( Symbols

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 21.6 Symbols
 Symbols in Scheme are widely used in three ways: as items of discrete
 data, as lookup keys for alists and hash tables, and to denote variable
    A "symbol" is similar to a string in that it is defined by a
 sequence of characters.  The sequence of characters is known as the
 symbol's "name".  In the usual case -- that is, where the symbol's name
 doesn't include any characters that could be confused with other
 elements of Scheme syntax -- a symbol is written in a Scheme program by
 writing the sequence of characters that make up the name, _without_ any
 quotation marks or other special syntax.  For example, the symbol whose
 name is "multiply-by-2" is written, simply:
    Notice how this differs from a _string_ with contents
 "multiply-by-2", which is written with double quotation marks, like
    Looking beyond how they are written, symbols are different from
 strings in two important respects.
    The first important difference is uniqueness.  If the same-looking
 string is read twice from two different places in a program, the result
 is two _different_ string objects whose contents just happen to be the
 same.  If, on the other hand, the same-looking symbol is read twice
 from two different places in a program, the result is the _same_ symbol
 object both times.
    Given two read symbols, you can use `eq?' to test whether they are
 the same (that is, have the same name).  `eq?' is the most efficient
 comparison operator in Scheme, and comparing two symbols like this is
 as fast as comparing, for example, two numbers.  Given two strings, on
 the other hand, you must use `equal?' or `string=?', which are much
 slower comparison operators, to determine whether the strings have the
 same contents.
      (define sym1 (quote hello))
      (define sym2 (quote hello))
      (eq? sym1 sym2) => #t
      (define str1 "hello")
      (define str2 "hello")
      (eq? str1 str2) => #f
      (equal? str1 str2) => #t
    The second important difference is that symbols, unlike strings, are
 not self-evaluating.  This is why we need the `(quote ...)'s in the
 example above: `(quote hello)' evaluates to the symbol named "hello"
 itself, whereas an unquoted `hello' is _read_ as the symbol named
 "hello" and evaluated as a variable reference ... about which more
 below ( Symbol Variables).


* Symbol Data                 Symbols as discrete data.
* Symbol Keys                 Symbols as lookup keys.
* Symbol Variables            Symbols as denoting variables.
* Symbol Primitives           Operations related to symbols.
* Symbol Props                Function slots and property lists.
* Symbol Read Syntax          Extended read syntax for symbols.
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