最近看ruby cookbook遇到这个用法，google一下，这里原文解释
http://stackoverflow.com/questions/18865860/enumerator-yielder-yield-vs-proc-yield
Enumerator yielder.yield VS Proc.yield
The yield statement has no receiver. Inside a method it means "Run the block right now". An error occurs if no block is attached. It is not always given an argument, because sometimes you just want to run the block.
就是说。有时候未必会传递一个block。仅仅是想运行一个block。但是未必会传递这个block，这时yielder.yield派上用场
例如
def foo
yield :bar
end
foo # LocalJumpError
foo { |x| puts x } # bar
Enumerator::Yielder
For a yielder, yield is almost always given an argument. That's because it means the same as << which is "The next time someone calls next on me, give them this value".
yield总是会得到一个参数， 因为<<和yield一样意思，下一次在我身上调用next方法，返回这个值
Enumerator.new { |yielder| yielder.yield 3 }.next # 3 #返回3
Enumerator.new { |yielder| yielder << 3 }.next # same thing
Proc
Procs and lambdas are basically functions. yield here means the same thing as call, which "Just call the function". You can give it an argument or not, depending on how the proc was defined. Nothing fancy here.
procs和lambda类似函数，yield和call作用一样，调用这个函数 你可以传递或者不传递参数，这个传不传按照proc怎么定义来看，
proc { |x| puts x }.yield(:bar) # bar
proc { |x| puts.x }.call(:bar) # same thing