Slow-Auto, Inconvenient-Semi

escaping false dichotomy with sanely-automatic derivation

Mateusz Kubuszok

About me

breaking things in Scala for 10 years
a little bit of open source - including co-authoring Chimney for 8 years
blog at Kubuszok.com
niche Things you need to know about JVM (that matter in Scala) ebook

Agenda

what is a type class
what is type class derivation
automatic and semi-automatic derivation a’la Circe
semi-automatic derivation a’la Jsoniter
sanely-automatic derivation a’la Chimney
does it matter to a library users how these approach differ

Examples

https://github.com/MateuszKubuszok/derivation-benchmarks

Type class

interface
with type paremeters
whose implementation can be automatically provided based on their type only

trait Encoder[A] {
  def apply(a: A): Json // <-- JSON as data
}
object Encoder {
  given encodeString: Encoder[String] = ...
  given encodeInt: Encoder[Int] = ...
  given encodeDouble: Encoder[Double] = ...
}

extension [A](value: A) {
  def asJson(using encoder: Encoder[A]): Json = encoder(value)
}

"value".asJson // using Encoder.encodeString
1024.asJson // using Encoder.encodeInt
3.13.asJson // using Encoder.encodeDouble

What if nobody wrote the implementation explicitly for my type?

case class Address(value: String)
case class User(name: String, address: Address)

Address("Paper St. 19").asJson // ???
User("John Smith", Address("Paper St. 19")).asJson // ???

No given instance of type Encoder[Address] was found for parameter encoder of
method asJson in object ...
No given instance of type Encoder[User] was found for parameter encoder of
method asJson in object ...

Type class derivation

(If you don’t understand this diagram, you probably haven’t spend 600h on a topic that most sane people avoid.)

Type class derivation.

We’ll have several of these pictures, which you can study at your own pace at home.

For now what I want you to remember:

derivation is about picking the implementations for some parts of your type, and combining them together into the implementation for the whole type
for case class es, it means that you have to have an implementation for each field’s type
for sealed trait s and enum s, it means that you have to have implementation for each subtype
because it combines the implementations from bottom-up someone, usually, the library’s author have to provide implementations for the smallest blocks, usualy primitives
and, because there is no magic, someone has to define how these small blocks can be combined, usually that’s also the author of the library
and if that sounds confusing, it only because you haven’t spend way too much time on this subject

Maybe some example would help.

Derivation a’la Circe

trait Encoder[A] {
  def apply(a: A): Json // <-- JSON as data
}

extension [A](value: A) {
  def asJson(using encoder: Encoder[A]): Json = encoder(value)
}

case class Address(value: String)
case class User(name: String, address: Address)

Address("Paper St. 19")
// { "value": "Paper St. 19" }
User("John Smith", Address("Paper St. 19"))
// { "name": "John Smith", "address": { "value": "Paper St. 19" } }

import MagicImportOfSomethingThatCreatesEncoders.given

Address("Paper St. 19").asJson // generates Encoder[Address] on demand
User("John Smith", Address("Paper St. 19")).asJson // ditto but for User

import ImportOfSomethingThatLetsYouCreateEncoders.deriveEncoder

given addressEncoder: Encoder[Address] = deriveEncoder[Address]
given userEncoder: Encoder[User] = deriveEncoder[User]

Address("Paper St. 19").asJson // using addressEncoder
User("John Smith", Address("Paper St. 19")).asJson // using userEncoder

As a reminder, we have this Encoder, which should turn your case class into JSON.

It has this nice extension method.

And we want to encode these `case class`es.

The library’s author has some assumption, like for instance, that each case class should be encoded as JSON object. Each field’s name would turn into that object’s key, and value, should be encoded, with the encoder for its type.

Automatic derivation assumes that all the missing implementations, that you have not provided yourself have an automatic fallback, often enabled with an import. You add that import, fallback becomes available in the implicit scope, and everything works. (Sometimes, this is implemented in the compation objects and then it cannot be disabled).

Semi-automatic derivation assumes that you want to define these implicits yourself, but you don’t want to write their implementation. It gives you some method, which would give you a new implementation, and even if there is implicit of a demanded type in scope, it ignores it. (So that you won’t end up with cyclical dependeny in the initialization).

If you will not write thise implicits/givens yourself, you’ll keep on getting implicit not found.

But let’s take a look a bit closer.

