Riccardo Odone

Posted on Sep 16, 2019 • Updated on Jun 27, 2020 • Originally published at odone.io

Mars Rover Kata in Haskell

#haskell #functional

You can keep reading here or jump to my blog to get the full experience, including the wonderful pink, blue and white palette.

Today we will TDD the Mars Rover Kata with the T in TDD meaning Type!

Red

The first step is to define all the types we need for our program to do it's job:

type Coord =
    (Int, Int)

type CoordDir =
    (Coord, Dir)

data Turn
    = L
    | R

data Dir
    = N
    | E
    | S
    | W

data Cmd
    = Turn Turn
    | Move

readMaybeT :: Read a => Text -> Maybe a

parsePlateauMax :: Text -> Maybe Coord

parseStartingCoordDir :: Text -> Maybe CoordDir

parseCommands :: Text -> Maybe [Cmd]

runCommands :: [Cmd] -> Coord -> CoordDir -> CoordDir

run :: Text -> Maybe CoordDir

Ideally we would use the functions above as follows:

main :: IO ()
main = do
    print $ parsePlateauMax "5 5"
    -- Just (5,5)

    print $ parseStartingCoordDir "1 2 N"
    -- Just ((1,2),N)

    print $ parseCommands "LMLMLMLMM"
    -- Just [Turn L,Move,Turn L,Move,Turn L,Move,Turn L,Move,Move]

    let commands = "5 5\n1 2 N\nLMLMLMLMM"
    maybe (print "ERR!") print $ run commands
    -- ((1,3),N)

Green

With the types in place it's enough to make it compile :D

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Data.Text (Text)
import qualified Data.Text as T (lines, words, unpack)
import Text.Read (readMaybe)

type Coord =
    (Int, Int)

type CoordDir =
    (Coord, Dir)

data Turn
    = L
    | R
    deriving (Show, Read)

data Dir
    = N
    | E
    | S
    | W
    deriving (Show, Read)

data Cmd
    = Turn Turn
    | Move
    deriving (Show)

readMaybeT :: Read a => Text -> Maybe a
readMaybeT = readMaybe . T.unpack

parsePlateauMax :: Text -> Maybe Coord
parsePlateauMax txt =
    case T.words txt of
        [x, y] -> (,) <$> readMaybeT x <*> readMaybeT y
        _      -> Nothing

parseStartingCoordDir :: Text -> Maybe CoordDir
parseStartingCoordDir txt = do
    (x, y, d) <- case T.words txt of
        [x, y, d] -> (,,) <$> readMaybeT x <*> readMaybeT y <*> readMaybeT d
        _      -> Nothing
    Just ((x, y), d)

parseCommands :: Text -> Maybe [Cmd]
parseCommands txt = toCmd `traverse` T.unpack txt
    where
        toCmd :: Char -> Maybe Cmd
        toCmd 'L' = Just (Turn L)
        toCmd 'R' = Just (Turn R)
        toCmd 'M' = Just Move
        toCmd _   = Nothing


runCommands :: [Cmd] -> Coord -> CoordDir -> CoordDir
runCommands []       _   coordDir             = coordDir
runCommands (c:cmds) max coordDir = runCommands cmds max $ newCoordDir c
    where
        newCoordDir (Turn L) = turnL coordDir
        newCoordDir (Turn R) = turnR coordDir
        newCoordDir Move     = move coordDir

        turnL (c, N) = (c, W)
        turnL (c, E) = (c, N)
        turnL (c, S) = (c, E)
        turnL (c, W) = (c, S)

        turnR (c, N) = (c, E)
        turnR (c, E) = (c, S)
        turnR (c, S) = (c, W)
        turnR (c, W) = (c, N)

        move ((x, y), N) = (wrap max (x, y+1), N)
        move ((x, y), E) = (wrap max (x+1, y), E)
        move ((x, y), S) = (wrap max (x, y-1), S)
        move ((x, y), W) = (wrap max (x-1, y), W)

        wrap (maxX, maxY) (x, y) = (wrap' maxX x, wrap' maxY y)

        wrap' max x | x < 0       = max + 1 + x
        wrap' max x | x > max     = x `rem` max
        wrap' max x | otherwise   = x


run :: Text -> Maybe CoordDir
run txt = do
    (max, coordDir, cmds) <- go $ T.lines txt
    Just $ runCommands cmds max coordDir
    where
        go [f, s, t] = (,,) <$> parsePlateauMax f <*> parseStartingCoordDir s <*> parseCommands t
        go _         = Nothing

main :: IO ()
main = do
    let commands = "5 5\n1 2 N\nLMLMLMLMM"
    print $ parsePlateauMax "5 5"
    print $ parseStartingCoordDir "1 2 N"
    print $ parseCommands "LMLMLMLMM"
    maybe (print "ERR!") print $ run commands