Scala tailrec annotation error

Question

I have a Java abstract class called ImmutableEntity and several subclasses that contain a class-level annotation called @DBTable. I am trying to search a class hierarchy for the annotation using a tail-recursive Scala method:

  def getDbTableForClass[A <: ImmutableEntity](cls: Class[A]): String = {
    @tailrec
    def getDbTableAnnotation[B >: A](cls: Class[B]): DBTable = {
      if (cls == null) {
        null
      } else {
        val dbTable = cls.getAnnotation(classOf[DBTable])
        if (dbTable != null) {
          dbTable
        } else {
          getDbTableAnnotation(cls.getSuperclass)
        }
      }
    }

    val dbTable = getDbTableAnnotation(cls)
    if (dbTable == null) {
      throw new
              IllegalArgumentException("No DBTable annotation on class " + cls.getName)
    } else {
      val value = dbTable.value
      if (value != null) {
        value
      } else {
        throw new
                IllegalArgumentException("No DBTable.value annotation on class " + cls.getName)
      }
    }
  }

When I compile this code, I am getting the error: "could not optimize @tailrec annotated method: it is called recursively with different type arguments". What is wrong with my inner method?

Thanks.

score 16 · Accepted Answer · answered Dec 23 '10 at 21:03

16

It's because of the way the compiler implements tail-recursion by loops. This is done as one step in a chain of transformations from Scala to Java bytecodes. Each transformation must produce a program that's again type-correct. However, it you can't change the type of variables in mid-loop execution, that's why the compiler could not expand into a type-correct loop.

answered Dec 23 '10 at 21:03

Martin Odersky

20,470
9
51
49

1

Thanks for the explanation. BTW, great language! – Ralph Dec 24 '10 at 13:03
1

I will also observe: what a fine, precise error message! "Could not optimize @tailrec annotated method: it is called recursively with different type arguments". Whoever is writing gems like that must be one cool cat. – psp Jan 10 '11 at 09:17

score 3 · Answer 2 · answered Dec 23 '10 at 22:06

May I suggest a more succinct version of the code?

def getDbTableForClass[A <: ImmutableEntity](cls: Class[A]): String = {
@tailrec
def getDbTableAnnotation[B >: A](cls: Class[B]): DBTable = cls match {
  case null => null
  case c if c.isAnnotationPresent(classOf[DBTable]) => c.getAnnotation(classOf[DBTable])
  case other => getDbTableAnnotation(other.getSuperclass)
}

getDbTableAnnotation(cls) match {
  case null => throw new IllegalArgumentException("No DBTable annotation on class " + cls.getName)
  case dbTable if dbTable.value ne null => dbTable.value
  case other => throw new IllegalArgumentException("No DBTable.value annotation on class " + cls.getName)
}

}

score 2 · Answer 3 · answered Dec 23 '10 at 17:39

2

Since the type parameter B and its bound aren't strictly required, you can use an existential type instead,

@tailrec
def getDbTableAnnotation(cls: Class[_]): DBTable = {
  ...
}

Scala accepts this definition for tail-recursive calls.

answered Dec 23 '10 at 17:39

Alex Boisvert

2,850
2
19
18

Thanks. I'll try that. Do you know why the first form was rejected? – Ralph Dec 23 '10 at 18:14
@Ralph: No, I don't know for sure. The @tailrec optimization converts the recursive function into a loop and with erasure I don't see how that could cause trouble. It could be a compiler implementation limitation or it could be that the spec doesn't allow it because the target platform may erase types like the JVM. – Alex Boisvert Dec 23 '10 at 18:31

Scala tailrec annotation error

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