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algebraische Datentyp:

data Tree k = Leaf { key  :: k }
   | Branch { left :: Tree k
            , right :: Tree k }

Kompositum:

interface Tree<K> { }
class Leaf<K>   implements Tree<K> { 
  Leaf(E key) { .. }
}
class Branch<K> implements Tree<K> {
  Branch(Tree<K> left, Tree<K> right) { .. }
}