Scala's tuple unwrapping nuance

Question

I\'ve noticed the following behavior in scala when trying to unwrap tuples into vals:

scala> val (A, B, C) = (1, 2, 3)
:5: error: not found: va         


        

Answer 1

Yes, and it gets worse:

val (i, j) : (Int, Int) = "Hello" -> "World"

The above will compile and fail at runtime with a ClassCastException. It is easy to forget that the (i, j) declaration is a pattern.

EDIT: for ziggystar, the Scala assignment rules state that in the statement:

val p = expr //or var

p can be either an identifier or a pattern (see section 15.7 of Programming in Scala, pp284). So for example, the following is valid:

val x :: y :: z :: rest = List(1, 2, 3, 4)

Taking this together with the fact that patterns are erased (i.e. parametric type information is unchecked) means that my original example will compile.