title: How does array.inject(:+) work?
body: <p>I know hash.map(&amp;:key) function like hash.map{|element| element.key} because it call the to_proc on the symbol :key. </p>
  
  <p>But why does array.inject(:+) work same as array.inject{|sum,x| sum + x }?</p>
  
  <p>Thank you.</p>