[Haskell-cafe] Re: instance Eq (a -> b)
Jonas Almström Duregård
jonas.duregard at gmail.com
Wed Apr 14 04:01:08 EDT 2010
> But if one did start considering bottom to be a value, one would have to
> distinguish different ones. For instance, (error "ABC") vs. (error
> "PQR"). Obviously this is not finite.
Nor is it computable, since it must distinguish terminating programs
from nonterminating ones (i.e. the halting problem).
On a side note, since "instance (Finite a, Finite b) => Finite (a ->
b)" should be possible, one can even compare some higher order
functions with this approach ;).
On 14 April 2010 09:29, Ashley Yakeley <ashley at semantic.org> wrote:
> On Wed, 2010-04-14 at 08:13 +0100, Thomas Davie wrote:
>> Your instances of Finite are not quite right:
>>
>> bottom :: a
>> bottom = doSomethingToLoopInfinitely.
>>
>> instance Finite () where
>> allValues = [(), bottom]
>
> Bottom is not a value, it's failure to evaluate to a value.
>
> But if one did start considering bottom to be a value, one would have to
> distinguish different ones. For instance, (error "ABC") vs. (error
> "PQR"). Obviously this is not finite.
>
> --
> Ashley Yakeley
>
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