Degrees minutes seconds parser in haskell, for parsing e.g. 37 deg 8' 21.26" N, 80 deg 34' 41.84" W

gps-parser.hs

latToDecimal :: DMSLat -> Double
latToDecimal (DMSLat dms direction) = 
  let decimal = dmsToDecimal
  in case direction of 
    North -> decimal
    South -> decimal * -1

lonToDecimal :: DMSLat -> Double
lonToDecimal (DMSLon dms direction) = 
  let decimal = dmsToDecimal
  in case direction of 
    East -> decimal
    West -> decimal * -1

-- | Converts degrees/minutes/seconds to decimal decimal format
dmsToDecimal :: DMS -> Double
dmsToDecimal (DMS d m s) = 
  d 
  + (m / 60) -- 60 minutes in an hour (degree)
  + (s / 3600) -- 3600 seconds in an hour (degree)

type Parser = Parsec Void Text

-- TODO: instead of parsing DMS, have exiftool output decimal degrees.


gpsPositionParser :: Parser (DMSLat, DMSLon)
gpsPositionParser = do
  lat <- parseLatitudeDMS
  _ <- string ", "
  lon <- parseLongitudeDMS
  pure (lat, lon)

data DMSLat = DMSLat DMS LatitudeDirection

parseLatitudeDMS :: Parser DMSLat
parseLatitudeDMS = do
  dms@(DMS d _ _) <- parseDegreesMinuteSeconds
  unless (d >= 0 && d <= 90) (fail $ "Invalid latitude degrees: " <> show d)

  direction <- parseLatitudeDirection
  
  pure $ DMSLat dms direction

data DMSLon = DMSLon DMS LongitudeDirection

parseLongitudeDMS :: Parser DMSLon
parseLongitudeDMS = do
  dms@(DMS d _ _) <- parseDegreesMinuteSeconds
  unless (d >= 0 && d <= 180) (fail $ "Invalid longitude degrees: " <> show d)

  direction <- parseLongitudeDirection

  pure $ DMSLon dms direction

data DMS = DMS Int Int Double

-- | Parses the DMS portion of a GPS position.
--
-- Validates degrees/seconds are within valid ranges, but the caller must validate degrees, whose range depends on latitude vs longitude
parseDegreesMinuteSeconds :: Parser (Int, Int, Double) -- Not sure what data type to use for this.
parseDegreesMinuteSeconds = do
  degrees <- intParser
  _ <- MPC.string " deg "
  minutes <- intParser
  unless (minutes >= 0 && minutes <= 59) (fail $ "Invalid minutes: " <> show minutes)
  _ <- MPC.string "' "

  seconds <- doubleParser
  unless (seconds >= 0 && seconds < 60) (fail $ "Invalid seconds: " <> show seconds)
  _ <- MPC.string "\" " -- Note: Parsing the trailing space after minutes

  pure $ DMS degrees minutes seconds

data LatitudeDirection = North | South
  deriving (Show)

data LongitudeDirection = East | West
  deriving (Show)

parseLatitudeDirection :: Parser Latitude
parseLatitudeDirection = do
  c <- MPC.upperChar
  case c of
    'N' -> pure North
    'S' -> pure South
    x -> fail $ "Invalid latitude:" <> [x]

parseLongitudeDirection :: Parser Longitude
parseLongitudeDirection = do
  c <- MPC.upperChar
  case c of
    'E' -> pure East
    'W' -> pure West
    x -> fail $ "Invalid longitude:" <> [x]

intParser :: Parser Int
intParser = do
  digitString <- concat <$> PC.some MPC.digitChar
  case readMay digitString of
    Nothing -> fail $ "Couldn't create an integer from a series of digits. This is likely a bug in the parser. Digits: " <> digitString
    Just i -> pure i

doubleParser :: Parser Double
doubleParser = do
  digits <- concat <$> PC.some MPC.digitChar
  -- In all 2904 data samples, across a range of phone manufacturers, it appeared all of them were to precisely 2 digits of precision
  -- Even edge cases like all zeroes.
  -- So, I'm requiring the period and trailing characters until that assumption is violated.
  -- Could arguably even parse this as Centi (Fixed E2), but didn't seem like that was helpful.
  period <- MPC.char '.'
  digits <- concat <$> PC.some MPC.digitChar
  let doubleString = digits <> [period] <> digits
  case readMay doubleString of
    Nothing -> fail $ "Couldn't create a double from a string. This is likely a bug in the parser. String: " <> digitString
    Just d -> pure d
    
    
-- "37 deg 8' 21.26\" N, 80 deg 34' 41.84\" W"
 -- "42 deg 1' 42.77\" N, 88 deg 8' 46.68\" W"
 -- "37 deg 34' 35.32\" N, 122 deg 19' 1.51\" W"
 -- "59 deg 14' 53.35\" N, 18 deg 5' 34.01\" E"
 -- "19 deg 26' 4.49\" N, 99 deg 11' 25.77\" W"
 -- "42 deg 19' 53.00\" N, 71 deg 25' 49.00\" W"
 -- "39 deg 13' 7.73\" N, 77 deg 15' 10.66\" W"
 -- "34 deg 26' 34.97\" S, 58 deg 38' 11.68\" W"
 -- "37 deg 16' 14.20\" N, 122 deg 2' 17.74\" W"
 -- "35 deg 5' 59.75\" N, 80 deg 40' 36.66\" W"
 -- "41 deg 52' 46.05\" N, 87 deg 38' 10.40\" W"
 -- "37 deg 45' 29.09\" N, 122 deg 26' 9.34\" W"
 -- "33 deg 56' 46.65\" N, 118 deg 24' 25.80\" W"
 -- "37 deg 56' 39.79\" S, 57 deg 35' 9.84\" W"
 -- "35 deg 46' 24.74\" N, 78 deg 47' 30.68\" W"
 -- "37 deg 21' 4.58\" N, 120 deg 36' 47.45\" W"
 -- "37 deg 37' 28.09\" N, 122 deg 3' 16.52\" W"
 -- "41 deg 30' 35.49\" N, 71 deg 35' 56.34\" W"
 -- "43 deg 18' 24.47\" N, 73 deg 38' 47.43\" W"
 -- "38 deg 55' 18.13\" N, 77 deg 2' 24.94\" W"
 -- "34 deg 40' 34.39\" N, 92 deg 16' 4.11\" W"
 -- "37 deg 46' 36.32\" N, 122 deg 25' 2.46\" W"