Ruby: What does the line "m = Hash.new {|h,k| h[k] = []}" accomplish that "Hash.new" doesn't?

Question

While studying this Railscast I came across the following bit of source code from Rack:

def self.middleware
  @middleware ||= begin
    m = Hash.new {|h,k| h[k] = []}
    m["deployment"].concat [
      [Rack::ContentLength],
      [Rack::Chunked],
      logging_middleware
    ]
    m["development"].concat m["deployment"] + [[Rack::ShowExceptions], [Rack::Lint]]
    m
  end
end

My question is about the third line. What does passing the block {|h,k| h[k] = []} to Hash.new accomplish? I tried it in IRB and it doesn't seem to do anything different from a regular Hash.new:

2.0.0p247 :003 > m1 = Hash.new
 => {} 
2.0.0p247 :004 > m2 = Hash.new{|h,k| h[k] = []}
 => {} 
2.0.0p247 :005 > m1 == m2
 => true

... but I'm going to guess that the guys who wrote Rack know more about Ruby than I do. What's the reasoning behind including that block?

Answer 1

It allows you to define default value as an array

h = Hash.new { |h, k| h[k] = [] }
h[:a]        # => {:a=>[]}
h[:b] << 123 # => {:a=>[], :b=>[123]}

More examples are here: Hash.new

Answer 2

This creates default object ([] in this case) as value for every key. It calls h1[:a] = [] implicitly resulting in addition of key in the hash.

h1 = Hash.new {|h,k| h[k] = []} 
h1[:a] # = []
h1[:a].push(1) # = [1]
h1[:b] # = []
h1 #= {:a=>[1], :b=>[]}

This returns a single default object as value for all keys.

h1 = Hash.new([])
h1[:a] # = []
h1[:a].push(1) # = [1]
h1[:b] # = [1]
h1 #= {}

This returns default object as value for all keys. But does not add key to the hash.

h1 = Hash.new{[]}
h1[:a] # = []
h1[:a].push(1) # = [1]
h1[:b] # = []
h1 #= {}