Originally published at https://programming-elm.com on May 20, 2019.
Back in September 2017, I presented the talk “Solving the Boolean Identity Crisis” at ElmConf. The talk highlights the downsides of using booleans in Elm code and offers ways to write clearer, more maintainable code. This post and the next couple of posts will share what I explored in that presentation. You can preview what’s to come by watching my talk on YouTube.
In this post, you will see how boolean function arguments obscure the intent of code. Then, you will learn how to replace boolean arguments with Elm’s custom types to write more understandable code.
Look at this function call to understand the problem with boolean arguments.
We pass in a string argument
"OGG" and a boolean argument
True to a
bookFlight function. If you encountered this in an Elm codebase, you
might wonder what the boolean argument does.
Boolean arguments hide the intent of code. We don’t know the significance of the
True value here without looking up the definition of
bookFlight. The boolean
argument makes this code harder to understand, especially as a newcomer
learning the codebase.
Looking up the definition, we find this. (I use
... to signify irrelevant
The boolean argument is called
isPremium, so it means the booking customer has
a premium status. We use an if-else expression to branch on
False, we’re not certain what status this customer has. We have
to assume that the customer has a “regular” status because the code makes that
implicit. We’ve lost the explicit
This code will present future problems if we need more than one customer status.
For example, let’s say we need to introduce a new economy status. We could
introduce another boolean argument called
After checking if
True, we check if
Otherwise, the implicit customer status is economy.
Now, function calls will look like this.
That’s even more confusing. We could easily mix up the order of the boolean
arguments too and accidentally book a customer with the wrong status. Also, we
could easily pass in two
True arguments. A customer can’t have both premium
and regular status. We have to let the first boolean argument
precedence in the if-else expression to deal with this invalid argument
We can clean up the
bookFlight function by replacing the boolean arguments
with an Elm custom type. Instead of hiding statuses behind boolean values, let’s
make them explicit. We can easily encode each type of status like so.
We add a
CustomerStatus custom type with three values, or constructors. Each
value perfectly encodes each status,
We can update the
bookFlight function like so.
bookFlight function makes it clear how to handle each customer status
without implicit if-else branching. Additionally, the compiler ensures we handle
each status. In the previous version of
bookFlight with two boolean arguments,
nothing would prevent us from accidentally forgetting to handle the
isRegular branch. The compiler would accept this code.
If we forgot the
Regular branch in the version with the
the code would not compile.
Would result in this compiler error:
Custom types provide compiler safety along with explicit code. Now calls to
bookFlight declare the intent of code because we pass in the
If we ran into the above function, we would more easily understand what’s happening. We’re booking a flight for a premium customer. We’ve made the code clearer and more maintainable.
What You Learned
In this post, you learned how boolean arguments can make code confusing and
unmaintainable by hiding the intent of code. You saw how replacing boolean
arguments with custom type values created better, safer code. Try this technique
out on your own Elm project. Find a function that accepts a boolean argument and
see if you can make a custom type that more explicitly encodes the meaning of
that boolean argument when it’s
To learn more about how to build Elm applications effectively, grab a copy of my book Programming Elm from The Pragmatic Programmers.