Re: [Haskell-cafe] Types depending on tuples.

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 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 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 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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