Thanks, that really cleared up a few of the questions I had about Jake McArthurs' code as well. The link you provided was a good read and shed some more light on the situation. The only bit I don't quite understand is why the following code implies an "or" relation in the type constraint of repr.
data Stroke = Line Point Point (forall repr. (CirclePen repr, RectPen repr) => repr) | Arc Point Point (forall repr. (CirclePen repr) => repr) | Spot Point (forall repr. (CirclePen repr, RectPen repr, ArbitraryPen repr) => repr)
To me this reads that repr should be both a CirclePen and a RectPen in order to satisfy the type constraint in the case of Line, but it seems that it is accepting a CirclePen or a RectPen (which is the desired behaviour, so I'm not complaining).