Automated Validation with Applicatives and Semigroups (Part 2 - Java)

In a previous post, I mentioned a small library for validating using an applicative functor pattern with a semigroup for accumulating errors using Haskell, Scala and Java programming languages. In this post, I will give a set up for an example usage, but not necessarily a complete example. This is left as an exercise and may be elaborated on in a future post.

I will start with Java. First we must decide how we are going to accumulate errors. I have decided to store them in a LinkedList. In more practical languages with a better collections library, we would probably use something else. We will use something else for Scala and Haskell (later!), or you could use the Functional Java extension. In the meantime, we will use java.util.LinkedList. We start by writing its Semigroup implementation:

public static <a> Semigroup<LinkedList<a>> LinkedListSemigroup() {
  return new Semigroup<LinkedList<a>>() {
    public LinkedList<a> append(final LinkedList<a> a1,
                                final LinkedList<a> a2) {
      final LinkedList<a> r = new LinkedList<a>(a1);
      r.addAll(a2);
      return r;
    }
  };
}

This is very straight-forward. In the previous example, I mentioned a class Person that is made of an age and a name. The age is an integer between 0 and 130 while the name is any string that starts with an upper-case character. Let’s write these data types:

// int wrapper between 0 and 130
class Age {
  private final int i;
  private Age(final int i) { this.i = i; }
  public int value() { return i; }
  public static Age age(final int i) {
    if(i <= 0 || i >= 130)
      throw new Error("out of range");
    else
      return new Age(i);
  }
}

And here is Name:

// String wrapper starts with upper-case
class Name {
  private final String s;
  private Name(final String s) { this.s = s; }
  public String value() { return s; }
  public static Name name(final String s) {
    if(s.isEmpty() || !Character.isUpperCase(s.charAt(0)))
      throw new java.lang.Error();
    else
      return new Name(s);
  }
}

I won’t bother writing the Person data type, but it is sufficient to say it will have an Age field and a Name field.

Now for validation. Suppose we have two string values; one each for age and name. We would like to check these for validation and return either one or more error messages (String) or a Person. More succinctly, we want a function with the type:

String -> String -> Validation<LinkedList<String>, Person>>

How are we going to achieve this function? First we must say how to create an Age from a String or an error message if we cannot:

String -> Validation<LinkedList<String>, Age>>

and same for Name

String -> Validation<LinkedList<String, Name>>

We also need a function to create a Person from an Age and a Name

Age -> Name -> Person

This is simple. It’s the Person constructor.

So to sum up, we need to write a function with this type:

Arguments:

F<String, Validation<LinkedList<string>, Age>>>

F<String, Validation<LinkedList<string>, Name>>>

F<Age, F<Name, Person>>

Semigroup<LinkedList<string>>

Return type:

F<String, F<String, <Validation<LinkedList<string>, Person>>>>

Of course such a function is likely to be polymorphic, rather than specifying concrete types. I recommend starting by writing a function with this type:

Arguments:

F<T, F<U, V>>

F<A, Validation<E, T>>

F<B, Validation<E, U>>

Return type:

F<A, F<B, Validation<E, V>>

This function can be written by using the constructs mentioned in the previous post. I’m out of breath. Java is incredibly laborious.

Scala and Haskell for another time!