When Does the Magic Comment Work, and When Does it Not?

1.26.2020

---

Adding # frozen_string_literal: true to a ruby file will prevent strings in that file from being modified and can improve performance. This Stack Overflow thread is as good of an introduction as any. But you can't just throw it in anywhere and expect it to work. There are rules, which are hard to discover, but they exist.

Putting the comment after some other code within the file, including within a class, will not freeze the strings:

class Foo
    # frozen_string_literal: true

    def run
        puts "foo" << "bar"
    end
end

Foo.new.run
# => foobar

puts "Hello World!"

# frozen_string_literal: true

class Foo
    def run
        puts "foo" << "bar"
    end
end

Foo.new.run
# => foobar

However, putting the comment after other comments will work:

# TODO: Implement "baz"
# frozen_string_literal: true

class Foo
    def run
        puts "foo" << "bar"
    end
end

Foo.new.run
# => can't modify frozen String (FrozenError)

Adding an extra hash (or any character) to the comment will not freeze the strings:

## frozen_string_literal: true

class Foo
    def run
        puts "foo" << "bar"
    end
end

Foo.new.run
# => foobar

# frozen_string_literal: true, also remember to get milk later

class Foo
    def run
        puts "foo" << "bar"
    end
end

Foo.new.run
# => foobar

But not using any spaces behaves properly:

#frozen_string_literal:true

class Foo
    def run
        puts "foo" << "bar"
    end
end

Foo.new.run
# => can't modify frozen String (FrozenError)

As does putting empty lines above the comment:

# frozen_string_literal: true

class Foo
    def run
        puts "foo" << "bar"
    end
end

Foo.new.run
# => can't modify frozen String (FrozenError)

There's likely a reason for all of this, both in a practical sense and a "it works this way because this is what the underlying source looks like," but it was fun to uncover the rules through trial and error. There are other magic comments, and they behave in a similar fashion, but frozen_string_literal is likely the most relevant for your day to day programming life.