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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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