Ok, I am still trying to understand kind errors and now have a very simple class and types: class MyClass a b where emptyVal::a b type MyType a = [a] type MyType2 = [] I can't figure out why some instance work and others don't. e.g. this one works: instance MyClass MyType2 a where emptyVal=[] But this one doesn't: instance MyClass (MyType a) a where emptyVal=[] and neither does this one: instance MyClass (MyType) a where emptyVal=[] How do I make (MyType a) work? For example, a real world example is: type MyType a = FiniteMap a String? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com On Tue, 23 Mar 2004, Jon Fairbairn wrote:
On 2004-03-23 at 16:58EST "S. Alexander Jacobson" wrote:
Implementing Reverse from before, I am running into this weird error:
type ReverseType a string = (string ->(string,a)) data Reverse a string = Reverse (ReverseType a string)
instance Monad (Reverse a s) where return x = Reverse (\text -> (text,x)) (Reverse p) >>= k = Reverse p3 where p3 s0 = p2 s1 where (Reverse p2) = k a (s1,a)=p s0
Produces the error:
Kind error: Expecting kind `* -> *', but `Reverse a s' has kind `*' When checking kinds in `Monad (Reverse a s)' In the instance declaration for `Monad (Reverse a s)'
I have no clue what this error message means.
Kinds are to types what types are to values. You've declared Reverse to have two arguments: it takes a type, then another type and returns a type, so its kind is * -> * -> *. (Reverse a) has kind * -> * and (Reverse a s) has kind *.
Now a monad is something that takes a type as an argument, so has kind * -> *, for example IO has kind * -> * -- you expect to see IO Something most places. So (Reverse a) could perhaps be a monad, but (Reverse a s) cannot be.
-- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk