Scala for comprehensions/loops and typed patterns

Question

According to section 6.19 of the Scala Language Specification this for loop:

for (e <-p) e\'

is translated to:

p <- e         


        

Answer 1

The translation you get from specification is actually

list.withFilter{case b: B => true; case _ => false}.foreach{case b: B => println(b)}

but it still compiles and works. It seems like Scala is losing the case and translating to

list.withFilter{case b: B => true; case _ => false}.foreach{b: B => println(b)}

which would give the same error.

This turns out to be a known and old bug: https://github.com/scala/bug/issues/900.

Workaround provided there:

object Typed { def unapply[A](a: A) = Some(a) }

for { Typed(b: B) <- list } println(b)