Inline

Inline Definitions

inline is a new soft modifier that guarantees that a definition will be inlined at the point of use. Example:

object Config:
   inline val logging = false

object Logger:

   private var indent = 0

   inline def log[T](msg: String, indentMargin: =>Int)(op: => T): T =
      if Config.logging then
         println(s"${"  " * indent}start $msg")
         indent += indentMargin
         val result = op
         indent -= indentMargin
         println(s"${"  " * indent}$msg = $result")
         result
      else op
end Logger

The Config object contains a definition of the inline value logging. This means that logging is treated as a constant value, equivalent to its right-hand side false. The right-hand side of such an inline val must itself be a constant expression. Used in this way, inline is equivalent to Java and Scala 2's final. Note that final, meaning inlined constant, is still supported in Scala 3, but will be phased out.

The Logger object contains a definition of the inline method log. This method will always be inlined at the point of call.

In the inlined code, an if-then-else with a constant condition will be rewritten to its then- or else-part. Consequently, in the log method above the if Config.logging with Config.logging == true will get rewritten into its then-part.

Here's an example:

var indentSetting = 2

def factorial(n: BigInt): BigInt =
   log(s"factorial($n)", indentSetting) {
      if n == 0 then 1
      else n * factorial(n - 1)
   }

If Config.logging == false, this will be rewritten (simplified) to:

def factorial(n: BigInt): BigInt =
   if n == 0 then 1
   else n * factorial(n - 1)

As you notice, since neither msg or indentMargin were used, they do not appear in the generated code for factorial. Also note the body of our log method: the else- part reduces to just an op. In the generated code we do not generate any closures because we only refer to a by-name parameter once. Consequently, the code was inlined directly and the call was beta-reduced.

In the true case the code will be rewritten to:

def factorial(n: BigInt): BigInt =
   val msg = s"factorial($n)"
   println(s"${"  " * indent}start $msg")
   Logger.inline$indent_=(indent.+(indentSetting))
   val result =
      if n == 0 then 1
      else n * factorial(n - 1)
   Logger.inline$indent_=(indent.-(indentSetting))
   println(s"${"  " * indent}$msg = $result")
   result

Note that the by-value parameter msg is evaluated only once, per the usual Scala semantics, by binding the value and reusing the msg through the body of factorial. Also, note the special handling of the assignment to the private var indent. It is achieved by generating a setter method def inline$indent_= and calling it instead.

Recursive Inline Methods

Inline methods can be recursive. For instance, when called with a constant exponent n, the following method for power will be implemented by straight inline code without any loop or recursion.

inline def power(x: Double, n: Int): Double =
   if n == 0 then 1.0
   else if n == 1 then x
   else
      val y = power(x, n / 2)
      if n % 2 == 0 then y * y else y * y * x

power(expr, 10)
// translates to
//
//    val x = expr
//    val y1 = x * x   // ^2
//    val y2 = y1 * y1 // ^4
//    val y3 = y2 * x  // ^5
//    y3 * y3          // ^10

Parameters of inline methods can have an inline modifier as well. This means that actual arguments to these parameters will be inlined in the body of the inline def. inline parameters have call semantics equivalent to by-name parameters but allow for duplication of the code in the argument. It is usually useful when constant values need to be propagated to allow further optimizations/reductions.

The following example shows the difference in translation between by-value, by-name and inline parameters:

inline def funkyAssertEquals(actual: Double, expected: =>Double, inline delta: Double): Unit =
   if (actual - expected).abs > delta then
      throw new AssertionError(s"difference between ${expected} and ${actual} was larger than ${delta}")

funkyAssertEquals(computeActual(), computeExpected(), computeDelta())
// translates to
//
//    val actual = computeActual()
//    def expected = computeExpected()
//    if (actual - expected).abs > computeDelta() then
//      throw new AssertionError(s"difference between ${expected} and ${actual} was larger than ${computeDelta()}")

Rules for Overriding

Inline methods can override other non-inline methods. The rules are as follows:

1. If an inline method f implements or overrides another, non-inline method, the inline method can also be invoked at runtime. For instance, consider the scenario:

   abstract class A:
      def f: Int
      def g: Int = f
   
   class B extends A:
      inline def f = 22
      override inline def g = f + 11
   
   val b = new B
   val a: A = b
   // inlined invocatons
   assert(b.f == 22)
   assert(b.g == 33)
   // dynamic invocations
   assert(a.f == 22)
   assert(a.g == 33)
   

   The inlined invocations and the dynamically dispatched invocations give the same results.

