[Haskell-cafe] Why is this strict in its arguments?

Luke Palmer lrpalmer at gmail.com
Wed Dec 5 07:38:05 EST 2007

On Dec 5, 2007 12:30 PM, Andrew Coppin <andrewcoppin at btinternet.com> wrote:
> Luke Palmer wrote:
> > On Dec 5, 2007 11:56 AM, Andrew Coppin <andrewcoppin at btinternet.com> wrote:
> >
> >> I was merely noting that questions of the form "is X decidable?" are
> >> usually undecidable. (It's as if God himself wants to tease us...)
> >>
> >
> > I take issue with your definition of "usually" then.
> >
> > Whenever "X is decidable" is undecidable, "'X is decidable' is decidable' is
> > decidable, namely false.  So there are at least as many decidable sentences
> > of the form "X is decidable" as there are undecidable ones.
> >
> Ouch... my head hurts.
> OK, well how about I rephrase it as "most 'interesting' questions about
> decidability tend to be undecidable" and we call it quits? ;-)

Nah, I was just performing a slight-of-hand on you.  Basically by
saying "X is decidable"
is undecidable, you were implying you could prove it.   Which you
usually can't.  Well,
rather, which you usually don't know if you can...


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