Open
Description
Assuming #146 gets adopted for conditionally setting message fields, we still don't have one thing we might desire: a simple idiom for deriving one message from another by copying most of the fields, which would ideally be expressed as
MyProto{f1: orig.f1, f2: orig.f2, ...}
Because we don't have the same value when reading an unset field as when writing it, we'd have to write
MyProto{f1: has(orig.f1) ? orig.f1 : unset, f2: has(orig.f2) ? orig.f2 : unset, ...}
which is substantially more verbose.
Instead, we can introduce utility functions which can accomplish the same thing more concisely, such as orig.copy(fields or field_mask) which would create a copy restricted to the listed subset of fields, orig.exclude(fields or field_mask) which would create a copy without the listed fields, and orig.merge({"f1": v1, ...}) which would overwrite select fields.