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Inline Functions and Implementing Algebraic Data Types

Inline Functions and Implementing Algebraic Data Types

Learn about the inline functions in Kotlin and how to implement Algebraic Data Types.

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We can think of inline functions as instructions for the compiler to copy and paste our code. Each time the compiler sees a call to a function marked with the inline keyword, it will replace the call with the concrete function body.

It makes sense to use the inline function if it’s a higher-order function that receives a lambda as one of its arguments. This is the most common use case where we’d want to use inline.

Let’s look at such a higher-order function and see what pseudocode the compiler will output.

First, here is the function definition:

inline fun logBeforeAfter(block: () -> String) {
    println("Before")
    println(block())
    println("After")
}
Log before and after executing a block of code

Here, we pass a lambda, or a block, to our function. This block simply returns the word "Inlining" as a String:

logBeforeAfter { "Inlining" }

If we were to view the Java equivalent of the decompiled bytecode, we would see that there’s no call to our makesSense function at all. Instead, we’d see the following:

String var1 = "Before"; <- Inline function call
System.out.println(var1);
var1 = "Inlining";
System.out.println(var1);
var1 = "After";
System.out.println(var1);
Prints three strings, demonstrating inline function call

Since the inline function is a copy/paste of our code, we shouldn’t use it if we have more than a few lines of code. It would be more efficient to have it as a regular function. But if we have ...