declare variable in custom control structure in scala - scala

Question

I am wondering if there is a way to create a temp variable in the parameter list of a custom control structure.
Essentially, I would like create a control structure that looks something like the
for loop where I can create a variable, i, and have access to i in the loop body only:
for(i<- 1 to 100) {
//loop body can access i here
}
//i is not visible outside
I would like to do something similar in my code. For example,
customControl ( myVar <- "Task1") {
computation(myVar)
}
customControl ( myVar <- "Task2") {
computation(myVar)
}
def customControl (taskId:String) ( body: => Any) = {
Futures.future {
val result = body
result match {
case Some(x) =>
logger.info("Executed successfully")
x
case _ =>
logger.error(taskId + " failed")
None
}
}
}
Right now, I get around the problem by declaring a variable outside of the custom control structure, which doesn't look very elegant.
val myVar = "Task1"
customControl {
computation(myVar)
}
val myVar2 = "Task2"
customControl {
computation(myVar2 )
}

Answer 1

You could do something like this:
import scala.actors.Futures
def custom(t: String)(f: String => Any) = {
Futures.future {
val result = f(t)
result match {
case Some(x) =>
println("Executed successfully")
x
case _ =>
println(t + " failed")
None
}
}
}
And then you can get syntax like this, which isn't exactly what you asked for, but spares you declaring the variable on a separate line:
scala> custom("ss") { myvar => println("in custom " + myvar); myvar + "x" }
res7: scala.actors.Future[Any] = <function0>
in custom ss
ss failed
scala> custom("ss") { myvar => println("in custom " + myvar); Some(myvar + "x") }
in custom ss
Executed successfully
res8: scala.actors.Future[Any] = <function0>
scala>

Answer 2

Note that the built-in for (x <- expr) body is just syntactic sugar for
expr foreach (x => body)
Thus it might be possible to achieve what you want (using the existing for syntax) by defining a custom foreach method.
Also note that there is already a foreach method that applies to strings. You could do something like this:
case class T(t: String) {
def foreach(f: String => Unit): Unit = f(t)
}
Note: You can also change the result type of f above from Unit to Any and it will still work.
Which would enable you to do something like
for (x <- T("test"))
print(x)
This is just a trivial (and useless) example, since now for (x <- T(y)) f(x) just abbreviates (or rather "enlongishes") f(y). But of course by changing the argument of f in the above definition of foreach from String to something else and doing a corresponding translation from the string t to this type, you could achieve more useful effects.