The king and the toaster.......

      Once upon a time, in a kingdom not far from here, a
     king summoned two of his advisors for a test. He showed them
     both a shiny metal box with two slots in the top, a control
     knob, and a lever. "What do you think this is?"
      One advisor, an engineer, answered first. "It is a
     toaster," he said. The king asked, "How would you design an
     embedded computer for it?" The engineer replied, "Using a
     four-bit microcontroller, I would write a simple program
     that reads the darkness knob and quantizes its position to
     one of 16 shades of darkness, from snow white to coal black.
     The program would use that darkness level as the index to a
     16-element table of initial timer values. Then it would turn
     on the heating elements and start the timer with the initial
     value selected from the table. At the end of the time delay,
     it would turn off the heat and pop up the toast.  Come back
     next week, and I'll show you a working prototype."
      The second advisor, a computer programmer, immediately
     recognized the danger of such short-sighted thinking. He
     said, "Toasters don't just turn bread into toast, they are
     also used to warm frozen waffles. What you see before you is
     really a breakfast food cooker. As the subjects of your
     kingdom become more sophisticated, they will demand more
     capabilities. They will need a breakfast food cooker that
     can also cook sausage, fry bacon, and make scrambled eggs. A
     toaster that only makes toast will soon be obsolete. If we
     don't look to the future, we will have to completely
     redesign the toaster in just a few years."
      "With this in mind, we can formulate a more intelligent
     solution to the problem. First, create a class of breakfast
     foods. Specialize this class into subclasses: grains, pork,
     and poultry. The specialization process should be repeated
     with grains divided into toast, muffins, pancakes, and
     waffles; pork divided into sausage, links, and bacon; and
     poultry divided into scrambled eggs, hard-boiled eggs,
     poached eggs, fried eggs, and various omelet classes."
      "The ham and cheese omelet class is worth special
     attention because it must inherit characteristics from the
     pork, dairy, and poultry classes. Thus, we see that the
     problem cannot be properly solved without multiple
     inheritance. At run time, the program must create the proper
     object and send a message to the object that says, 'Cook it
     yourself.' The semantics of this message depend, of course,
     on the kind of object, so they have a different meaning to a
     piece of toast than to scrambled eggs."
      "Reviewing the process so far, we see that the analysis
     phase has revealed that the primary requirement is to cook
     any kind of breakfast food. In the design phase, we have
     discovered some derived requirements. Specifically, we need
     an object-oriented language with multiple inheritance. Of
     course, users don't want the eggs to get cold while the
     bacon is frying, so concurrent processing is required, too."
      "We must not forget the user interface. The lever that
     lowers the food lacks versatility, and the darkness knob is
     confusing. Users won't buy the product unless it has a user-
     friendly, graphical interface. When the breakfast cooker is
     plugged in, users should see a cowboy boot on the screen.
     Users click on it, and the message 'Booting UNIX v8.3'
     appears on the screen. (UNIX 8.3 should be out by the time
     the product gets to market.) Users can pull down a menu and
     click on the foods they want to cook."
      "Having made the wise decision of specifying the
     software first in the design phase, all that remains is to
     pick an adequate hardware platform for the implementation
     phase. An Intel 80386 with 8MB of memory, a 30MB hard disk,
     and a VGA monitor should be sufficient. If you select a
     multitasking, object oriented language that supports
     multiple inheritance and has a built-in GUI, writing the
     program will be a snap. (Imagine the difficulty we would
     have had if we had foolishly allowed a hardware-first design
     strategy to lock us into a four-bit microcontroller!)"
      The king wisely had the computer programmer beheaded,
     and they all lived happily ever after.

Back to the archive...

This message was sent on 19 Jul 1996