Freezing Deep Ruby Data Structures Sunday, August 24, 2008

On one of my current ruby projects, I'm reading in a YML file and using the generated data structure as a hackish set of global configuation settings:

firm_1:
    departments:
        sales: 419
        executive: 999
        IT: 232
    locations:
        NY: 19
        WV: 27
        CA: 102
firm_2:
    ...

Because these should be treated as constants, they should not be overwritten (accidentally, of course). I wanted to go ahead and freeze them:

global_conf = YAML.load_file("...")
global_conf.freeze
global_conf['firm_1'] = {'foo' => 'bar'}
=> TypeError: can't modify frozen hash

But, as you probably know, Ruby's freeze doesn't affect the objects in a container.

global_conf['firm_1']['departments'] = {'foo' => 'bar'}
=> {"foo"=>"bar"}

That's bad.

So I hacked up a quick monkeypatch (or whatever the duck punchers call it these days) to recursively freeze containers:

#
#  allow us to freeze deep data structures by recursively freezeing each nested object
#
class Hash
    def deep_freeze # har, har ,har
        each { |k,v| v.deep_freeze if v.respond_to? :deep_freeze }
        freeze
    end
end
class Array
    def deep_freeze
        each { |j| j.deep_freeze if j.respond_to? :deep_freeze }
        freeze
    end
end

After loading these patches, calling deep_freeze does what we want:

global_conf = YAML.load_file("...")
global_conf.deep_freeze
global_conf['firm_1']['departments'] = {'foo' => 'bar'}
=> TypeError: can't modify frozen hash

Nice!