data-clump.md

Data Clump

What It Looks Like

A group of variables are all passed around together or used together.

def distance(x1, y1, x2, y2)
  Math.sqrt((x1 - x2) ** 2 + (y1 - y2) ** 2)
end

In this code, x1 and y1 form one data clump—they represent a point in the Cartesian plane. x2 and y2 form another clump.

Why It Hurts

When a method takes many positional arguments, calls to the method become difficult to read. It's too easy to pass arguments in the wrong order, or reference the wrong parameter within the method implementation.

How To Fix It

Instead of extracting individual fields from an object to pass them to a function, use Preserve Whole Object. If there is no such object, use Introduce Parameter Object.

def distance(a, b)
  Math.sqrt((a.x - b.x) ** 2 + (a.y - b.y) ** 2)
end

Next Steps

Often, refactoring this smell away is just a stepping stone to a bigger refactor. After Introduce Parameter Object, you may notice that the code has Feature Envy and should be a method on the parameter object.

After a sequence of such refactorings, the example code above might look like this:

class Point < Struct.new(:x, :y)
  def distance(other)
    dx = x - other.x
    dy = y - other.y
    Math.sqrt(dx ** 2 + dy ** 2)
  end
end