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(elisp)Streams Intro

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Introduction to Reading and Printing
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   "Reading" a Lisp object means parsing a Lisp expression in textual
form and producing a corresponding Lisp object.  This is how Lisp
programs get into Lisp from files of Lisp code.  We call the text the
"read syntax" of the object.  For example, the text `(a . 5)' is the
read syntax for a cons cell whose CAR is `a' and whose CDR is the
number 5.

   "Printing" a Lisp object means producing text that represents that
object--converting the object to its "printed representation" (Note:
Printed Representation).  Printing the cons cell described above
produces the text `(a . 5)'.

   Reading and printing are more or less inverse operations: printing
the object that results from reading a given piece of text often
produces the same text, and reading the text that results from printing
an object usually produces a similar-looking object.  For example,
printing the symbol `foo' produces the text `foo', and reading that text
returns the symbol `foo'.  Printing a list whose elements are `a' and
`b' produces the text `(a b)', and reading that text produces a list
(but not the same list) with elements `a' and `b'.

   However, these two operations are not precisely inverse to each
other.  There are three kinds of exceptions:

   * Printing can produce text that cannot be read.  For example,
     buffers, windows, frames, subprocesses and markers print as text
     that starts with `#'; if you try to read this text, you get an
     error.  There is no way to read those data types.

   * One object can have multiple textual representations.  For example,
     `1' and `01' represent the same integer, and `(a b)' and `(a .
     (b))' represent the same list.  Reading will accept any of the
     alternatives, but printing must choose one of them.

   * Comments can appear at certain points in the middle of an object's
     read sequence without affecting the result of reading it.