Novice: Understanding Constants - Definition and Redefinition

Ruby
1

Looking at the code from
here:Learn How to Blog and Build Websites for Profit!

I copied the Const class into my test module, and bolted it inside a
method. This then caused me a smallish problem, as I then received an
error when trying to set the constants from within a method.

After googling a bit, I found that I could set the constants from within
a method by using the ‘class.const_set’ method - as:

class Const
def get_const
CONST
end
CONST = OUTER_CONST + 1
end

def constants_test
STDOUT.puts "Constants Test (Const Is) : "+defined? Const
STDOUT.puts "Constants Test (Const.new.get_const):
"+Const.new.get_const.to_s
STDOUT.puts "Constants Test (Const::CONST): "+Const::CONST.to_s
STDOUT.puts "Constants Test (::OUTER_CONST): "+::OUTER_CONST.to_s
# Odd, I can set a constant (or even create a new one) like this
line
Const.const_set(“NEW_CONST”, 234)
# But the following line throws a constant redef error
#STDOUT.puts “Constants Test : (Const::NEW_CONST = 345
)”+(Const::NEW_CONST = 345).to_s
STDOUT.puts “Constants Test :
(Const::NEW_CONST)”+Const::NEW_CONST.to_s
end

What I’m then confused about is really two things:

  1. Why is it allowable to in effect modify a class? I can follow
    extending a class and adding additional features etc (although I haven’t
    got that far yet) - but I don’t quite grasp the concept of
    Const::NEW_CONST = value.

  2. I thought everything was in a method anyway - but why is it not
    allowed to do something like Const::NEW_CONST = value from within a
    method, but then allow Const.const_set(“CONSTANT”, value)?

2

Steve Tu wrote in post #1087312:

What I’m then confused about is really two things:

  1. Why is it allowable to in effect modify a class?

Because ruby allows you to change things at will. There are still other
ways to change a class other than the ways you have learned so far. For
instance a user can enter a name and you can add a method with that name
to a class. Another example: instance variables are private by default
in
ruby, but a programmer can access them at will. The idea is that if you
really want to break the rules–rather than by accident–ruby lets you.

  1. I thought everything was in a method anyway - but why is it not
    allowed to do something like Const::NEW_CONST = value

Well based on that logic, then because assignment to variables is
typically done with method names that end in ‘=’, for instance:

class Dog
def initialize(name)
@name = name
end

def name=(name)
@name = name
end

def name
@name
end
end

d = Dog.new “Joe”
puts d.name
d.name = ‘Al’
puts d.name

–output:–
Joe
Al

…then you might wonder
whether there is a method named “Const::NEW_CONST=” defined somewhere.

but then allow Const.const_set(“CONSTANT”, value)?

That’s just one example of many demonstrating how ruby lets you break
the rules if you really want to.