diff -r 3b1136fb6bee -r f0696713177b progs/fun/fun_parser.sc
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/progs/fun/fun_parser.sc	Sun Oct 25 01:25:01 2020 +0000
@@ -0,0 +1,173 @@
+// A parser for the Fun language
+//================================
+//
+// call with 
+//
+//     amm fun_parser.sc fact.fun
+//
+//     amm fun_parser.sc defs.fun
+//
+// this will generate a parse-tree from a list
+// of tokens
+
+import scala.language.implicitConversions    
+import scala.language.reflectiveCalls
+
+import $file.fun_tokens, fun_tokens._ 
+
+
+// Parser combinators
+//    type parameter I needs to be of Seq-type
+//
+abstract class Parser[I, T](implicit ev: I => Seq[_]) {
+  def parse(ts: I): Set[(T, I)]
+
+  def parse_single(ts: I) : T = 
+    parse(ts).partition(_._2.isEmpty) match {
+      case (good, _) if !good.isEmpty => good.head._1
+      case (_, err) => { 
+	println (s"Parse Error\n${err.minBy(_._2.length)}") ; sys.exit(-1) }
+    }
+}
+
+// convenience for writing grammar rules
+case class ~[+A, +B](_1: A, _2: B)
+
+class SeqParser[I, T, S](p: => Parser[I, T], 
+                         q: => Parser[I, S])(implicit ev: I => Seq[_]) extends Parser[I, ~[T, S]] {
+  def parse(sb: I) = 
+    for ((head1, tail1) <- p.parse(sb); 
+         (head2, tail2) <- q.parse(tail1)) yield (new ~(head1, head2), tail2)
+}
+
+class AltParser[I, T](p: => Parser[I, T], 
+                      q: => Parser[I, T])(implicit ev: I => Seq[_]) extends Parser[I, T] {
+  def parse(sb: I) = p.parse(sb) ++ q.parse(sb)   
+}
+
+class FunParser[I, T, S](p: => Parser[I, T], 
+                         f: T => S)(implicit ev: I => Seq[_]) extends Parser[I, S] {
+  def parse(sb: I) = 
+    for ((head, tail) <- p.parse(sb)) yield (f(head), tail)
+}
+
+// convenient combinators
+implicit def ParserOps[I, T](p: Parser[I, T])(implicit ev: I => Seq[_]) = new {
+  def || (q : => Parser[I, T]) = new AltParser[I, T](p, q)
+  def ==>[S] (f: => T => S) = new FunParser[I, T, S](p, f)
+  def ~[S] (q : => Parser[I, S]) = new SeqParser[I, T, S](p, q)
+}
+
+def ListParser[I, T, S](p: => Parser[I, T], 
+                        q: => Parser[I, S])(implicit ev: I => Seq[_]): Parser[I, List[T]] = {
+  (p ~ q ~ ListParser(p, q)) ==> { case x ~ _ ~ z => x :: z : List[T] } ||
+  (p ==> ((s) => List(s)))
+}
+
+case class TokParser(tok: Token) extends Parser[List[Token], Token] {
+  def parse(ts: List[Token]) = ts match {
+    case t::ts if (t == tok) => Set((t, ts)) 
+    case _ => Set ()
+  }
+}
+
+implicit def token2tparser(t: Token) = TokParser(t)
+
+implicit def TokOps(t: Token) = new {
+  def || (q : => Parser[List[Token], Token]) = new AltParser[List[Token], Token](t, q)
+  def ==>[S] (f: => Token => S) = new FunParser[List[Token], Token, S](t, f)
+  def ~[S](q : => Parser[List[Token], S]) = new SeqParser[List[Token], Token, S](t, q)
+}
+
+case object NumParser extends Parser[List[Token], Int] {
+  def parse(ts: List[Token]) = ts match {
+    case T_NUM(n)::ts => Set((n, ts)) 
+    case _ => Set ()
+  }
+}
+
+case object IdParser extends Parser[List[Token], String] {
+  def parse(ts: List[Token]) = ts match {
+    case T_ID(s)::ts => Set((s, ts)) 
