In Scala, yield
can work with for-loops; for example:
val ints: IndexedSeq[Int] = for(i <- 1 to 10) yield i
But I found that yield
can not work with while-loops, e.g. like:
while (resultSet.next()) yield new Row(resultSet)
Why is Scala designed like this?
I have searched on Google and stackoverflow, but could not find an answer.
Because a while loop is a java equivalent while loop, and the 'for loop' is translated to function call of: <IndexedSeq>.map
(if you use yield) or <IndexedSeq>.foreach
(if you don't care the result).
Example Scala Code:
class ForVsWhileLoop {
val dummy = for(i <- 1 to 10) yield i
var dummy2 = Seq.empty[Int]
var i = 0
while(i <= 10)
dummy2 :+= i
}
Compiles to (scala -Xprint:parse ForVsWhileLoop.scala):
[[syntax trees at end of parser]] // ForVsWhileLoop.scala
package <empty> {
class ForVsWhileLoop extends scala.AnyRef {
def <init>() = {
super.<init>();
()
};
// ***********************************************
// the 'for loop' is translated to a function call
val dummy = 1.to(10).map(((i) => i));
var dummy2 = Seq.empty[Int];
var i = 0;
// *******************
// classic while loop
while$1(){
if (i.$less$eq(10))
{
dummy2.$colon$plus$eq(i);
while$1()
}
else
()
}
}
}