Re: [Haskell-cafe] Class

On 11/2/06, Sebastian Sylvan wrote:

On 11/2/06, Slavomir Kaslev wrote:

...

Visible 'subclasses' Show. Doesn't this mean that Visible should be defined for all types with Show instances?

No, it just means that any type that you want to instantiate in Visible needs to also have a Show instance (is this a restriction you really want? See below).

Well, yes that's what I meant. I want class Visible for objects that have Show instance and I want to be able to use this Show instance in the definition of Visible. Again this is not some practical example, I am just trying to grasp Haskell classes and instances.

You could do something like

instance Show a => Visible a

(the default implementations that you provided in the class should be enough, you could possibly move the implementations to the "Show instance" and remove the Show super class from the Visible class - then you could make even non-Show types instances of Visible without having to define their instance for Show as well).

/S

-- Sebastian Sylvan +46(0)736-818655 UIN: 44640862

This doesn't work either: import List class Visible a where toString :: a -> String size :: a -> Int instance Show a => Visible a where toString = show size = length . show instance Visible a => Visible [a] where toString = concat . map toString size = foldl (+) 0 . map size vSort :: (Visible a, Ord a) => [a] -> String vSort = toString . List.sort s = vSort [1..3] gives: Illegal instance declaration for `Visible a' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Visible a' Failed, modules loaded: none. at instance Show a => Visible a where. Probably I should reconsider my expectations? How should something like this designed? -- Slavomir Kaslev