Hi Brian. Happy 2012!
I'm refering to the Computational Geometry: Quick Hull article in F#.NET Journal. Appreciate your futher mentoring.
Not sure what Gommer is refering to.

"A key aspect of the GUI is the ability to drag and drop points in the point set and watch the convex hull as it is recomputed in real time. The following drag value is the current state of the outstanding drag operation, if any, represented by the index of the point being dragged:

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let drag : int option ref = ref None;;

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val drag : int option ref = {contents = null;}

This design follows the mantra "make illegal states unrepresentable". Alternative designs in languages where this cannot be so easily expressed tend to use one boolean variable to indicate whether or not a drag is in progress and another integer variable to convey the index when meaningful. Using data structures such as the option type in F# precludes the possibility of non-sensical values being created and encountered at run-time, reducing the scope for errors and the number of run-time tests."

Don Syme, circa 2007, "... Indeed, computation expressions can be seen as a general monadic syntax for F#. ..."

Don had another blog where he noted the statefulness of mouse ...

I've been reading Tomas Petricek, et al's Rx. And other interactive manipulation blog articles, including recent comments on Time files and WP7 issues.

Some time ago, early days, I understood F# was not doing GUI so much; not sure what the state is now. Maybe it will become clearer soon with an Expert F# 3.0 book?

Thanks for your helpful insights as always. Art