2. Inline methods are effectively final.

3. Inline methods can also be abstract. An abstract inline method can be implemented only by other inline methods. It cannot be invoked directly:

   abstract class A:
      inline def f: Int
   
   object B extends A:
      inline def f: Int = 22
   
   B.f         // OK
   val a: A = B
   a.f         // error: cannot inline f in A.
   

Relationship to @inline

Scala 2 also defines a @inline annotation which is used as a hint for the backend to inline code. The inline modifier is a more powerful option:

• expansion is guaranteed instead of best effort,
• expansion happens in the frontend instead of in the backend and
• expansion also applies to recursive methods.

The definition of constant expression

Right-hand sides of inline values and of arguments for inline parameters must be constant expressions in the sense defined by the SLS §6.24, including platform-specific extensions such as constant folding of pure numeric computations.

An inline value must have a literal type such as 1 or true.

inline val four = 4
// equivalent to
inline val four: 4 = 4

It is also possible to have inline vals of types that do not have a syntax, such as Short(4).

trait InlineConstants:
   inline val myShort: Short

object Constants extends InlineConstants:
   inline val myShort/*: Short(4)*/ = 4

Transparent Inline Methods

Inline methods can additionally be declared transparent. This means that the return type of the inline method can be specialized to a more precise type upon expansion. Example:

class A
class B extends A:
   def m = true

transparent inline def choose(b: Boolean): A =
   if b then new A else new B

val obj1 = choose(true)  // static type is A
val obj2 = choose(false) // static type is B

// obj1.m // compile-time error: `m` is not defined on `A`
obj2.m    // OK

Here, the inline method choose returns an instance of either of the two types A or B. If choose had not been declared to be transparent, the result of its expansion would always be of type A, even though the computed value might be of the subtype B. The inline method is a "blackbox" in the sense that details of its implementation do not leak out. But if a transparent modifier is given, the expansion is the type of the expanded body. If the argument b is true, that type is A, otherwise it is B. Consequently, calling m on obj2 type-checks since obj2 has the same type as the expansion of choose(false), which is B. Transparent inline methods are "whitebox" in the sense that the type of an application of such a method can be more specialized than its declared return type, depending on how the method expands.

In the following example, we see how the return type of zero is specialized to the singleton type 0 permitting the addition to be ascribed with the correct type 1.

transparent inline def zero: Int = 0

val one: 1 = zero + 1

Transparent vs. non-transparent inline

As we already discussed, transparent inline methods may influence type checking at call site. Technically this implies that transparent inline methods must be expanded during type checking of the program. Other inline methods are inlined later after the program is fully typed.

For example, the following two functions will be typed the same way but will be inlined at different times.

inline def f1: T = ...
transparent inline def f2: T = (...): T

A noteworthy difference is the behavior of transparent inline given. If there is an error reported when inlining that definition, it will be considered as an implicit search mismatch and the search will continue. A transparent inline given can add a type ascription in its RHS (as in f2 from the previous example) to avoid the precise type but keep the search behavior. On the other hand, an inline given is taken as an implicit and then inlined after typing. Any error will be emitted as usual.

Inline Conditionals

An if-then-else expression whose condition is a constant expression can be simplified to the selected branch. Prefixing an if-then-else expression with inline enforces that the condition has to be a constant expression, and thus guarantees that the conditional will always simplify.

Example:

inline def update(delta: Int) =
   inline if delta >= 0 then increaseBy(delta)
   else decreaseBy(-delta)

A call update(22) would rewrite to increaseBy(22). But if update was called with a value that was not a compile-time constant, we would get a compile time error like the one below:

   |  inline if delta >= 0 then ???
   |  ^
   |  cannot reduce inline if
   |   its condition
   |     delta >= 0
   |   is not a constant value
   | This location is in code that was inlined at ...

In a transparent inline, an inline if will force the inlining of any inline definition in its condition during type checking.

Inline Matches

A match expression in the body of an inline method definition may be prefixed by the inline modifier. If there is enough static information to unambiguously take a branch, the expression is reduced to that branch and the type of the result is taken. If not, a compile-time error is raised that reports that the match cannot be reduced.

