[DaiPlusPlus]

> Also, mapping/transformation logic can often just be simple, pure, total functions

I find this is never the case - usually for the exact reason you inadvertently sold as some kind of "benefit": "it's reserving the right to change two pieces of data independently" - because when you need to map from, say `SignupRequest` to an `SavedAccount` object you'll encounter data-members required by `SavedAccount` which cannot be sourced from the `SignupRequest` object - for example, supposing the UX people come to us and say we need to split-up the registration form into 2 pages, such that most fields are still on page 1, but the password boxes are on page 2. Now you need to deal with how to safely persist data from page 1 for use in page 2 (so using hidden HTML inputs between pages won't work because that requires POST requests to work, but both pages should be able to be intiially-requested with a GET request.