This ticket covers an ongoing issue with the parser combinators. The last fix replaced the thread safety issue with a memory leak.That was unfortunate since it fairly easy to deal with the thread safety limitation. Resolving the memory leak requires more heroic efforts, illustrated by this code.
This issue has forced me to remain on Scala 2.9.x, which is becoming less tenable as time passes. The ticket implies that 2.11 will contain a fix, but there is no evidence of any change in the current code.
Reworking the code to use the phrase combinator side-steps the memory leak and allows an upgrade to Scala 2.10.x