[Haskell-cafe] Re: (flawed?) benchmark : sort

[Adrian Hey]

Denis Bueno wrote:
> On Mon, Mar 10, 2008 at 10:10 AM, Adrian Hey <ahey at iee.org> wrote:
>>  >>  The Eq instance you've given violates the law that (x == y) = True
>>  >>  implies x = y. Of course the Haskell standard doesn't specify this law,
>>  >>  but it should.
> 
> Unless I'm missing something obvious, the example Neil gave earlier
> should make it clear how impossible this requirement is:
> 
>   What if I had made the definition of Foo:
> 
>   data Foo = Foo Int (Int -> Int)
> 
> There is no way in general to decide the observational equivalence of
> two values of this data type (by reduction to the halting problem).
> Therefore it is impossible to write any function implementing such an
> equality test.

Did you read my original response to this example?

http://www.haskell.org/pipermail/haskell-cafe/2008-March/040356.html

Regards
--
Adrian Hey