Unroll provides an experimental @unroll annotation that can be applied to methods, classes, and constructors. @unroll generates unrolled/telescoping versions of the method, starting from the annotated parameter, which are simple forwarders to the primary method or constructor implementation. This allows you to maintain binary compatibility when adding a new default parameter, without the boilerplate of manually defining forwarder methods.

See the following PRs that demonstrate the usage of @unroll and the binary-compatibility boilerplate that can be saved:

• https://github.com/com-lihaoyi/mainargs/pull/113/files
• https://github.com/com-lihaoyi/mill/pull/3008/files
• https://github.com/com-lihaoyi/upickle/pull/555/files

In the past, evolving code in Scala while maintaining binary compatibility was a pain. You couldn't use default parameters, you couldn't use case classes. Many people fell back to Java-style builder patterns and factories with .withFoo everywhere to maintain binary compatibility. Or you would tediously define tons of binary compatibility stub methods that just copy-paste the original signature and forward the call to the new implementation.

In effect, you often gave up everything that made Scala nice to read and write, because the alternative was worse: every time you added a new parameter to a method, even though it has a default value, all your users would have to recompile all their code. And all their users would need to re-compile all their code, transitively. And so library maintainers would suffer so their users could have a smooth upgrading experience.

With @unroll, none of this is a problem anymore. You can add new parameters where-ever you like: method defs, classes, case classes, etc. As long as the new parameter has a default value, you can @unroll it to generate the binary-compatibility stub forwarder method automatically. Happy library maintainers, happy users, everyone is happy!

See this original discussion for more context:

• https://contributors.scala-lang.org/t/can-we-make-adding-a-parameter-with-a-default-value-binary-compatible

import scala.annotation.unroll

object Unrolled{
   def foo(s: String, n: Int = 1, @unroll b: Boolean = true, l: Long = 0) = s + n + b + l
}

Unrolls to:

import scala.annotation.unroll

object Unrolled{
   def foo(s: String, n: Int = 1, @unroll b: Boolean = true, l: Long = 0) = s + n + b + l

   def foo(s: String, n: Int, b: Boolean) = foo(s, n, b, 0)
   def foo(s: String, n: Int) = foo(s, n, true, 0)
}

import scala.annotation.unroll

class Unrolled(s: String, n: Int = 1, @unroll b: Boolean = true, l: Long = 0){
   def foo = s + n + b + l
}

Unrolls to:

import scala.annotation.unroll

class Unrolled(s: String, n: Int = 1, @unroll b: Boolean = true, l: Long = 0){
   def foo = s + n + b + l

   def this(s: String, n: Int, b: Boolean) = this(s, n, b, 0)
   def this(s: String, n: Int) = this(s, n, true, 0)
}

import scala.annotation.unroll

class Unrolled() {
   var foo = ""

   def this(s: String, n: Int = 1, @unroll b: Boolean = true, l: Long = 0) = {
      this()
      foo = s + n + b + l
   }
}

Unrolls to:

import scala.annotation.unroll

class Unrolled() {
   var foo = ""

   def this(s: String, n: Int = 1, @unroll b: Boolean = true, l: Long = 0) = {
      this()
      foo = s + n + b + l
   }

   def this(s: String, n: Int, b: Boolean) = this(s, n, b, 0)
   def this(s: String, n: Int) = this(s, n, true, 0)
}

import scala.annotation.unroll

case class Unrolled(s: String, n: Int = 1, @unroll b: Boolean = true){
  def foo = s + n + b
}

Unrolls to:

import scala.annotation.unroll

case class Unrolled(s: String, n: Int = 1, @unroll b: Boolean = true, l: Long = 0L){
   def this(s: String, n: Int) = this(s, n, true, 0L)
   def this(s: String, n: Int, b: Boolean) = this(s, n, b, 0L)
   
   def copy(s: String, n: Int) = copy(s, n, true, 0L)
   def copy(s: String, n: Int, b: Boolean) = copy(s, n, b, 0L)
   
   def foo = s + n + b
}
object Unrolled{
   def apply(s: String, n: Int) = apply(s, n, true, 0L)
   def apply(s: String, n: Int, b: Boolean) = apply(s, n, b, , 0L)
}

import scala.annotation.unroll

trait Unrolled{
  def foo(s: String, n: Int = 1, @unroll b: Boolean = true): String
}

object Unrolled extends Unrolled{
  def foo(s: String, n: Int = 1, b: Boolean = true) = s + n + b
}

Unrolls to:

trait Unrolled{
  def foo(s: String, n: Int = 1, @unroll b: Boolean = true): String
  def foo(s: String, n: Int = 1): String = foo(s, n, true)
}

object Unrolled extends Unrolled{
  def foo(s: String, n: Int = 1, b: Boolean = true) = s + n + b
}

Note that only the abstract method needs to be @unrolled, as the generated forwarder methods are concrete. The concrete implementation class or object does not need to @unroll its implementation method, as it only needs to implement the abstract @unrolled method and not the concrete forwarders.

