D's treatment of values versus side-effect free nullary functions

[Jim Balter]

"Peter Alexander" <peter.alexander.au at gmail.com> wrote in message 
news:i2er6d$1jkv$1 at digitalmars.com...
> On 24/07/10 12:43 PM, Jim Balter wrote:
>>> I thought that this was exactly the halting problem.
>>
>> A common misconception, but not so. The halting problem is to find an
>> algorithm that will, for every possible function and data as input,
>> correctly determine whether the function will terminate. Turing proved
>> that such an algorithm is impossible, but that's irrelevant to any
>> practical problem. Not only isn't it necessary to guarantee that the
>> functions you decide not to attempt CTFE on really don't terminate, but
>> no function in any real program looks anthing like the arcane function
>> that Turing proved will baffle your algorithm.
>
> Walter is right.
>

No, he's not, and I explained why, and you have failed to rebut anything I 
wrote.

> Yes, Turing did prove it using an unrealistic example, but the only reason 
> he did so was to keep the proof simple.

You have no absolutely no idea what you're talking about; Turing's proof is 
reductio ad absurdum based on a diagonalization argument -- the constructed 
undeterminable function is dependent on the algorithm purported to be able 
to decide it. See http://en.wikipedia.org/wiki/Halting_problem

> There are plenty of practical examples of functions which are difficult 
> (or impossible) to determine whether or not they terminate.

The halting problem proof isn't about "diffiicult", only "impossible", and 
you have failed to provide a function and a proof that it is *impossible* to 
determine whether it terminates.

> In particular, any recursion or unconstrained loops make it very difficult 
> to prove termination, and there's plenty of practical examples of those 
> e.g. quicksort.

Again, difficulty is not an issue, *impossibility* is -- *that* is what the 
halting problem is, and if you don't understand that then you don't 
understand the issue *at all*.