Thanks for all the suggestions .. Here's an example of a validation function with Reader Env .. validateOpenDate :: ZonedTime -> Reader Env (Validation [String] ZonedTime) validateOpenDate openDate = do now <- asks curTime if zonedTimeToUTC openDate > zonedTimeToUTC now then return $ Failure ["Account open date cannot be in the future"] else return $ Success openDate I have a few such functions and I wire them together to form a smart constructor that builds an ADT .. makeAccount :: String -> String -> ZonedTime -> IO (Validation [String] Account) makeAccount no name openDate = do env <- Env <$> getZonedTime flip runReader env $ do validAccountNo <- validateAccountNo no validAccountName <- validateAccountName name validOpeningDate <- validateOpenDate openDate return $ return $ Account <$> validAccountNo <*> validAccountName <*> validOpeningDate Does this look like an idiomatic implementation ? Is there any scope to make things better ? I compose applicatively as I need to accumulate all validation errors. And also Validation does not have a Monad .. regards. On Sat, Jan 4, 2020 at 9:20 PM Viktor Dukhovni wrote:

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. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Debasish Ghosh

Thanks for the pointer .. the documentation https://hackage.haskell.org/package/monad-validate-1.2.0.0/docs/Control-Mona... looks awesome. regards. On Sun, Jan 5, 2020 at 1:00 AM Alexis King wrote:

You might want to check out https://hackage.haskell.org/package/monad-validate, which provides a monad transformer version of a Validation-like type. You can stack it on top of IO and do whatever you want there. Errors are collected from multiple branches as long as you use applicative operations where possible, so you probably want to use ApplicativeDo. The monad laws are satisfied via a relaxed notion of equality; see the documentation for more info.

Disclaimer: I am the author of monad-validate.

Alexis

On Jan 4, 2020, at 08:58, Debasish Ghosh wrote:

Hi -

I am not an experienced Haskell programmer and would like to have some help with the following problem ..

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.

The question is how do I lift the result from IO into the Validation ? 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.

Any help will be appreciated ..

regards.

-- Debasish Ghosh

-- Debasish Ghosh http://manning.com/ghosh2 http://manning.com/ghosh Twttr: @debasishg Blog: http://debasishg.blogspot.com Code: http://github.com/debasishg