On Sat, Jan 04, 2020 at 08:28:50PM +0530, Debasish Ghosh wrote:
I am trying to validate a bunch of inputs and found the package Data.Validation. I am also using Data.Time for date and time. I have a function like this.
validateOpenDate :: ZonedTime -> Validation [String] ZonedTime
It's supposed to validate a date for which I need to fetch the current date and time. I can do that with getZonedTime :: IO ZonedTime. I want to check if the current time is less than the date time I pass to the function and return an appropriate Validation.
This function is not pure. It needs an external input that affects its output. Therefore, the honest type of this function is either: -- | Validate date relative to current time validateOpenDate :: ZonedTime -> IO (Validation [String] ZonedTime) or (with the caller doing the time lookup): -- | Validate date relative to supplied time validateOpenDate :: ZonedTime -> ZonedTime -> Validation [String] ZonedTime The "supplied time could be generalized to an "Environment" that carries whatever context information might be needed: type Env = Env { curTime :: !ZonedTime } With all the validation functions now: validateOpenDate :: ZonedTime -> Reader Env (Validation [String] ZonedTime) validateOpenData inputDate = do now <- asks curTime if inputDate > now then ... else ... and then import Control.Monad.Reader main :: IO () main = do env <- Env <$> getZonedTime flip runReader env $ do -- computations doing validation relative to the environment
The question is how do I lift the result from IO into the Validation?
The only way to do that without changing the output type is to lie, and use "unsafePerformIO" to extract the date inside a pure function. This is not recommended.
Or is there any alternative strategy that I should consider for this validation ? I have a number of other validations which follow the same strategy but don't have the IO. My final goal is to be able to compose all these validations applicatively to prepare a final ADT in a smart constructor.
You should be able "compose" the validations in the context of an environment. Either Monadically, or Applicatively. -- Viktor.