daniel.schoepe:
On Mon, Oct 26, 2009 at 11:09:20AM -0700, Don Stewart wrote:
This has been a long time coming. I think we first discussed this in April 2007.
Can you characterize what changes to the core (if any) were required to support this?
I added a `M.Map String StateExtension' field to XState to hold the state, where StateExtension is an existential data type which can hold values that are members of a type class. The string representation of the value's type is used as the key in the map. This can create problems when multiple modules store a common data type such as an Int. To prevent this I added instructions to create a separate data type for that. This should not create name conflicts as long as one automatically derives Typeable.
Ok. This same approach is used in Yi. It called it the "dynamic symbol table" approach in the Yi paper, iirc. It's a fine approach in a statically typed language.
Another thing I had to add was a list of type witnesses in XConf that is used to make values persistent between restarts when the type is an instance of Read. Currently this has to be explicitly specified when one makes a data type an instance of the type class since I didn't find a way to distinguish between Readable and non-Readable values automatically(not even with IncoherentInstances). I'd appreciate any suggestions on this issue though.
Finally, I made some modifications to the restart process to make values persistent.
The actual interface for using this feature is done in a contrib module, since it does not really belong in core.
Great!
This whole thing could have been implemented by (ab)using the layoutHook thus not needing any core changes, but this would be somewhat crude and since the additions to core are only about ~50 lines, including comments, I think this is the better way to realize this.
Ok.