It's more than just every line. Devel::Cover tests every line, every branch (if/else, etc.), and every conditional (A or B or C).

In my view it's great for reminding one to test edge cases. How often do we write something along the lines of: 

some_function() or die;
my $variable ||= 42;

[download]

How many test suites actually test that the die happens, or test the case where the variable doesn't get set in advance? In using Devel::Cover just a little, I've been more thoughtful about where and how I sprinkle defensive logic and I've also made a point of testing to ensure the defenses actually hold in practice.

-xdg

postscript: my "test that die happens" comment is poorly phrased, probably a bad example, and justly critiqued below by belg4mit. As adrianh and stvn suggest, the coverage test is really testing the or part and whether some_function is falsifiable, not whether die works. If the code coverage tells us that the die phrase is never executed, it's a reminder that we're making an assumption (that some_function might return false), but not testing that assumption. In practice, the coverage test for some_function itself would probably show that there are untested failure cases that explain why some_function never returns false.

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In reply to Re^3: Lessons learned from getting to 100% with Devel::Cover by xdg
in thread Lessons learned from getting to 100% with Devel::Cover by leriksen

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