For/Yield Approach in Scala

To write a for loop to work like a map method, add a yield statement to the end of the loop.

val result = for {

  c <- "hello, world"

  if c != 'l'

} yield c.toUpper

Adding yield to a for loop essentially places the result from each loop iteration into a temporary holding area. When the loop completes, all of the elements in the holding area are returned as a single collection.

Whereas the map or for/yield approaches are used to transform one collection into another, the foreach method is typically used to operate on each element without returning a result.

For example,
val p = (1 to 20).toList
for (k<-p if k %3 == 0) yield("<"+k+">")

vs.

p.filter(_ % 3 == 0).map(x=> "<"+x+">")

The conditioins in for comprehension are converted into a call to withFilter instead of filter. If filter were used, it would create an intermediate collection for each condition applied. The withFilter combinator delays evaluation until absolutely nessary. For a chain of withFilter calls, each withFilter invocation passes down a view to the next withFilter in line.