[Haskell-cafe] Building a tree?

wren ng thornton wren at freegeek.org
Wed Jun 10 20:23:08 EDT 2009

michael rice wrote:
> Here's a function from Data.Tree:
> unfoldTree :: (b -> (a, [b])) -> b -> Tree a
> Build a tree from a seed value
> Could someone please give me a brief example of usage.

In as far as understanding it, it may be easier if you look at the 
generalized version of the anamorphism:

     newtype Fix f = Fix { unFix :: f (Fix f) }

     type Tree a = Fix (Node a)

     data Node a b = Node a [b]

     instance Functor (Node a) where
         fmap f (Node a bs) = Node a (map f bs)

     unfoldTree f = Fix . fmap (unfoldTree f) . f

Live well,

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