59

This should be an easy one. How do I apply a function to a tuple in Scala? Viz:

 scala> def f (i : Int, j : Int) = i + j f: (Int,Int)Int scala> val p = (3,4) p: (Int, Int) = (3,4) scala> f p :6: error: missing arguments for method f in object $iw; follow this method with `_' if you want to treat it as a partially applied function f p ^ scala> f _ p :6: error: value p is not a member of (Int, Int) => Int f _ p ^ scala> (f _) p :6: error: value p is not a member of (Int, Int) => Int (f _) p ^ scala> f(p) :7: error: wrong number of arguments for method f: (Int,Int)Int f(p) ^ scala> grr!

Many thanks in advance.

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5 Answers 5

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80

In Scala 2.8 and newer:

scala> def f (i : Int, j : Int) = i + j f: (i: Int,j: Int)Int // Note the underscore after the f scala> val ff = f _ ff: (Int, Int) => Int = <function2> scala> val fft = ff.tupled fft: ((Int, Int)) => Int = <function1>

In Scala 2.7:

scala> def f (i : Int, j : Int) = i + j f: (Int,Int)Int // Note the underscore after the f scala> val ff = f _ ff: (Int, Int) => Int = <function> scala> val fft = Function.tupled(ff) fft: ((Int, Int)) => Int = <function>
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3 Comments

interesting to note, scala (2.11.0 here) will bring your untupled params up to a tuple if you do: fft.apply(1,2)
Good to know you can do this, but this seems not much shorter than just using function(tup._1,tup._2) in the first place
@AllenWang I think the important point is that tupled can be used for any arity. Good for maintainability.
18

Following up on the other answer, one could write (tested with 2.11.4):

scala> def f (i: Int, j: Int) = i + j f: (i: Int, j: Int)Int scala> val ff = f _ ff: (Int, Int) => Int = <function2> scala> val p = (3,4) p: (Int, Int) = (3,4) scala> ff.tupled(p) res0: Int = 7

See def tupled: ((T1, T2)) ⇒ R:

Creates a tupled version of this function: instead of 2 arguments, it accepts a single scala.Tuple2 argument.

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8

Scala 2.13

def f (i : Int, j : Int) = i + j import scala.util.chaining._ (3,4).pipe((f _).tupled) //res0: Int = 7
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Thanks for your answer, but adding some explanation perhaps could help more people
1 
scala> def f (i: Int, j: Int) = i + j f: (i: Int, j: Int)Int scala> val p = (3,4) p: (Int, Int) = (3,4) scala> val ft = (f _).tupled ft: ((Int, Int)) => Int = <function1> scala> ft apply(p) res0: Int = 7
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1

In Scala 3, you can use TupledFunction:

For a single use, you can do

summon[TupledFunction[(Int, Int) => Int, ((Int, Int)) => Int]].tupled(f)((2, 3))

To make it easier to use, you can use an extension (copied from Dotty's own documentation)

extension [F, T <: Tuple, R](f: F)(using tf: TupledFunction[F, T => R]) def tupled(t: T): R = tf.tupled(f)(t)

And then you can do f.tupled((2, 3)) to get 5.

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