Automatic derivation of Address

implicitly[Encoder[Address]] // <-- using Encoder[Address]

Semi-automatic derivation of Address

deriveEncoder[Address] // <-- creates new Encoder[Address]

Automatic derivation of User

implicitly[Encoder[User]] // <-- using Encoder[User]

Semi-automatic derivation of User

deriveEncoder[User] // <-- creates new Encoder[User]

OK, but where is the code?

Wouldn’t it be easier to understand with some examples?

1. We are focusing on user-side of the derivation story

2. Code is in the link

https://github.com/MateuszKubuszok/derivation-benchmarks

3. If you really need the derivation-internals-explanation-experience

Why people bother with semi-automatic derivation?

1. They want to make sure that they use the same implementation everywhere

2. "Speed"

// We're use Circe:
// trait Encoder[A] { ... } turns A -> Json
// trait Decoder[A] { ... } turns Json -> Either[Decoder.DecodingError, A]

case class Out(...) // <-- really big case class with nested case classes

// value -> Json -> value again
def roundTrip(out: Out): (Json, Either[Decoder.DecodingError, Out]) = {
  val json = out.asJson // <-- encode as Json using Encoder[Out]
  val parsed = json.as[Out] // <-- decode from Json using  Decoder[Out]
  json -> parsed
}

// Semi-automatic version will just have this:
implicit val in1Decoder: Decoder[In1] = deriveDecoder
implicit val in1Encoder: Encoder[In1] = deriveEncoder
implicit val in2Decoder: Decoder[In2] = deriveDecoder
implicit val in2Encoder: Encoder[In2] = deriveEncoder
implicit val in3Decoder: Decoder[In3] = deriveDecoder
implicit val in3Encoder: Encoder[In3] = deriveEncoder
implicit val in4Decoder: Decoder[In4] = deriveDecoder
implicit val in4Encoder: Encoder[In4] = deriveEncoder
implicit val in5Decoder: Decoder[In5] = deriveDecoder
implicit val in5Encoder: Encoder[In5] = deriveEncoder
implicit val outDecoder: Decoder[Out] = deriveDecoder
implicit val outEncoder: Encoder[Out] = deriveEncoder
// instead of automatic derivation import.

This shouldn’t be hard on compiler?

(less is better)

On Scala 2.13, not so bad. The compilation times are quite close, not deserving the bad press.

Of course, if we ignore the fact that both need at least 12 seconds on cold JVM to compile a single short file.

But on Scala 3 with automatic we have 46 seconds to compile a single file on cold JVM! On the other hand semi-auto works much faster!

What about runtime?

                     Scala 2   Scala 3  Units
compilation of      cold hot  cold hot
circeGenericAuto      14   4    46  16      s
circeGenericSemi      12   3    10   1      s
circeMagnoliaAuto     13   2    65  32      s
circeMagnoliaSemi     12   7    12   2      s
jsoniterScalaSanely    -   -     9   1      s
jsoniterScalaSemi     10   4     8   1      s

Scala 2.13.14

[info] Benchmark                          Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto    thrpt  10   7.319 ± 0.011  ops/ms
[info] JsonRoundTrips.circeGenericSemi    thrpt  10   6.775 ± 0.013  ops/ms

Scala 3.3.3

[info] Benchmark                            Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto     thrpt   10   0.490 ± 0.432  ops/ms
[info] JsonRoundTrips.circeGenericSemi     thrpt   10   4.607 ± 0.014  ops/ms

(more is better)

Auto vs Semi on Scala 2

PR #5649 - Faster compilation of inductive implicits (closed)
PR #6481 - Topic/inductive implicits 2.13.x (closed)
PR #6580 - Prune polymorphic implicits more aggressively (merged)
PR #7012 - Speed up implicit resolution by avoiding allocations when traversing TypeRefs in core (merged)
and more

             1) baseline - scalac 2.13.x  2) scalac 2.13.x with matchesPtInst
 HList Size
  50          4                            3
 100          7                            3
 150         15                            4
 200         28                            4
 250         48                            5
 300         81                            6
 350        126                            8
 400        189                           11
 450        322                           13
 500        405                           16         Compile time in seconds

Could something else improve performance?

