diff -r a80f0cf17f91 -r 5988e44ea048 parser2.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/parser2.scala	Wed Nov 14 08:45:30 2012 +0000
@@ -0,0 +1,141 @@
+:load matcher.scala
+
+// some regular expressions
+val LETTER = RANGE("abcdefghijklmnopqrstuvwxyz".toList)
+val ID = PLUS(LETTER)
+
+val DIGIT = RANGE("0123456789".toList)
+val NONZERODIGIT = RANGE("123456789".toList)
+val NUMBER = ALT(SEQ(NONZERODIGIT, STAR(DIGIT)), "0")
+
+val LPAREN = CHAR('(')
+val RPAREN = CHAR(')')
+
+val WHITESPACE = PLUS(RANGE(" \n".toList))
+val OPS = RANGE("+-*".toList)
+
+// for classifying the strings that have been recognised
+abstract class Token
+
+case object T_WHITESPACE extends Token
+case class T_NUM(s: String) extends Token
+case class T_ID(s: String) extends Token
+case class T_OP(s: String) extends Token
+case object T_LPAREN extends Token
+case object T_RPAREN extends Token
+case object T_IF extends Token
+case object T_THEN extends Token
+case object T_ELSE extends Token
+
+def tokenizer(rs: List[Rule[Token]], s: String) : List[Token] = 
+  tokenize(rs, s.toList).filterNot(_ match {
+    case T_WHITESPACE => true
+    case _ => false
+  })
+
+
+// lexing rules for arithmetic expressions
+val lexing_rules: List[Rule[Token]]= 
+  List(("if", (s) => T_IF),
+       ("then", (s) => T_THEN),
+       ("else", (s) => T_ELSE),
+       (NUMBER, (s) => T_NUM(s.mkString)),
+       (ID, (s) => T_ID(s.mkString)),
+       (WHITESPACE, (s) => T_WHITESPACE),
+       (LPAREN, (s) => T_LPAREN),
+       (RPAREN, (s) => T_RPAREN),
+       (OPS, (s) => T_OP(s.mkString)))
+
+
+// parse trees
+abstract class ParseTree
+case class Leaf(t: Token) extends ParseTree
+case class Branch(pts: List[ParseTree]) extends ParseTree
+
+def combine(pt1: ParseTree, pt2: ParseTree) = pt1 match {
+  case Leaf(t) => Branch(List(Leaf(t), pt2))
+  case Branch(pts) => Branch(pts ++ List(pt2))
+}
+
+// parser combinators
+abstract class Parser {
+  def parse(ts: List[Token]): Set[(ParseTree, List[Token])]
+
+  def parse_all(ts: List[Token]) : Set[ParseTree] =
+    for ((head, tail) <- parse(ts); if (tail == Nil)) yield head
+
+  def || (right : => Parser) : Parser = new AltParser(this, right)
+  def ~ (right : => Parser) : Parser = new SeqParser(this, right)
+}
+
+class AltParser(p: => Parser, q: => Parser) extends Parser {
+  def parse (ts: List[Token]) = p.parse(ts) ++ q.parse(ts)   
+}
+
+class SeqParser(p: => Parser, q: => Parser) extends Parser {
+  def parse(ts: List[Token]) = 
+    for ((head1, tail1) <- p.parse(ts); 
+         (head2, tail2) <- q.parse(tail1)) yield (combine(head1, head2), tail2)
+}
+
+class ListParser(ps: => List[Parser]) extends Parser {
+  def parse(ts: List[Token]) = ps match {
+    case Nil => Set()
+    case p::Nil => p.parse(ts)
+    case p::ps =>
+      for ((head1, tail1) <- p.parse(ts); 
+           (head2, tail2) <- new ListParser(ps).parse(tail1)) yield (Branch(List(head1, head2)), tail2)
+  }
+}
+
+case class TokParser(tok: Token) extends Parser {
+  def parse(ts: List[Token]) = ts match {
+    case t::ts if (t == tok) => Set((Leaf(t), ts)) 
+    case _ => Set ()
+  }
+}
+
+implicit def token2tparser(t: Token) = TokParser(t)
+
+case object IdParser extends Parser {
+  def parse(ts: List[Token]) = ts match {
+    case T_ID(s)::ts => Set((Leaf(T_ID(s)), ts)) 
+    case _ => Set ()
+  }
+}
+
+case object NumParser extends Parser {
+  def parse(ts: List[Token]) = ts match {
+    case T_NUM(s)::ts => Set((Leaf(T_NUM(s)), ts)) 
+    case _ => Set ()
+  }
+}
+
+lazy val E: Parser = (T ~ T_OP("+") ~ E) || T  // start symbol
+lazy val T: Parser = (F ~ T_OP("*") ~ T) || F
+lazy val F: Parser = (T_LPAREN ~ E ~ T_RPAREN) || NumParser
+   
+println(E.parse_all(tokenizer(lexing_rules, "1 + 2 + 3")))
+
+def eval(t: ParseTree) : Int = t match {
+  case Leaf(T_NUM(n)) => n.toInt
+  case Branch(List(t1, Leaf(T_OP("+")), t2)) => eval(t1) + eval(t2)
+  case Branch(List(t1, Leaf(T_OP("*")), t2)) => eval(t1) * eval(t2)
+  case Branch(List(Leaf(T_LPAREN), t, Leaf(T_RPAREN))) => eval(t) 
+}
+
+(E.parse_all(tokenizer(lexing_rules, "1 + 2 + 3"))).map(eval(_))
+(E.parse_all(tokenizer(lexing_rules, "1 + 2 * 3"))).map(eval(_))
+(E.parse_all(tokenizer(lexing_rules, "(1 + 2) * 3"))).map(eval(_))
+
+lazy val EXPR: Parser = 
+  new ListParser(List(T_IF, EXPR, T_THEN, EXPR)) || 
+  new ListParser(List(T_IF, EXPR, T_THEN, EXPR, T_ELSE, EXPR)) || 
+  IdParser
+ 
+println(EXPR.parse_all(tokenizer(lexing_rules, "if a then b else c")))
+println(EXPR.parse_all(tokenizer(lexing_rules, "if a then if x then y else c")))
+
+
+
+