Re: [Haskell-cafe] Class

On 11/2/06, Slavomir Kaslev wrote:

On 11/2/06, Sebastian Sylvan wrote:

...

On 11/2/06, Slavomir Kaslev wrote:

...

Little by little, I think I am getting closer.

class Show a => Visible a where toString :: a -> String toString = show size :: a -> Int size = length . show

Is just declaration, not definition. It doesn't define anything, even though it has default implementations for toString and size one still needs to define instance of it. Right?

As Sebastian Sylvan proposed, I probably need something like this:

class Visible a where toString :: a -> String size :: a -> Int

instance Show a => Visible a where toString = show size = length . show

But it seems ghc doesn't like instance definitions like 'instance Show a => Visible a where ...'. Why?

This is not Haskell98 since instance declarations of this form could cause the type checker to go into an infinite loop. This particular one is okay, though, but you have to start ghc with -fallow-undecidable-instances and -fglasgow-exts I'm afraid.

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

Thanks, Sebastian. That was helpful. Are there any papers on the subject?

The users guid to GHC explains it quite clearly, in my opinion, though I'm sure there are some papers on the subject as well (I can't think of any right now, though). /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862