Semantic blind spot in Ruby case statement

Semantic blind spot in Ruby case statement

Some time ago I’ve stumbled upon an article on case statements in Ruby. The author presents there an example of case statement with ranges:

case number
when (0..3)
  'low value'
when (4..7)
  'medium value'
when (8..10)
  'high value'
else
  'invalid value'
end

The ranges actually read well. I’d even write similar case statement myself. And yet an avid mutant user may tell you there’s a “flaw” hidden there. Can you spot it?

Let’s pick one branch of that conditional for a closer look. Be it this one:

when (8..10) then 'high value'

Assume we also have 100% line coverage, reported by simplecov for that example. That’s rather easy to achieve:

def test_high
  case_when = lambda do |number|
    case number
    when (0..3)
      'low value'
    when (4..7)
      'medium value'
    when (8..10)
      'high value'
    else
      'invalid value'
    end
  end

  assert_equal 'high value', case_when.call(8)
  assert_equal 'high value', case_when.call(9)
  assert_equal 'high value', case_when.call(10)
end

What would mutant report here, given that 100% line coverage?

Coverage:        88.00%

That drop of the coverage (as mutant sees it) can be attributed to conditions being shadowed by earlier branches. It doesn’t really matter if the lower-bound of the range in condition is a bit off. The test still pass.

   case number
   when (0..3)
     "low value"
   when (4..7)
     "medium value"
-  when (8..10)
+  when (1..10)
     "high value"
   else
     "invalid value"
   end

The mutant gem is a way to automatically detect such semantic gaps:

An automated code review tool, with a side effect of producing semantic code coverage metrics.

Think of mutant as an expert developer that simplifies your code while making sure that all tests pass.

You can perform such code mutations in small scale without mutant — manually. It’s a matter of changing the lower-bound in range condition and re-rerunning the tests.

def test_high
  case_when = lambda do |number|
    case number
    when (0..3)
      'low value'
    when (0..7)
      'medium value'
    when (0..10)
      'high value'
    else
      'invalid value'
    end
  end

  assert_equal 'high value', case_when.call(8)
  assert_equal 'high value', case_when.call(9)
  assert_equal 'high value', case_when.call(10)
end

So what’s the semantically reduced case statement, that passes under mutant’s scrutiny?

It appears to be this one:

case
when number < 0
  'invalid value'
when number <= 3
  'low value'
when number <= 7
  'medium value'
when number <= 10
  'high value'
else
  'invalid value'
end

Would you call it a good middle ground? Is the case statement still useful in this form?

You can find the code used in this post on my github.

Happy mutation testing!

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