This is not a valid type definition:
scala> type Addable = { def +(subject: Addable) }
<console>:4: error: illegal cyclic reference involving type Addable
type Addable = { def +(subject: Addable) }
Can this be expressed in scala?
Asked
Active
Viewed 1,717 times
7
Thomas Jung
- 32,428
- 9
- 84
- 114
3 Answers
5
No, it can't.
On page 40 of The Scala Language Specification Version 2.7:
However, it is a static error if a type alias refers recursively to the defined type constructor itself. That is, the type T in a type alias type t[tps] = T may not refer directly or indirectly to the name t.
Walter Chang
- 11,547
- 2
- 47
- 36
1
Here's what I did in a library I wrote, HTH:
trait Addable {
type AddableType <: Addable
def + (subject: AddableType): AddableType
}
trait Rational extends Addable {
type AddableType = Rational
override def + (subject: Rational): Rational
}
Yardena
- 2,837
- 20
- 17
-
The definition has to be a structural type as the underlining classes cannot be changed (like Int, Long). – Thomas Jung Aug 12 '09 at 05:11
-
2Hm, perhaps you could define a view (implicit def) to convert these classes to Addable? – Yardena Aug 12 '09 at 15:41
0
It seems this may be "fixed" in Scala 2.8:
skaffman
- 398,947
- 96
- 818
- 769
-
It does not work with the current nightly build. Welcome to Scala version 2.8.0.r18457-b20090810020144 (Java HotSpot(TM) Client V M, Java 1.6.0_12). Type in expressions to have them evaluated. Type :help for more information. scala> class A{ | type Addable = { def +(subject: Addable) } | }
:5: error: illegal cyclic reference involving method + type Addable = { def +(subject: Addable) } At least the error message changed :-). – Thomas Jung Aug 10 '09 at 07:52 -
Did you try the experimental command line option mentioned at the bottom of the above article? – skaffman Aug 10 '09 at 08:42
-