[Haskell-cafe] What are Kind errors and how do you fix them?

[Jon Fairbairn]

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 at cl.cam.ac.uk