Re: [Haskell-cafe] Converting EpochTime to Integer

Hi Bram, You can do case mtime of CTime t -> fromIntegral t Here `mtime :: EpochTime`, `t :: Int64` and the result is any `Num` you want, including `Integer`. CTime doesn't have an Integral instance itself, probably because division on times doesn't make sense. That's why you can't use `fromIntegral` directly. Regards, Erik On 16 September 2015 at 13:19, Bram Neijt 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 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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