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package examples;
abstract class Parsers[intype] {
abstract class Parser {
type Result = Option[intype];
def apply(in: intype): Result;
/*** p &&& q applies first p, and if that succeeds, then q
*/
def &&& (def q: Parser) = new Parser {
def apply(in: intype): Result = Parser.this.apply(in) match {
case None => None
case Some(in1) => q(in1)
}
}
/*** p ||| q applies first p, and, if that fails, then q.
*/
def ||| (def q: Parser) = new Parser {
def apply(in: intype): Result = Parser.this.apply(in) match {
case None => q(in)
case s => s
}
}
}
val empty = new Parser {
def apply(in: intype): Result = Some(in)
}
val fail = new Parser {
def apply(in: intype): Result = None
}
def opt(p: Parser): Parser = p ||| empty; // p? = (p | <empty>)
def rep(p: Parser): Parser = opt(rep1(p)); // p* = [p+]
def rep1(p: Parser): Parser = p &&& rep(p); // p+ = p p*
}
abstract class ListParsers[intype] extends Parsers[intype] {
def chr(p: char => boolean): Parser;
def chr(c: char): Parser = chr(d: char => d == c);
def letter : Parser = chr(Character.isLetter);
def digit : Parser = chr(Character.isDigit);
def ident : Parser = letter &&& rep(letter ||| digit);
def number : Parser = digit &&& rep(digit);
def list : Parser = chr('(') &&& listElems &&& chr(')');
def listElems : Parser = expr &&& (chr(',') &&& listElems ||| empty);
def expr : Parser = ident ||| number ||| list;
}
class ParseString(s: String) extends Parsers[int] {
val input = 0;
def chr(p: char => boolean) = new Parser {
def apply(in: int): Parser#Result =
if (in < s.length() && p(s charAt in)) Some(in + 1);
else None;
}
}
object Test {
def main(args: Array[String]): unit =
if (args.length == 1) {
val ps = new ListParsers[int] with ParseString(args(0));
ps.exprs(input) match {
case Some(n) =>
System.out.println("parsed: " + args(0).substring(0, n));
case None =>
System.out.println("nothing parsed");
}
} else System.out.println("usage: java examples.Test <expr-string>");
}
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