F# Parser Monad (http://www.cs.nott.ac.uk/~gmh/pearl.pdf)

// F# Parser Monad (http://www.cs.nott.ac.uk/~gmh/pearl.pdf)

type Parser<'a> = Parser of (string ->('a * string) list)

let extract(Parser(f)) = f

type ParserMonad() =
    member b.Bind(p, f) = Parser (fun cs ->
                                            let r = extract(p) cs
                                            let r' = List.map (fun (a,cs') -> extract(f a) cs') r
                                            List.concat r')
    member b.Return(x) = Parser (fun cs -> [x,cs])
    member b.Zero() = Parser (fun cs -> [])


let (++) p q = Parser(fun cs -> List.append (extract p cs) (extract q cs))
let (+++) p q = Parser(fun cs -> match (extract(p ++ q) cs) with
                                 | [] -> []
                                 | x::xs -> [x])

let parser = ParserMonad()