Code used with https://www.barry-moore-ii.com/blog/path-to-beginnery-in-functional-programming-with-haskell

v2.hs

import Data.List.Split (splitOn)

main :: IO ()
main = do
  contents <- getContents
  let path = minPath $ csv3ToRoadSecs (lines contents)
  let repr = pictoralRepr (pathStr path)
  -- Need to deal with last character
  let lastRepr = foldPaths repr (prev repr)
  print $ "Minimum distance travelled: " ++ show (pathDist path)
  print $ "Path travelled: " ++ pathStr path
  print "Take the following path:"
  print (top lastRepr)
  print (mid lastRepr)
  print (bot lastRepr)

type RoadSec = (Int, Int, Int)
type Path = (Int, String, Int)

csv3ToRoadSecs :: [String] -> [RoadSec]
csv3ToRoadSecs = map (toRoadSec . map (read::String->Int) . splitOn ",")

pathDist :: Path -> Int
pathDist (a, _, _) = a

pathStr :: Path -> String
pathStr (_, b, _) = reverse b

toRoadSec :: [Int] -> RoadSec
toRoadSec [x, y, z] = (x, y, z)
toRoadSec _ = error "RoadSec can only be built from 3 elements!"

minPath :: [RoadSec] -> Path
minPath ((a, b, c): xs) = foldl foldRoadSecs (min a b, takeWhich a b, c) xs

foldRoadSecs :: Path -> RoadSec -> Path
foldRoadSecs (m, p, c1) (a2, b2, c2)
  | head p == 'a' = (m + min a2 crossB, takeWhich a2 crossB ++ p, c2)
  | otherwise = (m + min b2 crossA, takeWhich crossA b2 ++ p, c2)
  where
    crossB = c1 + b2
    crossA = c1 + a2

takeWhich :: Int -> Int -> String
takeWhich a b
  | a < b = "a"
  | otherwise = "b"

type PictoralRepr = (String, String, String, String)

pictoralRepr :: String -> PictoralRepr
pictoralRepr (x:xs) = foldl foldPaths ([x], "", "", "") (tail . splitOn "" $ xs)

foldPaths :: PictoralRepr -> String -> PictoralRepr
foldPaths (p, t, m, b) n
  | p == "a" && n == "a" = (n, "|-" ++ t, "  " ++ m, "  " ++ b)
  | p == "a" && n == "b" = (n, "|-" ++ t, "| " ++ m, "| " ++ b)
  | p == "b" && n == "b" = (n, "  " ++ t, "  " ++ m, "|-" ++ b)
  | otherwise = (n, "| " ++ t, "| " ++ m, "|-" ++ b)

prev :: PictoralRepr -> String
prev (p, _, _, _) = p

top :: PictoralRepr -> String
top (_, t, _, _) = reverse t

mid :: PictoralRepr -> String
mid (_, _, m, _) = reverse m

bot :: PictoralRepr -> String
bot (_, _, _, b) = reverse b