Ah, it is a little bit confusing.
The type (forall repr. (CirclePen repr, RectPen repr) => repr) is
inhabited by a _represtentation_ that can be constructed both by
'circle' and by 'rectangle'. So we can use any of those two functions
to construct a value of that type. (Such type is FiniteDim for
example). The Arc constructor, on the other hand, accepts only values
of type (forall repr. (CirclePen repr) => repr). We don't know
anything about the concrete representation type, we only know that we
can construct it using 'circle'.
Hth
-dan
On Thu, Jan 16, 2014 at 6:16 PM, Luke Clifton
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).
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