All the compound data objects we have used so far were constructed ultimately from numbers. One of Scheme's strengths is working with arbitrary symbols as data.

In order to manipulate symbols we need a new element in our language: the ability to quote a data object. Suppose we want to construct the list (a b). We can't accomplish this with (list a b), because this expression constructs a list of the values of a and b rather than the symbols themselves. In Scheme, we refer to the symbols a and b rather than their values by preceding them with a single quotation mark:

 
 
(define a 1)
(define b 2)

 
 
(list a b)

 
 
(list 'a 'b)

 
 
(list 'a b)

In Scheme, any expression that is not evaluated is said to be quoted. This notion of quotation is derived from a classic philosophical distinction between a thing, such as a dog, which runs around and barks, and the word "dog" that is a linguistic construct for designating such things. When we use "dog" in quotation marks, we do not refer to some dog in particular but instead to a word. In language, quotation allow us to talk about language itself, and so it is in Scheme:

 
 
(list 'define 'list)

Quotation also allows us to type in compound objects, using the conventional printed representation for lists:

 
 
(car '(a b c))

 
 
(cdr '(a b c))

The full Scheme language contains additional features, such as mutation operations, vectors, and maps. However, the subset we have introduced so far provides a rich functional programming language capable of implementing many of the ideas we have discussed so far in this text.