[Haskell-cafe] where do I point the type annotations

18 May 2007

      I am playing with using SYB to make generic indexed collections.  The 
current code is this:

    data Syb = Syb [Dynamic] -- list of [Map val (Set a)] 

    empty item = Syb  $ gmapQ (toDyn . emp item) item
        where
        emp::x->y->Map.Map y (Set.Set x)
        emp x y = Map.empty

    insert x (Syb indices) = Syb $ zipWith f indices (gmapQ toDyn x)
        where
        f dynIndex dynAttr = toDyn $ Map.insert attr 
                             (maybe (Set.singleton x) (Set.insert x) $
                                    Map.lookup attr index) index
            where
            index = fromJust $ fromDynamic dynIndex
            attr = fromJust $ fromDynamic dynAttr

    e = empty i where i=i::Test
    t1 = Test "foo" 2
    c1 = insert t1 e

Which causes the following error (the line numbers are wrong because 
there was other code in the original):

      Ambiguous type variable `a' in the constraints:
        `Ord a' arising from use of `insert' at Main.hs:113:5-15
      `Typeable a' arising from use of `insert' at Main.hs:113:5-15
      Probable fix: add a type signature that fixes these type variable(s)

What am I doing wrong?  Is there a better way to define insert that does 
not have this problem?
Note: I also tried doing this so that each attribute tried to find a 
matching index using fromDynamic, but that gave me an error involving 
gmapQ not have an Ord constraint. 

-Alex-

[Haskell-cafe] where do I point the type annotations

Alex Jacobson