Re: Pointer function parameter

[Robert A Duff <bobduff@world.std.com>]

Alejandro Villanueva <190921@cepsz.unizar.es> writes:

> Robert A Duff wrote:
> 
> > Alejandro Villanueva <190921@cepsz.unizar.es> writes:
> >
> > > You are right! Turing machines can do ANYTHING, ...
> >
> > No, they can't!
> 
> wops! please, explain me that...

For your amusement, here's a list of some things a Turing machine (TM)
can't do:

A TM cannot calculate, in finite time, the last prime number, nor
produce the full decimal expansion of pi.

A TM cannot handle interrupts.

A TM cannot make a soft-boiled egg.  The machine has no peripheral
devices such as a stove, but more importantly, it has no concept of real
time (which is key for soft-boiled eggs).  In general, a TM can't run
real-time programs.

A TM cannot produce high quality original works of fiction.

A TM cannot convince me I should vote for Pat Buchanan.

A TM cannot write a large, useful, bug-free computer program.
And never will.

A TM cannot drive a GUI interface -- the machine has no bit-mapped
display, but more importantly, it has no concept of doing I/O as it goes
along.  That is, a TM's input is placed on the tape before the machine
starts, and you don't look at the output until it halts.  GUI's don't
fit into that I/O model.

Some computing devices, such as normal computers, and human brains,
*can* do some of these things!

Oh, one last thing: a TM cannot "exist".  (Does that fall under your
definition of "do ANYTHING"?)  A TM has an infinitely long tape, whereas
we can only build machines with finite memories.  (The hardware folks
are getting closer and closer to infinite-sized memories all the time,
but they still have a long way to go.  Besides, the software folks can
use it up faster than it grows.)

;-)

- Bob