When should I use an implicit class?

Question

For me, I would use an implicit class under the following scenarios:

• don't have access to the underlying type to be able to add the method I want.
• the method I want doesn't make sense in a "global" sense.
• i am splitting the functionality into another library of "extensions"
• actually converting to a new type adds semantic/readability value (the new type actually means something)

However, I am fairly new to Scala (<6 months) and I'm noticing the developers around me are using implicit classes when it breaks the scenarios above. When I asked why, the answer was "because that's what I've always done".

So my question is, is there an official recommendation for when one should use an implicit class over a normal function added to the class definition? (I couldn't find anything here: https://docs.scala-lang.org/overviews/core/implicit-classes.html)

Answer 1

As per the SIP,

Motivation for the implicit class was that the popular extension method pattern, sometimes called the Pimp My Library pattern was used in Scala to extend pre-existing classes with new methods, fields, and interfaces. There was also another common ‘extension’ use case known as type traits or type classes (see scala.math.Numeric). Type classes offered an alternative to pure inheritance hierarchies that was very similar to the extension method pattern. The main drawback to both of these techniques was that they suffered the creation of an extra object at every invocation to gain the convenient syntax. This made these useful patterns unsuitable for use in performance-critical code. In these situations it was common to remove use of the pattern and resort to using an object with static helper methods.

And implicit class syntax was thus added to solve these issues.