Understanding something from the perspective of something else

2019-10-20

Kotlin's extension functions are pretty cool. I used them at work recently, for something that I think exemplifies why they are so powerful and is an example of a good time to use them.

The context

I was working in a monolithic codebase, and needed to call several methods on a class that was owned by another team.

An analogy is that I needed to call methods on a class representing a Freezer to decide if I should offer customers the option to buy ice cream.

In the (view presenter) class my team owned I wanted to do something like:

if (freezer.isIcecreamAvailable()) {
  showIceCreamOptions()
} else {
  hideIceCreamOptions()
}

I needed to call several already existing methods on Freezer to know if ice cream was available.

class FreezerService {

  fun getTemperature(): FreezerTemperature

  fun isInStock(item: Item): Boolean

  // other methods

}

---

The problem

I wanted a method like:

fun isIceCreamAvailable(freezer: Freezer): Boolean {
  return freezer.isInStock(Item.ICE_CREAM) &&
         freezer.getTemperature() == FreezerTemperature.VALID
}

I thought about adding to the Freezer class but it was already over bloated, shared by lots of teams, and only my team was interested in selling ice cream.

I wanted this method to allow me to understand the Freezer from the perspective of selling ice cream. But I didn't want to add it to my class as it shouldn't know about the details of the Freezer.

I thought about adding a new class, but it would basically be an adapter for the Freezer, and didn't make sense when the other option was to add extension functions.

---

The solution

I added some extension functions for the Freezer:

fun Freezer.isIceCreamAvailable(): Boolean {
  return freezer.isInStock(Item.ICE_CREAM) &&
         freezer.getTemperature() == FreezerTemperature.VALID
}

Which I can call like any other method of the Freezer class:

freezer.isIceCreamAvailable()

---

Although I think it would be easy to overuse extension functions, there are a few reasons I think this is appropriate usage:

• They are not changing any state, only checking conditions
• The extension functions depend on the state of the Freezer only + allow us to understand it's state

When I told my friend Federico what I was doing, he said "ah, you're understanding the freezer from the perspective of the ice cream seller". I really like this way of phrasing it - thanks Federico for this blog title!