Magnolia

alternative to Shapeless/Mirrors
boasts about:
- better API
- better performance
- better compilation times
- better error messages when derivation fail

Error messages

Semi-automatic derivation

case class Street(name: Either[String, Nothing]) // <-- should not be able to derive name
case class Address(street: Street)
case class User(name: String, address: Address)

implicit val streetEncoder: Encoder[Street] = deriveEncoder
implicit val addressEncoder: Encoder[Address] = deriveEncoder
implicit val userEncoder: Encoder[User] = deriveEncoder

user.asJson

Shapeless' errors

could not find Lazy implicit value of type DerivedAsObjectEncoder[Street]
   implicit val streetEncoder: Encoder[Street] = deriveEncoder
                                                 ^

Mirrors' errors

  implicit val streetEncoder: Encoder[Street] = deriveEncoder
                                                ^^^^^^^^^^^^^
Failed to find an instance of Encoder[Either[String, Nothing]]

Magnolia’s errors

magnolia: could not find Encoder.Typeclass for type Either[String,Nothing]
     in parameter 'name' of product type Street
   implicit val streetEncoder: Encoder[Street] = EncoderSemi.derived
                                                             ^

Automatic derivation

case class Street(name: Either[String, Nothing])
case class Address(street: Street)
case class User(name: String, address: Address)

user.asJson

Shapeless/Mirrors/Magnolia

could not find implicit value for parameter encoder: Encoder[User]
     user.asJson
          ^

Round trip (reminder)

// Out - the outerermost of a deep nested, nasty case class structure
def roundTrip(out: Out): (Json, Result[Out]) = {
  val json = out.asJson // encode
  val parsed = json.as[Out] // decode
  json -> parsed
}

(less is better)

Perhaps at the times of Scala 2.12 the difference was bigger, but it seems that the compilation times on Scala 2.13 are close.

The small spike on hot JVM might be an error.

But something weird happens on Scala 3, Magnolia is always worse than Mirrors! Why?

Because on Scala 3 it was implemented with Mirrors so it adds its own overhead on top of Mirrors.

How about benchmarks?

                     Scala 2   Scala 3  Units
compilation of      cold hot  cold hot
circeGenericAuto      14   4    46  16      s
circeGenericSemi      12   3    10   1      s
circeMagnoliaAuto     13   2    65  32      s
circeMagnoliaSemi     12   7    12   2      s
jsoniterScalaSanely    -   -     9   1      s
jsoniterScalaSemi     10   4     8   1      s

Scala 2.13.14

[info] Benchmark                          Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto    thrpt  10   7.319 ± 0.011  ops/ms
[info] JsonRoundTrips.circeGenericSemi    thrpt  10   6.775 ± 0.013  ops/ms
[info] JsonRoundTrips.circeMagnoliaAuto   thrpt  10   7.689 ± 0.013  ops/ms
[info] JsonRoundTrips.circeMagnoliaSemi   thrpt  10   7.838 ± 0.013  ops/ms

Scala 3.3.3

[info] Benchmark                            Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto     thrpt   10   0.490 ± 0.432  ops/ms
[info] JsonRoundTrips.circeGenericSemi     thrpt   10   4.607 ± 0.014  ops/ms
[info] JsonRoundTrips.circeMagnoliaAuto    thrpt   10   0.077 ± 0.039  ops/ms
[info] JsonRoundTrips.circeMagnoliaSemi    thrpt   10   5.590 ± 0.013  ops/ms

(more is better)

Shapeless/Mirrors/Magnolia - different APIs, same approach.

Did anyone try something else?

Jsoniter Scala

prioritizes performance
no automatic derivation
no need to derive intermediate instances

How?

// Yes, only 1 codec, no need to manually derive implicits for nested cases
implicit val outCodec: JsonValueCodec[Out] =
  JsonCodecMaker.make(CodecMakerConfig.withAllowRecursiveTypes(true))

def roundTrip(out: Out): (String, Either[Throwable, Out]) = {
  val str = writeToString(out)
  val parsed = scala.util.Try(readFromString(str)).toEither
  str -> parsed
}

Recursive semi-automatic derivation

Recursive semi-automatic derivation

delegates everything to implicit search
types supported OOTB are handled via implicits in companion object

use implicit search only for overrides
types supported OOTB are handled by macro, implicit scope is empty by default

OK, but what does this gibberish mean for users?

