Am Montag 18 Januar 2010 19:09:23 schrieb david fries:
Hey everybody
I've been playing around with Parsec a little bit lately. I like it a lot, but now I've hit a bit of a challenge. Suppose I have to parse a variable length string representing a time interval. Depending on how many fields there are, the time is either interpreted as seconds, minutes and seconds or hours, minutes and seconds.
For example:
"... 31 ..." would be parsed as 31 seconds. "... 05:31 ..." would be parsed as 5 minutes and 31 seconds. "... 01:05:31 ..." would be parsed as 1 hour, 5 minutes and 31 seconds.
I've come up with the following solution using optionMaybe to deal with the problem:
data ElapsedTime = ElapsedTime { hours :: Int, minutes :: Int, seconds :: Int } deriving (Show, Eq, Ord)
p_elapsed_time :: CharParser () ElapsedTime p_elapsed_time = toElapsedTime <$> (optionMaybe p_Int) <*> (optionMaybe (char ':' *> p_Int)) <*> (optionMaybe (char ':' *> p_Int <* skipSpaces)) where toElapsedTime Nothing Nothing Nothing = ElapsedTime 0 0 0 toElapsedTime (Just s) Nothing Nothing = ElapsedTime 0 0 s toElapsedTime (Just m) (Just s) Nothing = ElapsedTime 0 m s toElapsedTime (Just h) (Just m) (Just s) = ElapsedTime h m s
p_elapsed_time = toElapsedTime <$> sepBy p_int (char ':') where toElapsedTime (h:m:s:_) = ElapsedTime h m s toElapsedTime [m,s] = ElapsedTime 0 m s toElapsedTime [s] = ElapsedTime 0 0 s toElapsedTime [] = ElapsedTime 0 0 0 You can replace the first pattern for toElapsedTime with [h,m,s] and add a failure case if sepBy p_int (char ':') parses more than three ints (or write a combinator that parses up to n results of p separated by sep). Or p_elapsed_time = do s <- p_int et <- cont_elapsed_time (ElapsedTime 0 0 s) cont_elapsed_time et@(ElapsedTime h m s) = do char ':' n <- p_int cont_elapsed_time (ElapsedTime m s n) <|> return et
Where p_Int simply parses a sequence of digits as an Int and skipSpaces does just that.
This works correctly, but it also feels kinda clumsy. For one the compiler rightly complains about non-exhaustive pattern matches in the definition of the toElapsedTime function, although I believe that's negligible in that particular case. Is there a better i.e. more elegant way to tackle such a problem?
regards, david