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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 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 can be written in a program to obtain the
corresponding object (see quote, Literal expressions).
External representations can also be used for input and output. The
procedure read (Input) parses external representations,
and the procedure write (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 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 Standard procedures.
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