Exploration Through Example

Example-driven development, Agile testing, context-driven testing, Agile programming, Ruby, and other things of interest to Brian Marick

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Wed, 31 Aug 2005

Pacific Northwest Software Quality Conference

I've long had affection for PNSQC. It's a regional conference, but the only evidence of that is its size. The content is more like what you'd expect from a national conference. It's in Portland, Oregon, which is a little bit of a hotbed of Agile: Ward Cunningham, Jim Shore, Rebecca Wirfs-Brock, Steven Newton - all these people are Portland locals (though I suppose Ward may be completely gone now). Of the six keynote and invited speakers, four are pretty firmly associated with Agile: Mike Cohn, Esther Derby, Linda Rising, and me.

They have a PDF brochure.

October tenth through the twelfth.

## Posted at 16:39 in category /conferences [permalink] [top]

Static typing, again

Sometimes when demoing Ruby, I get the reaction that runtime typing is too error-prone. I'm not alone in responding by saying that unit tests will catch the bugs a static typechecker would, plus others. So why use both?

That's something of a flip response, and it's not really true. Here's an example I stumbled over while working on a chapter of Scripting for Testers.

Here's code that sets a variable:

start_date = month_before(Time.now)

Here's one of the methods that consume it:

def svn_log(subsystem, start_date)
  credentials = "--username notme --password not-the-real-one"
  timespan = "--revision 'HEAD:{#{start_date}}'"
  root = "https://svn.pragprog.com/Bookshelf/titles/BMSFT/Book/code/"

  `svn log #{credentials} #{timespan} #{root}/#{subsystem}`
end

Here's how I called that method in a test:

svn_log('inventory', '2005-08-01')

The test passed. All the methods were tested, and all the tests passed. But the program failed:

$ ruby churn.rb
subversion/clients/cmdline/main.c:832: (apr_err=205000)
svn: Syntax error in revision argument 'HEAD:{Wed Aug 03 12:51:38 CDT 2005}'
subversion/clients/cmdline/main.c:832: (apr_err=205000)
svn: Syntax error in revision argument 'HEAD:{Wed Aug 03 12:51:38 CDT 2005}'
Changes since 2005-08-03:
inventory (-1)
churn (-1)


The problem is that I unit-tested svn_log with a string in the format Subversion expects, but month_before returns a Ruby Time object. When svn_log gets one of those, it formats it into the wrong format, which makes Subversion complain.

The root cause of the bug was my cretinous choice of a variable name. I should not have named a variable expected to contain a Time with the name "date", especially not when the idea "date" is part of the usage domain. Nevertheless, static typing would have caught it while dynamic typing did not.

However, it seems to me that such errors will be caught if the whole-system tests exercise every call in the app. Suppose you're doing TDD starting from business-facing tests. Every call in the app would have to be traceable to a test whose failure motivated (perhaps very indirectly) that call, therefore every call in the app would be exercised by the complete suite (modulo some smallish background rate of human coding/design error), therefore there is no need for typechecking. But I'm perhaps being too flip.

P.S. Reviewers of Scripting: the example has been slightly tweaked for this posting.

## Posted at 16:17 in category /agile [permalink] [top]