There are so many great ideas in engineering we can take home and apply to our own lives. Today I will talk about one of them: lazy evaluation.

The Concept

Lazy evaluation is simple to state: when given a task, you don’t start doing it right away. Instead, you keep accumulating tasks until you see an end-goal. At that point, you work backwards to do just enough work.

It’s simple, but it’s counter-intuitive. You have to wrestle with it a bit to understand it. Let me give an example from Spark, which is the poster-child of lazy evaluation strategies.

Suppose you want to select some rows in a table, filter based on a condition, then save as a table. In Spark, you could say:

df.select("*").filter("foo > 50").saveAsTable("finalTable")

Let’s break it down.

  1. Note the first function call. When Spark sees df.select("*"), does it fetch everything from the table? No! It only saves a note, and continues to look for an end-goal.

  2. You then specify a condition to filter upon: still not an end goal, so it adds to its notes and continues to wait.

  3. Then, it sees an output call, the saveAsTable(). Ah, here comes the end-goal.

Spark now works backward: it sees a filter. Before that, it sees a select("*"). It can push the condition backwards to the select, so that rows that do not match can be discarded immediately at read time. Quite efficient.

Now imagine if Spark worked like traditional eager programs. You would run out of memory right at the select("*") step, and the job would fail.

In real life

Lazy evaluation is a very powerful idea. It gives you a framework to know what to do, in the face of unlimited information: It says, you should not be eager.

We live in an age where data is infinite. There is a multitude of courses, books, websites on any important topic, and there are new things coming out every day. You cannot eagerly learn everything out there. There’s not enough time, and even if you did, as an individual you’re a weak learner.1

Instead, the lazy evaluation recipe is as follows:

  1. You should ask yourself, what is your end goal? What do you need to know or do, based on that end-goal, and what can you stop doing?

  2. Out of all the things you could be doing, you should select (or at least prioritize) those that will take you closer to your end-goal.

  3. Further, you should minimize or eliminate tasks that do not lead to your end goal, or at least you should be aware that they could be wasted effort.

Of course, it’s not always possible to know what can or cannot lead into the output. But it’s important to keep this in our mind, so that we can limit the damage if we stray from the path, by stopping such work after a pre-determined point in time.

Two Examples

This situation frequently happens during a career move. Interviews are daunting because they may choose to ask anything, so you should be prepared for everything, right? And also learn all the new things since you left school?

Instead, a different strategy is to look at job descriptions for the role or companies you would like to go to. You build a list of skills you need to know, and continue to work backward on acquiring those skills from the best possible resource in a reasonable amount of time.

A similar situation can occur in research-oriented projects, where it is easy to lose track of the deliverable in the name of research or trying new ideas. Especially in machine learning, a complex model with say 78% accuracy is not that much better than a simpler model with 75% accuracy.

Not a New Idea

The idea of lazy evaluation is not new. Relational databases have been building query plans since 1980s. The concept is part of the functional programming repertoire since 1970. Operating systems do not load the entire program into memory: they load the relevant pages as your program executes, a concept known as lazy loading.

Amazingly for a technical idea, lazy evaluation seems to be well-known in self help literature: Steven Covey said in his famous Seven Habits book, “Begin with the end in mind”. One would hope he had an intuition about lazy evaluation.

Lazy evaluation is one of those flywheel ideas. When you apply it, it should create less wasted work. This should create more time, and in this time, you probably apply the same idea to more projects and deliver them well. This is the flywheel.

In any case, don’t attempt to do a select("*") on an infinite knowledgebase.

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1 This is a concept in machine learning, where a bunch of different models can often do better than any single model. It is known as an ensemble. I like to think of Kaggle.com as an ensemble of humans, often coming up with ensembles of learning models.