Scala 2.8: _ behaviour changed?

Question

Using XScalaWT, this compiled under Scala 2.7:

class NodeView(parent: Composite) extends Composite(parent) { var nodeName: Label = null this.contains( label( nodeName = _ ) ) }

With 2.8.0 RC1, I get this error:

type mismatch; found : main.scala.NodeView required: org.eclipse.swt.widgets.Label

The types are:

label(setups: (Label => Unit)*)(parent: Composite) : Label contains(setups: (W => Unit)*) : W

So it looks like _ now binds to the outer function instead of inner.

Is this change intentional?

UPDATE: Here is a minimized example:

Scala 2.7.7:

scala> var i = 0 i: Int = 0 scala> def conv(f: Int => Unit) = if (_:Boolean) f(1) else f(0) conv: ((Int) => Unit)(Boolean) => Unit scala> def foo(g: Boolean => Unit) { g(true) } foo: ((Boolean) => Unit)Unit scala> foo(conv(i = _)) scala> i res4: Int = 1

Scala 2.8.0RC3:

scala> var i = 0 i: Int = 0 scala> def conv(f: Int => Unit) = if (_:Boolean) f(1) else f(0) conv: (f: (Int) => Unit)(Boolean) => Unit scala> def foo(g: Boolean => Unit) { g(true) } foo: (g: (Boolean) => Unit)Unit scala> foo(conv(i = _)) <console>:9: error: type mismatch; found : Boolean required: Int foo(conv(i = _)) ^ scala> foo(conv(j => i = j)) scala> i res3: Int = 1

Interestingly enough, this works:

scala> foo(conv(println _)) 1

I did post the error message: "type mismatch; found : main.scala.NodeView required: org.eclipse.swt.widgets.Label" — Alexey Romanov
– Alexey Romanov, Commented Apr 29, 2010 at 8:06

Alexey Romanov · Accepted Answer · 2010-06-08 18:07:47Z

Here is the answer I got from Lukas Rytz on the scala-user list:

Hi Alexey,

there has been a change in semantics when we introduced named arguments. An expression
foo(a = _) 
used to be parsed as follows:
foo(x => a = x) 
In 2.8, "a" is treated as a named argument, i.e. the expression is parsed as:
x => foo(a = x) 
I will add a migration warning for this change.

In some cases, you might get the old behaviour with curly braces instead.

Viktor Hedefalk · Accepted Answer · 2010-06-04 15:18:43Z

I noticed the same thing and asked on Dave's blog:

http://www.coconut-palm-software.com/the_new_visual_editor/doku.php?id=blog:simplifying_swt_with_scala#comment__930ba2f0a020203873d33decce01ebc2

No answer there yet though. Just as you're saying, it seems like the _ binds to the outer closure rather than the inner one.

Changing

nodeName = _

to

x => nodeName = x

fixes the problem.

Yes, but it hurts the readability of XScalaWT code for me quite a bit.
Yes, sure, and it's really weird that the semantics of _ changed that drastically. I would really like to understand what has happened too.
If you didn't see the answer yet, it isn't the semantics of _ which changed, but of =.

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