What Is it

A pair represents two values (they can be of any data type)

Representing Lists

Pairs can be used to represent lists. This is a list containing 1, 2, 3:

pair(1, pair(2, pair(3, null)))

null represents an empty list.

pair(x, y) can be written as [x, y]. so,

pair(1, pair(2, pair(3, null)))

can be written as [1, [2, [3, null]]]

These are the same

list(1, 2, 3)
pair(1, pair(2, pair(3, null)))
[1, [2, [3, null]]]

Same as box notation, but any sub-structure that is a list is formatted and printed as list(...), so

display_list(pair(pair(7,8), pair(1, pair(2, null))))
list([7, 8], 1, 2) // the pair [7, 8] is the first element in the list

draw_data(pair(1, pair(2, pair(3, null))))

will display:

const x = pair(1, 2)
head(x) // is 1
tail(x) // is 2

List length is counted starting from the tail (null) The length of a non-empty list is one more than the length of its tail