Add parser for ISO8601 durations

[nh2]

ISO8601 / RFC 339 has supports relative times (durations), quoting:

Durations:

   dur-second        = 1*DIGIT "S"
   dur-minute        = 1*DIGIT "M" [dur-second]
   dur-hour          = 1*DIGIT "H" [dur-minute]
   dur-time          = "T" (dur-hour / dur-minute / dur-second)
   dur-day           = 1*DIGIT "D"
   dur-week          = 1*DIGIT "W"
   dur-month         = 1*DIGIT "M" [dur-day]
   dur-year          = 1*DIGIT "Y" [dur-month]
   dur-date          = (dur-day / dur-month / dur-year) [dur-time]

   duration          = "P" (dur-date / dur-time / dur-week)

It would be nice if time could parse these.

[nh2]

To give an example of how it could be done, I made a gist that includes a standalone parser and formatter implemented with attoparsec: https://gist.github.com/nh2/16c84db9d10e8869d8ae

It closely resembles the ABNF grammar from the spec:

durSecond = DurSecond <$> (decimal <* char 'S')
durMinute = DurMinute <$> (decimal <* char 'M') <*> optional durSecond
durHour   = DurHour   <$> (decimal <* char 'H') <*> optional durMinute
durTime   = char 'T' *> ((DurTimeHour <$> durHour) <|>
                         (DurTimeMinute <$> durMinute) <|>
                         (DurTimeSecond <$> durSecond))
durDay    = DurDay    <$> (decimal <* char 'D')
durWeek   = DurWeek   <$> (decimal <* char 'W')
durMonth  = DurMonth  <$> (decimal <* char 'M') <*> optional durDay
durYear   = DurYear   <$> (decimal <* char 'Y') <*> optional durMonth
durDate   = (DurDateDay   <$> durDay   <*> optional durTime) <|>
            (DurDateMonth <$> durMonth <*> optional durTime) <|>
            (DurDateYear  <$> durYear  <*> optional durTime)

duration  = char 'P' *> ((DurationDate <$> durDate) <|>
                         (DurationTime <$> durTime) <|>
                         (DurationWeek <$> durWeek))

[AshleyYakeley]

There's no existing type in the library that can represent months or years. DiffTime and NominalDiffTime can only represent fixed amounts of time.

[nh2]

Heya Ashley!

That's right, new types would be needed for this.

[AshleyYakeley]

data CalendricalDuration = MkCalendricalDuration {
    cdMonths :: Int,
    cdTime :: NominalDiffTime
}

[AshleyYakeley]

I think if I were to go down this path, I'd want to sit down and implement ISO 8601 comprehensively.

[4e6]

I've recently discovered attoparsec-iso8601 package. It already has attoparsec parsers for iso8601 time. For me, it seems like a good place to put the duration parser implemented by @nh2 above.

[AshleyYakeley]

Fix merged.

[AshleyYakeley]

I think I should create a separate module for this.