Lists and Objects

Lists are a powerful feature in Scheme. Lists can express both: Data and Programs.

List handling

The following list functions can be used:

> '(1 2 3) "list literal"
= (1 2 3)

> (cons 0 '(1 2 3)) "prepend item to list"
= (0 1 2 3)

> (append '(1 2 3) 4 5) "append items to list"
= (1 2 3 4 5)

> (has? '(1 2 3) 2) "check if item is in list"
= true

> (filter '(1 2 3) (lambda (x) (< x 2))) "filter items that are smaller than 2"
= (1)

> (map '(1 2 3) (lambda (x) (* x 2))) "double all numbers"
= (2 4 6)

> (reduce '(1 2 3) + 0) "build sum by starting with 0 and reducing them over (+ 0 value)"
= 6

Associative Lists (Objects)

The following functions can be used:

> (set obj '("a" 1 "b" 2 "c" 3)) /* construct object from a list with alternating key-value */
= ("a" 1 "b" 2 "c" 3)

> (obj "a") /* extract a value */
= 1

> (set_assoc obj "a" "5") /* construct a new object with another value */
= ("a" "5" "b" 2 "c" 3)

> (set obj (set_assoc obj "a" "5")) /* modify obj */
= ("a" "5" "b" 2 "c" 3)


> (filter_assoc obj (lambda (key value) (not (equal? key "c")))) /* filter an object by a condition */
= ("a" 1 "b" 2)

> (map_assoc obj (lambda (key value) (* value 2))) /* double all values */
= ("a" 2 "b" 4 "c" 6)

> (reduce_assoc obj (lambda (accumulator key value) (+ accumulator value)) 0) /* sum up all values */
= 6

As you see in set_assoc, we have a pure functional approach here. Objects are not modified in-place like in languages like Java, JavaScript, Python, C++ or such, but rather we have to construct a modified copy of that value and overwrite it to the original value. The Scheme optimizer will detect if an in-place operation would be safe, too and can optimize this corner case but the language itself prioritizes safety.