src/Pure/System/scala.scala

--- a/src/Pure/System/scala.scala	Sun May 24 14:47:28 2020 +0200
+++ b/src/Pure/System/scala.scala	Sun May 24 19:45:42 2020 +0200
@@ -99,6 +99,9 @@
   /* registered functions */
 
   sealed case class Fun(name: String, apply: String => String)
+  {
+    override def toString: String = name
+  }
 
   lazy val functions: List[Fun] =
     Isabelle_System.services.collect { case c: Isabelle_Scala_Functions => c.functions.toList }.flatten