Friday, 5 September 2008

Scala: String vs RichString oddities

Update: In Scala 2.8, the below is no longer true. String.reverse now returns a String rather than a RichString:

scala> "a".reverse == "a"
res0: Boolean = true


In the Scala programming language, there is a class called RichString, that adds features to the underlying Java String. In the current version of Scala (, this leads to some odd behaviour:
"Im a string" == "Im a string".reverse.reverse
returns false, while
"Im a string" == "Im a string".reverse.reverse.toString
returns true!

Just to make your head spin, the following code does indeed work as expected:
val str :String = "Im a string".reverse.reverse
println(str == "Im a string") // prints "true"
val str = "Im a string".reverse.reverse
println(str == "Im a string") // prints "false"
does not.

The explanation is that String.reverse returns a RichString, and that == returns false when comparing a String and a RichString, even though it is the "same" string (as in the example above).

If I understand it correctly, this oddity will be fixed in future releases of Scala.

(And no, Scala's == is not the same as Java's ditto. It means "equal objects" rather than "refers to the same instance of an object".)

Scala mailing list item here.

No comments: