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parsing utctime with aeson

Tags:

haskell

aeson

I can't get aeson to parse an UTCTime value. I tried to encode one and feed it back, but that didn't work:

Prelude Data.Aeson Data.Time.Clock> getCurrentTime >>= (print . encode)
"\"2013-10-17T09:42:49.007Z\""
Prelude Data.Aeson Data.Time.Clock> decode "2013-10-17T09:42:49.007Z" :: Maybe UTCTime
Nothing
Prelude Data.Aeson Data.Time.Clock> decode "\"2013-10-17T09:42:49.007Z\"" :: Maybe UTCTime
Nothing

The FromJSON instance of the UTCTime type is the following (ref):

instance FromJSON UTCTime where
    parseJSON = withText "UTCTime" $ \t ->
        case parseTime defaultTimeLocale "%FT%T%QZ" (unpack t) of
          Just d -> pure d
          _      -> fail "could not parse ISO-8601 date"

following the format description found here, everything should be ok. What am I missing?

like image 461
Simon Bergot Avatar asked Oct 17 '13 09:10

Simon Bergot


1 Answers

Prelude Data.Aeson Data.Time> decode (encode [x]) :: Maybe [UTCTime]
Just [2013-10-17 10:06:59.542 UTC]

Note the "pitfalls" section in the haddocks:

Note that the JSON standard requires that the top-level value be either an array or an object. If you try to use decode with a result type that is not represented in JSON as an array or object, your code will typecheck, but it will always "fail" at runtime:

...

So stick to objects (e.g. maps in Haskell) or arrays (lists or vectors in Haskell):

like image 173
Thomas M. DuBuisson Avatar answered Sep 23 '22 21:09

Thomas M. DuBuisson