[Haskell-cafe] Criteria for determining if a recursive
function can be implemented in constant memory
Job Vranish
job.vranish at gmail.com
Tue Jul 6 09:53:26 EDT 2010
If your integers have a bounded size, then your Turing machine is not Turing
complete and can't run a Haskell interpreter.
You might be tempted to just make the numbers really big, but bounded, and
then say that you can still run most interesting programs while skirting the
issue of non computability. But you will find that all the problems that
are now theoretically solvable are still computationally intractable, and
you are practically back where you started.
This doesn't mean that you can't make programs that can determine very
interesting non-trivial properties of programs/functions, but it does mean
that there is no way to make them always work.
- Job
On Tue, Jul 6, 2010 at 9:37 AM, Steffen Schuldenzucker <
sschuldenzucker at uni-bonn.de> wrote:
>
> Forwarding this message to the list.
>
> No, I didn't think about the size of integers. For now, let all numbers
> have some bounded size.
>
>
> -------- Original Message -------- Subject: Re: [Haskell-cafe] Criteria
> for determining if a recursive function can be implemented in constant
> memory Date: Tue, 6 Jul 2010 13:25:57 +1200 From: Richard O'Keefe
> <ok at cs.otago.ac.nz> <ok at cs.otago.ac.nz> To: Steffen Schuldenzucker
> <sschuldenzucker at uni-bonn.de> <sschuldenzucker at uni-bonn.de>
>
> On Jul 6, 2010, at 12:23 AM, Steffen Schuldenzucker wrote:
> > Given the definition of a recursive function f in, say, haskell,
> > determine if f can be implemented in O(1) memory.
>
> How are you supposed to handle integer arithmetic?
>
> If you don't take the size of integers into account,
> then since a Turing machine can do any computation,
> it can run a Haskell interpreter, and since a Turing
> machine's tape can be modelled by a single integer
> (or more conveniently by two), any Haskell function
> can be implemented in O(1) Integers.
>
> If you do take the size of integers into account,
> then
> pow2 n = loop n 1
> where loop 0 a = a
> loop (m+1) a = loop m (a+a)
> requires O(n) memory.
>
>
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