Functional solving problems.

paths.txt

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+50  
+10  
+30  
+5  
+90  
+20  
+40  
+2  
+25  
+10  
+8  
+0

problems/road.hs

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+import Data.List
+
+data Section = Section { getA :: Int, getB :: Int, getC :: Int } deriving (Show)  
+type RoadSystem = [Section]
+
+heathrowToLondon :: RoadSystem  
+heathrowToLondon = [Section 50 10 30, Section 5 90 20, Section 40 2 25, Section 10 8 0]
+
+data Label = A | B | C deriving (Show)  
+type Path = [(Label, Int)]
+
+roadStep :: (Path, Path) -> Section -> (Path, Path)  
+roadStep (pathA, pathB) (Section a b c) =   
+    let priceA = sum $ map snd pathA  
+        priceB = sum $ map snd pathB  
+        forwardPriceToA = priceA + a  
+        crossPriceToA = priceB + b + c  
+        forwardPriceToB = priceB + b  
+        crossPriceToB = priceA + a + c  
+        newPathToA = if forwardPriceToA <= crossPriceToA  
+                        then (A,a):pathA  
+                        else (C,c):(B,b):pathB  
+        newPathToB = if forwardPriceToB <= crossPriceToB  
+                        then (B,b):pathB  
+                        else (C,c):(A,a):pathA  
+    in  (newPathToA, newPathToB)
+
+optimalPath :: RoadSystem -> Path  
+optimalPath roadSystem = 
+    let (bestAPath, bestBPath) = foldl roadStep ([],[]) roadSystem  
+    in  if sum (map snd bestAPath) <= sum (map snd bestBPath)  
+            then reverse bestAPath  
+            else reverse bestBPath
+
+groupsOf :: Int -> [a] -> [[a]]  
+groupsOf 0 _ = undefined  
+groupsOf _ [] = []  
+groupsOf n xs = take n xs : groupsOf n (drop n xs)
+
+main = do  
+    contents <- getContents  
+    let threes = groupsOf 3 (map read $ lines contents)  
+        roadSystem = map (\[a,b,c] -> Section a b c) threes  
+        path = optimalPath roadSystem  
+        pathString = concat $ map (show . fst) path  
+        pathPrice = sum $ map snd path  
+    putStrLn $ "The best path to take is: " ++ pathString  
+    putStrLn $ "The price is: " ++ show pathPrice

problems/rpn.hs

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+import Data.List  
+
+unsafeSolveRPN :: String -> Float
+unsafeSolveRPN = head . foldl foldingFunction [] . words  
+    where   foldingFunction (x:y:ys) "*" = (x * y):ys  
+            foldingFunction (x:y:ys) "+" = (x + y):ys  
+            foldingFunction (x:y:ys) "-" = (y - x):ys  
+            foldingFunction (x:y:ys) "/" = (y / x):ys  
+            foldingFunction (x:y:ys) "^" = (y ** x):ys  
+            foldingFunction (x:xs) "ln" = log x:xs  
+            foldingFunction xs "sum" = [sum xs]  
+            foldingFunction xs numberString = read numberString:xs
+