[Haskell-cafe] Restricted type classes

John Lato jwlato at gmail.com
Sun Sep 5 09:18:52 EDT 2010

On Sun, Sep 5, 2010 at 7:47 AM, Ivan Lazar Miljenovic <
ivan.miljenovic at gmail.com> wrote:

> On 5 September 2010 22:40, John Lato <jwlato at gmail.com> wrote:
> >
> > Having Pointed is categorically the right thing to do, which is why I
> argue
> > for its inclusion.  Also, I think it would be prudent to avoid a
> situation
> > with the possibility of turning into a rehash of the
> > Functor/Applicative/Monad mess.
> >
> > Are there any good reasons for not including it?  Just because we don't
> have
> > a use now doesn't mean it might not be useful in the future.
> Only reason I can think of: it's a pain to make useless class
> instances when there is no reason why they can't be combined (since
> you never make an instance of one without an instance of the other).

It's a one-time cost, though, so to me at least it's not a big deal.

> I _can_ think of a data type that could conceivably be an instance of
> Pointed but not Applicative: a BloomFilter (though there's not really
> any point in having a BloomFilter with only one value that I can see,
> but maybe someone can since there's the singletonB function).

Thanks for mentioning this.  Bloom filters certainly are an interesting
structure, in many ways.

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