The example below defines an inline method with a single inline match expression that picks a case based on its static type:

transparent inline def g(x: Any): Any =
   inline x match
      case x: String => (x, x) // Tuple2[String, String](x, x)
      case x: Double => x

g(1.0d) // Has type 1.0d which is a subtype of Double
g("test") // Has type (String, String)

The scrutinee x is examined statically and the inline match is reduced accordingly returning the corresponding value (with the type specialized because g is declared transparent). This example performs a simple type test over the scrutinee. The type can have a richer structure like the simple ADT below. toInt matches the structure of a number in Church-encoding and computes the corresponding integer.

trait Nat
case object Zero extends Nat
case class Succ[N <: Nat](n: N) extends Nat

transparent inline def toInt(n: Nat): Int =
   inline n match
      case Zero     => 0
      case Succ(n1) => toInt(n1) + 1

inline val natTwo = toInt(Succ(Succ(Zero)))
val intTwo: 2 = natTwo

natTwo is inferred to have the singleton type 2.

The scala.compiletime Package

The scala.compiletime package contains helper definitions that provide support for compile time operations over values. They are described in the following.

constValue and constValueOpt

constValue is a function that produces the constant value represented by a type.

import scala.compiletime.constValue
import scala.compiletime.ops.int.S

transparent inline def toIntC[N]: Int =
   inline constValue[N] match
      case 0        => 0
      case _: S[n1] => 1 + toIntC[n1]

inline val ctwo = toIntC[2]

constValueOpt is the same as constValue, however returning an Option[T] enabling us to handle situations where a value is not present. Note that S is the type of the successor of some singleton type. For example the type S[1] is the singleton type 2.

erasedValue

So far we have seen inline methods that take terms (tuples and integers) as parameters. What if we want to base case distinctions on types instead? For instance, one would like to be able to write a function defaultValue, that, given a type T, returns optionally the default value of T, if it exists. We can already express this using rewrite match expressions and a simple helper function, scala.compiletime.erasedValue, which is defined as follows:

erased def erasedValue[T]: T = ???

The erasedValue function pretends to return a value of its type argument T. In fact, it would always raise a NotImplementedError exception when called. But the function can in fact never be called, since it is declared erased, so can only be used at compile-time during type checking.

Using erasedValue, we can then define defaultValue as follows:

import scala.compiletime.erasedValue

inline def defaultValue[T] =
   inline erasedValue[T] match
      case _: Byte    => Some(0: Byte)
      case _: Char    => Some(0: Char)
      case _: Short   => Some(0: Short)
      case _: Int     => Some(0)
      case _: Long    => Some(0L)
      case _: Float   => Some(0.0f)
      case _: Double  => Some(0.0d)
      case _: Boolean => Some(false)
      case _: Unit    => Some(())
      case _          => None

Then:

val dInt: Some[Int] = defaultValue[Int]
val dDouble: Some[Double] = defaultValue[Double]
val dBoolean: Some[Boolean] = defaultValue[Boolean]
val dAny: None.type = defaultValue[Any]

As another example, consider the type-level version of toInt below: given a type representing a Peano number, return the integer value corresponding to it. Consider the definitions of numbers as in the Inline Match section above. Here is how toIntT can be defined:

transparent inline def toIntT[N <: Nat]: Int =
   inline scala.compiletime.erasedValue[N] match
      case _: Zero.type => 0
      case _: Succ[n] => toIntT[n] + 1

inline val two = toIntT[Succ[Succ[Zero.type]]]

erasedValue is an erased method so it cannot be used and has no runtime behavior. Since toIntT performs static checks over the static type of N we can safely use it to scrutinize its return type (S[S[Z]] in this case).

error

The error method is used to produce user-defined compile errors during inline expansion. It has the following signature:

inline def error(inline msg: String): Nothing

If an inline expansion results in a call error(msgStr) the compiler produces an error message containing the given msgStr.

import scala.compiletime.{error, code}

inline def fail() =
   error("failed for a reason")

fail() // error: failed for a reason

or

inline def fail(p1: => Any) =
   error(code"failed on: $p1")

fail(identity("foo")) // error: failed on: identity("foo")

The scala.compiletime.ops package

The scala.compiletime.ops package contains types that provide support for primitive operations on singleton types. For example, scala.compiletime.ops.int.* provides support for multiplying two singleton Int types, and scala.compiletime.ops.boolean.&& for the conjunction of two Boolean types. When all arguments to a type in scala.compiletime.ops are singleton types, the compiler can evaluate the result of the operation.

import scala.compiletime.ops.int.*
import scala.compiletime.ops.boolean.*

val conjunction: true && true = true
val multiplication: 3 * 5 = 15

Many of these singleton operation types are meant to be used infix (as in SLS §3.2.10).

