In this post I’d like to discover Scala 3 generic programming abilities. Scala 3 provides a lot of new features, and generic programming is the one of the areas where we have a lot of changes. I assume that you have used shapeless with Scala 2, but if not, I’ll try to explain things in this post. However I’d recommend to read this post before if you don’t even know what shapeless is.
Let’s take a look at tuples in Scala 3. In previous versions of Scala we had the famous Tuple1 .. Tuple22 classes
defined like:
final case class Tuple2[+T1, +T2](_1: T1, _2: T2)
extends Product2[T1, T2]
with Product
with SerializableThese classes are available in Scala 3 too, but there are another classes for tuples:
sealed trait Tuple extends Product
object EmptyTuple extends Tuple
trait NonEmptyTuple extends Tuple
case class [+H, +T <: Tuple] *:(head: H, tail: T) extends NonEmptyTupleI simplified the code a little bit, but the idea is the same. This is something new in the Scala library, but it looks
very familiar. Actually it’s a list with different types of its’ elements. This called the Heterogeneous list, or HList!
In Scala 2 we had HLists in shapeless, and also in some other libraries. Having heterogeneous lists in the standard library
makes a lot of sense, and now we don’t need any other HList implementation. Scala 3 tuples has a lot of useful functions, for
example they can be converted to List, be concatenated, zipped, etc.:
scala> val t = (5, "String", 3d, false)
val t: (Int, String, Double, Boolean) = (5,String,3.0,false)
scala> t.toList
val res0: List[Tuple.Union[t.type]] = List(5, String, 3.0, false)
scala> val x = t.drop(2)
val x: (Double, Boolean) = (3.0,false)
scala> t ++ x
val res1: Int *: String *: Double *:
scala.Tuple.Concat[Boolean *: scala.Tuple$package.EmptyTuple.type, x.type]
= (5,String,3.0,false,3.0,false)Moreover, Scala 3 provides mechanisms similar to shapeless Generics making possible to convert from algebraic data types
(if you are not familiar with algebraic data types, to understand further examples you might think that it’s just case classes)
to tuples and back. Let’s take a look at these features.
The Tuple companion object contains method fromProductTyped which allows us to construct tuple from a case class:
scala> case class Foo(a: String, b: Int)
// defined case class Foo
scala> Tuple.fromProductTyped(Foo("test", 5))
val res2: (String, Int) = (test,5)With this knowledge we can try to implement SqlSaver from “Getting started with shapeless” post for Scala 3. So, let’s do it.
The type class definition itself is unchanged:
trait SqlSaver[A] {
def save(statement: PreparedStatement, idx: Int)(a: A): Int
}However, there is a new syntax for implicits in Scala 3. When we need to use an implicit instance we use using keyword. So,
now the summoner method will look like:
object SqlSaver {
def apply[T](using ss: SqlSaver[T]): SqlSaver[T] = ss
}When we need to declare an instance of type class (or any other implicit), we use given keyword. With this new syntax instances of SqlSaver for
primitive types become:
object SqlSaver {
// ...
given SqlSaver[Int] = createSimpleSaver((a, s, i) => s.setInt(i, a))
given SqlSaver[String] = createSimpleSaver((a, s, i) => s.setString(i, a))
given SqlSaver[Double] = createSimpleSaver((a, s, i) => s.setDouble(i, a))
given SqlSaver[BigDecimal] = createSimpleSaver((a, s, i) => s.setBigDecimal(i, a.underlying))
given SqlSaver[LocalDateTime] =
createSimpleSaver((a, s, i) => s.setTimestamp(i, Timestamp.valueOf(a)))In this example we created anonymous instances, but it’s also possible to give names to the type class instances, e.g.:
given intSaver: SqlSaver[Int] = createSimpleSaver((a, s, i) => s.setString(i, a))Now, let’s implement SqlSaver instances for tuples. As it was before for HNil (empty HList), for the empty tuple it just does nothing:
given SqlSaver[EmptyTuple] = createSaver((_, _, i) => i)For non-empty tuple we need SqlSaver for head, to save left tuple member and SqlSaver for the tail, like we did before
for :: (non-empty HList):
given [H, T <: Tuple](using hSaver: SqlSaver[H], tSaver: SqlSaver[T]): SqlSaver[H *: T] =
new SqlSaver[H *: T] {
override def save(statement: PreparedStatement, idx: Int)(t: H *: T): Int = {
val next = hSaver.save(statement, idx)(t.head)
tSaver.save(statement, next)(t.tail)
}
}Here, we created instance for tuple dependent on the instances for the H and T via using keyword.
Finally, we can create instance for Products, which will convert a case class to tuple, and then call SqlSaver
for the tuple to really save the data. However, to do it we need to know the exact tuple type. For example, if the product is
case class Foo(a: String, b: Int) then the tuple type will be (String, Int), or that’s the same String *: Int *: EmptyTuple.
In shapeless for Scala 2, we used Generic to convert ADTs to and from HLists. It also was a type link between ADTs and their
HList representations. In Scala 3, we have class Mirror to connect products and coproducts with tuples both on the type and the value level.
To achieve that Mirror trait contains several type members. The MirroredElemTypes is a tuple type we are looking for. Bearing this
in mind, we can connect the mirror with the SqlSaver in the using part of the type class instance declaration:
import scala.deriving.Mirror
// ...
given [P <: Product](using m: Mirror.ProductOf[P],
ts: SqlSaver[m.MirroredElemTypes]
): SqlSaver[P] = new SqlSaver[P] {
override def save(statement: PreparedStatement, idx: Int)(t: P): Int =
ts.save(statement, idx)(Tuple.fromProductTyped(t))
}Here you can see another new cool feature of Scala 3. Previously we had to use Aux pattern to make types depend on each other. Now, we can just use type members in other function parameters or even as a result type.
Scala 3 brings us a lot of new features. Personally I like the way the language is evolving. Even if some things a bit controversial most of the stuff makes Scala more readable, and gives us tools to build standard solutions for standard problems. Typelevel programming in general is a complex topic, but the new code looks a bit simpler, and also it doesn’t require external dependencies.