Re: Persistence of limited tagged types

[Hyman Rosen <hyrosen@mail.com>]

Stephen Leake wrote:
> You will have to write a function that reads the external
> representation of the tag, allocates an object of the appropriate
> type, and returns the pointer. Yes, this function must be changed
> every time you add a new tag.

This is known as the Factory pattern. Instead of changing the
function for each new tag, you can have the package that defines
the type enroll a read method into a table of methods. Then the
factory reads the external tag representation, looks it up in
the table, and calls the method if it's present.