Type of "foreach" in Scala - scala

Question

I have example code:
def test = {
val l : Seq[(String, String)] = Seq()
val foo : (String, String) => Unit = {case (a, b)=>}
l.foreach[Unit](foo)
}
It gives me the next error:
Error:(8, 21) type mismatch;
found : (String, String) => Unit
required: ((String, String)) => Unit
l.foreach[Unit](foo)
As far as I understand foreach has a type (A=>U)=>Unit where:
A is template parameter of my collection and
U is template parameter of foreach itself
My question is why is ((String, String)) => Unit required? Where do the extra brackets come from?

Answer 1

If you check in the REPL, you'll see that foo is a <function2>, that is, a function that takes 2 argument:
scala> val foo : (String, String) => Unit = {case (a, b)=>}
foo: (String, String) => Unit = <function2>
.foreach however expects a function that takes a single arguement (of type A), which in your case is a Tuple2.
If you set foo to be a <function1>, then it works:
scala> val foo : ((String, String)) => Unit = {case (a, b)=>}
foo: ((String, String)) => Unit = <function1>
scala> val l : Seq[(String, String)] = Seq()
l: Seq[(String, String)] = List()
scala> l.foreach[Unit](foo)
scala>

Answer 2

Let's start from top.
You have
val l: Seq[(String, String)]
that is a Seq of tuples of two strings. So each of the elements of l is of type (String, String) which is a syntactic sugar for Tuple2[String, String].
Now you have
val foo: (String, String) => Unit
that is a function of two arguments each being String. Again, the type signature above is syntactic sugar for Function2[String, String, Unit].
Considering the above your code can be rewritten as follows:
def test = {
val l: Seq[Tuple2[String, String]] = Seq()
val foo: Function2[String, String, Unit] = {case (a, b)=>}
l.foreach[Unit](foo)
}
foreach expects a single argument function, i.e. a Function1[T, R], thus the type mismatch.

Answer 3

The type definition foo: (String, String) => Unit says that foo is a function which takes two parameters of type String. However, your sequence l: Seq[(String, String)] contains tuples of type type t = (String, String). Thus, calling l.foreach expects a function which is applicable to the tuple type t. Thus, it expects a function of type t => Unit, which is equivalent to ((String, String)) => Unit.