[Haskell-beginners] Problems defining a type with a multiplication function

[Amy de Buitléir]

I'm trying to define a Quaternion, which is sort of a four-element vector
with special rules for multiplication.

----- Quaternion.hs -----
data Quaternion = Quaternion Double Double Double Double
                  deriving (Show)

(*) :: (Quaternion a) => a -> a -> a
(*) (Quaternion w1 x1 y1 z1) (Quaternion w2 x2 y2 z2) = Quaternion w x y z
        where w = w1*w2 - x1*x2 - y1*y2 - z1*z2
              x = x1*w2 + w1*x2 + y1*z2 - z1*y2
              y = w1*y2 - x1*z2 + y1*w2 + z1*x2
              z = w1*z2 + x1*y2 - y1*x2 + z1*w2

----- end code -----

When I try to load this into ghci, I get:

Quaternion.hs:6:13:
    Ambiguous occurrence `*'
    It could refer to either `Main.*', defined at Quaternion.hs:5:0
                          or `Prelude.*', imported from Prelude

... and lots more messages like that. I understand roughly what the message
means, but I don't know how to tell it that when I use "*" within the
definition, I just want ordinary multiplication. Thanks in advance for any
help!

Amy
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