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import Control.Arrow
 
cross ((ax, ay), (bx, by), (cx, cy)) = 
    (bx - ax) * (cy - ay) - (cx - ax) * (by - ay)
 
groupInPairs (a:b:xs) = scanl (curry $ first snd) (a, b) xs
triangulate = uncurry (map . (uncurry . (,,))) $ (head &&& groupInPairs . tail)
task = (/ 2) . sum . map (abs . cross) . triangulate
 
main = print $ task [(0, 0), (0, 1), (1, 1), (1, 0)]
 

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:7:50:
    Couldn't match expected type `(a, c)'
           against inferred type `[a1] -> a1'
    In the first argument of `(&&&)', namely `head'
    In the second argument of `($)', namely
        `(head &&& groupInPairs . tail)'
    In the expression:
          uncurry (map . (uncurry . (,,))) $ (head &&& groupInPairs . tail)

Standard output is empty