What is Scala's yield?

Question

I understand Ruby and Python's yield. What does Scala's yield do?

Accepted Answer

It is used in sequence comprehensions (like Python's list-comprehensions and generators, where you may use `yield` too).

It is applied in combination with `for` and writes a new element into the resulting sequence.

Simple example (from scala-lang)

```
/** Turn command line arguments to uppercase */
object Main {
  def main(args: Array[String]) {
    val res = for (a <- args) yield a.toUpperCase
    println("Arguments: " + res.toString)
  }
}
```

The corresponding expression in F# would be

```
[ for a in args -> a.toUpperCase ]
```

or

```
from a in args select a.toUpperCase 
```

in Linq.

Ruby's `yield` has a different effect.

What is Scala's yield?

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