[DavidMcLaughlin]

> The first two uses of () are anonymous functions

No they're not. They are just containers for arguments to a function. In Scala whenever you call a function which takes as it's only parameter another function, you can even replace them entirely with curly brackets:

    scala> (1 to 5).map(_ * 5)
    res1: scala.collection.immutable.IndexedSeq[Int] = Vector(5, 10, 15, 20, 25)

is equivalent to:

    scala> 1 to 5 map { _ * 5 }
    res3: scala.collection.immutable.IndexedSeq[Int] = Vector(5, 10, 15, 20, 25)

The _ symbol is equivalent to the % in your Clojure example. So both examples above are equivalent to:

    scala> (1 to 5).map((x:Int) => x * 5)
    res2: scala.collection.immutable.IndexedSeq[Int] = Vector(5, 10, 15, 20, 25)

You don't need _any_ characters or visual indicators to declare an anonymous function. Example:

    scala> def apply(op: Int => Int, y: Int) = op(y)
    apply: (op: (Int) => Int, y: Int)Int

Can then be called as:

    scala> apply(_ * 10, 10)
    res4: Int = 100

Or:

    scala> apply((x:Int) => x * 10, 10)
    res5: Int = 100

Or even:

    scala> apply((_ * 10), 10)
    res6: Int = 100

Perhaps your original point stands that in Clojure you have _better_ visual clues for picking out anonymous functions at a glance, but I disagree that you even need them in the Scala code you posted.

[jimbokun]

"Perhaps your original point stands that in Clojure you have _better_ visual clues for picking out anonymous functions at a glance"

Yes, this is what I was trying to get at. Thanks for clarifying the examples, though. So, using _ in any expression makes that expression an anonymous function? Is _ by itself the identity function?

[DavidMcLaughlin]

> So, using _ in any expression makes that expression an anonymous function?

Ah, I should have said - the syntax above only works with one-liners. The following wouldn't work:

    scala> apply(val y = _ * 50; y * 100, 10)
    scala> apply((val y = _ * 50; y * 100), 10)
    scala> apply((x: Int) => val y = x * 50; y * 100, 10)

etc.

Instead you would need to explicitly surround the method body with curly brackets:

    scala> apply((x:Int) => { val y = x * 20; y }, 10)
    res18: Int = 200

And in this case, it's obvious what is happening.

So it's more like providing a one line expression where a function is expected makes it an anonymous function. The use of _ is simply a shortcut when you don't need to assign parameters to a local variable. It even works with multiple parameters. I.e. compare:

    scala> (1 to 5).foldLeft(0)((accumulator:Int,x:Int) => accumulator + x)
    res22: Int = 15

To:

    scala> (1 to 5).foldLeft(0)(_ + _)
    res19: Int = 15