( Symbol Data

Info Catalog ( Symbols ( Symbol Keys
 21.6.1 Symbols as Discrete Data
 Numbers and symbols are similar to the extent that they both lend
 themselves to `eq?' comparison.  But symbols are more descriptive than
 numbers, because a symbol's name can be used directly to describe the
 concept for which that symbol stands.
    For example, imagine that you need to represent some colours in a
 computer program.  Using numbers, you would have to choose arbitrarily
 some mapping between numbers and colours, and then take care to use that
 mapping consistently:
      ;; 1=red, 2=green, 3=purple
      (if (eq? (colour-of car) 1)
 You can make the mapping more explicit and the code more readable by
 defining constants:
      (define red 1)
      (define green 2)
      (define purple 3)
      (if (eq? (colour-of car) red)
 But the simplest and clearest approach is not to use numbers at all, but
 symbols whose names specify the colours that they refer to:
      (if (eq? (colour-of car) 'red)
    The descriptive advantages of symbols over numbers increase as the
 set of concepts that you want to describe grows.  Suppose that a car
 object can have other properties as well, such as whether it has or
    * automatic or manual transmission
    * leaded or unleaded fuel
    * power steering (or not).
 Then a car's combined property set could be naturally represented and
 manipulated as a list of symbols:
      (properties-of car1)
      (red manual unleaded power-steering)
      (if (memq 'power-steering (properties-of car1))
          (display "Unfit people can drive this car.\n")
          (display "You'll need strong arms to drive this car!\n"))
      Unfit people can drive this car.
    Remember, the fundamental property of symbols that we are relying on
 here is that an occurrence of `'red' in one part of a program is an
 _indistinguishable_ symbol from an occurrence of `'red' in another part
 of a program; this means that symbols can usefully be compared using
 `eq?'.  At the same time, symbols have naturally descriptive names.
 This combination of efficiency and descriptive power makes them ideal
 for use as discrete data.
Info Catalog ( Symbols ( Symbol Keys
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