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 scientist, 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 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 v.8.3' appears on the screen. (UNIX 8.3 should be
out by the time the product gets to the 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 scientist beheaded, and they all lived happily
ever after.
-----------------------------------------------------------------------------
For the Annals of Systems Programming:
In the future, the mobs of another French revolt were massacring their
intelligentsia in large numbers.
One day, they dragged three victims to the guillotine, a priest, a lawyer and a
systems programmer. The priest fell to his knees before the horrible device,
and the executioner asked him, "How do you want to go, face up or face down?"
The priest said, "I'd like to go face up, so I can look towards heaven". So,
they locked him into the machine, lowered the pillory over his neck, and pulled
the lever. The blade dropped, but jerked to a halt just before it touched his
Adam's apple. The crowd cried out, "It's a miracle from God. Let him go."
So the executioner released the priest. Next they brought up the lawyer, and
asked him which way he'd like to face. "I don't want to break a precedent, so
I'll go face up.", he replied. They snapped him into place and pulled the
lever. The blade flashed down, but again screeched to a stop just above his
neck. The crowd shouted, "Another miracle from God. Release the man." So the
executioner released the lawyer.
Finally, they brought the systems programmer to the guillotine, and asked him
which way he wanted to face. "I want to go face up so I can see how the mech-
anism works.", he said. They stuck him on the bench and locked him under the
pillory, but before the executioner could pull the lever to drop the blade, the
programmer cried, "Wait! Wait! I see the problem. There's a twisted line at
the pulley!"
And so it goes ...
Regards,
Slawek.
srogulsk@nospam.socs.uts.edu.au
sproguls@nospam.ghostgum.itd.uts.edu.au