Why can't i define a variable recursively in a code block?

Question

Why can\'t i define a variable recursively in a code block?

scala> {
     | val test: Stream[Int] = 1 #:: test
     | }
:9: error: forward          


        

Answer 1

The reason for this behavior depends on different val initialization times. If you type val x = 5 directly to the REPL, x becomes a member of an object, which values can be initialized with a default value (null, 0, 0.0, false). In contrast, values in a block can not initialized by default values.

This tends to different behavior:

scala> class X { val x = y+1; val y = 10 }
defined class X

scala> (new X).x
res17: Int = 1

scala> { val x = y+1; val y = 10; x } // compiles only with 2.9.0
res20: Int = 11

In Scala 2.10 the last example does not compile anymore. In 2.9.0 the values are reordered by the compiler to get it to compile. There is a bug report which describes the different initialization times.