Tail-recursion and scalaz promises

I am currently playing with Scalaz non-blocking futures aka. Promises. I am struggling to make the following function tail-recursive:

@tailrec开发者_开发问答
private def repeat( res: Promise[I] ):Promise[I] =
  res map p flatMap { 
    (b:Boolean) =>
      if( b ) repeat( res flatMap f ) else res
  }

where p is a predicate with type I=>Boolean and f is a concurrent function with type I=>Promise[I].

The method compiles without the annotation.

Any hints ? Thanks

Your method isn't recursive at all. res is a computation potentially running in another thread. res map p flatMap f will immediately return a promise as far as your method is concerned. The recurrence to repeat will occur in a different process.

In slightly more terse terms, Promise is a continuation monad, and flatMap calls are automatically translated to continuation-passing style for you.

Although this looks tail recursive because the call appears only once in code, you have more than one recursive call - one for each element inside your collection. At least that's what the compiler sees. (Supposing this is a flatMap on some collection; I have no idea what p does return)

You pass the recursion to somewhere as an anonymous function. No one knows how often it will be executed.