Scala: Default value for function parameter with generics

Question

I am having trouble for setting a default value for a function parameter when the types are generics.

My function signature looks like:

  def doStuff[ K, T ]( result: Future[ Seq[ T ] ] )
                     ( transform: T => Option[ K ] ): Future[ Either[ String, Option[ K ] ] ] = {
  }

And I am aware that I cat set a default value to a function parameter like:

  def doStuff(a: Int)
             (f: Int => Option[Int] = k => Option(k)): Future[ Either[ String, Option[ Int ] ] ] = {
  }

However I can't combine these generic types with a default function value

  def doStuff[ K, T ]( result: Future[ Seq[ T ] ] )
                     ( transform: T => Option[ K ] = k => Option(k)): Future[ Either[ String, Option[ K ] ] ] = {
  }

with an obvious error message: Option[T] does not conform expected Option[K]

my last resort is to pass class tag in for K and T and change the default parameter from k => Option(k) to

def doStuff[ K: ClassTag, T ]( result: Future[ Seq[ T ] ] )
                               ( transform: T => Option[ K ] = {
                                 case m: K => Option( m )
                                 case _ => None
                               } ): Future[ Either[ String, Option[ K ] ] ] = {
  }

but this approach will force me to pass in my generic parameters on function call.

Can anyone see any other approach?

for the default case K is Option[T] but otherwise no, there is no relationship at all, they can and will be totally different case classes — kali
– kali, Commented Feb 3, 2017 at 12:50
@kali is it necessary to have doStuff with two parameter lists? can it be with one? — zhelezoglo
– zhelezoglo, Commented Feb 3, 2017 at 15:00
If you can't think of a sensible default value, why do you want to assign one? — Jasper-M
– Jasper-M, Commented Feb 3, 2017 at 16:22
Or is your goal also to have K default to T when no second parameter is supplied? — Jasper-M
– Jasper-M, Commented Feb 3, 2017 at 16:39

zhelezoglo · Accepted Answer · 2017-02-03 19:29:36Z

1

I haven't achieved it with the signature you provided with two parameter lists. But I wonder if you can use it with one parameter list:

def toOption[R](x: R): Option[R] = Option(x)

def doStuff[K, T](result: Future[Seq[T]], transform: T => Option[K] = toOption[K] _): Future[Either[String, Option[K]]] = {
  result.map(r => Right[String, Option[K]](transform(r.head)))
}

// Then, if you need a function of the second parameter, you can use partially applied function:
val rf: (Int => Option[Int]) => Future[Either[String, Option[Int]]] = doStuff[Int, Int](Future(List(1, 2)), _)

// Or call with default argument:
val r: Future[Either[String, Option[Int]]] = doStuff[Int, Int](Future(List(1, 2)))

// These lines should print the same
println(rf(toOption))
println(r)

edited Feb 3, 2017 at 19:29

answered Feb 3, 2017 at 15:28

zhelezoglo

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1 Comment

kali Over a year ago

Hi, Thanks for the answer. If I am calling the function "with" the generic parameters (doStuff[Int, Int](.....)) then I'd rather use the "last resort" approach I mentioned in my question

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