I've written a date parser that parses different types of dates using Parsec.

Credit for the original problem goes to here.

The following formats are as such:

(month word) dd, yy
(month word) dd, yyyy

(month word) can be: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Note if is yyyy it is a full 4 digit year. If it is yy then it is only the last 2 digits of the year. Years only go between 1950-2049.

module Main where

import System.Environment
import Text.Printf

import Text.ParserCombinators.Parsec

type Day = Int
type Month = Int
type Year = Int

data Date = Date Year Month Day

instance Show Date where
    show (Date year month day) =
        printf "%04d-%02d-%02d" year month day

parseDay :: Parser Day
parseDay = do
    digits <- count 2 digit
    let day = read digits
    if day < 0 || day > 31
    then fail $ printf "day %02d not in range" day
    else return day

parseMonth :: Parser Month
parseMonth = do
    digits <- count 2 digit
    let month = read digits
    if month < 0 || month > 12
    then fail $ printf "month %02d not in range" month
    else return month

wordMonths :: [String]
wordMonths = [
    "Dec" ]

parseWordMonth :: Parser Month
parseWordMonth = choice $ map try parsers
        parseWordMonth :: String -> Month -> Parser Month
        parseWordMonth word month = do
            string word
            return month
        parsers :: [Parser Month]
        parsers = zipWith parseWordMonth wordMonths [1..12]

parseYear :: Parser Year
parseYear = do
    digits <- count 4 digit
    return $ read digits

parseTwoYear :: Parser Year
parseTwoYear = do
    digits <- count 2 digit
    let year = read digits
    return $ if year > 49
             then 1900 + year
             else 2000 + year

parseDate :: Parser Date
parseDate = do
    date <- choice $ map try [parseISODate, parseNiceDate, parseWeirdDate, parseWTFDate, parseWordyDate]
    return date
        -- yyyy-mm-dd
        parseISODate :: Parser Date
        parseISODate = do
            year <- parseYear
            char '-'
            month <- parseMonth
            char '-'
            day <- parseDay
            return $ Date year month day
        -- mm/dd/yy
        parseNiceDate :: Parser Date
        parseNiceDate = do
            month <- parseMonth
            char '/'
            day <- parseDay
            char '/'
            year <- parseTwoYear
            return $ Date year month day
        -- mm#yy#dd
        parseWeirdDate :: Parser Date
        parseWeirdDate = do
            month <- parseMonth
            char '#'
            year <- parseTwoYear
            char '#'
            day <- parseDay
            return $ Date year month day
        -- dd*mm*yyyy
        parseWTFDate :: Parser Date
        parseWTFDate = do
            day <- parseDay
            char '*'
            month <- parseMonth
            char '*'
            year <- parseYear
            return $ Date year month day
        -- (month word) dd, yyyy
        -- (month word) dd, yy
        parseWordyDate :: Parser Date
        parseWordyDate = do
            month <- parseWordMonth
            char ' '
            day <- parseDay
            string ", "
            year <- try parseYear <|> parseTwoYear
            return $ Date year month day

readDate :: String -> String
readDate xs = case parse parseDate "date" xs of
    Left err -> show xs ++ ": " ++ show err
    Right date -> show date

main :: IO ()
main = do
    contents <- getContents
    let dates = filter (/= "") (lines contents)
    mapM_ putStrLn $ map readDate dates

This is already quite polished but I wonder if more can be done to reduce the repetition. I'm looking for general advice on improving this parser (error handling, use of Parsec types, etc.). Perhaps there's a way to specify some sort of Date expression that could make this neater.


1 Answer 1


One interesting thing you could do is create the higher level date parser parseDate using the alternative instance on Parser with:

parseDate :: Parser Date
parseDate = do
  date <- parseISODate <|> parseNiceDate <|> parseWeirdDate <|> parseWTFDate <|> parseWordyDate
  return date

This really just makes it easy to read the code and is the general solution to combining monadic parsers.

The other way to do it without creating your own parser is to use something like the time package: http://hackage.haskell.org/package/time-1.5/docs/Data-Time-Format.html

Here you can define the parse strings:

yyyy-mm-dd                => "%Y-%m-%d"
mm/dd/yy                  => "%m/%d/%y"
mm#yy#dd                  => "%m#%y#%d"
dd*mm*yyyy                => "%d*%m*%Y"
(month word) dd, yy       => "%b %d, %y"
(month word) dd, yyyy     => "%b %d, %Y"

and then parse them in a similar way

dateFormats = ["%Y-%m-%d"
              ,"%b %d, %y"
              ,"%b %d, %Y"]

parseDate :: String -> Maybe Day
parseDate inp = choice $ map (\fmt -> parseTimeM True defaultTimeLocale fmt inp) dateFormats

and then you can print the day using the function in Data.Time.Format

printIso8601 :: Day -> IO ()
printIso8601 d = putStrLn $ formatTime defaultTimeLocale fmt d
  where fmt = iso8601DateFormat Nothing

If it wasn't something like time that has been done multiple times I think Parsec is the right answer for parsing and you use it well.

It definitely looks good to me. I'm not a fan of printf but that's just a preference.

  • \$\begingroup\$ I've found that the parseDate function won't work that way because it doesn't backtrack when something goes wrong. As for the time package, thanks for your suggestion! I'll reimplement the parser using that. \$\endgroup\$
    – wei2912
    Commented Nov 19, 2014 at 5:03

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