Darren New wrote:

> I realized why I thought trees in an OO language have child nodes with 
> null branches - It's OO. You have pointers to things. In Haskell, it's 
> expected that when you add a new leaf down at the bottom of the tree, 
> the root node changes. In OO, you don't want to have to reassign all 
> variables holding pointers to the root when you add a leaf. And that 
> applies recursively as well. Even balancing a tree, you tend to rotate 
> the child pointers rather than reassign the parent pointer, per se.

More succinctly: In OO, trees are usually mutable. In Haskell, trees are 
usually immutable.

> So that's at least part of it.

A bigger part of it is that, while essentially Haskell has pointers, 
null pointers are explicitly prohibited.

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