[Haskell-beginners] Typeclasses and generic types

[Christopher Howard]

I'm learning how to use typeclasses for the first time, and I tried
something like so:

[CODE]
data Point a = Point a a

getX (Point a _) = a
getY (Point _ a) = a

instance Eq Point where
  (==) a b = getX a == getX b && getY a == getY b
[/CODE]

But when I try to load this in GHCI I get:

[CODE]
    `Point' is not applied to enough type arguments
    The first argument of `Eq' should have kind `*',
    but `Point' has kind `* -> *'
    In the instance declaration for `Eq Point'
[/CODE]

This seems to be saying that Eq requires a concrete argument, which I
suppose I can understand.

I can fix this by changing "data Point a = Point a a" into "data Point =
Point Int Int" or something like that, but that seems kind of lame: what
if I want to leave my Point type more generic? (Maybe sometimes I'll
need a Point Int, and other times I'll need a Point Float, or some other
weird combination I haven't thought of yet.)

Instead of changing the "data" declaration, can I change the "instance"
declaration somehow so that it just defines an instance for a particular
Point sub-type? I tried some variations like "instance Eq (Point Float)"
but I haven't been able to come up with anything yet that satisfies the
compiler.

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