5

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.

Luigi Plinge
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    +1, and I just found another confusing compiler error when omit a pair of parentheses in expression `def f(xs: List[Int]): (Int, Int) => Int => Int = (xs :\ 0)` `found : (Int, Int) => Int => Int` `required: (Int, Int) => Int => Int` – 4e6 Oct 12 '11 at 06:00
  • Opened https://issues.scala-lang.org/browse/SI-5073 – Luigi Plinge Oct 14 '11 at 04:07

2 Answers2

2

Looks like a compiler bug. I've tested this expressions on different scala versions and what I've got for:

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

It behaves the same for 2.9.1.final and 2.8.2.final but for 2.7.7.final it fires different error message (Iterable vs. TraversableOnes), but I think it's because of collections library redesign in older versions.

def f(xs: List[Int]) = (0 /: xs) _
<console>:4: error: missing arguments for method /: in trait Iterable;
follow this method with `_' if you want to treat it as a partially applied function

Expression, that I mentioned in comment behaves different for different scala versions.

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

scala 2.9.1.final:

 found   : (Int, Int) => Int => Int
 required: (Int, Int) => Int => Int

Really confusing compiler message, definitely a bug.

scala 2.8.2.final:

 found   : => ((Int, Int) => Int) => Int
 required: (Int, Int) => (Int) => Int

Weird => in the beginning, in comparison to 2.7.7.final result looks like a regression.

scala 2.7.7.final:

 found   : ((Int, Int) => Int) => Int
 required: (Int, Int) => (Int) => Int

found is seemingly right but code is still not working.

I searched on scala bugtracker for similar issues but could not find anything suitable. Think it's enough to create a ticket (or two? looks like this two errors are not related).

4e6
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  • The first seems like a bug to me unless anyone can say differently; the second might be related to https://issues.scala-lang.org/browse/SI-5007 – Luigi Plinge Oct 12 '11 at 20:52
2

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.

psp
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