1 class SeqParser[I, T, S](p: => Parser[I, T], 

     2                          q: => Parser[I, S]) 

     3                              extends Parser[I, (T, S)] {

     4   def parse(sb: I) = 

     5     for ((head1, tail1) <- p.parse(sb); 

     6          (head2, tail2) <- q.parse(tail1)) 

     7             yield ((head1, head2), tail2)

     8 }

     9 

    10 class AltParser[I, T](p: => Parser[I, T], 

    11                       q: => Parser[I, T]) 

    12                           extends Parser[I, T] {

    13   def parse(sb: I) = p.parse(sb) ++ q.parse(sb)   

    14 }

    15 

    16 class FunParser[I, T, S](p: => Parser[I, T], f: T => S) 

    17   extends Parser[I, S] {

    18   def parse(sb: I) = 

    19     for ((head, tail) <- p.parse(sb)) 

    20       yield (f(head), tail)

    21 }

    22 

    23