Declarative Programming Recipes

If you've been using an imperative programming language such as Java, C#, or JavaScript, then you should consider the benefits of declarative programming when designing your next application. In this blog, I will explore the benefits of the following technologies:

• XML Schema
  
• ISO Schematron
  
• XForms/XBL (XML Binding Language)
  
• XSLT 2.0
  
• XQuery
  
• Atom Syndication
  
• AtomPub
  

The basic difference between declarative programming languages and imperative programming languages such as C# and Java is that the former specify the “what” (the intent) as opposed to the “how” (the algorithm). The following are some reasons to consider this new paradigm:

• Declarative programming languages are accessible to many non-programmers
  
• It’s possible to create a solution that is completely declarative (no Java, C#, JavaScript, or AJAX code)
  
• It facilitates the Model-Driven Architecture (MDA) software design approach
  
• When the data or content is managed in XML, a declarative programming model is a superior and simpler alternative to a conventional approach based on Java Server Faces (JSF), Spring, and Hibernate. For example, you get a more robust and simpler data and business rules validation mechanism and you can forgo ORM mapping altogether. However, declarative and imperative programming can co-exist in the same solution.
  

The following are some recipes for taking advantage of this new paradigm:

• Define your data structures and types in the XML Schema.
  
• Use XML Schema's xsd:appinfo element to:
  
  
  
  • Capture metadata such such as XML to RDBMS mapping information
    
  • Capture business rules using inline ISO Schematron rules
    
  • Control the labels, alerts, and appearances of UI components such as XForms controls.
    
  
  
• Generate your UI by transforming the XML Schema into XForms with XSLT.
  
• Alternatively, store your XML schema and other XML-formatted metadata artifacts in a native XML database and use XQuery to extract and manipulate the metadata during development and at run-time.
  
• Use XBL for enabling custom UI controls for your XForms. As an example, you can integrate a rich text editor using XBL and Dojo.
  
• Use ISO Schematron for business rules validation either post form-submission or by using XSLT to inject schematron rules into the XForms directly with the xforms:bind element.
  
• Use the RESTful or AtomPub API to a native XQuery-enabled XML database (such as Exist) to CRUD (create, read, update, delete) the data.
  
• If you're using SOAP-based web services, use XForms to send SOAP requests and display SOAP responses as well.
  
• Leverage Atom syndication for pushing updates to data consumers. In general, when creating your application, think "Syndication Bus".
  

Many software architects and developers are not yet aware of the power of this new paradigm. However, I believe that rather than using only familiar development frameworks, it’s important to always evaluate emerging alternatives and select the best approach even it involves a learning curve.