From: Joel VanderWerf <vjoel@...>
Date: 2004-01-30T07:55:19+09:00
Subject: fallbacks using contiuations


Here is an amusing little control structure that shows off ruby's 
continuations and is actually useful (of course, you might want to use 
something other than a global variable--that's just for simplicity):

# Yield to the associated block, if any. If the block raises an
# exception that matches +excep+, then resume execution at the
# previous successful fallback block.
#
def fallback(excep = StandardError)
   cont = nil
   callcc {|cont|}
   result = block_given? ? yield : nil
   $cont = cont # if it worked, remember what we did
   result
rescue excep
   $cont.call if $cont
   raise
end

if __FILE__ == $0

   first_time_C = true
   first_time_E = true

   puts "A"

   fallback do
     puts "B"
   end

   fallback do
     puts "C"
     if first_time_C
       first_time_C = false
       raise
     end
   end

   fallback do
     puts "D"
   end

   fallback do
     puts "E"
     if first_time_E
       first_time_E = false
       raise
     end
   end

   fallback do
     puts "F"
   end

end

__END__

Output is:

A
B
C
B
C
D
E
D
E
F