[tel]

Laziness in data structures has the biggest benefit in the spine. Leaf laziness is just more surface area to hide unexpected thunks. If you really want that, do something like

    data Box a = Box a

    type Lazier a = Tree (Box a)

Generally, I find that a little habit around leaf strictness ends up eliminating laziness concerns entirely until you get to explicit concurrent programming and need to think carefully about what thread is forcing what execution.