I have a pseudo-code like shown below. The ItemChurner.churn() is abstracted component which generates objects until x times, where x is unknown. :
def func: MyList = {
var list: MyList = MyList()
while(ItemChurner.canChurn) {
list = new MyList(ItemChurner.churn(), list)
}
list
}
Is there a way to avoid use of var?
If canChurn works as it should:
def func(churner: ItemChurner) = {
val iterator = new Iterator {
def hasNext = churner.canChurn
def next = churner.churn()
}
iterator.toList
}
About version (of the question) that contained catched exception check for churn():
If really expect some exceptions, what's the point of canChurn then?
Anyway, if you care about exceptions:
Iterator.continually(Try(churner.churn)).takeWhile(_.isSuccess).map(_.get).toList
This actually is not much precise, as churn might throw some other exception that has to be propagated, so here the scala's Exception helpers) come in hand:
def step = catching(classOf[NoMoreElementsException]) opt churner.churn()
Iterator.continually(step).takeWhile(_.nonEmpty).map(_.get).toList
Iterator.continually(if (churner.canChurn) Some(churner.churn()) else None).takeWhile(_.isEmpty).map(_.get).toList is a better choice as it relies only on the exposed part of churner capabilities.
Try might be slower, besides code that actually checks for exact exception that signalizes about no more elements is less elegant (might require Exception helpers)If you want to do using Simple recursion and avoid var. This is the general strategy used in functional programming.
Use Vector instead of List for effective append
def func[T](): List[T] = {
@tailrec
def helper(result: List[T]): List[T] = {
if (ItemChurner.canChurn) helper(result ++ List(ItemChurner.churn))
else result
}
helper(List.Empty[T])
}
Assuming ItemChurner.canChurn does not throw any exception. if it throws exceptions simply wrap it inside Try
Improving upon pamu's answer, you can do something on the lines of;
def func: MyList = {
def process(cur: MyList): MyList = {
if (ItemChurner.canChurn) process(new MyList(ItemChurner.churn(), cur))
else cur
}
process(new MyList())
}
Iteratorinterface likeval iterator = new Iterator {def hasNext = ItemChurner.canChurn; def next = ItemChurner.churn(), and theniterator.toListwhileloop with a try-catch? Shouldn'tItemChurnerreturnfalseif there aren't anymore elements?