External representations
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 6.4).
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 can be written in a program to obtain the
corresponding object (see quote, section 4.1.2).
External representations can also be used for input and output. The
procedure read (section 6.13.2) parses external
representations, and the procedure write (section 6.13.3)
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 is not always
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 6.
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