[Haskell-cafe] Converting EpochTime to Integer

[Patrick Chilton]

There's no need to go to Integer, just use this:

epochToJSON :: EpochTime -> Value
epochToJSON (CTime t) = toJSON t

On Wed, Sep 16, 2015 at 12:19 PM, Bram Neijt <bneijt at gmail.com> wrote:

> I'm sorry, but as a newby I can't seem to get it to work.
>
> If I use
>     return (mtime :: Integer)
> I get
>     src/Main.hs:21:13:
>         Couldn't match type ‘Foreign.C.Types.CTime’ with ‘Integer’
>         Expected type: Integer
>           Actual type: System.Posix.Types.EpochTime
>         In the first argument of ‘return’, namely ‘(mtime :: Integer)’
>         In a stmt of a 'do' block: return (mtime :: Integer)
>
> If I use
>     return (toInteger mtime)
> I get
>     src/Main.hs:21:13:
>         No instance for (Integral System.Posix.Types.EpochTime)
>           arising from a use of ‘toInteger’
>         In the first argument of ‘return’, namely ‘(toInteger mtime)’
>         In a stmt of a 'do' block: return (toInteger mtime)
>         In the expression:
>           do { status <- getFileStatus filepath;
>                let mtime = modificationTime status;
>                return (toInteger mtime) }
>
> What am I doing wrong here? How would I pattern match to convert the type?
>
> Greetings,
>
> Bram
>
> PS Code is still
> https://github.com/bneijt/httpmtimeline/blob/master/src/Main.hs#L21
> Versions
> cabal-install version 1.22.2.0
> using version 1.22.2.0 of the Cabal library
> The Glorious Glasgow Haskell Compilation System, version 7.10.1
>
>
>
>
>
> On Tue, Sep 15, 2015 at 6:48 PM, David Kraeutmann <kane at kane.cx> wrote:
> > EpochTime is an instance of Integral. You can just use toInteger ::
> > Integral a => a -> Integer.
> > On 9/15/2015 6:42 PM, Bram Neijt wrote:
> >> Dear Haskell cafe,
> >>
> >> I want to convert form
> >>
> >> mtime :: System.Posix.Types.EpochTime
> >>
> >> to something aeson can eat without having to define an instance. I
> >> choose Integer.
> >>
> >> My current solution[1] is just using (read . show). I know, ugly, but
> >> the only thing I could get going.
> >>
> >> What is the proper way to convert System.Posix.Types.EpochTime to
> >> Integer or something else aeson will automatically convert without
> >> making a ToJSON instance?
> >>
> >> As a beginner I wonder: is there a general way to go about finding
> >> conversion paths between types?
> >>
> >>
> >> Greetings,
> >>
> >> Bram Neijt
> >>
> >> [1] https://github.com/bneijt/httpmtimeline/blob/master/src/Main.hs#L21
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> >>
> >
> >
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