[Haskell-beginners] how to instance a class
tom.davie at gmail.com
Thu Feb 26 03:12:54 EST 2009
On 26 Feb 2009, at 09:02, Daneel Yaitskov wrote:
> But I can't instance my own class:
> class (Num a) => SVect a where
> (***) :: Num b => a -> b -> a
> instance (Num t) => SVect (Vertex3 t) where
> (Vertex3 x y z) *** c = Vertex3 (c*x) (c*y) (c*z)
> GHC posts about the error:
> Couldn't match expected type `b' against inferred type `t'
> `b' is a rigid type variable bound by
> the type signature for `***' at surface.hs:103:14
> `t' is a rigid type variable bound by
> the instance declaration at surface.hs:105:14
> In the second argument of `(*)', namely `x'
> In the first argument of `Vertex3', namely `(c * x)'
> In the expression: Vertex3 (c * x) (c * y) (c * z)
> I understand the "c" argument must have type as x,y and z, but
> I don't know what need to do.
I don't immediately see what is causing your error, but try
downloading the VectorSpace package off hackage – it may save you a
lot of wheel reinventing here.
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