[Haskell-cafe] Types depending on tuples.

[Mike Izbicki]

This works just fine for me:

class Vector a where
    toLists :: a -> [Double]

instance Vector (Double,Double,Double) where
    toLists (a,b,c)=[a,b,c]

On Tue, Aug 26, 2014 at 1:17 AM, Mike Meyer <mwm at mired.org> wrote:
> It's there, but in the text:
>
> class Vector a where
>    toLists :: a -> [Double]
>
>
> On Tue, Aug 26, 2014 at 2:59 AM, Mike Izbicki <mike at izbicki.me> wrote:
>>
>> You never actually provided the Vector type class, so it's difficult
>> to tell what exactly you're trying to do.
>>
>> On Tue, Aug 26, 2014 at 12:46 AM, Mike Meyer <mwm at mired.org> wrote:
>> > I'm doing some geometry in 2 and 3 dimensions. To do this, I've got
>> > Vector2d
>> > and Vector3d classes, which are basically just wrappers around 2 and
>> > 3-tuples of doubles:
>> >
>> > type Vector2d = (Double, Double)
>> > type Vector3d = (Double, Double, Double)
>> >
>> > I've also got a typeclass Vector so I can do polymorphic things on them:
>> >
>> > type Vector3d = (Double, Double, Double)
>> >
>> > (Ok, polymorphic thing).
>> >
>> > I now have a need to convert lists of Vector's to a list of lists of
>> > Doubles. I'd like that to be part of the Vector typeclass, but - well,
>> > declaring it is interesting.
>> >
>> > I can't just add a method "toLists :: a -> [Double]", because (according
>> > to
>> > ghc), there's no match between the expected type 'Double' and the actual
>> > type '[t1]'.
>> >
>> > Can I declare this typeclass?
>> >
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>> >
>
>