On Jan 25, 2007, at 2:08 , John Ky wrote:
On 1/25/07, Brandon S. Allbery KF8NH <[EMAIL PROTECTED]> wrote:
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
Would I be able to this?
getLeaves :: ANode -> [Leaf]
Leaf is a data constructor, not a type. Your second one:
getLeaves :: ANode -> [ANode]
is correct. If you want the type system to ensure they are only
leaves, then indeed you can't use this method.
(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
Okay, though it's a lot more wordy.
How so? You were declaring the class and instances anyway; I simply
defined a new method to go into it and renamed the constructor fields
to obey Haskell's rules, but you will probably be using the class
method so your code won't care about the latter.
--
brandon s. allbery [linux,solaris,freebsd,perl] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon university KF8NH
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
