Scala for-comprehension yielding result from multiple variable bindings

Question

I am very curious how Scala desugars the following for-comprehension:

for {
  a <- Option(5)
  b = a * 2
  c <- if (b == 10) Option(100) else None
} yield b + c

My difficulty comes from having both b and c in the yield, because they seem to be bound at different steps

Answer 1

You can even ask the compiler. The following command:

scala -Xprint:parser -e "for {
  a <- Option(5)
  b = a * 2
  c <- if (b == 10) Option(100) else None
} yield b + c"

yields this output

[[syntax trees at end of                    parser]] // scalacmd7617799112170074915.scala
package <empty> {
  object Main extends scala.AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };
    def main(args: Array[String]): scala.Unit = {
      final class $anon extends scala.AnyRef {
        def <init>() = {
          super.<init>();
          ()
        };
        Option(5).map(((a) => {
  val b = a.$times(2);
  scala.Tuple2(a, b)
})).flatMap(((x$1) => x$1: @scala.unchecked match {
          case scala.Tuple2((a @ _), (b @ _)) => if (b.$eq$eq(10))
  Option(100)
else
  None.map(((c) => b.$plus(c)))
        }))
      };
      new $anon()
    }
  }
}

Taking only the piece you are interested in and improving the readability, you get this:

Option(5).map(a => {
  val b = a * 2
  (a, b)
}).flatMap(_ match {
  case (_, b) => 
    if (b == 10)
      Option(100)
    else
      None.map(c => b + c)
})

Edit

As reported in a comment, literally translating from the compiler output seems to highlight a bug in how the desugared expression is rendered. The sum should be mapped on the result of the if expression, rather then on the None in the else branch:

Option(5).map(a => {
  val b = a * 2
  (a, b)
}).flatMap(_ match {
  case (_, b) => 
    (if (b == 10) Option(100) else None).map(c => b + c)
})

It's probably worth it to ask the compiler team if this is a bug.