Scheme

Overview

Questions

• How can I write a scheme program

Objectives

• Learn to write a scheme program

Introduction

---

We will follow the introduction to scheme available at https://try.gambitscheme.org/.

Basic

SCHEME

(display "hello world!\n")

SCHEME

(+ 6 1)

SCHEME

(expt 2 8)

SCHEME

(* 4 (atan 1))

SCHEME

(define pi (* 4 (atan 1)))
(define circum (lambda (r) (* 2 pi r)))
(circum 5)

SCHEME

(import (srfi 48))
(format #t "1+2=~s\n" (+ 1 2))

SCHEME

(import (srfi 19))  ;; for time procedures
(import (srfi 48))  ;; for format

(define (get-date)
  (time-utc->date (current-time time-utc)))

(format #t
        "current UTC time: ~a\n"
        (date->string (get-date)))

Types

SCHEME

(string-append "A" "BC")

SCHEME

(string-ref "XYZ" 2)  ;; indexing

SCHEME

(string-length "AB")  ;; get length

SCHEME

(char->integer #\A)   ;; conversion

SCHEME

(char<? #\A #\Z)      ;; comparison

SCHEME

(expt 2 75)           ;; 2^75

SCHEME

(exp (* 75 (log 2)))  ;; e^(75*ln(2))

SCHEME

(* (/ 5 3) (/ 9 2))   ;; 5/3 * 9/2

SCHEME

(+ (sqrt -6.25) 1)    ;; sqrt(-6.25)+1

SCHEME

(> 1e-2 1)            ;; 0.01 > 1 ?

SCHEME

(define lst (list 1 2))
lst

SCHEME

(cons 0 lst)         ;; add to front

SCHEME

(append lst '(3 4))  ;; concatenation

SCHEME

(car lst)            ;; get first

SCHEME

(cdr lst)            ;; remove first

SCHEME

(cdr (cdr lst))      ;; remove both

SCHEME

(length lst)         ;; get length

SCHEME

(define v (make-vector 5 42))
v

SCHEME

(vector-set! v 2 #t)  ;; mutation
v

SCHEME

(vector-ref v 2)      ;; indexing

SCHEME

(vector-length v)     ;; get length

Challenge 1: Can you do it?

What is the difference between a list and a vector in scheme?

Show me the solution

Lists are not indexed, one accesses the first and last items of a list. Vectors are indexed, any item can be accessed, but they are more expensive to process. Scheme is a programming language in the lisp family of programming languages (https://en.wikipedia.org/wiki/Lisp_(programming_language)), which extensively use lists.

SCHEME

(string->symbol "foo")  ;; conversion

SCHEME

(symbol->string 'foo)   ;; conversion

SCHEME

(define x (list '* 2 3 7))
x
(eval x)

Challenge 2: Can you do it?

Define an s-expression that uses the symbol + in a list that adds three numbers

Show me the solution

SCHEME

(define y (list '+ 237))
y
(eval y)

SCHEME

*

SCHEME

(* 2 3 7)

SCHEME

(apply * (list 2 3 7))

Challenge 3: Can you do it?

check if + is a procedure. Use + to add three numbers in a list.

Show me the solution

SCHEME

+
(apply + (list 2 3 7))

Key Points

• Scheme is a programming language that uses lists
• Scheme can be used to create very powerful compact programs
• Scheme has specifications which if implemented can be imported to give additional functionality such as formatted output
• Symbols can be used to create s-expressions that allow one to define and evaluate in separate steps