[Haskell-cafe] Typeclasses, data, constructors

[Sergiy Nazarenko]

Hi everyone!

I have data declaration like this:

data MyTable = MyTableOne Int String | MyTableTwo Int String String

and function that insert new row in tables:

newRow :: MyTable -> IO Int
newRow (MyTableOne fld1 fld2 ) = .....
newRow (MyTableTwo fld1 fld2 fld3 ) = ......

That work perfectly, I enjoy it. But now I want to get rows, and I don't
know what to do :( I want func something like this:

getRow :: Int -> [MyTable]
.... - what should I write here ?

Possibly use this function:

getRow 23 :: MyTableTwo
(like haskell function Read for example: read "7" :: Int)


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")]

But I have error:
Not in scope: type constructor or class `MyTableOne'
make: *** [Foo.so] Error 1
Not in scope: type constructor or class `MyTableTwo'
make: *** [Foo.so] Error 1

I ask you to suggest me right way in my efforts

Cheers!
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