Haskell and Time

I’ve found myself getting confused over the Data.Time modules a few times now. Today I wanted to do something that I ought to be simple both to do (it was) and find out how to do (it wasn’t, at least not for me). I thought I’d write it down, because I’m bound to forget about it otherwise, and it might be helpful for some other poor soul out there.

I wanted to store a date in a database (SQLite) using Haskell (HDBC). Since I’m only interested in the date and not the time I chose to represent it in Haskell as a Date.Time.Calendar.Day. For Day 0 means 1858-11-17. HDBC on the other hand needs the date in seconds since epoch (1970-01-01 00:00 UTC), which conveniently can be represented with a single Integer. Time.Data.Clock.POSIX contains a few handy functions to convert between POSIXTime (NominalDiffTime) and UTCTime. However, there are no special functions for converting between an Integer and NominalDiffTime. The way I found was to do it via the class types that NominalDiffTime implements.

First I thought that Enum might offer the solution, but my experiments with that was a bit confusing:

> pt <- getPOSIXTime
> fromEnum pt
> toEnum 4118657661830593344 :: POSIXTime 

Not really the result I had hoped for. No, instead the function to use to get the Integer is floor, and the only way I found to convert a NominalDiffTime from an Integer is to go via a Ratio:

> fromRational (1221603766 % 1) :: POSIXTime

Have I missed something or is this the best way to achieve this?

Micha Bartoszkiewicz

You can use fromInteger from the Num class:

> fromInteger 42 :: POSIXTime


fromIntegral 1221603766 :: POSIXTime seems to work too, though it isn’t much shorter.


@Micha & Andy: thanks for pointing that out.


I don’t understand how to do this conversion, please, explain:

fromInteger 43 :: POSIXTime


getPOSIXTime >>= (return .  fromInteger)

    Couldn't match expected type 'time-'
           against inferred type 'Integer'

or another example

Prelude Data.Time.Clock.POSIX> a <- getPOSIXTime
Prelude Data.Time.Clock.POSIX> :type a
a :: POSIXTime
Prelude Data.Time.Clock.POSIX> fromInteger a

    Couldn't match expected type 'Integer'
           against inferred type 'POSIXTime'


@Mechanic, I think you’ve got it the wrong way around. One would use fromInteger to convert an integer to POSIXTime.

In both your examples you are trying pass a POSIXTime to fromInteger, as expected you get type errors when you do that.

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