given:
val m = Map[String, Int]("a" -> 1, "b" -> 2, "c" -> 3)
m.foreach((key: String, value: Int) => println(">>> key=" + key + ", value=" + value))
why does the compiler complain
error: type mismatch
found : (String, Int) => Unit
required: (String, Int) => ?
Answer
I'm not sure about the error, but you can achieve what you want as follows:
m.foreach(p => println(">>> key=" + p._1 + ", value=" + p._2))
That is, foreach takes a function that takes a pair and returns Unit, not a function that takes two arguments: here, p has type (String, Int).
Another way to write it is:
m.foreach { case (key, value) => println(">>> key=" + key + ", value=" + value) }
In this case, the { case ... } block is a partial function.