Hi Christopher, Try this instead: instance Eq (Point a) where .. Eq needs to take a type of kind `*', meaning the type itself doesn't have any type variables left. `Point' by itself is a type 'constructor', if you will, that needs another type to complete it (thus you can imagine it's like a 'type function' of kind `* -> *', meaning you need to give one type (`*') to yield a type `*'). You can just use `a' (or whatever) to represent any type filled in there, however. HTH! Arlen On Mon, 2011-05-09 at 19:25 -0800, Christopher Howard wrote:
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.