map error when applying on list of tuples in scala

Question

If applying map method to a list of tuple in Scala, it complains error as below:

scala> val s = List((1,2), (3,4))
s: List[(Int, Int)] = List((1,2), (3,4))

scala> s.map((a,b) => a+b)
<console>:13: error: missing parameter type
Note: The expected type requires a one-argument function accepting a 2-Tuple.
    Consider a pattern matching anonymous function, `{ case (a, b) =>  ... }`
    s.map((a,b) => a+b)
            ^
<console>:13: error: missing parameter type
    s.map((a,b) => a+b)

But if I apply similar map method to list of Int, it works fine:

scala> val t = List(1,2,3)
t: List[Int] = List(1, 2, 3)

scala> t.map(a => a+1)
res14: List[Int] = List(2, 3, 4)

Anyone knows why it is? Thanks.

Yuval Itzchakov · Accepted Answer · 2016-11-15 18:42:52Z

8

Scala dosen't deconstruct tuples automatically. You'll need to either use curly brackets:

val s = List((1,2), (3,4))
val result = s.map { case (a, b) => a + b }

Or use a single parameter of type tuple:

val s = List((1,2), (3,4))
val result = s.map(x => x._1 + x._2)

Dotty (the future Scala compiler) will bring automatic deconstruction of tuples.

answered Nov 15, 2016 at 18:42

Yuval Itzchakov

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