Scala 2: generic function that returns another generic function

Question

How can you return a generic function from another generic function? This doesn't work

def gen1[A](a: A) = [B](b: B) => ???

But i don't want to define the second generic in the first function like this

def gen1[A, B](a: A) = (b: B) => ???

Is it possible?

Error here:

illegal start of simple expression
[error]   def gen1[A](a: A) = [B](b: B) => ???
[error]                       ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed

What I'm trying to do is the equivalent of dotty's:

def gen1 = [A] => (a: A) => [B] => (b: B) => (a, b)

But can't find documentation for it in scala 2

Have you tried? What error do you encounter? Try to specify the return type. — cchantep
– cchantep, Commented Mar 20, 2021 at 10:27
"But i don't want to define the second generic in the first function" why? Sounds a little bit like a xy problem to me. — Guru Stron
– Guru Stron, Commented Mar 20, 2021 at 10:31
Well what I'm asking for is "How to define a curried function that has a generic in the returned function without having to declare all the generics in the main function". I can find documentation on how to do it in dotty, but not scala 2. Don't really know what u are talking about with "xy problem" — rockson
– rockson, Commented Mar 20, 2021 at 10:47
The reason you can find it in dotty, but on in scala 2 is obviously because it exists in dotty, but does not in scala 2 :) Because scala 2 is not dotty. The definitive sign of an xy problem is a statement like "I know I can do X but I don't want to do it, because I want to do Y". You are fixated on what you think is a solution, that does not really solve anything. Having said that, you can do what you "want" in scala 2, I just have no idea why you would want to do it: def foo() = { def bar[B](b: B) = ??? ; bar _ } — Dima
– Dima, Commented Mar 20, 2021 at 11:43
And why would you need to know why I have to do it? It's like saying that If I ask how to screw a screw I need to explain what I'm building. Makes no sense. Also your solution doesn't give the same result as dotty's one. scastie.scala-lang.org/QdcL9FZkRSKeZk1OXlyrvg This will require Nothing as parameter in the returned function. — rockson
– rockson, Commented Mar 20, 2021 at 12:26

Jasper-M · Accepted Answer · 2021-03-20 13:15:04Z

2

Polymorphic functions don't exist in Scala 2. What you can do to get a similar effect is define a custom class with a polymorphic apply method.

class Gen1PartiallyApplied[A](a: A){
  def apply[B](b: B) = ???
}
def gen1[A](a: A) = new Gen1PartiallyApplied(a)

gen1[Int](42)[String]("foo")

answered Mar 20, 2021 at 13:15

Jasper-M

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2 Comments

rockson Over a year ago

Exactly what I was looking for. No need to specify "WHY", either provide the alternative to Polymorphic functions or say that's not possible in scala 2 THat's it, don't know why people were running in circles. Thank you sir

Luis Miguel Mejía Suárez Over a year ago

@rockson sorry people wanted to help you instead of just saying "is not possible" (which was actually said).

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