diff -r 47a299e7010f -r 8d0af38389bc progs/comb2.scala
--- a/progs/comb2.scala	Sun Jul 28 01:00:41 2019 +0100
+++ b/progs/comb2.scala	Sun Jul 28 14:24:46 2019 +0100
@@ -1,28 +1,28 @@
-// A parser and evaluator for the while language
+// A parser and interpreter for the While language
 // 
 
 import scala.language.implicitConversions
 import scala.language.reflectiveCalls
 
 
-abstract class Parser[I <% Seq[_], T] {
+abstract class Parser[I, T](implicit ev: I => Seq[_]) {
   def parse(ts: I): Set[(T, I)]
 
   def parse_all(ts: I) : Set[T] =
     for ((head, tail) <- parse(ts); if (tail.isEmpty)) yield head
 }
 
-class SeqParser[I <% Seq[_], T, S](p: => Parser[I, T], q: => Parser[I, S]) extends Parser[I, (T, S)] {
+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 ((head1, head2), tail2)
 }
 
-class AltParser[I <% Seq[_], T](p: => Parser[I, T], q: => Parser[I, T]) extends Parser[I, T] {
+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 <% Seq[_], T, S](p: => Parser[I, T], f: T => S) extends Parser[I, S] {
+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)
 }
@@ -48,7 +48,7 @@
 
 implicit def string2parser(s : String) = StringParser(s)
 
-implicit def ParserOps[I<% Seq[_], T](p: Parser[I, T]) = new {
+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)
@@ -141,7 +141,7 @@
 // an interpreter for the WHILE language
 type Env = Map[String, Int]
 
-def eval_aexp(a: AExp, env : Env) : Int = a match {
+def eval_aexp(a: AExp, env: Env) : Int = a match {
   case Num(i) => i
   case Var(s) => env(s)
   case Aop("+", a1, a2) => eval_aexp(a1, env) + eval_aexp(a2, env)
@@ -174,4 +174,6 @@
 
 def eval(bl: Block) : Env = eval_bl(bl, Map())
 
+// parse + evaluate fib program; then lookup what is
+// stored under the variable result 
 println(eval(Block.parse_all(fib).head)("result"))