[Haskell-cafe] Leaves of a Tree

[Neil Mitchell]

Hi Tom

> data Tree = Branch Tree Tree | Leaf Int deriving (Eq, Ord)

> leaves :: Tree -> Set Int
> leaves (Leaf n) = singleton n
> leaves (Branch left right) = union (leaves left) (leaves right)

The standard method for a traversal over leaves with accumulation is:

leaves :: Tree -> Set Int
leaves x = f []
   where
       f (Leaf n) rest = n : rest
       f (Branch l r) rest = f l (f r rest)

This makes the construction of the list quite cheap.

Then you can do the fromList trick, and that might give you a speed up.

Thanks

Neil