Why are implicit conversion deprecated in scala?

Question

Taken from "Scala with cats" (page 18):

Implicit Conversions
When you create a type class instance constructor using an implicit def, be sure to mark the parameters to the method as implicit parameters. Without this keyword, the compiler won’t be able to fill in the parameters during implicit resolution. implicit methods with non‐implicit parameters form a different Scala pattern called an implicit conversion. This is also different from the previous section on Interface Syntax, because in that case the JsonWriter is an implicit class with extension methods. Implicit conversion is an older programming pattern that is frowned upon in modern Scala code. Fortunately, the compiler will warn you when you do this. You have to manually enable implicit conversions by importing scala.language.implicitConversions in your file

Can anyone please sum up well why implicit conversion are deprecated? What were there limit or issues? Why the approach with implicit parameters is better?

Note I know how to use the modern approach well, including chaining implicit and all. I am just curious as to what was the issue and why it was deprecated.

Implicit derivation is very different from implicit conversions the former is a logic proof when you can say something like I can derive the Monoid[List[Map[String, Option[Int]]]] because I can derive the Monoid of List because I can derive the one of Map because I can derive the one of Option because I know there is one for Int - The latter, is just a way of saying I will turn an A into a B every time a B is needed but the code produce an A, in general, such pattern leads to code that is hard to read (understand), hard to maintain and can hide costly oeprations. — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Dec 5, 2020 at 15:39
However, there are indeed "valid use cases". But, IMHO, those a little and should be reserved when there is no other cleaner solution. For an in-depth discussion about the topic read: contributors.scala-lang.org/t/… — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Dec 5, 2020 at 15:40
Implicit conversions are not deprecated docs.scala-lang.org/tour/implicit-conversions.html dotty.epfl.ch/docs/reference/contextual/conversions.html — Dmytro Mitin
– Dmytro Mitin, Commented Dec 5, 2020 at 16:59
There are many difficulties with type inference in presence of implicit conversions stackoverflow.com/questions/63964610/… stackoverflow.com/questions/62630439/… stackoverflow.com/questions/62205940/… stackoverflow.com/questions/62751493/… ... — Dmytro Mitin
– Dmytro Mitin, Commented Dec 5, 2020 at 17:11
... stackoverflow.com/questions/63002466/… stackoverflow.com/questions/63697952/… — Dmytro Mitin
– Dmytro Mitin, Commented Dec 5, 2020 at 17:11

Andrew Norman · Accepted Answer · 2021-02-05 17:47:16Z

Martin Odersky, the inventor of Scala, has indicated that implicits (and implicit conversions) are being deprecated in scala 3.1 and will eventually be removed from the language altogether.

the implicit functionality will be replaced with Extension Methods and Givens. Extension Methods and Givens provide a more tighter functional solution that doesn't introduce the unsuspecting and hidden side effects that implicits cause. Odersky now views implicits as a “recipe for disaster" and are "too implicit" which was his motivation to replace their functionality in 3.x.

https://www.slideshare.net/Lightbend/scala-3-is-coming-martin-odersky-shares-what-to-know

https://hub.packtpub.com/is-scala-3-0-a-new-language-all-together-martin-odersky-its-designer-says-yes-and-no/

Since then I found out why! I will post the specific answer. But there is a thread in google where they emphasize moving away from implicit conversion to context bound and the reason for this is clear ! None the less thanks for the links

steinybot · Accepted Answer · 2023-10-11 22:18:01Z

As of Scala 3.3, implicit conversions are not deprecated.

... in Scala 2, implicit conversions were also used to provide additional members to closed classes (see Implicit Classes). In Scala 3, we recommend to address this use-case by defining extension methods instead of implicit conversions (although the standard library still relies on implicit conversions for historical reasons).

Implicit conversions for extending closed classes have been replaced by extension methods but implicit conversions still exist and have their use.

There are a couple of reasons to avoid implicit conversions:

It can be hard to read code that implicitly converts a value of one type to another. Often it is better to be explicit and convert it manually.
If an API expects you to make use of implicit conversions then it can be difficult to understand the error messages when the implicit conversion is not in scope.

Because implicit conversions can have pitfalls if used indiscriminately the compiler warns in two situations:

when compiling a Scala 2 style implicit conversion definition.

at the call site where a given instance of scala.Conversion is inserted as a conversion.

To turn off the warnings take either of these actions:

Import scala.language.implicitConversions into the scope of:

a Scala 2 style implicit conversion definition

call sites where a given instance of scala.Conversion is inserted as a conversion.

Invoke the compiler with -language:implicitConversions

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