14.2 Writing Tests with Test::Simple

The Test::Simple module is included with the Perl distribution, starting in Perl 5.8.^[7]

^[7] Older Perl versions back to 5.004_03 can install the same module from the CPAN.

Test::Simple automates the boring task of writing "ok 1", "ok 2", "ok 3", and so on, in your program. Test::Simple exports one subroutine, called (appropriately) ok. It's best illustrated by example. For the earlier code, you can rewrite it as:

use Test::Simple tests => 4;

ok(1 + 2 =  = 3, '1 + 2 =  = 3');
ok(2 * 4 =  = 8, '2 * 4 =  = 8');
my $divide = 5 / 3;
ok(abs($divide - 1.666667) < 0.001, '5 / 3 =  = (approx) 1.666667');
my $subtract = -3 + 3;
ok(($subtract eq "0" or $subtract eq "-0"), '-3 + 3 =  = 0');

Ahh. So much simpler. The use not only pulls the module in but also defines the number of tests. This generates the 1..4 header. Each ok test evaluates its first argument. If the argument is true, it prints the proper ok message. If not, it prints the proper not ok message. For this particular example, the output looks like:^[8]

^[8] Don't be misled when reading the mathematics of the output. The first number and the dash on each ok line are just labels; Perl isn't telling you that 1 - 1 + 2 = = 3!

1..4
ok 1 - 1 + 2 =  = 3
ok 2 - 2 * 4 =  = 8
ok 3 - 5 / 3 =  = (approx) 1.666667
ok 4 - -3 + 3 =  = 0

The ok N messages are followed with the labels given as the second parameters. This is great for identifying each test, especially because the numbers 1 through 4 don't appear in the original test anymore. The test harness ignores this information, unless you invoke make test with make test TEST_VERBOSE=1, in which case, the information is displayed for each test.

What if a test fails? If you change the first test to 1 + 2 = = 4, you get:

1..4
not ok 1 - 1 + 2 =  = 4
#     Failed test (1.t at line 4)
ok 2 - 2 * 4 =  = 8
ok 3 - 5 / 3 =  = (approx) 1.666667
ok 4 - -3 + 3 =  = 0
# Looks like you failed 1 tests of 4.

The ok 1 became not ok 1. But also notice the extra message indicating the failed test, including its file and line number. Messages preceded by a pound-sign comment marker are merely comments, and are (mostly) ignored by the test harness.

For many people, Test::Simple is simple enough to use for a wide range of tests. However, as your Perl hackery evolves, you'll want to step up to the next level of Perl testing hackery as well, with Test::More.