(less is better)

Jsonier Scala beaten all of the other approaches: Shapeless, Mirrors, Magnolia, whether automatic or semiautomatic. 10 seconds on cold JVM going down to 4 seconds on Scala 2.13. 8 seconds down to 1 on Scala 3. And we are only taking about the compilation time, not the actual performance!

                     Scala 2   Scala 3  Units
compilation of      cold hot  cold hot
circeGenericAuto      14   4    46  16      s
circeGenericSemi      12   3    10   1      s
circeMagnoliaAuto     13   2    65  32      s
circeMagnoliaSemi     12   7    12   2      s
jsoniterScalaSanely    -   -     9   1      s
jsoniterScalaSemi     10   4     8   1      s

Scala 2.13.14

[info] Benchmark                          Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto    thrpt  10   7.319 ± 0.011  ops/ms
[info] JsonRoundTrips.circeGenericSemi    thrpt  10   6.775 ± 0.013  ops/ms
[info] JsonRoundTrips.circeMagnoliaAuto   thrpt  10   7.689 ± 0.013  ops/ms
[info] JsonRoundTrips.circeMagnoliaSemi   thrpt  10   7.838 ± 0.013  ops/ms
[info] JsonRoundTrips.jsoniterScalaSemi   thrpt  10  20.081 ± 0.151  ops/ms

Scala 3.3.3

[info] Benchmark                            Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto     thrpt   10   0.490 ± 0.432  ops/ms
[info] JsonRoundTrips.circeGenericSemi     thrpt   10   4.607 ± 0.014  ops/ms
[info] JsonRoundTrips.circeMagnoliaAuto    thrpt   10   0.077 ± 0.039  ops/ms
[info] JsonRoundTrips.circeMagnoliaSemi    thrpt   10   5.590 ± 0.013  ops/ms
[info] JsonRoundTrips.jsoniterScalaSemi    thrpt   10  21.480 ± 0.070  ops/ms

(more is better)

But can it be automatic?

Automatic derivation a’la Chimney

Solution

trait TypeClass[A] extends TypeClass.AutoDerived[A] { ... }
object TypeClass {

  // semi-automatic derivation of TypeClass[A]
  inline def derived[A]: TypeClass[A] = ${ derivedImpl[A] }

  trait AutoDerived[A] { ... }
  object AutoDerived extends AutoDerivedLowPriorityImplicits
  trait AutoDerivedLowPriorityImplicits {

    // automatic derivation of TypeClass.AutoDerived[A]
    inline given derived[A]: AutoDerived[A] = ${ derivedImpl[A] }
  }
}

extension [A](value: A)
  // uses TypeClass[A] defined by user manually or with TypeClass.derived,
  // falling back on automatic derivation
  def method(using TypeClass.AutoDerived[A]) = ...

// allowed to try summoning TypeClass[Sth].
// NOT allowed to try summoning TypeClass.AutoDerived[Sth]!
def derivedImpl[A: Type]: Expr[TypeClass[A]] = ...

(Disclaimer: understanding this code is not necessary to understand its implications on the next slides)

(Solutions for New Prioritization of Givens in Scala 3.7 in a moment)

Can we test it outside Chimney?

Yes.

Sanely-automatic derivation

I implemented wrapper around Jsoniter (on Scala 3-only) which works like this:

import jsonitersanely.* // <-- 1 import, like with std automatic derivation

def roundTrip(out: Out): (String, Either[Throwable, Out]) = {
  val str = write(out)
  val parsed = scala.util.Try(read[Out](str)).toEither
  str -> parsed
}

How does it compare to Circe or normal Jsoniter Scala?

(less is better)

I would say "yes". Sanely-automatic derivation has amost the same compilation times as Jsoniter, much, much faster than Circe, no matter which apporach.

                     Scala 2   Scala 3  Units
compilation of      cold hot  cold hot
circeGenericAuto      14   4    46  16      s
circeGenericSemi      12   3    10   1      s
circeMagnoliaAuto     13   2    65  32      s
circeMagnoliaSemi     12   7    12   2      s
jsoniterScalaSanely    -   -     9   1      s
jsoniterScalaSemi     10   4     8   1      s

Scala 2.13.14

[info] Benchmark                          Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto    thrpt  10   7.319 ± 0.011  ops/ms
[info] JsonRoundTrips.circeGenericSemi    thrpt  10   6.775 ± 0.013  ops/ms
[info] JsonRoundTrips.circeMagnoliaAuto   thrpt  10   7.689 ± 0.013  ops/ms
[info] JsonRoundTrips.circeMagnoliaSemi   thrpt  10   7.838 ± 0.013  ops/ms
[info] JsonRoundTrips.jsoniterScalaSemi   thrpt  10  20.081 ± 0.151  ops/ms

