A PlanarFe Adventure

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When Constants Are NOT Constant

The speed of light.

Avogadro’s number.

Planck’s constant.

The mass of an electron.

I thought that I understood constants. Unchanging. Immutable. Then Ruby went and fucked it up.

My understanding of constants in Ruby was that they are represented by a capitalized bareword (often all upper case) which, once defined, would cause Ruby to pitch a fit if you were to try to reassign its value. Makes sense, right? So when I saw this I was a bit confused:

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class Shoe
  BRANDS = []

  def initialize(brand)
    @brand = brand
    if BRANDS.include?(@brand) == false
    BRANDS << @brand
    end
  end
end

Wait.. whaaaaaa????

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BRANDS << @brand

Aren’t we changing what BRAND represents? Yes and no. We are NOT changing the object to which the bareword “BRANDS” points. We ARE changing the contents of that array, or what I formerly thought of as the value for “BRANDS”.

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BRANDS = []
BRANDS.object_id #=> 70132206506080
BRANDS << "LA Gear"
BRANDS << "FILA"
BRANDS #=> ["LA Gear", "FILA"]
BRANDS.object_id #=> 70132206506080

But, wait! There’s more!

So, while the value of PI might be the same in your home as it is in Intercourse, Pennsylvania, Fucking, Austria, and Wetwang, Yorkshire UK (all real places, btw), a constant may not be the same everywhere in your code. Let’s try this:

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PIE = ["mmmmmm","piiiiiiiieeeeeee"]

class TastyStuff
  PIE = ["Pecan", "Apple", "Pumpkin"]

  def initialize(name)
    if PIE.include?(name)
      puts "Nom!"
    end
  end
end

my_treat = TastyStuff.new("Pecan") #=> "Nom!"

PIE #=> ["mmmmmm","piiiiiiiieeeeeee"]

So now we have two “PIE"s? Hello Namespacing! How’re the wife and kids?!

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PIE = ["mmmmmm","piiiiiiiieeeeeee"]

class TastyStuff
  PIE = ["Pecan", "Apple", "Pumpkin"]

  def initialize(name)
    if PIE.include?(name)
      puts "Nom!"
    end
  end
end

my_treat = TastyStuff.new("Pecan") #=> "Nom!"

PIE #=> ["mmmmmm","piiiiiiiieeeeeee"]
TastyStuff::PIE #=> ["Pecan", "Apple", "Pumpkin"]

What happens if we get rid of the “PIE” inside the class?

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PIE = ["Pecan", "Apple", "Pumpkin"]

class TastyStuff
  def initialize(name)
    if PIE.include?(name)
      puts "Nom!"
    end
  end
end

my_treat = TastyStuff.new("Pecan") #=> "Nom!"

PIE #=> ["Pecan", "Apple", "Pumpkin"]

But if we reverse it…

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class TastyStuff
  PIE = ["Pecan", "Apple", "Pumpkin"]

  def initialize(name)
    if PIE.include?(name)
      puts "Nom!"
    end
  end
end

my_treat = TastyStuff.new("Pecan") #=> "Nom!"

PIE #=> uninitialized constant PIE (NameError)

Sooooooo… now we can’t call “PIE” from outside of the class? It only works from within the class? I guess we could call it a ‘Class Constant’? Yeah, let’s call it a Class Constant because, well, that’s what it’s called.

Let’s summarize:

  • Constants can only be set to point to an object once.
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LYRICS = ["Nah Nah Nah Nah"]
LYRICS = ["Hey Hey Hey, Goodbye"]
#=>2: warning: already initialized constant LYRICS
#=>1: warning: previous definition of LYRICS was here
  • The object to which a constant points CAN be changed.
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LYRICS = ["Nah Nah Nah Nah"]
LYRICS << "Hey Hey Hey, Goodbye"
#=> ["Nah Nah Nah Nah" , "Hey Hey Hey, Goodbye"]
  • Constants have a scope / are namespaced.
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LYRICS = ["Nah Nah Nah Nah"]

class Tunes
  LYRICS = ["Hey Hey Hey, Goodbye"]
end

LYRICS #=> ["Nah Nah Nah Nah"]
Tunes::LYRICS #=> ["Hey Hey Hey, Goodbye"]