Functional programming treats the function as its most fundamental unit of abstraction. In functional languages, functions are first-class citizens, which means they can be passed as arguments to other functions along with being normally evaluated to compute a value. On the other hand, the Java programming language belongs to the object-oriented paradigm, in which the fundamental unit of abstraction is the object, and values are computed by asking questions of objects by invoking their methods.
DJ mixes the functional and object-oriented paradigms by adding functions to the Java programming language as first-class citizens, letting us express programs in a succinct, functional style, without losing all the benefits of the object-oriented paradigm. Also, DJ is fully compatible with Java RMI, so using the power of functional abstraction we can drastically simplify the construction of type-safe mobile applications and middleware.
The functional abstractions of DJ allow a programmer to compose different small (or large) pieces of program functionality together in a type-safe way by expressing them in terms of frozen expressions. Consider the following canonical example that prints a message:
class Main {
public static void main(String[] args) throws Exception {
(-> void) hello = freeze() { System.out.println("Hello, world!); };
defrost(hello);
}
}
A more complicated example, using higher-order functions (functions that take functions as parameters) is shown below:
class Map {
public static void main(String[] args) throws Exception {
// Create the map function
((int -> int),int[] -> int[]) map = freeze((int -> int) f, int[] arr) {
int[] ret = new int[3];
for (int i = 0; i < arr.length; i++)
ret[i] = defrost(arr[i], f);
return ret;
};
// Function to increment a value
(int -> int) inc = freeze(int f) {
return ++f;
};
// Input data
int[] t = {1, 2, 3};
// Apply the function
int[] q = defrost(inc, t, map);
for (int i=0; i < q.length; i++)
System.out.println(q[i]);
// Yields "2,3,4"
}
}
DJ builds on the Polyglot Extensible Compiler Framework and is implemented by translating DJ source into code that can be compiled by the standard Java compiler. This allows DJ code to be executed by any standard Java Virtual Machine requiring no modifications, except the installation of a small run-time helper library. Key contributions include:
This contains all the reduction and typing rules for the language.
Currently only available for Linux/cygwin. DJ is released under the terms of the Lesser GNU GPL.
Please visit our SourceForge files page to download DJ.
polyglot.jar.polyglot.jar, java_cup.jar, JFlex.jar and bcel-5.1.jar into the lib subdirectory of the DJ source distribution.ant dj-jar to build a dj.jar file.$ ./djc Hello.dj $ java Hello