diff -r 8067d0a8ba04 -r 1b718d6065c2 progs/comb1.scala
--- a/progs/comb1.scala	Tue Jul 30 20:19:40 2019 +0100
+++ b/progs/comb1.scala	Tue Jul 30 23:44:56 2019 +0100
@@ -1,11 +1,13 @@
 import scala.language.implicitConversions
 import scala.language.reflectiveCalls
 
-/* Note, in the lectures I did not show the implicit type consraint
- * I => Seq[_], which means that the input type 'I' needs to be
- * a sequence. */
+/* Note, in the lectures I did not show the implicit type 
+ * constraint IsSeq, which means that the input type 'I' needs 
+ * to be a sequence. */
 
-abstract class Parser[I, T](implicit ev: I => Seq[_]) {
+ type IsSeq[A] = A => Seq[_]
+
+abstract class Parser[I : IsSeq, T] {
   def parse(ts: I): Set[(T, I)]
 
   def parse_all(ts: I) : Set[T] =
@@ -13,20 +15,20 @@
         if (tail.isEmpty)) yield head
 }
 
-class SeqParser[I, T, S](p: => Parser[I, T], 
-                         q: => Parser[I, S])(implicit ev: 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(sb: I) = 
     for ((head1, tail1) <- p.parse(sb); 
          (head2, tail2) <- q.parse(tail1)) yield ((head1, head2), tail2)
 }
 
-class AltParser[I, T](p: => Parser[I, T], 
-                      q: => Parser[I, T])(implicit ev: 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(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] {
+class FunParser[I : IsSeq, T, S](p: => Parser[I, T], 
+                                 f: T => S) extends Parser[I, S] {
   def parse(sb: I) = 
     for ((head, tail) <- p.parse(sb)) yield (f(head), tail)
 }