[Haskell-cafe] Types depending on tuples.

Mike Meyer mwm at mired.org
Tue Aug 26 07:46:30 UTC 2014

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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