There's a third way, too, and I haven't seen anybody mention it yet
I've noticed it, but there are some problems with this
representation, so I decided not to mention it. It's OK as far as we
don't want functions working on two areas - I don't see, how we can
implement, say, intersect :: Shape -> Shape -> Bool in this way.
However, it's a useful pattern.
(apologies if I just missed it). You can provide an explicit record
of the relevant "member functions", and "instantiate" it in different
ways. E.g.
data Shape = Shape { area :: Int }
square x = Shape (x^2)
rectangle x y = Shape (x*y)
circle r = Shape (pi*r^2)
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