Build an activity from a post, using its associated like and comments.

buildActivity.scala

type CommentsOrLikes = Either[Comments, Likes]

def buildActivity(post: Post): Promise[Activity] = {

    val likeUrl = LIKE_URL % (post.id, target.token)
    val commentsUrl = COMMENT_URL % (post.id, target.token)

    /*Construct paging enumerators, mapping each value to either Left or Right*/
    val comments = pagingEnumerator(commentsUrl).map(Left.apply) 
    val likes = pagingEnumerator(likeUrl).map(Right.apply) 
       
    //Enumerate likes and comments in parallel with the 'interleave' function
    val content:Enumerator[CommentsOrLikes] = likes interleave comments

    /*Initial value for fold*/
    val activity = Activity(Nil, post, Nil)
    
    /*Fold over each enumerated value, building the activity as we go*/
    val activityIterateePromise = 
        content |>> Iteratee.fold[CommentsOrLikes, Activity](activity) {        
            case Left(comments) => activity copy (comments = activity.comments ++ comments)
            case Right(likes)   => activity copy (likes = activity.likes ++ likes)        
        }

    /* Finally, activityIterateePromise will be a Promise[Iteratee[Activity]], 
     * which we need to turn into an Iteratee[Activity] and then run it to 
     * actually build our activity */
    Iteratee.flatten(activityIterateePromise).run    
}