Why is there a difference in behavior between these two pattern matches in a for comprehension?

Question

Consider this Map[String, Any]:

val m1 = Map((\"k1\" -> \"v1\"), (\"k2\" -> 10))

Now let\'s write a for:

Answer 1

Well, the latter example doesn't work because it isn't spec'ed to. There's some discussion as to what would be the reasonable behavior. Personally, I'd expect it to work just like you. The thing is that:

val v: String = (10: Any) // is a compile error
(10: Any) match {
    case v: String =>
} // throws an exception

If you are not convinced by this, join the club. :-) Here's a workaround:

for (va @ (v: String) <- l1) println(v)

Note that in Scala 3, you can:

for (case v: String <- l1) println(v)

Answer 2

The main reason for the speced behavior is that we want to encourage people to add type annotations, for clarity. If in for comprehensions, they get potentially very costly filter operations instead, that's a trap we want to avoid. However, I agree that we should make it easier to specify that something is a pattern. Probably a single pair of parens should suffice.

val x: String = y       // type check, can fail at compile time
val (x: String) = y     // pattern match, can fail at run time

for (x: String <- ys)   // type check, can fail at compile time
for ((x: String) <- ys) // pattern match, can filter at run time