Scala parser combinators: how to return the content of intermediate parsers when combined with “into”?

Question

Here is what I am trying to do:

def parser = parser_a >> {
  case a => val c = compute(a) ; parser_b(c)
} ^^ {
  case a ~ b => (a, b)
}

Of course it won't work, since the function after the ^^ operator only gets the result of parser_b . How can I keep the result of parser_a ?

Answer 1

You can use the fact that parsers are monadic to write this as follows:

val parser = for {
  a <- parser_a
  b <- parser_b(compute(a))
} yield (a, b)

Alternatively you could change the following line in your solution (note that success here is just a less specific version of the general monadic return ).

  case a => val c = compute(a) ; success(a) ~ parser_b(c)

I personally find the for -comprehension a little clearer in this case, though.