On Thursday, January 25, 2007 7:08 AM, John Ky wrote:
On 1/25/07, Brandon S. Allbery KF8NH <[EMAIL PROTECTED]> wrote:
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
Would I be able to this?
getLeaves :: ANode -> [Leaf]
If not, is it the case that people generally don't bother and do this
instead?
getLeaves :: ANode -> [ANode]
As has been pointed out, Leaf is a data constructor not a type so you'd have
to use [ANode].
Inspired by the problem I tried a GADT:
data IsLeaf
data IsBranch
data ANode a where
Branch :: String -> String -> [forall b. ANode b] -> ANode IsBranch
Leaf :: String -> String -> ANode IsLeaf
getLeaves :: ANode IsBranch -> [ANode IsLeaf]
getLeaves (Branch _ _ ls) = leaves ls
leaves :: [forall b. ANode b] -> [ANode IsLeaf]
leaves (l@(Leaf _ _) : ls) = l : leaves ls
leaves (Branch _ _ ls : lls) = leaves ls ++ leaves lls
but unfortunately the above code generates the following error by GHC6.6:
Couldn't match expected type `forall b. ANode b'
against inferred type `ANode a'
In the pattern: Leaf _ _
In the pattern: (l@(Leaf _ _)) : ls
In the definition of `leaves':
leaves ((l@(Leaf _ _)) : ls) = l : (leaves ls)
Just out of curiosity, does anyone know why the above code doesn't compile
ie why is the inferred type for the pattern:
Leaf _ _
(ANode a) and not (ANode IsLeaf)?
Thanks, Brian.
--
http://www.metamilk.com
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