Since type aliases have the same precedence rules as their term-level equivalents, the operations compose with the expected precedence rules:

import scala.compiletime.ops.int.*
val x: 1 + 2 * 3 = 7

The operation types are located in packages named after the type of the left-hand side parameter: for instance, scala.compiletime.ops.int.+ represents addition of two numbers, while scala.compiletime.ops.string.+ represents string concatenation. To use both and distinguish the two types from each other, a match type can dispatch to the correct implementation:

import scala.compiletime.ops.*

import scala.annotation.infix

type +[X <: Int | String, Y <: Int | String] = (X, Y) match
   case (Int, Int) => int.+[X, Y]
   case (String, String) => string.+[X, Y]

val concat: "a" + "b" = "ab"
val addition: 1 + 1 = 2

Summoning Implicits Selectively

It is foreseen that many areas of typelevel programming can be done with rewrite methods instead of implicits. But sometimes implicits are unavoidable. The problem so far was that the Prolog-like programming style of implicit search becomes viral: Once some construct depends on implicit search it has to be written as a logic program itself. Consider for instance the problem of creating a TreeSet[T] or a HashSet[T] depending on whether T has an Ordering or not. We can create a set of implicit definitions like this:

trait SetFor[T, S <: Set[T]]

class LowPriority:
   implicit def hashSetFor[T]: SetFor[T, HashSet[T]] = ...

object SetsFor extends LowPriority:
   implicit def treeSetFor[T: Ordering]: SetFor[T, TreeSet[T]] = ...

Clearly, this is not pretty. Besides all the usual indirection of implicit search, we face the problem of rule prioritization where we have to ensure that treeSetFor takes priority over hashSetFor if the element type has an ordering. This is solved (clumsily) by putting hashSetFor in a superclass LowPriority of the object SetsFor where treeSetFor is defined. Maybe the boilerplate would still be acceptable if the crufty code could be contained. However, this is not the case. Every user of the abstraction has to be parameterized itself with a SetFor implicit. Considering the simple task "I want a TreeSet[T] if T has an ordering and a HashSet[T] otherwise", this seems like a lot of ceremony.

There are some proposals to improve the situation in specific areas, for instance by allowing more elaborate schemes to specify priorities. But they all keep the viral nature of implicit search programs based on logic programming.

By contrast, the new summonFrom construct makes implicit search available in a functional context. To solve the problem of creating the right set, one would use it as follows:

import scala.compiletime.summonFrom

inline def setFor[T]: Set[T] = summonFrom {
   case ord: Ordering[T] => new TreeSet[T](using ord)
   case _                => new HashSet[T]
}

A summonFrom call takes a pattern matching closure as argument. All patterns in the closure are type ascriptions of the form identifier : Type.

Patterns are tried in sequence. The first case with a pattern x: T such that an implicit value of type T can be summoned is chosen.

Alternatively, one can also use a pattern-bound given instance, which avoids the explicit using clause. For instance, setFor could also be formulated as follows:

import scala.compiletime.summonFrom

inline def setFor[T]: Set[T] = summonFrom {
   case given Ordering[T] => new TreeSet[T]
   case _                 => new HashSet[T]
}

summonFrom applications must be reduced at compile time.

Consequently, if we summon an Ordering[String] the code above will return a new instance of TreeSet[String].

summon[Ordering[String]]

println(setFor[String].getClass) // prints class scala.collection.immutable.TreeSet

Note summonFrom applications can raise ambiguity errors. Consider the following code with two givens in scope of type A. The pattern match in f will raise an ambiguity error of f is applied.

class A
given a1: A = new A
given a2: A = new A

inline def f: Any = summonFrom {
   case given _: A => ???  // error: ambiguous givens
}

summonInline

The shorthand summonInline provides a simple way to write a summon that is delayed until the call is inlined.

transparent inline def summonInline[T]: T = summonFrom {
   case t: T => t
}

Reference

For more information about the semantics of inline, see the Scala 2020: Semantics-preserving inlining for metaprogramming paper.

For more information about compiletime operation, see PR #4768, which explains how summonFrom's predecessor (implicit matches) can be used for typelevel programming and code specialization and PR #7201 which explains the new summonFrom syntax.