Scala generic Function vs Generic Method

Question

So far i was under the impression that the only way to define Generic function in scala, was using Method e.g.

def myToString[A](value: A) = {println(value)}

However i figured the following way:

val myToStringFun: (T forSome {type T}) => Unit = value => println(value)

Is there something that i am not seeing or is that the pattern to write generic function in Scala without resorting to method ?

I never seen that pattern before, and only came up with it, based my learning around Higher-Kinded Type and the notion of existential ...

Please share your thoughts or wisdom on this....

EDIT1:

If the above is correct, why is it that this pattern is not used and people systematically resort to Method for generic function. Is it just a ease of notation

EDIT2:

If a function (T forSome {type T}) => Unit <=> Any => Unit

It seems however that

val mySeqToString: (Seq[T] forSome {type T}) => String = {
    case head +: tail => s"$head +: " + seqToString(tail)
    case Nil => "Nil"
}

is equivalent to

def seqToString[T](seq: Seq[T]): String = seq match {
    case head +: tail => s"$head +: " + seqToString(tail)
    case Nil => "Nil"
}

correct ?

Answer 1

Type A in your function is a standard generic type. You can do many things with it, such as demand a typeclass instance:

import cats.implicits._
def myToString[A: cats.Show](value: A) = { println(value.show) }

On the other hand, type (T forSome {type T}) is an existential type. You might recognize it by its more popular shorthand notation, _ (e.g. List[_]). Not much you can do with that one. If you check the type of value in

val myToStringFun: (T forSome { type T }) => Unit = value => println(value)

you will notice that it's Any. BTW, using existential types via forSome is being dropped.

Functions are monomorphic in Scala, unlike methods that are polymorphic. Personally, I think this article(s) provides a great explanation on that topic.