Why in this example, no error is thrown and b ends up holding default value?
scala> val b = a; val a = 5
b: Int = 0
a: Int = 5
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1 Answer
When you do this in the REPL, you are effectively doing:
class Foobar { val b = a; val a = 5 }
b and a are assigned to in order, so at the time when you're assigning b, there is a field a, but it hasn't yet been assigned to, so it has the default value of 0. In Java, you can't do this because you can't reference a field before it is defined. I believe you can do this in Scala to allow lazy initialisation.
You can see this more clearly if you use the following code:
scala> class Foobar {
println("a=" + a)
val b = a
println("a=" + a)
val a = 5
println("a=" + a)
}
defined class Foobar
scala> new Foobar().b
a=0
a=0
a=5
res6: Int = 0
You can have the correct values assigned if you make a a method:
class Foobar { val b = a; def a = 5 }
defined class Foobar
scala> new Foobar().b
res2: Int = 5
or you can make b a lazy val:
scala> class Foobar { lazy val b = a; val a = 5 }
defined class Foobar
scala> new Foobar().b
res5: Int = 5
answered Jan 18, 2012 at 10:25
Matthew Farwell
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3 Comments
Rogach
Shouldn't scalac issue a warning in such situations? Since it seems to be a sure bug and will probably lead to funny bugs sometimes.
Matthew Farwell
@Rogach it could be a warning in the Scala compiler I suppose, but it may well be useful, esp. for lazy evaluation. More in the realms of a style checker such as scalastyle (github.com/scalastyle/scalastyle)
Daniel C. Sobral
@Rogach Detecting such situations reliably is hard, if not impossible. Scalac will probably have warning for simple cases.