[Haskell-cafe] Ignorant begginer question

[Stefan Holdermans]

Maurício,

> What is wrong with this code?

I'm sorry, I don't have time to tell the whole story on kinds right 
now, so you'll have to do it with this practical solution. Maybe 
someone else will jump in and give you the grand tour on kinds.

Your problem right now is that the type Complex takes (needs) a type 
argument. Its definitions is (module strictness flags):

   data Complex a = a :+ a

So, what you have to do, is applying Complex to a type argument.

For instance,

   roots :: (Complex Double, Complex Double, Complex Double)
              -> (Complex Double, Complex Double)

or

   roots :: (Complex Float, Complex Float, Complex Float)
              -> (Complex Float, Complex Float)

or, more general, and therefore probably better,

   roots :: (RealFloat a) => (Complex a, Complex a, Complex a)
              -> (Complex a, Complex a);

HTH,

Stefan


On Dec 23, 2004, at 6:09 PM, Maurício wrote:

>   Guys,
>
>   What is wrong with this code?
>
> **************
> import Complex
> roots :: (Complex, Complex, Complex) -> (Complex, Complex);
> roots (a,b,c) = (x1,x2) where { x1 = (b*b + (sqrt_delta))/(2*a); x2 = 
> (b*b - (sqrt_delta))/(2*a); sqrt_delta = sqrt 4*a*c}
> **************
>
>   I load it into GHCi and get:
>
> **************
> Kind error: `Complex' is not applied to enough type arguments
> In the type: (Complex, Complex, Complex) -> (Complex, Complex)
> While checking the type signature for `roots'
> Failed, modules loaded: none.
> **************
>
>   Thanks,
>   Maurício
>
>
>
>
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