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After going through some examples on the web I realize that there is a way to write an anonymous function without the underscore when only a single arg. Also, I'm experimenting with the "span" method on List, which I never knew existed. Anyway, here is my REPL session:

scala> val nums = List(1, 2, 3, 4, 5)
nums: List[Int] = List(1, 2, 3, 4, 5)

scala> nums span (_ != 3)
res0: (List[Int], List[Int]) = (List(1, 2),List(3, 4, 5))

scala> nums span (3 !=)
res1: (List[Int], List[Int]) = (List(1, 2),List(3, 4, 5))

So far so good. But when I try to use the "less than" operator:

scala> nums span (_ < 3)
res2: (List[Int], List[Int]) = (List(1, 2),List(3, 4, 5))

scala> nums span (3 <)
res3: (List[Int], List[Int]) = (List(),List(1, 2, 3, 4, 5))

Why is this behaving differently?

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  • 3
    The (3 <) syntax is transformed into a function through eta expansion. It isn't a topic much discussed on S.O., that I can recall. You could always search about it, and, if you don't find anything, ask someone to explain. Commented Jul 6, 2011 at 17:19
  • @Daniel - thanks, that term was useful for searching, and I found this link Commented Jul 8, 2011 at 12:16

3 Answers 3

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10 
scala> nums span (_ < 3)
res0: (List[Int], List[Int]) = (List(1, 2),List(3, 4, 5))

scala> nums span (3 >)
res1: (List[Int], List[Int]) = (List(1, 2),List(3, 4, 5))

3 < is a shortcut to 3 < _, which creates a partially applied function from method call.

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2 Comments

Now I realized my thinking was upside-down for a while. Accepting this answer, I really needed that explanation on the last line. if you can point me to some reference documentation explaining that particular syntax, that would be great ! Thanks.
Check out args.foreach(println) example at Step 7 of this tutorial
3

It's behaving correctly:

scala> nums span (3 < _)
res4: (List[Int], List[Int]) = (List(),List(1, 2, 3, 4, 5))

The predicate is false for the first element of the list, so the first list returned by span is empty.

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1 
scala> nums span (3 < _) 
res0: (List[Int], List[Int]) = (List(),List(1, 2, 3, 4, 5))
// is equivalent to 
scala> (nums takeWhile{3 < _}, nums.dropWhile{3 < _}) 
res1: (List[Int], List[Int]) = (List(),List(1, 2, 3, 4, 5))

where the predicate is false already for the first element(1) therefore nums.takeWhile{false} results in the empty list List()

For the second part nothing is dropped because the predicate is false already for the first element(1) and therefore the nums.dropWhile{false} is the whole list List(1, 2, 3, 4, 5).

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