The printed representation of an object is the format of the
output generated by the Lisp printer (the function prin1) for
that object. The read syntax of an object is the format of the
input accepted by the Lisp reader (the function read) for that
object. See section Reading and Printing Lisp Objects.
Most objects have more than one possible read syntax. Some types of object have no read syntax, since it may not make sense to enter objects of these types directly in a Lisp program. Except for these cases, the printed representation of an object is also a read syntax for it.
In other languages, an expression is text; it has no other form. In Lisp, an expression is primarily a Lisp object and only secondarily the text that is the object's read syntax. Often there is no need to emphasize this distinction, but you must keep it in the back of your mind, or you will occasionally be very confused.
Every type has a printed representation. Some types have no read
syntax--for example, the buffer type has none. Objects of these types
are printed in hash notation: the characters `#<' followed by
a descriptive string (typically the type name followed by the name of
the object), and closed with a matching `>'. Hash notation cannot
be read at all, so the Lisp reader signals the error
invalid-read-syntax whenever it encounters `#<'.
(current-buffer)
=> #<buffer objects.texi>
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (see section Evaluation). However,
evaluation and reading are separate activities. Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later. See section Input Functions, for a description of
read, the basic function for reading objects.
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