HackerRank Calculate Perimeter - Functional Programming - Ad hoc

perimeter.hs

-- Calculate the perimeter delineated by the list of points
--
-- Input from standard in is as follows:
--
-- 4
-- 0 0
-- 0 1
-- 1 1
-- 1 0
--
-- First line is the number of points that follow
--
-- Output is as follows:
--
-- 4.0
--
-- That is, the calculation of the perimeter described by the input
--

import           Control.Monad
import           System.IO

distance :: (Floating a) => (a, a) -> (a, a) -> a
distance (ax, ay) (bx, by) = sqrt (((bx - ax) ** 2) + ((by - ay) ** 2))

perimeter [] = -1
perimeter [i] = 0
perimeter [a, b] = distance a b
perimeter (ah:at) = distance ah (head at) + perimeter at

main = do
    input <- getLine
    inputs <- replicateM (read input) getLine
    let dataset = map (\x -> map (\w -> read w :: Int) (words x)) inputs
    let points = map (\x -> (fromIntegral (head x), fromIntegral (last x))) dataset
    let polygon = points ++ [ head points ]
    print $ perimeter polygon