diff -r 46eee459a999 -r 66adcae6c762 progs/parser-combinators/comb2.sc
--- a/progs/parser-combinators/comb2.sc	Thu Oct 05 14:36:54 2023 +0100
+++ b/progs/parser-combinators/comb2.sc	Fri Oct 13 15:07:37 2023 +0100
@@ -18,6 +18,8 @@
 
 case class ~[+A, +B](x: A, y: B)
 
+type IsSeq[I] = I => Seq[_]
+
 abstract class Parser[I, T](using is: I => Seq[_])  {
   def parse(in: I): Set[(T, I)]  
 
@@ -29,22 +31,22 @@
 // parser combinators
 
 // alternative parser
-class AltParser[I, T](p: => Parser[I, T], 
-                      q: => Parser[I, T])(using I => Seq[_]) extends Parser[I, T] {
+class AltParser[I : IsSeq, T](p: => Parser[I, T], 
+                              q: => Parser[I, T]) extends Parser[I, T] {
   def parse(in: I) = p.parse(in) ++ q.parse(in)   
 }
 
 // sequence parser
-class SeqParser[I, T, S](p: => Parser[I, T], 
-                         q: => Parser[I, S])(using I => Seq[_]) extends Parser[I, ~[T, S]] {
+class SeqParser[I : IsSeq, T, S](p: => Parser[I, T], 
+                                 q: => Parser[I, S]) extends Parser[I, ~[T, S]] {
   def parse(in: I) = 
     for ((hd1, tl1) <- p.parse(in); 
          (hd2, tl2) <- q.parse(tl1)) yield (new ~(hd1, hd2), tl2)
 }
 
 // map parser
-class maparser[I, T, S](p: => Parser[I, T], 
-                         f: T => S)(using I => Seq[_]) extends Parser[I, S] {
+class maparser[I : IsSeq, T, S](p: => Parser[I, T], 
+                                f: T => S) extends Parser[I, S] {
   def parse(in: I) = for ((hd, tl) <- p.parse(in)) yield (f(hd), tl)
 }
 
@@ -90,7 +92,7 @@
 
 
 // more convenient syntax for parser combinators
-extension [I, T](p: Parser[I, T])(using I => Seq[_]) {
+extension [I : IsSeq, T](p: Parser[I, T]) {
   def ||(q : => Parser[I, T]) = new AltParser[I, T](p, q)
   def ~[S] (q : => Parser[I, S]) = new SeqParser[I, T, S](p, q)
   def map[S](f: => T => S) = new maparser[I, T, S](p, f)