By-name type parameters

Question

Imagine I have the following class definition:

class Foo[T]

and I want to do the following

def bar(x:Foo[ =>Int ]):Int = ???

But compiler fails with "no by-name parameter type allowed here"

How can I use a by-name type as type parameter for a generic method?

score 10 · Accepted Answer · answered Nov 16 '12 at 12:06

10

You’ll have to provide your own lazy wrapper. Something like this:

class Lazy[T](wrp: => T) {
  lazy val value: T = wrp
}

and then:

def bar(x: Foo[Lazy[T]]): Int = ???

answered Nov 16 '12 at 12:06

what about `class Lazy[T](val value: =>T)`? No need to repeat the value holder. – pedrofurla Nov 16 '12 at 12:08
3

@pedrofurla. not allowed (val parameters may not be call by name). And if it was, at what times would value be supposed to be initialized? – Didier Dupont Nov 16 '12 at 12:18
1

Weird. Didn't know that. I supposed they would be init-ed at first call. – pedrofurla Nov 16 '12 at 12:27

1 Answers1