Default_proc

I’ve seen this here and there:

hsh = {}
(hsh[:key] ||= []) << data

Also this:

hsh = {}
hsh.key?(:key) ? (hsh[:key] << data) : (hsh[:key] = [data])

But you can define your hash with a default proc

hsh = Hash.new {|h,k| h[k] = [] }
hsh[:key] << data

You can even set the default_proc after the fact..

hsh = {}
hsh.default_proc = proc {|h,k| h[k] = [] }
hsh[:key] << data