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 Ruby user's guideClass constants 

A constant has a name starting with an uppercase character. It should be assigned a value at most once. In the current implementation of ruby, reassignment of a constant generates a warning but not an error (the non-ANSI version of eval.rb does not report the warning):

ruby>fluid=30
   30
ruby>fluid=31
   31
ruby>Solid=32
   32
ruby>Solid=33
   (eval):1: warning: already initialized constant Solid
   33

Constants may be defined within classes, but unlike instance variables, they are accessible outside the class.

ruby> class ConstClass
    |   C1=101
    |   C2=102
    |   C3=103
    |   def show
    |     print C1," ",C2," ",C3,"\n"
    |   end
    | end
   nil
ruby> C1
ERR: (eval):1: uninitialized constant C1
ruby> ConstClass::C1
   101
ruby> ConstClass.new.show
101 102 103
   nil

Constants can also be defined in modules.

ruby> module ConstModule
    |   C1=101
    |   C2=102
    |   C3=103
    |   def showConstants
    |     print C1," ",C2," ",C3,"\n"
    |   end
    | end
   nil
ruby> C1
ERR: (eval):1: uninitialized constant C1
ruby> include ConstModule
   Object
ruby> C1
   101
ruby> showConstants
101 102 103
   nil
ruby> C1=99  # not really a good idea
   99
ruby> C1
   99
ruby> ConstModule::C1  # the module's constant is undisturbed ...
   101
ruby> ConstModule::C1=99 
ERR: (eval):1: compile error
(eval):1: parse error
ConstModule::C1=99
                ^
ruby> ConstModule::C1  # .. regardless of how we tamper with it.
   101


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