DDevelopment & Design

Validation with warnings in scala with scalaz

  • byAjk
  • August 26, 2014
  • 2 minute read
Monad are containers with ‘special powers’, when it comes to applying function over its content.Validation special power is propagating Failure over validation process.If you are not familiar with scalaz.Validation I urge you to read this example,…

Monad are containers with ‘special powers’, when it comes to applying function over its content.
Validation special power is propagating Failure over validation process.

If you are not familiar with scalaz.Validation I urge you to read this example, which shows how to use Validation: A Tale of 3 Nightclubs

Basically validation looks like this:

Scalaz.Validation uses idiomatic scala way to compose monads by For Comprehension.

Concrete validation method, returning scalaz.Validation instances looks like this:

Scalaz provide helper methods for wrapping values into Failure or Success.

To sum it up. Validation is a an elegant way to handle application validation logic.

However it’s not enough.

Our business rules require application logic’s to perform validation with warnings, which should not propagate as failures, but rather propagate independently of Success/Failure types.

We liked monad approach to data validation so we wanted to keep it that way.

Let me introduce Validation with warnings

What it does is basically wrapping scalaz.Validation into another type responsible for carrying warnings over validation process

Thank to scala type inference our validation code look’s just the same, but now for expression operates on ValidationWithWarnings type rather than Validation.

OK, but what about validation code? We created similar helper methods for wrapping validation into ValidationWithWarnings and wrapping values directly into warnings.

One could inline warning in for loop:

Or use it in validation method:

And of course chain it in for-loop:

Applicative

We support scalaz.Applicative, so it’s possible to take few validations and apply them to function if all elements are successes, collecting any errors and warnings if present.

Summing

Similarly to scalaz.Validation, we also support summing values, if value type has Semigroup typeclass:

Repository

Code with examples in test files can be found at https://github.com/Ajk4/ValidationWithWarnings

Q&A

Why not use Writer Monad?
– Same reason why we prefer Validation over Either with left/right projection. It’s more direct and descriptive.

Why validation nel underhood?
– It suited our business needs best.

Validation is not a Monad!
– True. 

— Previous article

Multi phased processing in scala

Next article —

Warsjawa – hold your own workshop

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DDevelopment & Design

Simple trick to DRY your Grails controller

Grails controllers are not very DRY. It's easy to find duplicated code fragments in default generated controller. Take a look at code sample below. It is duplicated four times in show, edit, update and delete actions:

class BookController {
def show() {
def bookInstance = Book.get(params.id)
if (!bookInstance) {
flash.message = message(code: 'default.not.found.message', args: [message(code: 'book.label', default: 'Book'), params.id])
redirect(action: "list")
return
        }
        [bookInstance: bookInstance]
    }
}

Why is it duplicated?

There is a reason for that duplication, though. If you move this snippet to a method, it can redirect to "list" action, but it can't prevent controller from further execution. After you call redirect, response status changes to 302, but after method exits, controller still runs subsequent code.

Solution

At TouK we've implemented a simple trick to resolve that situation:

  1. wrap everything with a simple withStoppingOnRender method,
  2. whenever you want to render or redirect AND stop controller execution - throw EndRenderingException.

We call it Big Return - return from a method and return from a controller at once. Here is how it works:

class BookController {
def show(Long id) {
withStoppingOnRender {
Book bookInstance = Book.get(id)
validateInstanceExists(bookInstance)
            [bookInstance: bookInstance]
        }
    }

protected Object withStoppingOnRender(Closure closure) {
try {
return closure.call()
        } catch (EndRenderingException e) {}
    }

private void validateInstanceExists(Book instance) {
if (!instance) {
flash.message = message(code: 'default.not.found.message', args: [message(code: 'book.label', default: 'Book'), params.id])
redirect(action: "list")
throw new EndRenderingException()
        }
    }
}

class EndRenderingException extends RuntimeException {}

Example usage

For simple CRUD controllers, you can use this solution and create some BaseController class for your controllers. We use withStoppingOnRender in every controller so code doesn't look like a spaghetti, we follow DRY principle and code is self-documented. Win-win-win! Here is a more complex example:

class DealerController {
@Transactional
def update() {
withStoppingOnRender {
Dealer dealerInstance = Dealer.get(params.id)
validateInstanceExists(dealerInstance)
validateAccountInExternalService(dealerInstance)
checkIfInstanceWasConcurrentlyModified(dealerInstance, params.version)
            dealerInstance.properties = params
saveUpdatedInstance(dealerInstance)
redirectToAfterUpdate(dealerInstance)
        }
    }
}