For me the section Pattern Matching and record classes is missing a discussion about the fact that even if the pattern uses a binding, the value of that binding is not accessed (so the corresponding record accessor is not called) until the binding is used at least once.
As attractive as that seems, I don't think we want to go there. There are two big problems here.
1. Specifying in terms of accessors constrains our ability to evolve the language -- in a direction we know we want to go, and soon. Just as records get the obvious constructor if you don't give them one, they also get the obvious _deconstructor_ if you don't give them one. But surely, we might want to allow people to explicitly specify the deconstructor, just as they can explicitly specify the constructor. (The obvious implementation delegates to the accessors.) More importantly, we want a "record pattern" to appeal to a deconstructor, so it is useful for all classes, not just records, and we want the semantics of such patterns to be consistent across records and classes, to permit compatible refactoring. So a record pattern really has to be a degenerate case of a deconstructor pattern. Giving the accessors more billing than they deserve in this story just distracts from the real star, which is deconstructors.
2. Specifying that we do not call the accessor unless / until the binding is used effectively creates an expectation of laziness, which is cool but which will be extremely difficult to maintain when we get to more sophisticated patterns. And if record patterns are lazy but everything else is not, that's just confusing. (And, worse, we are spending our laziness budget where it matters least -- record accessors are trivially inlined away 99% of the time, so the benefit of laziness for records is almost zero.) Instead, we should specify that there is no guarantee about the timing (or number of invocations) of the accessor, and let the compiler generate the best code it can.
(I spent a lot of time trying to figure out how we can preserve the option for laziness in complex patterns, and it generally falls apart about when it would start to be valuable.)
