def foo(value)
  "#{value}foo"
end
[1,2].map &method(:foo)
#=> ["1foo", "2foo"]

def foo(value)
  "#{value[0]}foo"
end
[[1,2],[3,4]].map &method(:foo)
# ArgumentError: wrong number of arguments (2 for 1)
#   from (irb):13:in `foo'
#   from (irb):13:in `to_proc'
#   from (irb):14:in `map'
#   from (irb):14
#   from :0