[Haskell-cafe] Problem with type families

[Daniel Fischer]

Am Montag 21 September 2009 23:15:48 schrieb Victor Nazarov:
> I've tried to reimplement code presented in the following blog post:
> http://cdsmith.wordpress.com/2009/09/20/side-computations-via-type-classes/
>
> But I've got stuck with the following error:
>
> Triangulable.hs:41:8:
>     `addRows' is not a (visible) method of class `Triangulable'
>
> Triangulable.hs:43:8:
>     `addCols' is not a (visible) method of class `Triangulable'
>
> I think the minimal example to reproduce this error is the following:
>
> ...
> class Triangulable m
>   where type Elem m
>         (@@)     :: m -> (Int, Int) -> Elem
>         size     :: m -> Int
>         swapRows :: Int -> Int -> m -> m
>         swapCols :: Int -> Int -> m -> m
>         addRow   :: (Num Elem) => Elem -> Int -> Int -> m -> m
>         addCol   :: (Num Elem) => Elem -> Int -> Int -> m -> m
>
> newtype ListMatrix a = ListMatrix [[a]]
>
> instance Num a => Triangulable (ListMatrix a)
>   where type Elem (ListMatrix a) = a
>         (ListMatrix xs) @@ (i, j) = xs !! i !! j
>         size (ListMatrix xs) = size xs
>         swapRows m n (ListMatrix xs) = ListMatrix $ swap m n xs
>         swapCols m n (ListMatrix xs) = ListMatrix $ map (swap m n) xs
>         addRows q m n (ListMatrix xs) = ListMatrix $ modifyNth n
> (zipWith comb (xs !! m))
>           where comb a b = q * a + b
>         addCols q m n (LisMatrix xs) = ListMatrix $ map (\row ->
> modifyNth n (comb (row !! m)) row)
>           where comb a b = q * a + b
> ...
>
> How can I fix this?

Shouldn't the methods be called the same in the class definition and instance declaration?

So

instance ...
    where
   ...
    addRow q m n (ListMatrix xs) = ...
    addCol q m n (ListMatrix xs) = ..

>
> --
> Victor Nazarov