Scala 3: inline keyword
In Scala 3, the inline keyword introduces powerful metaprogramming capabilities that allow for compile-time evaluation and optimization of code. This feature provides various benefits, including improved performance and reduced boilerplate code. Here’s a detailed explanation of how inline works, along with examples.
What is inline?
The inline keyword in Scala 3 can be used in two main contexts:
1. Inline Methods: Methods that are expanded at the call site, enabling optimizations and compile-time computations.
2. Inline Parameters: Parameters that can be evaluated at compile-time, allowing for greater flexibility and efficiency.
When a method is marked as inline, the Scala compiler will replace calls to that method with the method’s body at the call site. This can eliminate the overhead of method calls and enable further compile-time optimizations.
inline def square(x: Int): Int = x * x
def areaOfSquare(side: Int): Int = square(side)
In this example, the square method is marked as inline. When areaOfSquare is called, the square method call is replaced with its body:
def areaOfSquare(side: Int): Int = side * side
- Performance: Eliminates the overhead of method calls.
- Optimization: Allows the compiler to perform additional optimizations that wouldn’t be possible if the method were not inlined.
- Compile-time Evaluation: Enables certain computations to be done at compile-time, reducing runtime computation.
Inline parameters are parameters that can be evaluated at compile-time. They are often used in combination with inline methods to enable more flexible and efficient code. Example of Inline Parameters
inline def power(x: Int, inline n: Int): Int =
if n == 0 then 1
else x * power(x, n - 1)
def calculatePower(): Int = power(2, 3)
In this example, n is an inline parameter. The compiler can unroll the recursion at compile-time, resulting in more efficient code:
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def calculatePower(): Int = 2 2 2
Inline methods and parameters can be combined with inline conditionals for more sophisticated compile-time logic.
inline def max(x: Int, y: Int): Int =
inline if x > y then x else y
def findMax(): Int = max(5, 3)
In this example, the inline if allows the condition to be evaluated at compile-time:
def findMax(): Int = 5
Inline methods can be used to define macros, enabling complex compile-time code generation. This is a more advanced use of inline and can replace some of the use cases previously covered by macros in Scala 2.
Example of a Simple Macro
import scala.compiletime._
inline def assert(condition: => Boolean): Unit =
inline if condition then ()
else error("Assertion failed")
def checkCondition(): Unit = assert(2 + 2 == 4)
In this example, the assert method will be evaluated at compile-time. If the condition is not met, it will produce a compile-time error.
The inline keyword in Scala 3 introduces powerful capabilities for compile-time computation and optimization:
1. Inline Methods: Methods marked as inline are expanded at the call site, reducing overhead and enabling optimizations.
2. Inline Parameters: Parameters that are evaluated at compile-time, allowing for more efficient and flexible code.
3. Inline Conditionals: Allows for compile-time evaluation of conditionals, further optimizing the code.
4. Macros: Advanced use of inline methods for compile-time code generation, replacing some use cases of Scala 2 macros.
These features make Scala 3 a more powerful and efficient language, allowing developers to write high-performance, low-overhead code with advanced metaprogramming capabilities.