[Haskell-cafe] No instance for (Read POSIXTime)

[dokondr]

Yitz, thanks for the detailed answer!
May be I should have formulated my question differently. All I actually
need is some way to get *seconds* since epoch from the  system, so I could
manipulate them as integers.
Correct me if I am wrong, but UTCTime does not help here.
The only way I found to get seconds form epoch using existing today GHC
libraries is that:

utcStr = "Wed, 07 Dec 2011 10:10:05 +0000"
posixSecondsIntg = read posixSecondsStr :: Integer
posixSecondsStr =  init $ show (cvtUTCtoSec  utcStr) -- throw away 's' from
posix seconds string, s\
uch as "1235657s"

cvtUTCtoSec utcTimeStr = utcTimeToPOSIXSeconds utcTime where
        utcTime = fromJust $ tryParseTime utcTimeStr

timeFormat1 = "%a, %d %b %Y %T %z"
timeFormat2 = "%m/%e/%Y %l:%M:%S %p"

tryParseTime :: String -> Maybe UTCTime
tryParseTime timeStr = tryFormat (parseTime defaultTimeLocale timeFormat1
timeStr :: Maybe UTCTime)
   where
     tryFormat time
        | time == Nothing = parseTime defaultTimeLocale timeFormat2 timeStr
:: Maybe UTCTime
        | otherwise = time


Not a very easy way, isn't it?

On Sun, Dec 11, 2011 at 9:22 PM, Yitzchak Gale <gale at sefer.org> wrote:

> dokondr wrote:
> > When I try to read POSIXTime...
> > No instance for (Read POSIXTime)...
> > What should I do to provide Read instance for POSIXTime?
>
> Short answer: if you are thinking about this as a moment
> in time that could be parsed from the usual kind of
> string representation for that, you probably want to
> use UTCTime in your data type, not POSIXTime.
>
> If you really, really want to represent it internally
> as POSIXTime, then you should read those strings
> as UTCTime and then convert them to POSIXTime
> to store in your data type. I.e., in that case don't
> make your data type an instance of Read.
>
> POSIXTime is just a type alias for NominalDiffTime,
> i.e., a quantity of time between two moments.
>
> This is what the Show instance looks like:
>
> Prelude Data.Time> realToFrac 1000000 :: NominalDiffTime
> 1000000s
>
> By convention, the Read instance would expect a
> string in that format. Generally people aren't interested
> in that, so there is no Read instance.
> Even if you did want to parse that, you would just
> parse it as a number and then use realToFrac, as I
> did above.
>
> The rule of thumb is: always represent moments in
> time as a UTCTime.
>
> Regards,
> Yitz
>
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