[Haskell-beginners] Tyepclasses and creating 'Num' instances

[Alex Hammel]

Be careful with that. I seem to recall a StackOverflow post where a really
confusing bug was eventually tracked down to a declaration of

instance Num (Double,Double)

somewhere in an imported package.

On Mon, Feb 23, 2015 at 1:03 PM, Michael Orlitzky <michael at orlitzky.com>
wrote:

> On 02/23/2015 03:46 PM, Stuart Hungerford wrote:
> >
> > instance (AddSemigroup a, AddSemigroup b) => AddSemigroup (a, b)
> >
> > GHC says it "Could not deduce (Num (a, b))" in this situation, which
> > seems fair enough so I tried:
> >
> > instance (AddSemigroup a, Num a, AddSemigroup b, Num b) => AddSemigroup
> (a, b)
>
> Since you've defined the "AddSemigroup plus" to be the "Num plus", when
> you attempt to make (a,b) an instance of AddSemigroup, GHC tries to use
> the "Num plus" on them; i.e.
>
>   (x1,y1) + (x2,y2) = ???
>
> Since (a,b) isn't an instance of Num, it doesn't know what to do here.
> It's obvious that you want,
>
>   (x1,y1) + (x2,y2) = (x1+x2, y1+y2)
>
> (which is fine, since both 'a' and 'b' are instance of Num), but you
> could just as well declare,
>
>   (x1,y1) + (x2,y2) = (x1*x2, y1*y2)
>
> The compiler doesn't know, so declaring 'a' and 'b' as instances of Num
> isn't enough. You really have to tell it how to add the pair.
>
>
> >
> > With the same result.  Separate instances seem to work though:
> >
> > instance (Num a, Num b) => Num (a, b)
> >
> > instance (AddSemigroup a, AddSemigroup b) => AddSemigroup (a, b)
> >
>
> Now it knows how to add (x1,y1) and (x2,y2), since (a,b) is an instance
> of Num. The compiler won't infer the pair instance from the individual
> ones though.
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20150223/ea2dc781/attachment.html>