Start with `loop { puts "Looping forever!" }`. The braces supply a code block, and `loop` repeatedly runs it. Inside an iterator you write yourself, `yield` makes that handoff: execution moves from the running method to its block, then resumes in the method immediately after `yield`. The method has not returned. section 6.3.1 section 6.3.2
A small iterator
`def my_loop; yield while true; end` can be called as `my_loop { puts "My-looping forever!" }`. Each pass runs the block and returns control to `my_loop`, which yields again. A block belongs to the method call, but is separate from its argument list: a call can have arguments, a block, both, or neither. section 6.3.2 section 6.3.3
An iterator can pass values into its block. For example, `5.times { |i| puts i }` supplies `0` through `4` to `i`; the whole call returns `5`. This separates two events that are easy to confuse: one method invocation can yield many times, but returns once when it finishes. Without an attached block, a method has nothing to yield to. section 6.3.5 section 6.3.3
Listing 6.4 yield at work in Integer#my_times
class Integer
def my_times
c = 0
puts "c = 0"
puts "until c == #{self}..." 1
until c == self
yield c
c += 1
end
self 2
end
end
result = 5.my_times {|i| puts "I'm on iteration #{i}!" }
puts result
Follow `my_times` through one pass: `yield c` gives the current count to the block; when the block finishes, `c += 1` runs. After the loop, `self` makes the integer receiver the method’s return value. section 6.3.5
Values travel back the other way, too: an expression such as `acc << yield(self[c])` appends the block’s result to an array. That is the key distinction between `each`, which returns its receiver, and `map`, which collects block results into a new array. A custom `my_map` can use `yield` directly or build on `my_each`. section 6.3.7
Yielding also works outside ordinary iteration. A `BasicObject` subclass can use `method_missing` to build an indented list, yielding to nested blocks as it increases the indentation level and continuing afterward to decrease it. When the block is optional, that implementation checks for one before yielding; `BasicObject` does not provide `block_given?`. section 13.3.2
