The Pitfall of Implicit Returns

Implicit returns are a feature in some languages. They have recently bitten me, so here's my opinion.

Statements, Expressions, and Returns

Before diving into implicit returns, we must explain two programming concepts influencing them. A lot of literature is available on the subject, so I'll paraphrase one of the existing definitions:

An expression usually refers to a piece of code that can be evaluated to a value. In most programming languages, there are typically three different types of expressions: arithmetic, character, and logical.

A statement refers to a piece of code that executes a specific instruction or tells the computer to complete a task.

— Expression vs. Statement

Here's a Kotlin snippet:

val y = 10                //1
val x = 2                 //1

x + y                     //2

println(x)                //1

1. Statement, executes the assignment "task"
2. Expression, evaluates to a value, e.g., 12

Functions may or may not return a value. When they do, they use the return keyword in most programming languages. It's a statement that needs an expression.

In Kotlin, it translates to the following:

fun hello(who: String): String {
    return "Hello $who"
}

In this regard, Kotlin is similar to other programming languages with C-like syntax.

Implicit Returns

A couple of programming languages add the idea of implicit returns: Kotlin, Rust, Scala, and Ruby are the ones I know about; each has different quirks.

I'm most familiar with Kotlin: you can omit the return keyword when you switch the syntax from a block body to an expression body. With the latter, you can rewrite the above code as the following:

fun hello(who: String): String = "Hello $who"

Rust also allows implicit returns with a slightly different syntax.

fn hello(who: &str) -> String {
    return "Hello ".to_owned() + who;          //1
}

fn hello_implicit(who: &str) -> String {
    "Hello ".to_owned() + who                  //2
}

1. Explicit return
2. Transform the statement in expression by removing the trailing semicolon — implicit return

Let's continue with Kotlin. The expression doesn't need to be a one-liner. You can use more complex expressions:

fun hello(who: String?): String =
    if (who == null) "Hello world"
    else "Hello $who"

The Pitfall

I was writing code lately, and I produced something akin to this snippet:

enum class Constant {
    Foo, Bar, Baz
}


fun oops(constant: Constant): String = when (constant) {
    Constant.Foo -> "Foo"
    else -> {
        if (constant == Constant.Bar) "Bar"
        "Baz"
    }
}

Can you spot the bug?

Let's use the function to make it clear:

fun main() {
    println(oops(Constant.Foo))
    println(oops(Constant.Bar))
    println(oops(Constant.Baz))
}

The results are:

Foo
Baz
Baz

The explanation is relatively straightforward. if (constant == Constant.Bar) "Bar" does nothing. The following line evaluates to "Bar"; it implicitly returns the expression. To fix the bug, we need to add an else to transform the block into an expression:

if (constant == Constant.Bar) "Bar"
else "Baz"

Note that for simpler expressions, the compiler is smart enough to abort with an error:

fun oops(constant: Constant): String =
    if (constant == Constant.Bar) "Bar"      //1
    "Baz"

1. 'if' must have both main and 'else' branches if used as an expression

Conclusion

Implicit return is a powerful syntactic sugar that allows for more concise code. However, concise code doesn't necessarily imply being better code. I firmly believe that explicit code is more maintainable in most situations.

In this case, I was tricked by my code!

Beware of implicit returns.

Go Further

• Expression vs. Statement
• Rust, Ruby, and the Art of Implicit Returns
• To be clear, Rust does not have "implicit returns"
• On the merits of verbosity and the flaws of expressiveness