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toRecord, fromRecord, and exportsThe toRecord method uses a feature called exports, which is a list of
2-tuples of the format Name#Type, where both Name and Type are atoms.
The former is the attribute name to be saved, and the latter is the type
of the attribute. Note that these types are specific to MOZ, and not Oz
itself. The Type information is used to determine when special
processing is needed to be able to safely save an attribute to disk.
For example, toRecord uses the Type field to look for object
references, which need special processing.
It is very important that the export list contain only the names
of attributes that contain non-stateful data, or else the save will
fail. Stateful data is data that is specific to a particular invocation
of a running program, such as open file handles. Oz programmers also
refer to stateful data as 'resources'.
Simple records, atoms, Oz names, and procedures are all safe.
Threads, file handles, and any other value that isn't guaranteed to be
persistent, are all unsafe to save. Any attribute that stores this form
of data either must not be named in the export list, or there
must be special code in toRecord to deal with that Type, such as
there is for the type called objectRef, which is for references
to objects.
There is also an analogous featExports list for saving the value
of features. Since features are write-once values, an object that has a
featExports list should only have fromRecord called on it once. Any
subsequent calls once the features have been initialized will fail.