Alessandro Lacava

on Designing and Developing Software

Scala DSL for Currency-related Operations

A simple internal DSL in Scala for money-related operations

Source code: Scala DSL for money-related operations

This Domain-Specific Language (DSL) lets you perform operations among different currencies, by transparently doing all internal conversions. The conversion map is injected implicitly by the client code.

Usage Example

Here’s a simple usage example:

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import com.lambdista.money._
import com.lambdista.money.Currency._

object Main {

  def main(args: Array[String]): Unit = {

    implicit val conversion: Conversion = Map(
      (GBP, EUR) -> 1.270,
      (EUR, USD) -> 1.268,
      (GBP, USD) -> 1.611
    )

    val sumAndConversion1 = 100.001(USD) + 200(EUR) to GBP
    println(s"sumAndConversion1: $sumAndConversion1")

    val sumAndConversion2: Money = 100(USD) + 210.4(EUR) to EUR
    println(s"sumAndConversion2: $sumAndConversion2")

    val sum = 100.001(USD) + 200(EUR)
    val simpleConversion = sum(GBP)
    println(s"simpleConversion: $simpleConversion")

    val sumWithSimpleNumber = 100(USD) + 23.560
    println(s"sumWithSimpleNumber: $sumWithSimpleNumber")

    val multiplicationWithSimpleNumber = 100(USD) * 23
    println(s"multiplicationWithSimpleNumber: $multiplicationWithSimpleNumber")

    val usd = Currency("USD")

    val multiplication = 100(usd) * 23(EUR)
    println(s"multiplication: $multiplication")

    val divisionWithSimpleNumber = 100(USD) / 23
    println(s"divisionWithSimpleNumber: $divisionWithSimpleNumber")

    val comparison = 100(USD) > 90(EUR)
    println(s"100 USD > 90 EUR? $comparison")

  }

}

As you can see the client code just needs two simple imports and an implicit value of type Conversion in order to use the DSL. The operations shown in the previous code are only a few among the available ones. Have a look at the Money class for a complete coverage.

You can find the source code here: Scala DSL for money-related operations

Try-Success-Failure API for Java 8

Try-Success-Failure API: Java implementation of the famous Scala counterpart

Source code: Try-Success-Failure API for Java

This API is a Java implementation of Scala Try API, originally implemented by the guys at Twitter and later added to the Scala Standard Library.

The Try type represents a computation that may fail. If the computation is successful returns the value wrapped in a Try.Success otherwise returns the java.lang.Exception wrapped in a Try.Failure.

To use Try you need to call the Try.apply(FailableSupplier) method passing in a lambda with the same signature used for a common java.util.function.Supplier. Indeed FailableSupplier is just a java.util.function.Supplier with a throws Exception added to its get method.

For example, Try can be used to perform division on a user-defined input, without the need to do explicit exception-handling in all of the places that an exception might occur.

An important property of Try shown in the divide method of the MainExample class is its ability to pipeline (chain if you prefer) operations, catching exceptions along the way thanks to its flatMap method. If you are not a seasoned functional programming geek concepts such as flatMap/map might not be easy to grasp at first. However you’ll get used to them and, in the end, you’ll love them. Moreover you’re going to encounter these methods more and more often since some important Java 8 classes already implement them (e.g. java.util.Optional and java.util.stream.Stream. Anyway for the moment just take for granted that to pipeline more than two operations, say N, you just need to chain them by using N - 1 flatMap calls and a last call to map. E.g.: Suppose you have 3 variables (x, y and z) being of type Try and you just want to sum them up. The code you need for doing that is the following:

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x.flatMap(a -> y.flatMap(b -> z.map(c -> a + b + c)))

Apart from map and flatMap, Try has many other useful methods. See the TryTest class for a thorough coverage of all methods.

Usage example

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System.out.println("Integer division");
System.out.println("Enter the dividend press Return and then enter the divisor: ");
Scanner dividend = new Scanner(System.in);
Scanner divisor = new Scanner(System.in);

Try<Integer> num = Try.apply(dividend::nextInt);
Try<Integer> denom = Try.apply(divisor::nextInt);

Try<Integer> result = num.flatMap(x -> denom.map(y -> x / y));
Try<String> resultTryStr = result.map(i -> "The result of division is: " + i);
String resultStr = resultTryStr.getOrElse("You must've divided by zero or entered something that's not an Int. Try again!");
System.out.println(resultStr);

In the previous example if you enter two valid integers with the second one–the divisor–being different from zero then the code prints out The result of division is: $RESULT, where $RESULT is the division between the first and the second number. On the other hand, if you either enter non valid integers–such as a string–or the second number is zero then you’ll get the message You must've divided by zero or entered something that's not an Int. Try again! printed out.

Anyway, as I already said, see the TryTest class for a thorough coverage of all methods.

Final Notes

Any criticism/suggestion is more than welcome!

Source code: Try-Success-Failure API for Java

Combine JavaScript and a Template Engine for Flexible Web Apps

Combine the power and simplicity of the TrimPath template engine with JavaScript and Ajax to develop next-generation web applications.

Template engines (TEs) can be very useful in web development scenarios where you need to generate and format text automatically according to specific processing rules. These engines can also help you build your applications based on the Model-View-Controller (MVC) pattern, making them more robust and maintainable than applications based on spaghetti code. Most programming languages provide built-in or third-party TEs. Java, for example, has Velocity and FreeMarker, among others. For PHP, Smarty is the most used TE.

