Question about something in Hudak's book

[Tom Pledger]

Ludovic Kuty writes:
 :
 | Is it an idiom or some sort of optimization ?

It's more to do with the particular algorithm for finding the area of
a convex polygon.  Try working through the calculation of the area of
this kite.

    Polygon [(0, 0), (1, 0), (2, 2), (0, 1)]

I think the two versions of the area function will give different
results, and that this will highlight the reason for v1 being matched
separately.