scala variable number by name parameters [duplicate] - scala

Question

This question already has answers here:
Scala variable argument list with call-by-name possible?
(2 answers)
Closed 6 years ago.
I am trying to implement a control flow structure which can accept a variable number of by-name parameters.
See CalculateGroup method and its use.
I was trying to follow this post, but still have some issues
As I can see from the error, I suspect I need to define a type annotation predicate in CalculateGroup function?
Here is current code:
def compare[T : Numeric](x: T)(y: T) : Boolean = implicitly[Numeric[T]].gt( x, y )
val items = compare[Double](10) _
val assertionsEnabled = true
def Calculate( predicate: => Boolean ) =
if (assertionsEnabled && !predicate)
throw new AssertionError
Calculate{
items(5)
}
def CalculateGroup( list: (predicate: => Boolean) *) =
{
list.foreach( (p : (predicate: => Boolean) ) => {
if (assertionsEnabled && !predicate)
throw new AssertionError
})
}
CalculateGroup{
items(5),
items(3),
items(8)
}
Error details:
scala ControlFlow.scala
/Users/pavel/Documents/ControlFlow/ControlFlow.scala:36: error: ')' expected but ':' found.
def CalculateGroup( list: (predicate: => Boolean) *) =
^
/Users/pavel/Documents/ControlFlow/ControlFlow.scala:68: error: ')' expected but '}' found.
}
^
two errors found

Answer 1

You cannot use by-name var args, you could use a lazy collection like Iterator or maybe Stream:
def compare[T : Numeric](x: T)(y: T) : Boolean = implicitly[Numeric[T]].gt( x, y )
val items = compare[Double](10) _
val assertionsEnabled = true
def Calculate(predicate: => Boolean) =
if (assertionsEnabled && !predicate)
throw new AssertionError
Calculate{
items(5)
}
def CalculateGroup(list: Iterator[Boolean]) =
{
list.foreach { (p : Boolean ) =>
if (assertionsEnabled && !p) {
throw new AssertionError
}
}
}
CalculateGroup{Iterator(
items(5),
items(3),
items(8)
)}

Answer 2

You have a syntax problem... you are placing a colon in front of the word predicate in the signature of the method CalculateGroup and in the foreach. Just remove them and it should compile.
just remove it and know that the word predicate is not alias for a variable, but it should be the name of a class. So it's better if you capitalize it. Contrary to the case of your methods, which shouldn't be capitalized.
Update
To have multiple by-name parameters just do this:
def CalculateGroup( list: (=> Boolean) *) =
{
list.foreach( (p : (=> Boolean) ) => {
if (assertionsEnabled && !p)
throw new AssertionError
})
}