Scala 3.3.3

[info] Benchmark                            Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto     thrpt   10   0.490 ± 0.432  ops/ms
[info] JsonRoundTrips.circeGenericSemi     thrpt   10   4.607 ± 0.014  ops/ms
[info] JsonRoundTrips.circeMagnoliaAuto    thrpt   10   0.077 ± 0.039  ops/ms
[info] JsonRoundTrips.circeMagnoliaSemi    thrpt   10   5.590 ± 0.013  ops/ms
[info] JsonRoundTrips.jsoniterScalaSemi    thrpt   10  21.480 ± 0.070  ops/ms
[info] JsonRoundTrips.jsoniterScalaSanely  thrpt   10  21.408 ± 0.070  ops/ms

(more is better)

But Jsoniter parsing String s vs Circe parsing Json might be apples vs oranges.

Can we have some more fair comparison?

More fair comparison

trait FastShowPretty[A] {

  def showPretty(
    value:   A,
    sb:      StringBuilder,
    indent:  String = "  ",
    nesting: Int = 0
  ): StringBuilder
}

implicit class FastShowPrettyOps[A](private val value: A) {

  def showPretty(indent: String = "  ", nesting: Int = 0)(
    implicit fsp: FastShowPretty[A]
  ): String =
    fsp.showPretty(value, new StringBuilder, indent, nesting).toString()
}

case class Street(name: String)
case class Address(street: Street)
case class User(name: String, address: Address)

println(User("John", Address(Street("Paper St"))).showPretty())

User(
  name = "John",
  address = Address(
    street = Street(
      name = "Paper St"
    )
  )
)

automatic and semi-automatic derivation using Shapeless (Scala 2)
automatic and semi-automatic derivation using Mirror s (Scala 3)
automatic and semi-automatic derivation using Magnolia (Scala 2 & 3)
sanely-automatic derivation with macros and Chimney macro commons (Scala 2 & 3)

(less is better)

It seems that some results are the same like with JSONs experiments.

Scala 2.13 approaches are close.

Semi-automatic results on Scala 3 are slightly better.

Automatic results on Scala 3 are much worse.

Naive macro implementation, isn’t very bad, especially considering how convenient it is, but it seems that it’s slower to compile than semi-automatic.

                            Scala 2   Scala 3  Units
compilation of             cold hot  cold hot
showGenericProgrammingAuto   15   5    53  29      s
showGenericProgrammingSemi   10   2    10   2      s
showMagnoliaAuto             10   1    43  15      s
showMagnoliaSemi             10   2     9   1      s
showSanely                   14   4    16   5      s

Scala 2.13.14

[info] Benchmark                                Mode  Cnt  Score   Error   Units
[info] ShowOutputs.showGenericProgrammingAuto  thrpt   10  2.651 ± 0.012  ops/ms
[info] ShowOutputs.showGenericProgrammingSemi  thrpt   10  2.829 ± 0.033  ops/ms
[info] ShowOutputs.showMagnoliaAuto            thrpt   10  3.621 ± 0.017  ops/ms
[info] ShowOutputs.showMagnoliaSemi            thrpt   10  3.745 ± 0.028  ops/ms
[info] ShowOutputs.showSanely                  thrpt   10  2.202 ± 0.359  ops/ms

Scala 3.3.3

[info] Benchmark                                Mode  Cnt  Score   Error   Units
[info] ShowOutputs.showGenericProgrammingAuto  thrpt   10  0.156 ± 0.013  ops/ms
[info] ShowOutputs.showGenericProgrammingSemi  thrpt   10  3.492 ± 0.013  ops/ms
[info] ShowOutputs.showMagnoliaAuto            thrpt   10  0.090 ± 0.023  ops/ms
[info] ShowOutputs.showMagnoliaSemi            thrpt   10  3.918 ± 0.012  ops/ms
[info] ShowOutputs.showSanely                  thrpt   10  2.204 ± 0.396  ops/ms

(more is better)

But wait.

Jsoniter had one more trick. It "caches" subroutines as def s.

Would that make a difference?

