Scala implicit with variable argument list

Question

I have library method taking variable argument list and producing data

class Data
def process(elems: String*): Data = new Data

and I want my strings to be implicitly converted to Data

implicit def strToData(ts: String): Data = process(t)

so I can write something like

val data: Data = "one"

but I want tuples of strings to be implicitly converted too. I've added another implicit

implicit def strsToData(ts: String*): Data = process(ts: _*)

it compiles fine, but conversion fails

val data: Data = ("one", "two")
val dat3: Data = ("one", "two", "three")
val dat4: Data = ("one", "two", "three", "four")

with

found   : Seq[java.lang.String]
required: this.Data
val data: Data = Seq("one", "two")

Is there any way to convert tuples implicitly, or a reason why it can be achieved?

Update: Tuples can be of any arity.

Are you sure Tuples can be of any arity? I think max is 22 (see scala-lang.org/api/current/scala/Tuple23.html vs scala-lang.org/api/current/scala/Tuple22.html) — Martijn
– Martijn, Commented Jul 27, 2012 at 13:44

Nikita Volkov · Accepted Answer · 2012-07-27 12:31:04Z

ts: String* is not a tuple, but an Iterable. You can't convert iterable to tuple, because tuple is a static type and its arity is resolved at compile time. Emil H answered how you can do an implicit conversion from tuple.

Transparent implicit conversions from one type to another type is a common pitfall and are greatly discouraged. Instead you should apply a "wrapper" approach, or use "Value Classes" since Scala 2.10:

In Scala 2.9:

implicit def stringStringTupleExtender (ts : (String, String)) =
  new {
    def data = process(ts)
  }

In Scala 2.10:

implicit class StringStringTupleExtender (val ts : (String, String)) extends AnyVal {
  def data = process(ts)
}

Then you'll use it like so:

val data : Data = ("sdf", "lsdfj").data
val data1 : Data = "sdf".data // if you do the same for String

If you're looking for dynamically resolving any input collection, then, guess what, you should use a collection, not a tuple.

In Scala 2.9

implicit def seqExtender (ts : Seq[String]) =
  new {
    def data = process(ts)
  }

Usage:

val data : Data = Seq("sdf", "lsdkfjsdl", "ldsfjk").data

Ptharien's Flame · Accepted Answer · 2012-07-27 15:11:27Z

3

While all the warnings in @NikitaVolkov's answer apply in double force, you can write a version that accepts any arity of tuple, as well as any case class:

implicit def prod2data(Product p): process((p.productIterator.toSeq):_*)

answered Jul 27, 2012 at 15:11

Ptharien's Flame

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Comments

Emil L · Accepted Answer · 2012-07-27 10:48:01Z

2

You would need to create a implicit conversion from a tuple to Data:

implicit def strTplToData(ts:(String, String)) = process(ts._1, ts._2)

Then you could do:

val data: Data = ("one", "two")

answered Jul 27, 2012 at 10:48

Emil L

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2 Comments

lambdas Over a year ago

Is there another solution than to write function for every tuple arity?

Emil L Over a year ago

I do not think there is an elegant way of achieving that simply.

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