Scala类型参数被推断为元组

Question

I suddenly came across this (unexpected to me) situation:

def method[T](x: T): T = x

scala> method(1)
res4: Int = 1

scala> method(1, 2)
res5: (Int, Int) = (1,2)

Why in case of two and more parameters method returns and infers a tuple but throwing error about parameter list? Is it by intention? Maybe this phenomenon has a name?

Answer 1

Here is the excerpt from scala compiler :

/** Try packing all arguments into a Tuple and apply `fun'
 *  to that. This is the last thing which is tried (after
 *  default arguments)
 */
def tryTupleApply: Option[Tree] = ...

And here is related issue: Spec doesn't mention automatic tupling

It all means that in the above written example (type-parameterized method of one argument) scala tries to pack parameters into tuple and apply function to that tuple. Further from this two short pieces of information we may conclude that this behaviour not mentioned in language specification , and people discuss to add compiler warnings for cases of auto-tupling. And that this may be called auto-tupling .

Answer 2

% scala2.10 -Xlint

scala> def method[T](x: T): T = x
method: [T](x: T)T

scala> method(1)
res1: Int = 1

scala> method(1, 2)
<console>:9: warning: Adapting argument list by creating a 2-tuple: this may not be what you want.
        signature: method[T](x: T): T
  given arguments: 1, 2
 after adaptation: method((1, 2): (Int, Int))
              method(1, 2)
                    ^
res2: (Int, Int) = (1,2)