The many ways to write the signature of a function in Scala

Main.scala

object Main extends App {
    // lambda function type are specified, type signature is inferred (works for both val and def)
    val map1 = (f: (Int) => Int, l: Seq[Int]) => { for (x <- l) yield f(x) }

    // lambda function types and result type are specified, type signature is inferred (works for both val and def)
    val map2 = (f: (Int) => Int, l: Seq[Int]) => { for (x <- l) yield f(x) } : Seq[Int]

    // type signature is specified, lambda function parameter types inferred (works for both val and def)
    val map3: ((Int) => Int, Seq[Int]) => Seq[Int] = (f, l) => { for (x <- l) yield f(x) }

    // // type signature and lambda function parameter types are specified (works for both val and def)
    val map4: ((Int) => Int, Seq[Int]) => Seq[Int] = (f: (Int) => Int, l: Seq[Int]) => { for (x <- l) yield f(x) }

    // type signature, result, and lambda function parameter types are specified (works for both val and def)
    val map5: ((Int) => Int, Seq[Int]) => Seq[Int] = (f: (Int) => Int, l: Seq[Int]) => { for (x <- l) yield f(x) } : Seq[Int]

    // method definition with type signature inline with function parameters, result type is inferred
    def map6(f: (Int) => Int, l: Seq[Int]) = { for (x <- l) yield f(x) }

    // method definition with type signature inline with function parameters, result type is specified
    def map7(f: (Int) => Int, l: Seq[Int]): Seq[Int] = { for (x <- l) yield f(x) }

    // "best" definition of map, using generics and inline type signature
    def map8[A, B](f: (A) => B, l: Seq[A]): Seq[B] = { for (x <- l) yield f(x) }

    // test map functions
    val double: (Int) => Int = (x: Int) => x * 2
    val mySeq = 1 to 10
    println(map1(double, mySeq))
    println(map2(double, mySeq))
    println(map3(double, mySeq))
    println(map4(double, mySeq))
    println(map5(double, mySeq))
    println(map6(double, mySeq))
    println(map7(double, mySeq))
    println(map8(double, mySeq))
}