Objects that respond to
to_proc can be converted to procs with the
& operator (which will also allow them to be passed as blocks).
The class Symbol defines
#to_proc so it tries to call the corresponding method on the object it receives as parameter.
p [ 'rabbit', 'grass' ].map( &:upcase ) # => ["RABBIT", "GRASS"]
Method objects also define
output = method( :p ) [ 'rabbit', 'grass' ].map( &output ) # => "rabbit\ngrass"