Illustration of the talk "Safety Nets: learn to code with confidence" thanks to Git history
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Safety Nets: learn to code with confidence

This repository contains the code illustrating the talk given by me (Christophe Philemotte) at RedDotRubyConference 2014 and 2014: Safety Nets: Learn to code with confidence.

Each commit corresponds to a step in the illustration and will allow you to reproduce it step by step.

A simple invoicing application

As illustration, we'll implement a simple invoicing application. This is a common use case that everybody should easily relate to. Please consider that it's not a real application. It's oversimplified as the goal is to support the concept of safety nets without digging too deep in each kind of. As consequence, some design could be certainly be improved.


A simple invoice

An invoice is composed of several items in a certain quantity and with a certain price. You can add an item to an invoice. You can get the total of an invoice.

Promote total local variable to member attribute

Then, we think it would be better to make total local variable a member attribute.

But the test fails. We miss something when doing the change.

  1) Failure:
Invoice#test_0003_returns a correct total even after updating a previously added item [spec/invoice_spec.rb:30]:
Expected: 105
  Actual: 195.0

Fix the failure

We miss to reset the total before calculate it. By doing it, the tests then pass again.

Static Analysis

Manage European VAT

Then, as the invoice application is mainly used by Europeans, we decide to manage VAT. We add then the necessary logic for: we need a country code and a VAT rate.


With Flog we can detect that our very naive implementation is complicated, i.e. it's hard to understand, then to change.

$ flog lib/invoice.rb 
    27.4: flog total
     5.5: flog/method average

    17.2: Invoice#total                    lib/invoice.rb:13

A quote

We need now to issue quote as some customers would like to know the price before purchasing. A quote is very similar to an invoice except that we set the total in advance and it doesn't have items.


Copy-pasting is acceptable when working on an implementation. Eventually, we need to get rid of any duplicates. It's something we can easily forget. Thanks to Flay, we stay aware of all the duplicates present in the codebase.

$ flay lib
Total score (lower is better) = 116

1) IDENTICAL code found in :if (mass*2 = 116)

A little refactoring

We decide to extract the calculation of the subtotal, before removing the duplicates. We do that by adding a dedicated private method. The tests are green.

But tests don't catch everything

In the new method, we did a few errors that we can detect with ruby -w:

$ ruby -w lib/invoice.rb 
lib/invoice.rb:41: warning: shadowing outer local variable - total
lib/invoice.rb:40: warning: assigned but unused variable - total

Rubocop can also detect those problems and even more:

$ rubocop lib/invoice.rb 
Inspecting 1 file


lib/invoice.rb:1:1: C: Missing top-level class documentation comment.
class Invoice
lib/invoice.rb:13:3: C: Method has too many lines. [14/10]
  def total
lib/invoice.rb:18:11: C: Use %w or %W for array of words.
    elsif ['IT','FR','NL','LU','DE'].include?(@country_code)
lib/invoice.rb:18:16: C: Space missing after comma.
    elsif ['IT','FR','NL','LU','DE'].include?(@country_code)
lib/invoice.rb:18:21: C: Space missing after comma.
    elsif ['IT','FR','NL','LU','DE'].include?(@country_code)
lib/invoice.rb:18:26: C: Space missing after comma.
    elsif ['IT','FR','NL','LU','DE'].include?(@country_code)
lib/invoice.rb:18:31: C: Space missing after comma.
    elsif ['IT','FR','NL','LU','DE'].include?(@country_code)
lib/invoice.rb:40:5: W: Useless assignment to variable - total.
    total = 0
lib/invoice.rb:41:26: W: Shadowing outer local variable - total.
    @items.reduce(0) do |total, item|
lib/invoice.rb:42:7: W: Useless assignment to variable - total. Use just operator +.
      total += item.price * item.quantity

1 file inspected, 10 offenses detected

Let's fix already some flaws detected by Rubocop

Thanks to Rubocop, we can make the code better. Less risk of bug.

$ rubocop lib/invoice.rb 
Inspecting 1 file


lib/invoice.rb:1:1: C: Missing top-level class documentation comment.
class Invoice
lib/invoice.rb:13:3: C: Method has too many lines. [14/10]
  def total

1 file inspected, 2 offenses detected

Code Review

Even if static analysis can attract our attention on sensible points in our codebase, they don't give you the solution.

However they can feed a Code Review which is a good time to spread knowledge and discuss design and refactoring.

Removing Duplicates

By extracting the calculation of the VAT rate in a dedicated module, we can remove the duplicates:

$ flay lib/
Total score (lower is better) = 0

Reducing Complexity

By extracting the test of European countries into a dedicate class, we can spread the complexity that was already reduced in the previous step:

34.7: flog total
 2.9: flog/method average

 7.0: Invoice#calculate_subtotal       lib/invoice.rb:28
 6.7: Invoice#initialize               lib/invoice.rb:5
 5.7: Invoice#total                    lib/invoice.rb:16
 4.8: Vat::rate                        lib/vat.rb:2

Reducing more

We can try to rearrange the condition, but in the end it doesn't improve the score:

New Version

$ flog -d lib/vat.rb 
     9.1: flog total
     3.0: flog/method average

     5.9: Vat::rate                        lib/vat.rb:2
     3.1:   branch
     1.4:   is_valid?
     1.2:   !
     1.2:   europe?
     1.1:   belgium?
     1.0:   assignment

Old Version:

4.8: Vat::rate                        lib/vat.rb:2
3.3:   branch
1.2:   is_valid?
1.1:   europe?
1.0:   belgium?
1.0:   assignment

Boolean expression could be argued as complex, as it is not straightforward. In the end, it depends on you. I find the last version clearer, but it's true the previous one has the benefits to explicit the cases.