[Haskell-cafe] Trouble understanding records and existential types

[Brandon S. Allbery KF8NH]

On Jan 24, 2007, at 19:34 , John Ky wrote:

> class Node -- yadda yadda
>
> data Branch = Branch { name :: String, description :: String,  
> children :: [AnyNode] }
> data Leaf = Leaf { name :: String, value :: String }
>
> The problem here is I can't use the same 'name' field for both  
> Branch and Leaf.  Ideally I'd like the name field in the Node  
> class, but it doesn't seem that Haskell classes are for that sort  
> of thing.

I'm probably missing something, but:

(a) Why not:

data ANode = Branch { name :: String, description :: String,  
children :: [AnyNode] }
            | Leaf { name :: String, value :: String } -- this reuse  
is legal
-- leaving Node available if you still need it

(b) I think you *can* do this with a class:

class Node a where
   name :: a -> String

data Branch = Branch { brName :: String, ... }
data Leaf = Leaf { lName :: String, ... }

instance Node Branch where
   name = brName

instance Node Leaf where
   name = lName

-- 
brandon s. allbery    [linux,solaris,freebsd,perl]     allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH