Perl Hacks

Hack #61. Test with Specifications

Let the computer write your tests.

Writing tests is a great way to gain confidence in your code. Each test you write makes a tiny claim about what your code ought to do. When it passes, you have clear evidence to support the claim. If you write enough tests to make a cohesive suite, the tiny claims within the suite combine to imply a general claim that your code works properly.

There are times, however, when writing a suite of tests is the hard way to make a general claim. Sometimes, the claim you want to make seems so simple, yet the tests you have to write seem so voluminous. For these times, it would be nice to be able to turn the process around. Instead of writing tests to make a claim, why not make the claim outright and let the computer write the tests for you? That's the idea behind specification-based testing. The Test::LectroTest family of modules brings this idea to Perl.

The Hack

To make claims about your code, you define properties that say that your code must behave in particular ways for a general spectrum of conditions. Then LectroTest automatically gathers evidence to support or refute your claims by executing large, random test suites that it generates on the fly. When it finishes, it prints the results in standard TAP format, just as the other Perl testing tools do.

Suppose you need to test Perl's sqrt function, which you expect to compute the square root of its argument. The first step is to figure out what sqrt ought to do. From ...