Applicative parser example in Scala

Question

This is a new version of my previous question

We can define a parser as type Parser[A] = String => List[(A, String)]. The parser takes an input string an

Answer 1

<*> (terrible name) apparently has this signature:

<*>[B](f: F[(A) ⇒ B]): F[B]

So let's just chase the types through, thinking about what a parser should do - helped by the fact that List already implements flatMap:

def <*>[A, B](fa: Parser[A], fab: Parser[(A) => B]) =
  new Parser[B] {
    def apply(s: String) =
      for {
        (a, rem1) ← fa(s)
        (ab, rem2) ← fab(rem1)
      } yield (ab(a), rem2)
  }

That looks like a sensible implementation - parse a first, then parse ab from the remainder, and then we have our result.

That example is too symbolic for me and I don't know Haskell - if you can find documentation for the <$ and <|> then I'll give it a go.