18.hs

main = print output

sumRow sums@(s1:srest) (r1:rrest) = (s1+r1) : f sums rrest
    where f [y] [x] = [x+y]
          f (y1:y2:yrest) (x:xrest) = max (x+y1) (x+y2):f (y2:yrest) xrest

triangle = [[75],
            [95,64],
            [17,47,82],
            [18,35,87,10],
            [20,04,82,47,65],
            [19,01,23,75,03,34],
            [88,02,77,73,07,63,67],
            [99,65,04,28,06,16,70,92],
            [41,41,26,56,83,40,80,70,33],
            [41,48,72,33,47,32,37,16,94,29],
            [53,71,44,65,25,43,91,52,97,51,14],
            [70,11,33,28,77,73,17,78,39,68,17,57],
            [91,71,52,38,17,14,91,43,58,50,27,29,48],
            [63,66,04,68,89,53,67,30,73,16,69,87,40,31],
            [04,62,98,27,23,09,70,98,73,93,38,53,60,04,23]]

output = maximum$foldl1 sumRow triangle