haskell - Is there a better way to convert from UTCTime to EpochTime? -

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i want set file's modification time time got exif data.

to time exif, found :

graphics.exif.gettag :: exif -> string -> io (maybe string)

to set file modification time, found :

system.posix.files.setfiletimes :: filepath -> epochtime -> epochtime -> io ()

assuming find time in exif, need convert string epochtime.

  • with parsetime can utctime.
  • with utctimetoposixseconds can posixtime
  • with posixtime can more or less epochtime

to convert utctime epochtime typechecks, i'm not sure it's correct :

fromintegral . fromenum . utctimetoposixseconds $ etime

this part of function gettime return time exif data, if present, otherwise file's modification time :

gettime (path,stat) =  let ftime                 = modificationtime $ stat      err (someexception _) = return ftime  time <- liftio $ handle err $    exif <- exif.fromfile path    let getexiftime = maybet . liftio . exif.gettag exif    res <- runmaybet $      tmp <- msum . map getexiftime $ [ "datetimeoriginal","datetimedigitized", "datetime" ]      maybet . return . parsetime defaulttimelocale "%y:%m:%d %h:%m:%s" $ tmp    case res of      nothing    -> return ftime      etime -> return . fromintegral . fromenum . utctimetoposixseconds $ etime  return (path,time)

my question is

is there better/simpler way convert time ? ( maybe using different libaries )

data.time best supported time library, agree choice of using parse string representation of date , time out of exif data.

are sure need set modification time of file? that's unusual. if so, yes, you'll need use system.posix libraries on posix system.

if need read modification of file, better off using more generic function system.directory.getmodificationtime. unfortunately, function uses non-standard time library, system.time long-deprecated old-time package in case. still need similar machinations.

your conversion posixtime epochtime happens ok in particular case, in general it's not ideal way go.

the epochtime type, a.k.a time_t type c, not support direct way constructed in haskell without going via integral value, though not integral depending on operating system. go via c using ffi if potential fractions of second important you. here that's not important, because you're getting seconds %s format parameter can have no fractional part. anyway. still need kind of rounding or truncating non-integral utctime type epochtime.

you're using enum instance of posixtime rounding/truncating you, decides to. again, in particular case doesn't matter, because happen know value integral. in general, it's better specify explicitly using floor, ceiling, or round. e.g.,

return $ maybe ftime (frominteger . round . utctimetoposixseconds) etime

(note don't need write out case explicitly, can use maybe function prelude.)

notice explicitly using frominteger push conversion through integer type. if want go via int instead (watch out "year 2038 problem" on 32-bit machines), define separate conversion function make clear:

  return $ maybe ftime utctimetoepochtime etime ...  -- convert utctime epochtime via int utctimetoepochtime :: utctime -> epochtime utctimetoepochtime = fromintegral . tosecs       tosecs :: utctime -> int     tosecs = round . utctimetoposixseconds

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