scheme John McCarthy, developed the ideas during the Dartmouth Summer Research Project on Artificial Intelligence, 1956. First implementation on the IBM 704 John McCarthy published “Recursive functions of symbolic expressions and their computation by machine” in Communications of the Association for Computing Machinery in 1960. CSE 341 -- S. Tanimoto Introduction

History of Lisp & Scheme (continued) 1970s: advanced dialects -- MacLisp, InterLisp; Lisp machines (Symbolics, Inc.; Lisp Machines, Inc.; Xerox; Texas Instruments) 1975: Scheme. Late 1970s: Portable Standard Lisp, XLISP. 1984. Common Lisp. Use of Lisp as internal scripting languages: Gnu Emacs, AutoCAD. 1987 CLOS = Common Lisp Object System. 1991 IEEE Standard approved for Scheme. 1991 Revised4 Report for Scheme. 1994 ANSI Standard Lisp. 1990s - Scheme as scripting lang. in the GIMP. CSE 341 -- S. Tanimoto Introduction

Interacting with Scheme Interaction takes place in a Listener Window. Scheme runs an endless “READ-EVAL-PRINT loop” for interacting with the user: READ EVAL PRINT CSE 341 -- S. Tanimoto Introduction

Interacting with Scheme (continued) > (+ 3 5) 8 > (* 2.5 (+ 2 2)) 10.0 > (define x 5) > (sqrt x) 2.236068 > (* x x) 25 CSE 341 -- S. Tanimoto Introduction

CSE 341 -- S. Tanimoto Introduction Scheme’s Syntax Scheme uses one fundamental syntactic construct, the “symbolic expression” or S-expression. Any “atom” such as a number or a symbol, is an S-expression. Any parenthesized list of S-expressions, separated by whitespace, is an S-expression. A procedural form is a list of the form (procedure-name arg1 arg2 ... argN) This is sometimes called parenthesized prefix form. (+ 3 5) (* 2.5 (+ 2 2 7)) (sqrt x) CSE 341 -- S. Tanimoto Introduction

CSE 341 -- S. Tanimoto Introduction Parentheses in Scheme Every parenthesis in an S-expression has significance. (a b c) is different from (a (b c)) or ((a b) c). To call a zero-argument function, put its name in parentheses: (read) But not ((read)) The empty list is denoted ( ). Parentheses should always be balanced. (+ 3 (* 4 5) is an incomplete S-expression. Misplaced paren’s are a leading cause of beginners’ Scheme programming errors. CSE 341 -- S. Tanimoto Introduction

CSE 341 -- S. Tanimoto Introduction In-Class Exercise Convert the following mathematical expressions into Scheme’s parenthesized prefix form. a. 5 + 9 b. 5 x + 9 y + z c. 100 w / 7 mod 255 d. P and Q or R e. y = f(x + 1) CSE 341 -- S. Tanimoto Introduction

CSE 341 -- S. Tanimoto Introduction Defining a function > (* 5 5 5) 125 > (define (cube n) (* n n n)) > (cube 2) 8 > (cube 15.001) 3375.6753 CSE 341 -- S. Tanimoto Introduction

CSE 341 -- S. Tanimoto Introduction Symbolic Values Symbols are like identifiers, but they are commonly used as values as well as variables. > (define x ’pizza) > x PIZZA > (define pizza ’pepperoni) > pizza PEPPERONI > (eval x) CSE 341 -- S. Tanimoto Introduction

More on Evaluation of Symbols > (define x ’y) > (define y ’z) > (define z ’x) > x Y > (eval x) Z > (eval (eval x)) X CSE 341 -- S. Tanimoto Introduction

CSE 341 -- S. Tanimoto Introduction Values of Symbols A symbol without a value in the current context is said to be “unbound”. The value of a symbol can be any Scheme object, including a number, another symbol, a procedure, a list, and array, etc. A symbol can have several local (“lexical”) values and one global value. However, symbols belong to “packages”, and two different packages can have symbols with the same name. CSE 341 -- S. Tanimoto Introduction