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Ruby Programming
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Catch and Throw

While the exception mechanism of raise and rescue is great for abandoning execution when things go wrong, it's sometimes nice to be able to jump out of some deeply nested construct during normal processing. This is where catch and throw come in handy.

catch (:done)  do
  while gets
    throw :done unless fields = split(/\t/)
    songList.add(Song.new(*fields))
  end
  songList.play
end

catch defines a block that is labeled with the given name (which may be a Symbol or a String). The block is executed normally until a throw is encountered.

When Ruby encounters a throw, it zips back up the call stack looking for a catch block with a matching symbol. When it finds it, Ruby unwinds the stack to that point and terminates the block. If the throw is called with the optional second parameter, that value is returned as the value of the catch. So, in the previous example, if the input does not contain correctly formatted lines, the throw will skip to the end of the corresponding catch, not only terminating the while loop but also skipping the playing of the song list.

The following example uses a throw to terminate interaction with the user if ``!'' is typed in response to any prompt.

def promptAndGet(prompt)
  print prompt
  res = readline.chomp
  throw :quitRequested if res == "!"
  return res
end

catch :quitRequested do   name = promptAndGet("Name: ")   age  = promptAndGet("Age:  ")   sex  = promptAndGet("Sex:  ")   # ..   # process information end

As this example illustrates, the throw does not have to appear within the static scope of the catch.


Ruby Programming
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