How to specify Scala 3 `scala.Conversion` when the types being converted take an arbitrary type parameter - Stack Overflow

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From the Scala 3 docs on implicit conversions:

In Scala 3, an implicit conversion from type S to type T is defined by a given instance of type scala.Conversion[S, T]. For compatibility with Scala 2, it can also be defined by an implicit method.

For example, this code defines an implicit conversion from Int to Long:

given int2long: Conversion[Int, Long] with
  def apply(x: Int): Long = x.toLong

This would be the preferred syntax in Scala 3, while the old Scala 2 syntax would be:

implicit def int2long(x: Int): Long = x.toLong

However, there's a case the old Scala 2 syntax supports, that I can not see how write in the new Scala 3 syntax: what if the types being converted take an arbitrary type parameter? ie you need to transform a Foo[A] to a Bar[A]?

In Scala 2 this would be:

implicit def foo2bar[A](x: Foo[A]): Bar[A] = ???

In Scala 3, where would the type parameter A be identified?

For instance, this definitely doesn't work:

given [A] foo2bar: Conversion[Foo[A], Bar[A]] with
  def apply(x: Int): Long = ???

From the Scala 3 docs on implicit conversions:

In Scala 3, an implicit conversion from type S to type T is defined by a given instance of type scala.Conversion[S, T]. For compatibility with Scala 2, it can also be defined by an implicit method.

For example, this code defines an implicit conversion from Int to Long:

given int2long: Conversion[Int, Long] with
  def apply(x: Int): Long = x.toLong

This would be the preferred syntax in Scala 3, while the old Scala 2 syntax would be:

implicit def int2long(x: Int): Long = x.toLong

However, there's a case the old Scala 2 syntax supports, that I can not see how write in the new Scala 3 syntax: what if the types being converted take an arbitrary type parameter? ie you need to transform a Foo[A] to a Bar[A]?

In Scala 2 this would be:

implicit def foo2bar[A](x: Foo[A]): Bar[A] = ???

In Scala 3, where would the type parameter A be identified?

For instance, this definitely doesn't work:

given [A] foo2bar: Conversion[Foo[A], Bar[A]] with
  def apply(x: Int): Long = ???

• scala
• implicit-conversion
• implicit
• scala-3
• type-parameter

Share Improve this question Follow edited Mar 21 at 10:50 Dmytro Mitin 51.8k33 gold badges3232 silver badges7373 bronze badges asked Mar 21 at 10:13 Roberto TyleyRoberto Tyley 25.4k1212 gold badges7575 silver badges105105 bronze badges

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1 Answer 1

Sorted by: Reset to default 6

given foo2bar[A]: Conversion[Foo[A], Bar[A]] with
  def apply(x: Foo[A]): Bar[A] = ???

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As mentioned in the comments by @Dmytro, the name foo2bar can also be ommitted.

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