Guard Clauses and the Single Responsibility Principle (SRP)

Question

Currently reading the fantastic book 'Practical Object Orientated Design In Ruby' and working through a simple kata to really practice some of the principals it discusses.

I have a method that does the following:

def release_bike
  capacity_empty_error if empty?
  bike
end

This 'releases a bike object' from my DockingStation class (I am aware that the bike object should be an argument, but for the time being I just want the method to return the bike object, not remove it).

The #capacity_empty_error does the following:

def capacity_empty_error
  raise "Docking Station is empty."
end

And #empty? looks like:

def empty?
  bike == nil
end

Where bike is a wrapper method for my instance variable @bike. The current capacity of the the DockingStation class is therefore 1, since the code assumes it is full when #bike is set to something (I plan to add proper capacity later).

So hopefully that explains my code, please ask questions and suggest improvements if not, the question I want to ask is:

I feel that the guard clause line in #release_bike:

capacity_empty_error if empty?

is one responsibility for the method and the return of bike on the next line down is a second responsibility. Tis then obviously breaks SRP, but I cannot see how to use a guard clause except as adding it to an existing method as a second responsibility.

Can it be done? How have others achieved it?

I look at it this way: the contract for `release_bike` is, "I release the bike and return it". But it can't do that if the rack is empty. SRP actually requires that the method deal with that case itself, which it chooses to do by raising an exception... you are free to see it differently, of course. — Andy Jones, Apr 19 '16 at 08:04

score 1 · Accepted Answer · answered Apr 18 '16 at 11:20

What about moving functionality to other classes/modules?

module BikeChecker
  def capacity_empty_error
    raise "Docking Station is empty."
  end

  def release_bike
    capacity_empty_error if empty?
  end

  def empty?
    false
  end
end

class YourClass
  include BikeChecker

  def release_bike
    super
    bike
  end

  def empty?
    bike == nil
  end
end

You can give up the #empty? method from BikeChecker, i left it here for better understanding

Ahh! I have never seen the use of super, thank you very much Igor. — Joe, Apr 18 '16 at 20:59

Guard Clauses and the Single Responsibility Principle (SRP)

1 Answers1