(less is better)

It seems that caching results of the derivation, inside the same macro, is not that difficult, and 1 non-invasive PR later, we run the numbers again.

We beat all the other approaches. It’s the fasted thing to compile. How about benchmarks?

                            Scala 2   Scala 3  Units
compilation of             cold hot  cold hot
showGenericProgrammingAuto   15   5    53  29      s
showGenericProgrammingSemi   10   2    10   2      s
showMagnoliaAuto             10   1    43  15      s
showMagnoliaSemi             10   2     9   1      s
showSanely                    6   1     7   1      s

Scala 2.13.14

[info] Benchmark                                Mode  Cnt  Score   Error   Units
[info] ShowOutputs.showGenericProgrammingAuto  thrpt   10  2.651 ± 0.012  ops/ms
[info] ShowOutputs.showGenericProgrammingSemi  thrpt   10  2.829 ± 0.033  ops/ms
[info] ShowOutputs.showMagnoliaAuto            thrpt   10  3.621 ± 0.017  ops/ms
[info] ShowOutputs.showMagnoliaSemi            thrpt   10  3.745 ± 0.028  ops/ms
[info] ShowOutputs.showSanely                  thrpt   10  4.811 ± 0.026  ops/ms

Scala 3.3.3

[info] Benchmark                                Mode  Cnt  Score   Error   Units
[info] ShowOutputs.showGenericProgrammingAuto  thrpt   10  0.156 ± 0.013  ops/ms
[info] ShowOutputs.showGenericProgrammingSemi  thrpt   10  3.492 ± 0.013  ops/ms
[info] ShowOutputs.showMagnoliaAuto            thrpt   10  0.090 ± 0.023  ops/ms
[info] ShowOutputs.showMagnoliaSemi            thrpt   10  3.918 ± 0.012  ops/ms
[info] ShowOutputs.showSanely                  thrpt   10  4.800 ± 0.042  ops/ms

(more is better)

Bonus: debugging

case class Street(name: Either[String, Nothing]) // <-- this should fail the derivation
case class Address(street: Street)
case class User(name: String, address: Address)

// scalacOptions += "-Xmacro-settings:fastshowpretty.logging=true"
def printObject(out: User): String = out.showPretty()

[error] .../ShowSanely.scala:12:54: Failed to derive showing for value : example.ShowSanely.User:
[error] No build-in support nor implicit for type scala.Nothing
[error]   def printObject(out: User): String = out.showPretty()
[error]                                                      ^

