Domain-Specific Languages: A Small Introduction

Domain-specific languages (DSLs), also known as micro- languages or little languages, are programming languages designed to focus on a particular domain. Well-known DSLs include regular expressions, markdown, extensible markup language (XML), and structured query language (SQL). General-purpose languages (GPLs) have a wider scope. They provide a set of processing capabilities applicable to different problem domains. Mainstream GPLs are Java, C/C++, Python, and Scala.

To better understand the differences between DSLs and GPLs, consider the following example. The C program- ming language is a GPL. It provides the basic forms for abstractions and computation. What happens if someone wants to define a matrix of integers in C? An array of pointers must be declared like the following:

int **matrix;

To access the values of the matrix, the programmer will have to write complex pointer arithmetic statements. If one attempts to implement an algorithm for the multiplication of matrices, a function must be defined that accepts the two matrices as parameters and returns the result.

int **multiply(int **m_first, int **m_sec);

More advanced languages such as C++ and Java pro- vide more advanced methods to create abstractions; thus there are classes and interfaces. A typical implementation of the matrix multiplication algorithm would have created a class named Matrix with a method called multiply. For example, in Java, the code would look like the following:

public class Matrix {
   public void mupltiply(Matrix m) { ... }
}

This approach has many benefits if compared to the C version. The domain abstraction, which is the matrix, is declared directly as a type. In addition, it also contains the method multiply, which is closer to the reality of the mathematical domain.

With modern programming languages, it is easy to create complex libraries that declare and implement the abstractions of specific domains, but there is a barrier; the syntax of the language must be always used.

Consider now octave or mathematica, a language created specifically to deal with this algorithm implementation. These DSLs are used massively for simulations and mathematical modelling. Does anyone consider mathematica’s language to develop a web server or a database management system? Those languages focus on the mathematical domain only. Outside it, they have no meaning. The languages of mathematica and octave are DSLs.

The rest of this entry is structured as follows; first, a brief glimpse on DSL advantages and disadvantages is presented, along with a basic terminology. Three popular DSLs are also presented, with small practical examples of their use. The next section emphasizes on DSL design and implementation patterns. Finally, the entry concludes with the analysis of various works on programming language embeddings and the basic elements on how all these meth- ods can be combined to enhance the overall DSL design and implementation process.

Rest of the entry can be found in the latest volume (2016) of Encyclopedia of Computer Science and Technology (Taylor & Francis)

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