RE: Re[2]: [Haskell-cafe] confusion about 'instance'....

-----Original Message----- From: Luke Palmer [mailto:lrpalmer@gmail.com] Sent: 10 January 2008 14:12 To: Nicholls, Mark Cc: Bulat Ziganshin; haskell-cafe@haskell.org Subject: Re: Re[2]: [Haskell-cafe] confusion about 'instance'....

On Jan 10, 2008 2:04 PM, Nicholls, Mark wrote:

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I can translate OO into mathematical logic pretty easily, I was

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to do the same thing (informally of course) with Haskell....but

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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

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