scala - could not optimize @tailrec annotated method loop: it contains a recursive call not in tail position -

i have following recursive function want use tail recursion on it. compiler complains implementation error:error:(79, 7) not optimize @tailrec annotated method loop: contains recursive call not in tail position n match { ^

is because of loop assumes it's not in tail position?

def dsl[n,e](qnodes:qnodelike[n,e]*) = {     val markers = scala.collection.mutable.map.empty[string, n]     @tailrec     def loop(n:qnodelike[n,e]):unit = {       n match {         case qnode(head, kids:seq[halfedgelike[e,n]]) => {            for(kid <- kids){             kid match {               case emptyhalfedge() =>               case halfedge(e, n) => loop(n)             }           }         }          case qnodemarker(head, marker, kids:seq[halfedgelike[e,n]]) => {           markers.update(marker,head)           for(kid <- kids){             kid match {               case emptyhalfedge() =>               case halfedge(e, n) => loop(n)             }           }         }       }     }      loop(qnodes.head)   } 

yes, that's right. make tail recursive, should use explicit accumulator passed recursion.

however, unless have very deep , narrow trees, unlikely need tail recursive optimization, run time grow large before end stack overflow.

here's rough idea of how make tail optimized:

@tailrec def loop(n:list[qnodelike[n,e]]):unit = {   n match {     case qnode(head, kids:seq[halfedgelike[e,n]]) :: rem => {       kids match {         case nil =>         case emptyhalfedge() :: rem2 => loop(rem2 ::: rem)         case halfedge(e, n) :: rem2 => loop(n :: rem2 ::: rem)       }     }      case qnodemarker(head, marker, kids:seq[halfedgelike[e,n]]) :: rem => {       markers.update(marker,head)       kids match {         case nil =>         case emptyhalfedge() :: rem2 => loop(rem2 ::: rem)         case halfedge(e, n) :: rem2 => loop(n :: rem2 ::: rem)       }     }      case nil =>   } }