Inline Methods
Scala 3 inline.
Inline Methods is a free Scala for Backend Engineering & Functional Programming lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Scala for Backend Engineering & Functional Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What Is inline?
The inline keyword in Scala 3 tells the compiler to expand a definition at the call site during compilation. It is the entry point to metaprogramming, before full macros.
An Inline Method
Mark a method inline and its body is substituted wherever it is called. This can remove call overhead and enable compile-time specialization.
inline def square(x: Int): Int = x * x
val r = square(5) // compiles to 5 * 5Inline Parameters
An inline parameter forces its argument to be inlined too. If the argument is a constant, the compiler can fold it into a literal.
inline def power(x: Int, inline n: Int): Int =
if (n == 0) 1 else x * power(x, n - 1)Constant Folding
When inline parameters are literals, branches and recursion can be fully resolved at compile time, leaving only the final expression in the bytecode.
inline def power(x: Int, inline n: Int): Int =
if (n == 0) 1 else x * power(x, n - 1)
val c = power(2, 3) // expands to 2 * 2 * 2 * 1inline if and inline match
Inside an inline method, inline if and inline match are resolved at compile time when the scrutinee is statically known, eliminating dead branches.
inline def label(inline b: Boolean): String =
inline if (b) "yes" else "no"Transparent Inline
transparent inline lets the method's return type be refined per call site — the compiler may infer a more specific type than the declared one.
transparent inline def first(inline b: Boolean): Any =
inline if (b) 1 else "x" // type can be Int or Stringcompiletime.error
The scala.compiletime package offers tools like error to fail compilation with a custom message — useful for validating inline arguments.
import scala.compiletime.error
inline def positive(inline n: Int): Int =
inline if (n <= 0) error("n must be positive") else nInline Values
An inline val is a true compile-time constant. It can be used where literals are required, such as in other inline computations.
inline val MaxRetries = 3
inline def retries: Int = MaxRetriesPerformance vs Code Size
Inlining trades code size for speed and compile-time guarantees. Overusing it on large bodies bloats bytecode, so reserve it for small, hot, or specialization-critical methods.
Runnable Without Inline
Here is the same squaring logic as an ordinary self-contained program. The inline version would produce identical results with the multiplication expanded at the call site.
object Main {
def square(x: Int): Int = x * x
def main(args: Array[String]): Unit = {
println(square(5)) // 25
}
}Inline Is the Foundation
Macros build on inline methods: an inline def can call a macro to generate code. Mastering inline first makes the macro layer much easier.
Quick Check
What does adding transparent to an inline method change?
Recap
You learned Scala 3 inlining:
inline defexpands at the call siteinlineparameters and constant foldinginline if/inline matchtransparent inline,inline val,compiletime.error
Next: macro basics.
Frequently asked questions
Is the “Inline Methods” lesson free?
Yes — the full text of “Inline Methods” is free to read here on the web, and the Scala for Backend Engineering & Functional Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Scala for Backend Engineering & Functional Programming course, upgrade to CoddyKit PRO.
What will I learn in “Inline Methods”?
Scala 3 inline. You practise Scala for Backend Engineering & Functional Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Scala for Backend Engineering & Functional Programming?
No prior experience is required. Scala for Backend Engineering & Functional Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Inline Methods” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Scala for Backend Engineering & Functional Programming lesson?
Yes. Every Scala for Backend Engineering & Functional Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Inline Methods
- Macros Basics
- Quotes and Splices
- Practical Macros