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Where do conditionals come from?

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Where do conditionals come from?

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Given that we want to reduce the number of conditional branches in our code, I wonder whether it is possible to catalogue all of the reasons they exist, so that we might then in turn list a few ways to eliminate each category.

So to that end I’ve listed out those origins of conditionals that I could think of. Is this a fool’s errand? Possibly, but let’s give it a try anyway. Please comment if you can spot any that I have missed, and let’s see if we can grow a “complete” list…

  1. Checking a value returned to me from code I own
    This is the essence of the Null Check smell, and more generally Connascence of Meaning. For example, see this (intentionally bad) code from William C. Wake’s Refactoring Workbook:
    public class Report {
      public static void report(Writer out, List<Machine> machines, Robot robot) throws IOException
      {
        //...
    
        out.write("Robot");
        if (robot.location() != null)
          out.write(" location=" + robot.location().name());
    
        if (robot.bin() != null)
          out.write(" bin=" + robot.bin());
    
        //...
      }
    }
    
  2. Checking a value returned to me from code I do not own
    For example, in Rails projects we often see code of this kind:
    if User.exists?(:user_id => current_user.id)
      # ...
    else
      # ...
    end
    
  3. Checking a parameter passed to me by code I own
    Whenever I write a method that accepts a Boolean parameter, I open myself up to the Control Couple smell. Here’s an example from Piotr Solnica’s blog:
    def say(sentence, loud = false)
      if loud
        puts sentence.upcase
      else
        puts sentence
      end
    end
    
  4. Checking a parameter passed to me by code I do not own
    Here’s an example from the jQuery documentation:
    <script>
    var xTriggered = 0;
    $( "#target" ).keypress(function( event ) {
      if ( event.which == 13 ) {
         event.preventDefault();
      }
      xTriggered++;
      var msg = "Handler for .keypress() called " + xTriggered + " time(s).";
      $.print( msg, "html" );
      $.print( event );
    });
    </script>
    
  5. Checking my own state or attributes
    Here’s an (intentionally bad) example from Martin Fowler’s Refactoring:
    class Employee {
      private int _type;
      static final int ENGINEER = 0;
      static final int SALESMAN = 1;
      static final int MANAGER = 2;
    
      Employee (int type) {
        _type = type;
      }
    
      int payAmount() {
        switch (_type) {
          case ENGINEER:
            return _monthlySalary;
          case SALESMAN:
            return _monthlySalary + _commission;
          case MANAGER:
            return _monthlySalary + _bonus;
          default:
            throw new RuntimeException("Incorrect Employee");
        }
      }
    }
    
  6. Checking a value I set previously in this method
    For example:
    public String format(List&lt;String&gt; items) {
      StringBuffer s = new StringBuffer();
      boolean first = true;
      for (String item : items) {
        if (first)
          first = false;
        else
          s.append(", ");
        s.append(item);
      }
      return s.toString();
    }
    
  7. Checking the type of an exception thrown by code I own
    Here’s an example from the PHP language documentation:
    <?php
    function inverse($x) {
        if (!$x) {
            throw new Exception('Division by zero.');
        }
        return 1/$x;
    }
    
    try {
        echo inverse(5) . "\n";
        echo inverse(0) . "\n";
    } catch (Exception $e) {
        echo 'Caught exception: ',  $e->getMessage(), "\n";
    }
    
    // Continue execution
    echo "Hello World\n";
    ?>
    
  8. Checking the type of an exception thrown by code I do not own
    We often find code such as this in Rails controllers:
    def delete
      schedule_id = params[:scheduleId]
      begin
        Schedules.delete(schedule_id)
      rescue ActiveRecord::RecordNotFound
        render :json => "record not found"
      rescue ActiveRecord::ActiveRecordError
        # handle other ActiveRecord errors
      rescue # StandardError
        # handle most other errors
      rescue Exception
        # handle everything else
      end
      render :json => "ok"
    end
    

Is this list anything like complete? Can it ever be?

I wonder if a useful next step might be to write down some general strategies for removing each of these kinds of conditonal…?

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