[Haskell-cafe] What are Kind errors and how do you fix them?
S. Alexander Jacobson
alex at alexjacobson.com
Tue Mar 23 18:21:41 EST 2004
So if I want to use Monad, I have to have Reverse
only work with Strings and not some data type
that might be a String?
Is there a workaround that would allow me to
preserve flexibility around the target datatype?
Otherwise I'll rename Forward and Reverse to
toString and fromString, but the notion of being
able to guarantee a roundtrip in and out of an
arbitrary datatype seems useful and powerful.
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 at cl.cam.ac.uk
S. Alexander Jacobson mailto:me at alexjacobson.com
More information about the Haskell-Cafe