add_keys.rb

require 'transproc'

include Transproc::Helper

def source_fn(source)
  t(:add_keys, [:source]) >> t(:map_array, t(:map_value, :source, proc { source }))
end

fb_source = source_fn(:facebook)
tw_source = source_fn(:twitter)

puts fb_source.call([{ msg: 'Foo' }, { msg: 'Bar' }]).inspect
puts tw_source.call([{ msg: 'Foo' }, { msg: 'Bar' }]).inspect

# [{:source=>:facebook, :msg=>"Foo"}, {:source=>:facebook, :msg=>"Bar"}]
# [{:source=>:twitter, :msg=>"Foo"}, {:source=>:twitter, :msg=>"Bar"}]