I almost always learn something new when hanging out in the #ruby-lang IRC channel at freenode. Or at least I’m reminded of something that I used to know but have since forgotten.

There was some discussion today of how constants work in Ruby and I realized that I didn’t quite understand the difference between a constant and a “frozen” object. In Ruby, a constant is variable whose name begins with a capital letter, e.g.

irb(main):001:0> ANSWER = "Forty-Two" => "Forty-Two"

Now, as Aredridel noted, constants are just constant references to objects. In other words, attempting to change the object to which ANSWER refers (i.e. by assigning some new value to ANSWER) will result in a warning:

irb(main):002:0> ANSWER = "Forty-Three" (irb):2: warning: already initialized constant ANSWER => "Forty-Three"

Note that Ruby doesn’t enforce the “constant reference” rule — it did go ahead and bind ANSWER to a different String — but it does at least warn you about what you’re doing.

So the basic purpose of using constants is to be aware of when those variables are bound to different objects. That doesn’t mean, however, that we can’t change things about the objects to which those constants are bound. Continuing with the previous example, we can make a quick check of the object’s identity:

irb(main):003:0> ANSWER.objectid => 22455208

And then call some method (of the mutator persuasion) on the object that will change its properties:

irb(main):004:0> ANSWER.upcase! => "FORTY-THREE"

And a subsequent check will confirm that ANSWER still refers to the same String that it did before we called upcase!:

irb(main):005:0> ANSWER.objectid => 22455208

Now, if we want to actually lock down the object itself, so that it can’t be changed anymore, the method we need to call is freeze:

irb(main):006:0> ANSWER.freeze => "FORTY-THREE"

Trying to modify a frozen object results in a TypeError:

irb(main):011:0> ANSWER.downcase! TypeError: can't modify frozen string from (irb):11:in `downcase!' from (irb):11

Hopefully the process of writing this long-winded explanation will help me to remember the distinction between constants and frozen objects in the future. Hope it helped you, too.