The printed representation of an object is the format of the
output generated by the Lisp printer (the function prin1) for
that object. Every data type has a unique printed representation.
The read syntax of an object is the format of the input accepted
by the Lisp reader (the function read) for that object. This
is not necessarily unique; many kinds of object have more than one
syntax. See Reading and Printing Lisp Objects.
In most cases, an object’s printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in hash notation, which consists of the characters ‘#<’, a descriptive string (typically the type name followed by the name of the object), and a closing ‘>’. (This is called “hash notation” because it begins with the ‘#’ character, known as “hash” or “number sign”). For example:
(current-buffer)
⇒ #<buffer objects.texi>
Hash notation cannot be read at all, so the Lisp reader signals the
error invalid-read-syntax whenever it encounters ‘#<’.
We describe the read syntax and the printed representation of each Lisp data type where we describe that data type, in the following sections of this chapter. For example, see String Type, and its subsections for the read syntax and printed representation of strings; see Vector Type for the same information about vectors; etc.
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.
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 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 Input Functions, for a description of
read, the basic function for reading objects.