[info] .../ShowSanely.scala:12:54: Logs:
[info]  - Started derivation for value : example.ShowSanely.User
[info]  - Attempting rule ImplicitRule
[info]  - Skipped summoning example.showmacros.FastShowPretty[example.ShowSanely.User]
[info]  - Attempting rule CachedDefRule
[info]  - Attempting rule BuildInRule
[info]  - Attempting rule ProductRule
[info]  - Checking if def for example.ShowSanely.User exists
[info]  - Started deriving def for example.ShowSanely.User
[info]    - Started derivation for string : java.lang.String
[info]    - Attempting rule ImplicitRule
[info]    - Attempting rule CachedDefRule
[info]    - Attempting rule BuildInRule
[info]    - Successfully shown java.lang.String: sb.append("\"").append(string).append("\"")
[info]    - Started derivation for address : example.ShowSanely.Address
[info]    - Attempting rule ImplicitRule
[info]    - Attempting rule CachedDefRule
[info]    - Attempting rule BuildInRule
[info]    - Attempting rule ProductRule
[info]    - Checking if def for example.ShowSanely.Address exists
[info]    - Started deriving def for example.ShowSanely.Address
[info]      - Started derivation for street : example.ShowSanely.Street
[info]      - Attempting rule ImplicitRule
[info]      - Attempting rule CachedDefRule
[info]      - Attempting rule BuildInRule
[info]      - Attempting rule ProductRule
[info]      - Checking if def for example.ShowSanely.Street exists
[info]      - Started deriving def for example.ShowSanely.Street
[info]        - Started derivation for either : scala.util.Either[java.lang.String, scala.Nothing]
[info]        - Attempting rule ImplicitRule
[info]        - Attempting rule CachedDefRule
[info]        - Attempting rule BuildInRule
[info]        - Attempting rule ProductRule
[info]        - Attempting rule SumTypeRule
[info]        - Checking if def for scala.util.Either[java.lang.String, scala.Nothing] exists
[info]        - Started deriving def for scala.util.Either[java.lang.String, scala.Nothing]
[info]          - Started derivation for left : scala.util.Left[java.lang.String, scala.Nothing]
[info]          - Attempting rule ImplicitRule
[info]          - Attempting rule CachedDefRule
[info]          - Attempting rule BuildInRule
[info]          - Attempting rule ProductRule
[info]          - Checking if def for scala.util.Left[java.lang.String, scala.Nothing] exists
[info]          - Started deriving def for scala.util.Left[java.lang.String, scala.Nothing]
[info]            - Started derivation for string : java.lang.String
[info]            - Attempting rule ImplicitRule
[info]            - Attempting rule CachedDefRule
[info]            - Attempting rule BuildInRule
[info]            - Successfully shown java.lang.String: sb.append("\"").append(string).append("\"")
[info]          - Cached result of def for scala.util.Left[java.lang.String, scala.Nothing]
[info]          - Successfully shown scala.util.Left[java.lang.String, scala.Nothing]: show_nothing$u005D(left, nesting)
[info]          - Started derivation for right : scala.util.Right[java.lang.String, scala.Nothing]
[info]          - Attempting rule ImplicitRule
[info]          - Attempting rule CachedDefRule
[info]          - Attempting rule BuildInRule
[info]          - Attempting rule ProductRule
[info]          - Checking if def for scala.util.Right[java.lang.String, scala.Nothing] exists
[info]          - Started deriving def for scala.util.Right[java.lang.String, scala.Nothing]
[info]            - Started derivation for nothing : scala.Nothing
[info]            - Attempting rule ImplicitRule
[info]            - Attempting rule CachedDefRule
[info]            - Attempting rule BuildInRule
[info]          - Cached result of def for scala.util.Right[java.lang.String, scala.Nothing]
[info]        - Cached result of def for scala.util.Either[java.lang.String, scala.Nothing]
[info]      - Cached result of def for example.ShowSanely.Street
[info]    - Cached result of def for example.ShowSanely.Address
[info]  - Cached result of def for example.ShowSanely.User
[info]   def printObject(out: User): String = out.showPretty()
[info]                                                      ^

What changed?

Changes to givens (3.7.0)

// an example of mature, production-grade library using these stuffs:
import io.scalaland.chimney.dsl._
import io.scalaland.chimney.Transformer

case class Foo()
case class Bar(a: Int)
object Bar {
  given t: Transformer[Foo, Bar] = _ => Bar(0)
}

Foo().transformInto[Bar]

// before 3.7.0
Bar(0)

Now:
Ambiguous given instances: both given instance t in object Bar
and method deriveAutomatic in trait TransformerAutoDerivedCompanionPlatform
match type io.scalaland.chimney.Transformer.AutoDerived[Foo, Bar]
of parameter transformer of method transformInto
in package io.scalaland.chimney.dsl

However

// New utility!
Expr.summonIgnoring[A](symbolsVarArgs*)

// When looking for implicits
//   io.scalaland.chimney.Transformer.derive
// will be ignored - only user-provided implicits are considered!
Expr.summonIgnoring[Target](
  Symbol.classSymbol("io.scalaland.chimney.Transformer")
    .companionModule
    .methodMember("derive").head
)

So in general

trait TypeClass[A] {
  // methods
}
object TypeClass {

  given inline derive[A]: TypeClass[A] =
    ${ macros[A] } // Derive code recursively using Expr.summonignoring
                   // to avoid calling itself: users' implicits only!
}

Benchmarks for 2.13.20 vs 3.7.0-RC1

(less is better)

Automatic derivation of Magnolia and Mirrors switched places, everythine else is the same.

