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Validations with TotallyLazy

22 Jul 2015

TotallyLazy is a library that gives you some tools to do functional programming on the JVM and particularly in Java, like: Option/Maybe, Either, persistent data structures, Sequence operations, etc. Unfortunately, even if quite useful and nice to use this library is also practically undocumented (and no, I don't consider raw code as sufficient documentation).

Since I've been using this library for some time now, in my pet projects and also using it in production, I'd like to share a thing or two that I've learned along the way. In particular, I'd like to show you how to do validations using the primitives offered by TotallyLazy.

What does it means to validate?

TotallyLazy model a validation like a function on some data type that can either:

Be successful.
Be a failure with a list of error messages.

More precisely, Validator is a Predicate<T> with an additional method ValidationResult validate(T instance). A ValidationResult is a type which can be successful or failed, and in the latter case can contain a list of error messages (as Strings).

In Haskell we'd have something like this:

data ValidationResult = Successful | Failed [String]
validate :: a -> ValidationResult

Ok, how do I do that?

In this case, an example can be much more informative that a lot of words. Let's take for example the validation of an email.

An Email in this example is a classic POJO, that I express here as an Haskell record because it's much more concise and the point is not the structure of this class. The only peculiarity of this class is that every field is optional.

data Email = Email {fromAddress :: Maybe String,
                    toAddress :: Maybe String,
                    subject :: Maybe String,
                    body :: Maybe String,
                    host :: Maybe String}

First of all, we want to write a Validator for checking to validate that an optional String field must contain a value:

/**
 * Builds a {@link Validator} that checks that a given {@link Option} contains
 * a value.
 *
 * @param name Name of the datum
 * @return {@link Validator}
 */
private Validator<Option<?>> notEmpty(final String name) {
  return new Validator<Option<?>>() {
    @Override
    public ValidationResult validate(Option<?> x) {
      return matches(x)
          ? ValidationResult.constructors.pass()
          : ValidationResult.constructors.failure("Parameter " + name
              + " missing");
    }

    @Override
    public boolean matches(Option<?> x) {
      return x.isDefined();
    }
  };
}

We've defined a creation method that builds and returns a Validator<Option<?>> that validates an optional value by checking if it actually contains a value. Note that since Validator is also a Predicate, we define the corresponding #matches() method and use that in our #validate().

Next, we need to define an ad-hoc validation for the host: it's optional (in fact ignored) if the system is configured to use a mock mailer. Let's assume that we a method that says if it's configured the mock mailer or not, and write this validation:

/**
 * Builds a validator that checks that the host is present (unless the SMTP
 * server is a mock).
 *
 * @return {@link Validator}
 */
private Validator<Option<String>> validHost() {
  return new Validator<Option<String>>() {
    @Override
    public ValidationResult validate(Option<String> host) {
      return matches(host)
          ? ValidationResult.constructors.pass()
          : ValidationResult.constructors.failure("Host is missing");
    }

    @Override
    public boolean matches(Option<String> host) {
      return host.isDefined() || mailServerIsAMock();
    }
  };
}

Finally, we can use this Validators to "declaratively" validate an entire Email value as follows:

public ValidationResult validateEmailParameters(Email email) {
  return sequence(notEmpty("from").validate(email.getFromAddress()),
                  notEmpty("to").validate(email.getToAddress()),
                  notEmpty("subject").validate(email.getSubject()),
                  notEmpty("body").validate(email.getBody()),
                  validHost().validate(email.getHost()))
         .reduce(ValidationResult.functions.merge());
}

Note that #notEmpty() is defined in very general terms and can be reused in many more circumstances.

And now?

This is just a quick overview on the validation primitives offered by TotallyLazy. A good next step would be looking at all the fine combinators for Validators.

Validations with TotallyLazy

What does it means to validate?

Ok, how do I do that?

And now?

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