第31页 Designing Beautiful APIs

#flatten Method
def distance(*args)
     distance(args.flatten) 
end
[1,2,3,[4]].flatten 
     => [1, 2, 3, 4]

#zip method
%w[F HS FS HB].zip((0..4).to_a)
     => [["F", 0], ["HS", 1], ["FS", 2], ["HB", 3]]

(0..6).to_a 
     =>[0, 1, 2, 3, 4, 5, 6]

[1,2, 3, 4].all? {|x| Numeric=== x}
     => true
[1,2,3,4].all? {|x| x>2}
     =>false

#raise ArgumentError .. 

foo = Hash.new {|k,v| k[v] = []} #init value tyep.
foo[:bid]
     =>[]
ft = Hash.new #doesn't init value type
ft[:bid]
     => nil