[Haskell-cafe] confusion about 'instance'....

[Nicholls, Mark]

Someone said something about having 2 instances of the type in the
typeclass.....maybe I misinterpreted it.


> -----Original Message-----
> From: Luke Palmer [mailto:lrpalmer at gmail.com]
> Sent: 10 January 2008 14:12
> To: Nicholls, Mark
> Cc: Bulat Ziganshin; haskell-cafe at haskell.org
> Subject: Re: Re[2]: [Haskell-cafe] confusion about 'instance'....
> 
> On Jan 10, 2008 2:04 PM, Nicholls, Mark <Nicholls.Mark at mtvne.com>
wrote:
> > I can translate OO into mathematical logic pretty easily, I was
trying
> > to do the same thing (informally of course) with Haskell....but
things
> > are not quite what they appear....not because of some OO hang up
(which
> > I probably have many)...but because of what "type class" actually
means.
> 
> But you can think of a type class as a set of types!  The problem is
that
> if we allow certain kinds of instances (such as the Foo instance I
gave
> earlier) then the set is allowed to be non-recursive (only recursively
> enumerable), so determining whether a particular type is a member of
it
> would be undecidable.
> 
> Luke