Question

redefining some constant in Ruby (ex. FOO = 'bar') generates the warning already initialized constant.

I’m trying to write a sort of ReallyConstants module, where this code should have this behaviour:

ReallyConstants.define_constant(:FOO, 'bar') #=> sets the constant ReallyConstants::FOO to 'bar'
ReallyConstants::FOO #=> 'bar'
ReallyConstants::FOO = 'foobar' #=> this should raise an Exception

that is, constant redefinition should generate an exception.

Is that possible?

Answer 1

Just scope your constant within a module and then use Module#freeze to prohibit further modification of the module.

E.g.

module Really
  CONSTANT = :foo
  freeze
end

Really::CONSTANT = :bar
-> RuntimeError: can't modify frozen Module

Note that this doesn’t speak to the mutability of the value assigned to the constant. For that, use Object#freeze.

This approach will bite you hard in environments where your code is reloaded, like in a Rails application. There, you’ll have to jump through an additional hoop, checking whether the module has been defined yet before defining it.

Generally, defensive programming in Ruby is more trouble than it’s worth. What’s your concern?