[Haskell-cafe] classes inheritance seems not to work.

[Sebastian Sylvan]

On 2/18/06, asker <spamisnothx-askme at yahoo.es> wrote:
>
> Hi,
>
> I'm trying to make three haskell classes and an instance this way:
> ------------------------------------------------------------------------------
> class (Eq a) => Graph a where
>  vert :: [a]
>  ady :: a -> [a]
>
> class (Graph a) => Paths a where
>  gps :: a -> a -> [[a]]
>
> class (Paths a) => Minimum a where
>  mgps :: a -> a -> [a]
>
> instance Minimum Char where
>  vert = ['a' .. 'f'] -- This is the line of the error message.
>  ady 'a' = ['b', 'c']
>  ady 'b' = ['d', 'c', 'e']
>  ady 'd' = ['f']
>  gps = bepgr -- bepgr is defined and implemented in other code's part
>  mgps = head gps
> ------------------------------------------------------------------------------
>
>
>
>
> When I try to run this I get: "ERROR: filepath:line -No member vert in class
> "Minimum"" I don't know why does this happen, am I doing something wrong?
>

The class Minimum doesn't have a member 'vert', Graph does, though.

You need to instantiate your data type in all three classes
separately. So three separate "instance" declarations, one for each
class.


--
Sebastian Sylvan
+46(0)736-818655
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