On Monday, 29 July 2013 at 11:46:05 UTC, Leandro Motta Barros wrote:
This may be off topic, but here I go anyway...

Back in the school days, I joked that the Halting Problem is actually
easy to solve with a Turing Machine. Since a Turing Machine is a
theoretical device that exists only in our imagination, we can just suppose that it is infinitely fast. So, we simply have to load our
program in the machine and run it. If the machine doesn't stop
immediately, it means that it will run forever.

And what does this have to do with DMD?

Well, I kinda have the same feeling when using it. For my ~10kloc project, I still haven't felt a real need to use a real build system.
I just dmd *.d. If any measurable time passes without any error
message appearing in the console, I know that my compiled successfully
(and it is the linker that is running now).

BTW, 10kloc is not such a large codebase, but this is with DMD 2.063
anyhow, before those improvents ;-)

LMB




The halting problem isn't about something taking an infinite amount of time but about the decidability of it.

For example, we can write a program that will take forever but if it is known to do so and will never halt, then there is nothing special about it.

For example, for(;;); in an infinite loop and will never halt(except when you turn the power off ;) but it's halting state is completely known.

Halting problems are much more complex.

Even something like

for(;;)
{
   if (random() == 3) break;
}

is decidable(it will halt after some time).

I would write a program that is undecidable but the margin is too short! ;)

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