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I'm having some trouble wrapping my head around this passage of your ZuriHac-2015 talk:

Only the second, as inserting into an empty tree can be done without comparing the value being inserted. For example, this expression

insert (1 `div` 0) Leaf

does not raise a division-by-zero expression but

insert (1 `div` 0) (Node 2 Leaf Leaf)

does.

Obviously, a divide-by-zero exception will be raised in both cases when those expressions are evaluated in GHCi, because the latter forces complete evaluation in order to print the results.

In what situation can this discrepancy in behaviour between the two expressions be observed / reproduced?