Hi! 👋
I’m trying to get through the exercises in the Binary Tree example here:
http://elm-lang.org/examples/binary-tree.
I’ve got to the 4-th and it seems to me like it’s impossible to solve (with
the understanding I have now). 🤔
So, here it goes:
(4) Write a general fold function that acts on trees. The fold
> function does not need to guarantee a particular order of
> traversal.
>
> fold : (a -> b -> b) -> b -> Tree a -> b
…and here is my attempt to solve it:
fold : (a -> b -> b) -> b -> Tree a -> b
> fold f z tree =
> case tree of
> Empty ->
z
>
Node v left right ->
> let
> z_ = f v z
> l_ = fold f z left
> r_ = fold f z right
> in
> {- TODO: figure out how to combine the 3 values of type b -}
> f v z_
The issue I’m stuck with in the last case —​ Node v left right -> — is that
I now got 3 values of type b which I can’t fold into the final result: f has
the type of (a -> b -> b), so if the only value of type a here is z, the
most I can do is one folding operation, but I get another b as a result. 😶
My question is: How can I fold the 3 values? Or is this approach workable
at all? What am I missing? 🤔
Cheers! 👋
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