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. -Alex- 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 [EMAIL PROTECTED] > > _________________________________________________________________ S. Alexander Jacobson mailto:[EMAIL PROTECTED] tel:917-770-6565 http://alexjacobson.com _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
