Alex, AJ> Ok, I am still trying to understand kind errors and now have AJ> a very simple class and types: Okay, let's have a look. You declare
class MyClass a b where emptyVal :: a b
In the type signature for emptyVal the type variable a is applied to b. So, we infer a :: k -> l b :: k for kinds k and l. Applying the default binding k = * to k and l (so that k = * and l = *) yields a :: * -> * b :: * So far, so good. Then you declare
type MyType a = [a] type MyType2 = []
Since [] :: * -> *, we infer MyType :: * -> * MyType2 :: * -> * Then you write
instance MyClass MyType2 b where emptyVal = []
(For clarity, I write b where you used a.) Recall that we inferred that the first argument (a) of MyClass should have kind * -> *, while the second argument (b) should have kind *. Here, you've substituted MyType2 :: * -> * for a. So the kinds match and everything is okay. But then:
instance MyClass (MyType b) b where emptyVal = []
The second argument (b) should have kind *. Since, no further information is available, we thus assume b :: *. MyType has kind * -> * and is applied to b; so, (MyType b) :: *. However, we expected the first argument of MyClass to have kind * -> *. So, this instance declaration won't compile. When you write
instance MyClass MyType b where emptyVal= []
the kinds are okay, but this won't compile since type synonyms are not allowed to be applied partially. Finally, assuming FiniteMap :: * -> * -> *, from
type StringMap a = FiniteMap a String
we infer StringMap :: * -> *. Since, synonyms have to be applied fully, you can only substitute StringMap for the second argument of MyClass. HTH, Stefan
-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of S. Alexander Jacobson Sent: Saturday, March 27, 2004 5:09 AM To: Jon Fairbairn Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] What are Kind errors and how do you fix them?
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
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