[Haskell-cafe] Parsing LocalTime from Unix seconds

[Marc Busqué]

Thanks Alexander for your answer.

I opened an issue in `time` repository:

https://github.com/haskell/time/issues/104

It seems it is intended behaviour, but I think it is inconsistent and it
makes difficult to parse from a string when you don't know beforehand
the format it has.

In the issue link there is the developed version of my answer... :)

Marc Busqué
http://waiting-for-dev.github.io/about/

On Fri, 14 Sep 2018, Alexander V Vershilov wrote:

> Hi Marc,
>
> The best way of answering such questions is to check the source code.
> Hackage provides
> a nice way of doing that -  click on 'Source' near the instance that
> you are interested in:
>
> https://hackage.haskell.org/package/time-1.6.0.1/docs/Data-Time-Format.html#t:ParseTime
>
> And you'll see the implementation
>
> ```
>
> instance ParseTime LocalTime where
>    buildTime l xs = LocalTime <$> (buildTime l xs) <*> (buildTime l xs)
> ```
>
> That builds time from `Day` and `TimeOfDay` passing your parse string
> to each of those.
> Then you can check ParseTime instance of Day:
>
> https://hackage.haskell.org/package/time-1.6.0.1/docs/src/Data.Time.Format.Parse.html#line-331
>
> I'm not providing it here, as it's quite big, but the main point is
> that `s` is ignored so in that case
> Day appear to be:
>
> ```
>            rest (YearMonth m:_) = let
>                d = safeLast 1 [x | MonthDay x <- cs]
>                in fromGregorianValid y m d
> ```
> with y=m=d=1
>
> if you continue the process for TimeOfDay you'll find that `s` is
> ignored there as well, and
> `midnight = TimeOfDay 0 0 0` is returned in that case.
>
> So it appeared that LocalTime consists of the components that ignore
> your parse string and return
> default value instead.
>
> I don't know if that is intended behaviour or not, but for me it makes
> more sense to parse to UTCTime/POSIXTime
> and then convert into LocalTime, in case if you get seconds as input.
>
> Hope that helps.
>
>
> On Thu, 6 Sep 2018 at 13:42, Marc Busqué <marc at lamarciana.com> wrote:
>>
>> In GHCi
>>
>> ```
>> :m +Data.Time
>> parseTimeM True defaultTimeLocale "%s" "1535684406" :: Maybe UTCTime
>> -- => Just 2018-08-31 03:00:06 UTC
>> parseTimeM True defaultTimeLocale "%s" "1535684406" :: Maybe LocalTime
>> -- => Just 1970-01-01 00:00:00
>> ```
>>
>> Why? ¯\(°_o)/¯
>>
>> Marc Busqué
>> http://waiting-for-dev.github.io/about/_______________________________________________
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>
>
>
> -- 
> Alexander
>