[Haskell-cafe] wierd type errros with MPTCs
daniel.is.fischer at web.de
Thu Jan 27 14:55:51 EST 2005
Am Donnerstag, 27. Januar 2005 18:53 schrieb S. Alexander Jacobson:
> This code gives me a Kind error because IVal isn't
> applied to enough type arguments.
> import qualified Set
> class Table table indexVal | indexVal->table where
> --insertIndex::item->indexVal item -> table item ->table item
> union::table item -> table item -> table item
> --union t1 t2 = t1
> data DBTable item = DBTable
> data IVal item = Name item
> instance Table DBTable (IVal ) where
> Weirdly, when I uncomment the insertIndex
> function, things work. But, if I then uncomment
I'm not sure about this, but I believe that the type-variable indexVal in the
class definition is defaulted to kind *.
Now IVal has kind * -> *, so the instance declaration doesn't kind-match.
If you uncomment insertIndex, ghc can see that indexVal must have kind * -> *,
so things work.
> the default implementation of union, I get:
> No instance for (Table DBTable indexVal)
> arising from use of `Main.$dmunion' at example.hs:13
> In the definition of `union': union = Main.$dmunion
> In the definition for method `union'
> In the instance declaration for `Table DBTable IVal'
> I don't know what this error even means. But it
> goes away if I put the union implementation in the
> instance rather than in the class.
I don't see what's wrong there either.
Maybe consulting the user's guide will help.
> Bot these error messages seem unreasonable. Can
> someone clarify?
a) I got it right,
b) it helped.
> Note: I am using GHC 6.2.2
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