The whole concept of ‘immutable data’ (and the more general idea of a formula-reduction approach to computation) is a fundamental dead-end. This is because physical machinery inescapably relies on mutable storage – and the reasonably-efficient use thereof.
The approach taken by Curtis is a result of his mis-spent youth among academics, who accept the costs of this ‘hair shirt’ approach to computation because it allows them to write ‘proofs’ (not to be confused with actual mathematical proofs. The very notion of proving any important property of a computational system as a whole, rather than of an abstract algorithm taken individually, is laughable – because real-world situations do not give you the kind of hard priors required to rise to the level of an honest mathematical proof.)
A. Perlis taught that ‘one cannot transition from the informal to the formal by formal means.’ And indeed you can’t.
The ‘proofs’ so beloved by FP academics are written in order to participate in a bizarre grant embezzlement scheme, rather than for any practical purpose. They are fruits of a poison tree, just the same as anything created by Lysenkoist biologists.