Ruby 2.5 introduces FrozenError class

Recently Ruby 2.5 was released

Ruby 2.5 introduces FrozenError class which is a subclass of RuntimeError class. This change is introduced to differentiate exceptions raised from attempting to modify a frozen object from generic exceptions. Let’s take few examples to learn about the change.

String
Prior to Ruby 2.5

2.4.0 :001 > NAME = 'Atul'.freeze
 => "Atul"
2.4.0 :002 > NAME << 'JOY'
RuntimeError: can't modify frozen String
 from (irb):2
 from /Users/atul/.rvm/rubies/ruby-2.4.0/bin/irb:11:in `<main>'

Ruby 2.5

2.5.0 :001 > NAME = 'Atul'.freeze
 => "Atul" 
2.5.0 :002 > NAME << 'Joy'
Traceback (most recent call last):
        2: from /home/atul/.rvm/rubies/ruby-2.5.0/bin/irb:11:in `<main>'
        1: from (irb):2
FrozenError (can't modify frozen String)

Array

Prior to Ruby 2.5

2.4.0 :001 > a = [1, 2, 3]
 => [1, 2, 3]
2.4.0 :002 > a.freeze
 => [1, 2, 3]
2.4.0 :003 > a << 4
RuntimeError: can't modify frozen Array
 from (irb):3
 from /Users/atul/.rvm/rubies/ruby-2.4.0/bin/irb:11:in `<main>'

Ruby 2.5

2.5.0 :001 > a = [1, 2, 3]
 => [1, 2, 3] 
2.5.0 :002 > a.freeze
 => [1, 2, 3] 
2.5.0 :003 > a << 4
Traceback (most recent call last):
        2: from /home/atul/.rvm/rubies/ruby-2.5.0/bin/irb:11:in `<main>'
        1: from (irb):3
FrozenError (can't modify frozen Array)

Hash

Prior to Ruby 2.5

2.4.0 :001 > h = {a: 1}
 => {:a=>1}
2.4.0 :002 > h.freeze
 => {:a=>1}
2.4.0 :003 > h.delete(:a)
RuntimeError: can't modify frozen Hash
 from (irb):3:in `delete'
 from (irb):3
 from /Users/atul/.rvm/rubies/ruby-2.4.0/bin/irb:11:in `<main>'

Ruby 2.5

2.5.0 :001 > h = {a: 1}
 => {:a=>1} 
2.5.0 :002 > h.freeze
 => {:a=>1} 
2.5.0 :003 > h.delete(:a)
Traceback (most recent call last):
        3: from /home/atul/.rvm/rubies/ruby-2.5.0/bin/irb:11:in `<main>'
        2: from (irb):3
        1: from (irb):3:in `delete'
FrozenError (can't modify frozen Hash)

Object

Prior to Ruby 2.5

2.4.0 :001 > o = Object.new.freeze
 => #<Object:0x007fce5919ad50>
2.4.0 :002 > o.taint
RuntimeError: can't modify frozen Object
 from (irb):2:in `taint'
 from (irb):2
 from /Users/atul/.rvm/rubies/ruby-2.4.0/bin/irb:11:in `<main>'

Ruby 2.5

2.5.0 :001 > o = Object.new.freeze
 => #<Object:0x000055c909144ce0> 
2.5.0 :002 > o.taint
Traceback (most recent call last):
        3: from /home/atul/.rvm/rubies/ruby-2.5.0/bin/irb:11:in `<main>'
        2: from (irb):2
        1: from (irb):2:in `taint'
FrozenError (can't modify frozen Object)