gistfile1.rb

# using each with object, no return necessary
irb(main):003:0>  [:first_name, :last_name].each_with_object({}) {|x, m| m[x] = x }
=> {:first_name=>:first_name, :last_name=>:last_name}
irb(main):004:0> 

# using inject
# doesn't work, without expressly returning memo
irb(main):001:0> [:first_name, :last_name].inject({}){|m, x| m[x] = x }
NoMethodError: undefined method `[]=' for :first_name:Symbol
	from (irb):1:in `block in irb_binding'
	from (irb):1:in `each'
	from (irb):1:in `inject'
	from (irb):1
	from /Users/jed/.rbenv/versions/1.9.3-p194/bin/irb:12:in `<main>'

irb(main):002:0> [:first_name, :last_name].inject({}){|m, x| m[x] = x ; m}
=> {:first_name=>:first_name, :last_name=>:last_name}

# using tap
irb(main):004:0> [:first_name, :last_name].inject({}) {|m, x| m.tap { m[x] = x } }
=> {:first_name=>:first_name, :last_name=>:last_name}