[Haskell-cafe] Re: Positive integers

Aaron Denney wnoise at ofb.net
Wed Mar 29 13:08:00 EST 2006

On 2006-03-27, Dylan Thurston <dthurston at barnard.edu> wrote:
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> On Mon, Mar 27, 2006 at 05:02:20AM -0800, John Meacham wrote:
>> well, in interfaces you are going to end up with some specific class or
>> another concretely mentioned in your type signatures, which means you
>> can't interact with code that only knows about the alternate class. like
>> genericLength :: Integral a =3D> [b] -> a
>> if you have a different 'Integral' you can't call genericLength with it,
>> or anything built up on genericLength. basically there would be no way
>> for 'new' and 'old' polymorphic code to interact.=20
> I think the idea would be that the source for genericLength would
> compile using either class hierarchy with no change.  For the case of
> genericLength, this is true for the proposed alternate prelude Hennig
> Theilemann pointed to.  It would be mostly true in general for that
> proposal, with the exception that you would sometimes need to add Show
> or Eq instances.


>> the inability to evolve the class hierarchy is a serious issue, enough
>> that it very well could be impractical for haskell' unless something
>> like class aliases were widely adopted.
> I think that as long as you're not defining classes source compatibility
> would not be hard.  Of course you couldn't hope to link code written
> with one hierarchy against another.

Wouldn't instance declaration also be problematic?

(And yes, we desperately need something like class aliases.)

Aaron Denney

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