Scientific Interface Definition Language (SIDL)
In order to support multi-language interoperability, Babel relies on the specification of interfaces in the Scientific Interface Definition Language (SIDL) (pronounced SIGH-dull). SIDL is similar to COM and CORBA IDLs, but was designed with an emphasis on scientific computing. Specifically, SIDL supports dynamic multi-dimensional arrays and has built-in complex numbers. SIDL also supports interface contracts, which define properties that must hold true during execution -- before and/or after method invocation -- when contract enforcement is enabled. It will acquire a set of directives to aid in the description of massively parallel distributed objects.
When it comes to deciding what programming idioms to support across all languages and which ones to reject, SIDL strikes a careful balance between minimalism and completeness. It is not a lowest common denominator solution. SIDL is minimal to keep the learning curve as low as possible. It is complete so developers do not feel constrained in how to express their solutions.
SIDL is object-oriented. Its object model closely resembles that of Java and Objective C. In this model there is single inheritance of implementation and multiple inheritance of interfaces. It supports the typical notions of virtual, static, and final methods. SIDL also provides a basic set of features by defining and implementing the basic types for interfaces, classes and exceptions. All types implicitly inherit from these basic types.
The most important concept to grasp about SIDL is that SIDL only defines a public interface that other programs may use to access your code. As a result, all methods defined as part of a SIDL file are public; if you do not want a method to be globally usable, simply do not define it in your SIDL file. Furthermore, all object and class data is implicitly private. There is no way to declare or define data in a SIDL file. Instead, any data required for your code should be declared in the implementation language files. This way, the languages that use your code through Babel may create your objects and pass them around just like any normal piece of data, but they may only access the data through the provided interface.
SIDL also has a complete set of fundamental data types, from booleans to double precision complex numbers. It also supports more sophisticated types such as enumerations, strings, objects, and dynamic multi-dimensional arrays.
SIDL is still a work in progress. Of particular research interest are directives that will be added for parallel distributed object interaction and features to specify computational quality of service semantics associated with the interfaces.