dist :: Floating a => (a, a) -> (a, a) -> a
dist (i, j) (m, n) = sqrt ((i-m)^2   (j-n)^2)

zip' :: Floating a => [a] -> [(a,a)]
zip' []        =  []
zip' (x:[])    =  []
zip' (x:y:ys)  =  ((x,y):zip' ys)

perimeter :: Floating a => [(a, a)] -> a
perimeter [] = 0
perimeter (x:[]) = 0
perimeter (x:y:ys) = dist x y   perimeter (y:ys)

so suppose we have a list lst = [ (x1,y1) , (x2, y2) , (x3, y3), (x4, y4) ]. I know what my perimeter function does is basically:

dist (x1,y1) (x2,y2)   perimeter [(x2, y2),(x3, y3),(x4,y4)]
dist (x1,y1) (x2,y2)   dist (x2,y2) (x3,y3)   perimeter [(x3,y3),(x4,y4)]
dist (x1,y1) (x2,y2)   dist (x2,y2) (x3,y3)   dist (x3,y3) (x4, y4) perimeter [(x4,y4)]

now in order to make the perimeter complete, ideally somehow if we're given the original list x:y:ys then perimeter(z:[]) = dist x z (in other words, head of original list last of original list). So in our specific case, we'd end with:

 dist a b   dist b c   dist c d   dist d a

but I'm at a loss on how to get this to work. I'm sure it's something very simple. Any hints or suggestions would be much appreciate