1. Only the first parameter list of multi-parameter list methods (i.e. curried or taking implicits) can be unrolled. This is an implementation restriction that may be lifted with a bit of work

2. As unrolling generates synthetic forwarder methods for binary compatibility, it is possible for them to collide if your unrolled method has manually-defined overloads

3. Unrolled case classes are only fully binary compatible in Scala 3, though they are almost binary compatible in Scala 2. Direct calls to unapply are binary incompatible, but most common pattern matching of case classes goes through a different code path that is binary compatible. In practice this should be sufficient for 99% of use cases, but it does means that it is possible for code written as below to fail in Scala 2 if a new unrolled parameter is added to the case class Unrolled.

def foo(t: (String, Int)) = println(t)
Unrolled.unapply(unrolled).map(foo)

unapply is not a binary compatibility issue in Scala 3, even when called directly, due to Option-less Pattern Matching

Unroll is tested via a range of test-cases: classMethod, objectMethod, etc. These are organized in build.sc, to take advantage of the build system's ability to wire up compilation and classpaths

Each of these cases has three versions, v1 v2 v3, each of which has different numbers of default parameters

For each test-case, we have the following tests:

1. unroll[<scala-version>].tests[<test-case>]: Using java reflection to make sure the correct methods are generated in version v3 and are callable with the correct output

2. unroll[<scala-version>].tests[<test-case>].{v1,v2,v3}.test: Tests compiled against the respective version of a test-case and running against the same version

3. unroll[<scala-version>].tests[<test-case>].{v1v2,v2v3,v1v3}.test: Tests compiled an older version of a test-case but running against newer version. This simulates a downstream library compiled against an older version of an upstream library but running against a newer version, ensuring there is backwards binary compatibility

4. unroll[<scala-version>].tests[<test-case>].{v1,v2,v3}.mimaReportBinaryIssues: Running the Scala MiMa Migration Manager to check a newer version of test-case against an older version for binary compatibility

You can also run the following command to run all tests:

./mill -i -w "unroll[_].tests.__.run"         

This can be useful as a final sanity check, even though you usually want to run a subset of the tests specific to the scala-version and test-case you are interested in.

trait Upstream{ // v1
   def foo(s: String, n: Int = 1)
}

trait Upstream{ // v2
   // source
   def foo(s: String, n: Int = 1, @unroll b: Boolean = true)
   
   // generated
   def foo(s: String, n: Int = 1, b: Boolean = true) = foo(s, n)
   def foo(s: String, n: Int)
}

trait Upstream{ // v3
   // source
   def foo(s: String, n: Int = 1, @unroll b: Boolean = true, @unroll l: Long = 0)
   
   // generated
   def foo(s: String, n: Int = 1, b: Boolean = true, l: Long = 0) = fooDown(s, n, b)
   def foo(s: String, n: Int, b: Boolean) = foo(s, n)
   def foo(s: String, n: Int)
}

trait Downstream extends Upstream{ // v1 
   final def foo(s: String, n: Int = 1) = println(s + n)
}

trait Downstream extends Upstream{ // v2
   // source
   def foo(s: String, n: Int = 1, @unroll b: Boolean = true) = println(s + n + b)
   
   // generated
   def foo(s: String, n: Int = 1, b: Boolean = true) = println(s + n + b)
   def foo(s: String, n: Int) = foo(s, n, true)
}

trait Downstream extends Upstream{ // v3
   // source
   def foo(s: String, n: Int = 1, @unroll b: Boolean = true, @unroll l: Long = 0) = println(s + n + b + l)
   
   // generated
   def foo(s: String, n: Int = 1, b: Boolean = true, l: Long = 0) = println(s + n + b + l)
   def foo(s: String, n: Int, b: Boolean) = foo(s, n, b, 0)
   def foo(s: String, n: Int) = foo(s, n, true, 0)
}