Why don't other programming languages have ranges?

[Walter Bright]

dsimcha wrote:
> But the point is that redundancy is probably the **cheapest, most efficient** way
> to get ultra-high reliability.

It also works incredibly well. Airliners use a dual path system, which means 
that no single failure can bring it down. If it didn't work, the skies would be 
raining airplanes.

Triple path systems add a great deal of cost, with only a negligible increase in 
safety.


> Yes, cost matters even when people's lives are at stake.

If the FAA required a triple path system in airliners, likely the airplane would 
be so heavy it could barely lift itself, let alone have a payload.


> If, on the other hand, you have redundancy, you're at least as strong as your
> strongest link because only one system needs to work.  Assuming the systems were
> designed by independent teams and have completely different weak points, we can
> assume their failures are statistically independent.  Assume we have m redundant
> systems each with probability p_s of failing in some way or another.  Then the
> probability of the whole thing failing is:
> 
> product_i = 1 to m(p_s), or the probability that ALL of your redundant systems
> fail.  Assuming they're all decent and designed independently, with different weak
> points, they probably aren't going to fail at the same time.
> 
> For example, if each redundant system really sucks and has a 5% chance of failure,
> then the probability that they both fail and you're up the creek is only 0.25%.

Yup. The important thing to make this work is to avoid coupling, where a failure 
in one path causes the other path to fail as well. This can be tricky to get right.

Coupling is why a process attempting to diagnose and fix its own bugs is doomed.