An important concept in Scheme (and Lisp) is that of the external representation of an object as a sequence of characters. For example, an external representation of the integer 28 is the sequence of characters “28”, and an external representation of a list consisting of the integers 8 and 13 is the sequence of characters “(8 13)”.
The external representation of an object is not necessarily unique. The integer 28 also has representations “#e28.000” and “#x1c”, and the list in the previous paragraph also has the representations “( 08 13 )” and “(8 . (13 . ()))” (see section see Pairs and lists).
Many objects have standard external representations, but some, such as procedures, do not have standard representations (although particular implementations may define representations for them).
An external representation may be written in a program to obtain the corresponding object (see ‘quote’, section see Literal expressions).
External representations can also be used for input and output. The procedure ‘read’ (section see Input) parses external representations, and the procedure ‘write’ (section see Output) generates them. Together, they provide an elegant and powerful input/output facility.
Note that the sequence of characters “(+ 2 6)” is not an external representation of the integer 8, even though it is an expression evaluating to the integer 8; rather, it is an external representation of a three-element list, the elements of which are the symbol + and the integers 2 and 6. Scheme’s syntax has the property that any sequence of characters that is an expression is also the external representation of some object. This can lead to confusion, since it may not be obvious out of context whether a given sequence of characters is intended to denote data or program, but it is also a source of power, since it facilitates writing programs such as interpreters and compilers that treat programs as data (or vice versa).
The syntax of external representations of various kinds of objects accompanies the description of the primitives for manipulating the objects in the appropriate sections of chapter Standard procedures.