Go to developer.com to read the rest of the article: Combine JavaScript and a Template Engine for Flexible Web Apps.

Script.aculo.us Controls: Do Your Web Users a Favor

Learn how to integrate Script.aculo.us web controls into your web applications to make the end-user experience more pleasant.

In a previous DevX article, I wrote about the JavaScript framework Prototype, which aims to ease the development of dynamic web applications. That article ended with just a few words about Script.aculo.us, which is a fantastic UI library based on Prototype. This article gives Script.aculo.us its due by examining the web controls it provides: autocompleters, sliders, and in-place editors.

Script.aculo.us is a pretty big library, so no single article can cover it completely. However, the knowledge you will gain will enable you to leverage the power of Script.aculo.us web controls to improve the end-user experience.

Go to DevX to read the rest of the article: Script.aculo.us Controls: Do Your Web Users a Favor.

Will Your Next Web Application Be a Google Gadget?

Learn how to leverage the Google Gadget API to make your web applications more reachable. As an example, you’ll see how to build a real-world gadget that fetches and displays the DevX feeds.

Since AJAX first appeared, developer interest in JavaScript has increased exponentially—and toolsets for building functionality in JavaScript have burgeoned as well. One of the latest tools for JavaScript-related development is Google Gadgets. Many web sites already provide gadgets built with the Google tools, and you can, too. This article shows you how to develop a Google Gadget that fetches DevX RSS feeds and displays them to the user.

Go to DevX to read the rest of the article: Will Your Next Web Application Be a Google Gadget?.

The Productivity Perks Behind Prototype’s Popularity

Learn how to leverage the popular Prototype JavaScript framework to speed up your AJAX-based development. You’ll see how to use JavaScript in an object-oriented way.

The Prototype home page claims that “Prototype is a JavaScript framework that aims to ease the development of dynamic Web applications.” By using many object-oriented paradigms such as class-driven development and inheritance to enable JavaScript application development, Prototype completely accomplishes its aim. In fact, this framework contains a lot of useful functionalities, which you cannot help but apply to every application you develop once you’ve started using them.

Prototype’s main claim to fame is its rich AJAX (Asynchronous JavaScript and XML) library, which simplifies the now popular Web development technique for making non-blocking calls to the server through JavaScript. (It topped the recent Ajaxian.com 2006 Survey as the most popular AJAX framework.) Of course, everything you can do with Prototype you also can do using plain JavaScript—upon which Prototype is built. But why would you want to complicate your life? For example, which would you rather type to accomplish the same task: document.getElementById(“myElement”) or $(“myElement”)? If you answered $(“myElement”), then read the remainder of this article for an in-depth discussion of some useful Prototype functions—$ is just one.

Go to DevX to read the rest of the article: The Productivity Perks Behind Prototype’s Popularity.

Speed Up Your AJAX-based Apps With JSON

Find out how to improve your Web application performance by leveraging AJAX and JSON. In particular, you’ll see the advantages of using JSON over XML as a lightweight JavaScript data-interchange format.

JSON stands for JavaScript Object Notation and is a lightweight data-interchange format. Because it is text-based it is easy for humans to read and write, and it has a regular syntax that’s easy to parse programmatically. JSON is basically a subset of JavaScript and, as you’ll see, is even easier to parse than XML.

Go to DevX to read the rest of the article: Speed Up Your AJAX-based Apps with JSON.

Build Brilliant Client/Server Apps With J2ME, PHP, and MySQL

In this article you’ll see how to make the three technologies work together by developing a simple and useful application that lets you update a multi-user blog and download the latest posts from it.

Write once, run anywhere. This Java motto seems to hold true even for mobile devices. Indeed, since it was born J2ME has had an ever-increasing impact on mobile development. Some may argue that J2ME is not as portable as other Java technologies, but even so, the result obtained on different mobile devices is, in most cases, still acceptable.

Go to DevX to read the rest of the article: Build Brilliant Client/Server Apps with J2ME, PHP, and MySQL.

Producing Professional MSDN-style Documentation With .NET and NDoc

Tired of trying to keep your documentation synchronized with the source? You can help alleviate the problem by creating professional documentation directly from the built-in XML documentation features of the C# language using the brilliant NDoc open-source application.

The .NET framework provides a nice way to insert XML documentation tags inside C# source code. These tags can then be extracted to an XML file, and using NDoc, transformed into fully-functional MSDN-style documentation.

Go to DevX to read the rest of the article: Producing Professional MSDN-style Documentation with .NET and NDoc.

Obtaining Wireless News With J2ME and PHP

J2ME and PHP are both powerful technologies used to develop applications for mobile devices and Web applications, respectively. In this article, you’ll see how to make them work together by developing a simple and useful application that retrieves the latest news from the Web and displays that on a mobile device.

The Java 2 Platform, Micro Edition (J2ME) is targeted at cell phones, smart cards, pagers, and other consumer devices. J2ME technology consists of a virtual machine and a set of APIs suitable for tailored runtime environments for these devices. PHP, on the other hand, is a widely used server-based language to build Web applications. But these two radically different technologies work very well together. In this article you’ll see how they can interact via HTTP (Hyper Text Transfer Protocol). Of course, this is not intended to be a thorough explanation of how HTTP works—you only need to know that HTTP is a request/response protocol. That simply means that the client application performs a request and the server application returns a response.

Go to DevX to read the rest of the article: Obtaining Wireless News with J2ME and PHP.