On 27 July 2011 10:45, Sergiy Nazarenko
Hi everyone! Hi,
I have data declaration like this: I try to use typeclasses, class GetRow a where hasID :: Int -> IO a instance GetRow MyTableOne where hasID myid = return [(MyTableOne 1 "name")] instance GetRow MyTableTwo where hasID myid = return [(MyTableTwo 1 "name" "path")]
The issue here seems to be that you are trying to define a class instance for /constructors/, not types. I don't really understand what it is you are trying to do, but perhaps you could treat MyTableOne and MyTableTwo as distinct types. Something like: \begin{code} data MyTableOne = MyTableOne Int String data MyTableTwo = MyTableTwo Int String String instance GetRow MyTableOne where ... instance GetRow MyTableTwo where ... \end{code} You might even use a class for tables, so you can define arbitrary table structures which all support a common interface (e.g., row fetching). Hope this helps. Regards, Joachim