> Instead of allocating Expr objects willy-nilly on the heap, we’ll pack them into a single, contiguous array.

This happens naturally if you bump-allocate them in a garbage-collected run-time, particularly under a copying collector. Free lists also tend to co-locate because they are produced during sweep phases of GC which run through heaps in order of address.

Don't make me bring out the L word for the billionth time.

> A flat array of Exprs can make it fun and easy to implement hash consing

OK, it's not a case of L-ignorance, just willful neglect.