Scheme in 5 Minutes

Expressions

Every Scheme program is an expression. An expression in parentheses calls a function: the first element is the function, the rest are arguments. Nesting works the way you'd expect.

(+ 1 2)

(* 3 4)

; Nesting: (2 * 3) + 1
(+ (* 2 3) 1)

Define

Use define to bind a name to a value or to define a function.

(define x 5)
(display x)

(define (square x) (* x x))

(display (square 5))

Lists

Lists are the core data structure. Quote a list with ' to prevent evaluation. car returns the first element, cdr returns the rest, cons adds an element to the front.

(display '(1 2 3)) (newline)

(display (car '(1 2 3))) (newline)

(display (cdr '(1 2 3))) (newline)

(display (cons 0 '(1 2 3)))

Conditionals

if takes a test, a then-branch, and an else-branch. cond handles multiple branches.

(display (if (> 3 2) 'yes 'no))

(define (classify n)
  (cond
    ((< n 0) 'negative)
    ((= n 0) 'zero)
    (else 'positive)))

(display (classify -1)) (newline)
(display (classify 0)) (newline)
(display (classify 7))

Higher-order functions

Functions that take other functions as arguments. map applies a function to every element. filter keeps elements that pass a test.

(define (square x) (* x x))

(display (map square '(1 2 3))) (newline)

(display (filter odd? '(1 2 3 4 5)))

Lambda

lambda creates an anonymous function. You can pass it directly without naming it first.

; A lambda that squares its argument
(display ((lambda (x) (* x x)) 4)) (newline)

; Passing a lambda to map
(display (map (lambda (x) (+ x 10)) '(1 2 3)))

Let

let creates local bindings. Each binding is a name-value pair. The bindings are only visible inside the body.

(let ((x 3) (y 4))
  (display (+ x y)))