scala - tail recursion fails (possibly because of implicit cons conversion) -

i have believe simple tail-recursive function. however, @tailrec tells me otherwise.

  @tailrec   def _next(continue : string, current : list[long], first : boolean ) : stream[long]  = {     current match {       case head :: tail => head #:: _next(continue, tail, false) //this line breaks tailrec       case nil if first => empty       case _ => {         val (nc, nl) = getids(continue)         _next(nc, nl, true)       }     }   } 

which presents me with

[error] not optimize @tailrec annotated method _next: contains recursive call not in tail position [error]  case head :: tail => head #:: _next(continue, tail, false) [error]                                     ^ 

it has implicit notification receive eclipse: - implicit conversions found: _next(continue, tail, false) => conswrapper(_next(continue, tail, false)), unfortunately, isn't helping me resolve issue.

how can fix this, and, brownie points, did go wrong thinking tail-recurse?

the problem last operation in code not call _next, stream cons operation #::.

one solution use streambuilder build stream , keep streambuilder accumulator variable.

  @tailrec   def _next(continue : string, current : list[long], first : boolean, acc: stream.streambuilder[long]) : stream[long]  = {     current match {       case head :: tail =>         acc += head         _next(continue, tail, false, acc)       case nil if first => acc.result       case _ => {         val (nc, nl) = getids(continue)         _next(nc, nl, true, acc)       }     }   } 

this not particularly efficient - streambuilder more appropriate if add entire collections using ++= instead of +=. reason, consider changing code this:

  @tailrec   def _next(continue : string, current : list[long], first : boolean, acc: stream.streambuilder[long]) : stream[long]  = {     current match {       case nil =>         acc.result       case list =>         acc += list         val (nc, nl) = getids(continue)         _next(nc, nl, true, acc)     }   }