(lispref.info)Streams Intro

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Introduction to Reading and Printing

   "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, reading the text `(a . 5)'
returns 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.  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 are `a' and `b'.

   However, these two operations are not precisely inverses.  There are
two kinds of exceptions:

   * Printing can produce text that cannot be read.  For example,
     buffers, windows, subprocesses and markers print into 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.

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