                     Scala 2   Scala 3  Units
compilation of      cold hot  cold hot
circeGenericAuto       3   1    15   9      s
circeGenericSemi       3   1     2   0      s
circeMagnoliaAuto      3   1    23  17      s
circeMagnoliaSemi      3   0     3   0      s
jsoniterScalaSanely    -   -     3   0      s
jsoniterScalaSemi      2   0     2   0      s

Scala 2.13.20

[info] Benchmark                          Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto   thrpt   10   8.392 ± 0.137  ops/ms
[info] JsonRoundTrips.circeGenericSemi   thrpt   10   8.815 ± 0.121  ops/ms
[info] JsonRoundTrips.circeMagnoliaAuto  thrpt   10   9.469 ± 0.165  ops/ms
[info] JsonRoundTrips.circeMagnoliaSemi  thrpt   10   9.443 ± 0.143  ops/ms
[info] JsonRoundTrips.jsoniterScalaSemi  thrpt   10  24.615 ± 0.101  ops/ms

Scala 3.7.0-RC1

[info] Benchmark                            Mode  Cnt   Score   Error   Units
[info] JsonRoundTrips.circeGenericAuto     thrpt   10   3.394 ± 1.267  ops/ms
[info] JsonRoundTrips.circeGenericSemi     thrpt   10   8.047 ± 0.096  ops/ms
[info] JsonRoundTrips.circeMagnoliaAuto    thrpt   10   0.128 ± 0.062  ops/ms
[info] JsonRoundTrips.circeMagnoliaSemi    thrpt   10   6.176 ± 0.104  ops/ms
[info] JsonRoundTrips.jsoniterScalaSanely  thrpt   10  24.460 ± 0.149  ops/ms
[info] JsonRoundTrips.jsoniterScalaSemi    thrpt   10  24.396 ± 0.116  ops/ms

(less is better)

Automatic derivation of Magnolia and Mirrors switched places, everythine else is the same.

                            Scala 2   Scala 3  Units
compilation of             cold hot  cold hot
showGenericProgrammingAuto    3   1     7   3      s
showGenericProgrammingSemi    2   0     2   0      s
showMagnoliaAuto              2   0    14   8      s
showMagnoliaSemi              2   0     2   0      s
showSanely                    1   0     2   0      s

Scala 2.13.20

[info] Benchmark                                Mode  Cnt  Score   Error   Units
[info] ShowOutputs.showGenericProgrammingAuto  thrpt   10  5.584 ± 0.067  ops/ms
[info] ShowOutputs.showGenericProgrammingSemi  thrpt   10  5.953 ± 0.057  ops/ms
[info] ShowOutputs.showMagnoliaAuto            thrpt   10  6.702 ± 0.106  ops/ms
[info] ShowOutputs.showMagnoliaSemi            thrpt   10  6.605 ± 0.289  ops/ms
[info] ShowOutputs.showSanely                  thrpt   10  8.201 ± 0.078  ops/ms

Scala 3.7.0-RC1

[info] Benchmark                                Mode  Cnt  Score   Error   Units
[info] ShowOutputs.showGenericProgrammingAuto  thrpt   10  3.687 ± 1.588  ops/ms
[info] ShowOutputs.showGenericProgrammingSemi  thrpt   10  6.291 ± 0.062  ops/ms
[info] ShowOutputs.showMagnoliaAuto            thrpt   10  0.265 ± 0.194  ops/ms
[info] ShowOutputs.showMagnoliaSemi            thrpt   10  8.316 ± 0.067  ops/ms
[info] ShowOutputs.showSanely                  thrpt   10  9.191 ± 0.081  ops/ms

Summary

Last year’s survey showed that 53% of developers complained about compile times. That the effort should be made to make the compiler faster.

Perhaps, that’s not the compiler. Perhaps, we just did the derivation the wrong way.

We could see that through great effort automatic-derivation was optimized to be as performant as semi-automatic one on Scala 2.13. It took several years.

It hasn’t yet happened on Scala 3.

And then we could beat all that effort with a single PR to a macro, that just doesn’t follow the popular conventions. For me it means that the conventions are at fault.

It doesn’t mean that Shapeless/Mirrors/Magnolia/all the current inline def and compiletime ops effort was in vain - if you take a look at libraries implemented with these tools and libraries implemented with macros, you can think that many of our popular libraries wouldn’t be faster without Shapeless.

They would never have been created in the first place. These tools make it much easier to start learning about metaprogramming and even with them it’s difficult.

But if we want to have user friendly libraries in Scala - and I know we all do - we should start challenging the current solutions.

So we should start giving - in a polite and respectful way - feedback to library maintainers that we can do better. That it’s the libraries that can make compilation time shorter, generated code faster, and error messages better.

And I’m fine if that won’t happen with the sanely-automatic derivation I invented, as long as thing will improve.

Thank you!

https://github.com/MateuszKubuszok/derivation-benchmarks