7.2 cons

The cons function constructs lists; it is the inverse of car and cdr. For example, cons can be used to make a four element list from the three element list, (fir oak maple):

(cons 'pine '(fir oak maple))

After evaluating this list, you will see

(pine fir oak maple)

appear in the echo area. cons causes the creation of a new list in which the element is followed by the elements of the original list.

We often say that ‘cons puts a new element at the beginning of a list; it attaches or pushes elements onto the list’, but this phrasing can be misleading, since cons does not change an existing list, but creates a new one.

Like car and cdr, cons is non-destructive.