Question

In other words, is there a good reason why this shouldn’t compile?

def f(xs: List[Int]) = xs.foldLeft(0) _  // OK
def f(xs: List[Int]) = (xs :\ 0) _       // OK
def f(xs: List[Int]) = (0 /: xs) _

<console>:15: error: missing arguments for method /: in trait TraversableOnce;
follow this method with `_' if you want to treat it as a partially applied function

Here are some workarounds:

def f(xs: List[Int]) = xs./:(0) _
def f(xs: List[Int]): ((Int, Int) => Int) => Int = (0 /: xs)

but my question is mainly about the proper syntax in general.

Answer 1

I fixed this just now, but I can’t check it in yet because it requires amending the specification.

scala> def f(xs: List[Int]) = (0 /: xs) _
f: (xs: List[Int])(Int, Int) => Int => Int

scala> f(1 to 10 toList)
res0: (Int, Int) => Int => Int = <function1>

scala> res0(_ + _)
res1: Int = 55

The problem is that the spec defines “e1 op e2” if op is right-associative to be { val x=e1; e2.op(x ) } for reasons which are not apparent to me, since the simpler e2.op(e1) solves this issue among others, like https://issues.scala-lang.org/browse/SI-1980. I will make inquiries.