Validation with warnings in scala with scalaz

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. 

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

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.