I agree that "concisely updating immutable data structures" is the easiest way to show how this is useful. let x = (set `bar.quux` new_quux) y instead of

I agree that "concisely updating immutable data structures" is the easiest way to show how this is useful. let x = (set `bar.quux` new_quux) y instead of let x = { foo: y.foo, bar: {baz: y.bar.baz, quux: new_quux} } If the language allows, both expressions could be fully type-checked at compile time.