了解解析器组合器〜输出

Question

Reading DSLs in Action , I saw this code for the Sequential Combinator :

def ~ [U](p: => Parser[U]): Parser[~[T, U]] =
(for(a <- this; b <- p) yield new ~(a,b)).named("~")

What is the meaning of the return type: Parser[~[T, U]] ?

Is the return type a Parser containing a type that's the result of applying ~ to arguments of type T and U ?

Answer 1

The type ~ is basically a tuple type that allows an infix notation. So the returned parser in the question parses a type that is isomorphic to (T,U) . The infix notation allows to write the following:

def intParser: Parser[Int] = ???
def intPair: Parser[~[Int,Int]] = intParser ~ intParser
def product: Parser[Int] = intPair ^^ {case f1 ~ f2 => f1 * f2}

Or in one line, as it's usually done:

def product: Parser[Int] = intParser ~ intParser ^^ {case f1 ~ f2 => f1 * f2}

Or maybe a bit more sane, with the numbers separated by a * :

def product: Parser[Int] = intParser ~ ("*" ~> intParser) ^^ {case f1 ~ f2 => f1 * f2}

Note that ~> drops the left hand side.