+    case _ => Set ()
+  }
+}
+
+
+
+// Abstract syntax trees for the Fun language
+abstract class Exp extends Serializable 
+abstract class BExp extends Serializable 
+abstract class Decl extends Serializable 
+
+case class Def(name: String, args: List[String], body: Exp) extends Decl
+case class Main(e: Exp) extends Decl
+
+case class Call(name: String, args: List[Exp]) extends Exp
+case class If(a: BExp, e1: Exp, e2: Exp) extends Exp
+case class Write(e: Exp) extends Exp
+case class Var(s: String) extends Exp
+case class Num(i: Int) extends Exp
+case class Aop(o: String, a1: Exp, a2: Exp) extends Exp
+case class Sequence(e1: Exp, e2: Exp) extends Exp
+case class Bop(o: String, a1: Exp, a2: Exp) extends BExp
+
+
+
+// Grammar Rules for the Fun language
+
+// arithmetic expressions
+lazy val Exp: Parser[List[Token], Exp] = 
+  (T_KWD("if") ~ BExp ~ T_KWD("then") ~ Exp ~ T_KWD("else") ~ Exp) ==>
+    { case _ ~ x ~ _ ~ y ~ _ ~ z => If(x, y, z): Exp } ||
+  (M ~ T_SEMI ~ Exp) ==> { case x ~ _ ~ y => Sequence(x, y): Exp } || M
+lazy val M: Parser[List[Token], Exp] =
+  (T_KWD("write") ~ L) ==> { case _ ~ y => Write(y): Exp } || L
+lazy val L: Parser[List[Token], Exp] = 
+  (T ~ T_OP("+") ~ Exp) ==> { case x ~ _ ~ z => Aop("+", x, z): Exp } ||
+  (T ~ T_OP("-") ~ Exp) ==> { case x ~ _ ~ z => Aop("-", x, z): Exp } || T  
+lazy val T: Parser[List[Token], Exp] = 
+  (F ~ T_OP("*") ~ T) ==> { case x ~ _ ~ z => Aop("*", x, z): Exp } || 
+  (F ~ T_OP("/") ~ T) ==> { case x ~ _ ~ z => Aop("/", x, z): Exp } || 
+  (F ~ T_OP("%") ~ T) ==> { case x ~ _ ~ z => Aop("%", x, z): Exp } || F
+lazy val F: Parser[List[Token], Exp] = 
+  (IdParser ~ T_LPAREN ~ ListParser(Exp, T_COMMA) ~ T_RPAREN) ==> 
+    { case x ~ _ ~ z ~ _ => Call(x, z): Exp } ||
+  (T_LPAREN ~ Exp ~ T_RPAREN) ==> { case _ ~ y ~ _ => y: Exp } || 
+  IdParser ==> { case x => Var(x): Exp } || 
+  NumParser ==> { case x => Num(x): Exp }
+
+// boolean expressions
+lazy val BExp: Parser[List[Token], BExp] = 
+  (Exp ~ T_OP("==") ~ Exp) ==> { case x ~ _ ~ z => Bop("==", x, z): BExp } || 
+  (Exp ~ T_OP("!=") ~ Exp) ==> { case x ~ _ ~ z => Bop("!=", x, z): BExp } || 
+  (Exp ~ T_OP("<") ~ Exp)  ==> { case x ~ _ ~ z => Bop("<",  x, z): BExp } || 
+  (Exp ~ T_OP(">") ~ Exp)  ==> { case x ~ _ ~ z => Bop("<",  z, x): BExp } || 
+  (Exp ~ T_OP("<=") ~ Exp) ==> { case x ~ _ ~ z => Bop("<=", x, z): BExp } || 
+  (Exp ~ T_OP("=>") ~ Exp) ==> { case x ~ _ ~ z => Bop("<=", z, x): BExp }  
+
+lazy val Defn: Parser[List[Token], Decl] =
+   (T_KWD("def") ~ IdParser ~ T_LPAREN ~ ListParser(IdParser, T_COMMA) ~ T_RPAREN ~ T_OP("=") ~ Exp) ==>
+     { case _ ~ y ~ _ ~ w ~ _ ~ _ ~ r => Def(y, w, r): Decl }
+
+lazy val Prog: Parser[List[Token], List[Decl]] =
+  (Defn ~ T_SEMI ~ Prog) ==> { case x ~ _ ~ z => x :: z : List[Decl] } ||
+  (Exp ==> ((s) => List(Main(s)) : List[Decl]))
+
+
+
+// Reading tokens and Writing parse trees
+
+import ammonite.ops._
+
+def parse_tks(tks: List[Token]) : List[Decl] = 
+  Prog.parse_single(tks)
+
+@doc("Parses a file.")
+@main
+def main(fname: String) : Unit = {
+  val tks = tokenise(os.read(os.pwd / fname))
+  println(parse_tks(tks))
+}
+
+