Menu
I've got some code like this:
def foo (s: => Any) = println(s)
But when I want to transform this to an argument list with variable length, it won't compile anymore (tested on Scala 2.10.0-RC2):
def foo (s: => Any*) = println(s)
What must I write, that it works like this?
553
Call by-name evaluation is similar to call by-value, but it has the advantage that a function argument won't be evaluated until the corresponding value is used inside the function body. Both strategies are reduced to the final value as long as: The reduced expression consists of pure functions.
By-name parameters are evaluated every time they are used. They won't be evaluated at all if they are unused. This is similar to replacing the by-name parameters with the passed expressions. They are in contrast to by-value parameters. To make a parameter called by-name, simply prepend => to its type.
In Java and Scala, certain builtin (a/k/a primitive) types get passed-by-value (e.g. int or Int) automatically, and every user defined type is passed-by-reference (i.e. must manually copy them to pass only their value).
In other words, the call by value function arguments are evaluated once before entering the function, but the call by name function arguments are evaluated inside the function only when they are needed. Hope this helps!
You have to use zero-argument functions instead. If you want, you can
implicit def byname_to_noarg[A](a: => A) = () => a
and then
def foo(s: (() => Any)*) = s.foreach(a => println(a()))
scala> foo("fish", Some(7), {println("This still happens first"); true })
This still happens first
fish
Some(7)
true
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
