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.

Tags:
scala

Ajk

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Warsjawa – hold your own workshop

Phonegap / Cordova and cross domain ssl request problem on android.

byPaweł Byszewski
July 29, 2013

In one app I have participated, there was a use case:

User fill up a form.
User submit the form.
System send data via https to server and show a response.

During development there wasn’t any problem, but when we were going to release production version then some unsuspected situation occurred. I prepare the production version accordingly with standard flow for Android environment:

ant release
align
signing

During conduct tests on that version, every time I try to submit the form, a connection error appear. In that situation, at the first you should check whitelist in cordova settings. Every URL you want to connect to, must be explicit type in:

res/xml/cordova.xml

If whitelist looks fine, the error is most likely caused by inner implementation of Android System. The Android WebView does not allow by default self-signed SSL certs. When app is debug-signed the SSL error is ignored, but if app is release-signed connection to untrusted services is blocked.

Workaround

You have to remember that secure connection to service with self-signed certificate is risky and unrecommended. But if you know what you are doing there is some workaround of the security problem. Behavior of method

CordovaWebViewClient.onReceivedSslError

must be changed.

Thus add new class extended CordovaWebViewClient and override ‘onReceivedSslError’. I strongly suggest to implement custom onReceiveSslError as secure as possible. I know that the problem occours when app try connect to example.domain.com and in spite of self signed certificate the domain is trusted, so only for that case the SslError is ignored.

public class MyWebViewClient extends CordovaWebViewClient {

   private static final String TAG = MyWebViewClient.class.getName();
   private static final String AVAILABLE_SLL_CN 
                               = "example.domain.com";

   public MyWebViewClient(DroidGap ctx) {
       super(ctx);
   }

   @Override
   public void onReceivedSslError(WebView view, 
                                  SslErrorHandler handler, 
                                  android.net.http.SslError error) {

       String errorSourceCName = error.getCertificate().
                                  getIssuedTo().getCName();

       if( AVAILABLE_SLL_CN.equals(errorSourceCName) ) {
           Log.i(TAG, "Detect ssl connection error: " + 
                       error.toString() + 
                       „ so the error is ignored”);

           handler.proceed();
           return;
       }

       super.onReceivedSslError(view, handler, error);
   }
}

Next step is forcing yours app to use custom implementation of WebViewClient.

public class Start extends DroidGap
{
   private static final String TAG = Start.class.getName();

   @Override
   public void onCreate(Bundle savedInstanceState)
   {
       super.onCreate(savedInstanceState);
       super.setIntegerProperty("splashscreen", R.drawable.splash);
       super.init();

       MyWebViewClient myWebViewClient = new MyWebViewClient(this);
       myWebViewClient.setWebView(this.appView);

       this.appView.setWebViewClient(myWebViewClient);
       
       // yours code

   }
}

That is all ypu have to do if minSdk of yours app is greater or equals 8. In older version of Android there is no class

android.net.http.SslError

So in class MyCordovaWebViewClient class there are errors because compliator doesn’t see SslError class. Fortunately Android is(was) open source, so it is easy to find source of the class. There is no inpediments to ‘upgrade’ app and just add the file to project. I suggest to keep original packages. Thus after all operations the source tree looks like:

Class SslError placed in source tree.

Now the app created in release mode can connect via https to services with self-signed SSl certificates.

DDevelopment & Design

Grails render as JSON catch

byTomasz Kalkosiński
November 21, 2012

One of a reasons your controller doesn't render a proper response in JSON format might be wrong package name that you use. It is easy to overlook. Import are on top of a file, you look at your code and everything seems to be fine. Except response is still not in JSON format.

Consider this simple controller:

class RestJsonCatchController {
def grailsJson() {
render([first: 'foo', second: 5] as grails.converters.JSON)
    }

def netSfJson() {
render([first: 'foo', second: 5] as net.sf.json.JSON)
    }
}

And now, with finger crossed... We have a winner!

$ curl localhost:8080/example/restJsonCatch/grailsJson
{"first":"foo","second":5}
$ curl localhost:8080/example/restJsonCatch/netSfJson
{first=foo, second=5}

As you can see only grails.converters.JSON converts your response to JSON format. There is no such converter for net.sf.json.JSON, so Grails has no converter to apply and it renders Map normally.

Conclusion: always carefully look at your imports if you're working with JSON in Grails!

Edit: Burt suggested that this is a bug. I've submitted JIRA issue here: GRAILS-9622 render as class that is not a codec should throw exception

DDevelopment & Design

Simple trick to DRY your Grails controller

byTomasz Kalkosiński
June 5, 2013

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:

wrap everything with a simple withStoppingOnRender method,
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)
        }
    }
}