Skip to main content




A unit testing and regressing testing framework is builtin to the Rascal language


To write a test in Rascal, use the test modifier on a parametrized Function declaration:

test bool integerAdditionIsCommutative(int a, int b) {        
return a + b == b + a;

test bool integerMultiplicationIsCommutative(int a, int b)
= a * b == b * a;
  • ❶ a function header defines test parameters for which a few hundred examples will be randomly generated. The name of the function services as the name of the test
    • You can call a test function like any other function
    • Test functions are not to be overloaded. Use unique names for unique tests.
  • ❷ the function returns true or false as the "test", but you can also not throw an exception for success and Throw one for failure.
  • ❸ all sorts of Function are allowed.
  • For complex parameter types like Algebraic Data Types also random inputs will be generated that satisfy the grammar.
  • For functions with multiple parameters, sometimes the random code generator will provide structurally equal parameters for successive arguments if the types allow this, or even values shared in memory.
  • Also the "random" input generator may specialize towards patterns that are found in the bodies of the functions-under-test, to achieve coverage faster. In other words, neither coverage nor uniform randomness is guaranteed. Expect something in between.

Running Tests

This command will run all tested in the imported modules and also any locally defined tests:


Then there is the RascalJUnitTestRunner and RascalJUnitParallelRecursiveTestRunner Java classes which you can use to annotate your own test class which will then load the given test modules:

package org.rascalmpl.test.basic;

import org.junit.runner.RunWith;
import org.rascalmpl.test.infrastructure.RascalJUnitTestPrefix;
import org.rascalmpl.test.infrastructure.RascalJUnitTestRunner;

@RunWith(RascalJUnitTestRunner.class) ❶
@RascalJUnitTestPrefix("lang::rascal::tests::basic") ❷
public class RunRascalTestModules { }
  • ❶ selects the test runner (parallel or not)
  • ❷ selects the root package to search for modules and tests
  • Then use JUnit configuration in Eclipse, VScode or Maven to activate the tests at the right time.
  • Reporting goes through standard JUnit reporting, so this integrates well with IDEs and continuous integration systems like Jenkins and GitHub Actions.