How to write Arbitrary instances that depend on current time?

[saurabhnanda]

I need to write Arbitrary instances for fields of type UTCTime, where the values are something that make sense for the problem domain, for example:

• Current time
• Current time minus X hours, where X is a small integer
• Current time plus X hours, where X is a small integer
• Midnight OR midnight +/- 10 minutes, in UTC, of any date within (currentTime, currentTime + 1 year)
• Midnight OR midnight +/- 10 minutes, in a random timezone, of any date within (currentTime, currentTime + 1 year)
• Any start OR end of week boundary within (currentTime, currentTime + 1 year)
• Any start OR end of month boundary within (currentTime, currentTime + 1 year)

Related questions:

• How does one do IO inside an Arbitrary instance, i.e. inside the Gen monad?
• How does one pass a "special value" (in this case the currentTime) to an Arbitrary instance?

[phadej]

IMHO this kind of questions are perfect fit for StackOverflow.

Very brief answer: Getting current time is a "side-effect", factor it out.

• Parametrise the unit at test:

foo  :: MonadTime m => m Res
foo' :: Monad m => m UTCTime -> m Res

• Generate intervals, or moments:

data Offset
    = Now
    | HoursInFuture (Small Int)
    | NextMidning
    ...

applyOffset :: Offset -> UTCTime -> UTCTime

• You cannot have IO inside Arbitrary instance. You can have IO inside Property though. See Test.QuickCheck.Monadic
• Don't try to pass special value to instance. Generate a change, and apply it in the Property.

[saurabhnanda]

Okay, taking this to StackOverflow

[nick8325]

@phadej's suggestion of defining an Offset type is the approach I would try first.

Alternatively, you can avoid the Arbitrary class and define your generator as a function of type UTCTime -> Gen UTCTime, and then use forAll or forAllShrink in your property.

You can do I/O in a property by using the ioProperty combinator. For example:

prop_whatever x y z =
  ioProperty $ do
     time <- getCurrentTime
     return $ ... rest of property goes here ...

[brandon-leapyear]

✨ This is an old work account. Please reference @brandonchinn178 for all future communication ✨

---

I've also historically done

arbitraryMyType :: Foo -> Bar -> Gen MyType

forAll (arbitraryMyType foo bar) $ \myType -> ...

(I know I'm posting on a really old issue. Maybe this issue should be closed?)

[MaximilianAlgehed]

Closing this as the discussion has moved to Stack Overflow