Re: Collecting values from Functors?

Rather than using fmap, why not use gmap in GHC. Using an appropriate generic traversal scheme, say listify, all the code you write is the following expression:

listify (const True) mytree :: [Int]

if you are looking for all the Ints in mytree = Fork (Leaf 42) (Fork (Leaf 88) (Leaf 37)) So this would give you [42,88,37].

What happens if the tree contains additional integers to record, say, balancing information. The information a functor provides is vital here. Take as the simplest example newtype Weighted a = WithWeight (Int, a) Without the functor definition there is no way to distinguish the two Ints in Weighted Int. Cheers, Ralf