1.2.6 External Representations

An important concept in Scheme 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 . ( )))’.

Many objects have standard external representations, but some, such as procedures and circular data structures, 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 Quoting).

External representations can also be used for input and output. The procedure read parses external representations, and the procedure write 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 that evaluates 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 data as programs.