Help need "Cannot use type synonym in instance head"

Ashley Yakeley
Sat, 13 Apr 2002 21:31:13 -0700

At 2002-04-13 21:22, Benny wrote:

>type Poly = [Float]
>addPoly x y = zipWith (+) x y
>instance (Float a) => Num (Poly a) where
>               (+) = addPoly
>I got the error message "Cannot use type synonym in instance head" when I 
>was trying to compile the codes above. Can anyone tell me why and how to 
>solve it?

There are at least two errors here. Firstly the context of your instance 
"(Float a) =>" is wrong. "Float" is not a class. Secondly, "(Poly a)" is 
meaningless, since Poly is a type and does not take any parameter.

Ashley Yakeley, Seattle WA