diff -r 53f08d873e09 -r 7af2eea19646 solutions/cw4/lexer.sc
--- a/solutions/cw4/lexer.sc	Sun Sep 17 19:12:57 2023 +0100
+++ b/solutions/cw4/lexer.sc	Tue Sep 19 09:54:41 2023 +0100
@@ -1,5 +1,5 @@
-import scala.language.implicitConversions    
-import scala.language.reflectiveCalls
+// Lexer from CW2 with additions to the rules
+//============================================
 
 // Rexp
 abstract class Rexp
@@ -27,7 +27,7 @@
 case class Rec(x: String, v: Val) extends Val
 
 
-// Convenience typing
+// Convenience for typing
 def charlist2rexp(s : List[Char]): Rexp = s match {
   case Nil => ONE
   case c::Nil => CHAR(c)
@@ -37,13 +37,13 @@
 implicit def string2rexp(s : String) : Rexp = 
   charlist2rexp(s.toList)
 
-implicit def RexpOps(r: Rexp) = new {
+extension (r: Rexp) {
   def | (s: Rexp) = ALT(r, s)
   def % = STAR(r)
   def ~ (s: Rexp) = SEQ(r, s)
 }
 
-implicit def stringOps(s: String) = new {
+extension (s: String) {
   def | (r: Rexp) = ALT(s, r)
   def | (r: String) = ALT(s, r)
   def % = STAR(s)
@@ -82,7 +82,7 @@
   case RECD(_, r1) => der(c, r1)
   case RANGE(s) => if (s.contains(c)) ONE else ZERO 
   case PLUS(r1) => SEQ(der(c, r1), STAR(r1))
-  case OPTIONAL(r1) => ALT(der(c, r1), ZERO)
+  case OPTIONAL(r1) => der(c, r1)
   case NTIMES(r, i) => 
     if (i == 0) ZERO else SEQ(der(c, r), NTIMES(r, i - 1))
 }
@@ -119,7 +119,7 @@
   case RECD(x, r) => Rec(x, mkeps(r))
 
   case PLUS(r) => Stars(List(mkeps(r)))   // the first copy must match the empty string
-  case OPTIONAL(r) => Right(Empty)
+  case OPTIONAL(r) => if (nullable(r)) Stars(List(mkeps(r))) else Stars(Nil) 
   case NTIMES(r, i) => Stars(List.fill(i)(mkeps(r)))
 }
 
@@ -136,7 +136,7 @@
 
   case (RANGE(_), Empty) => Chr(c)
   case (PLUS(r), Sequ(v1, Stars(vs))) => Stars(inj(r, c, v1)::vs)
-  case (OPTIONAL(r), Left(v1)) => Left(inj(r, c, v1))
+  case (OPTIONAL(r), v1) => Stars(List(inj(r, c, v1)))
   case (NTIMES(r, n), Sequ(v1, Stars(vs))) => Stars(inj(r, c, v1)::vs)
 }
 
@@ -199,20 +199,21 @@
 def lexing_simp(r: Rexp, s: String) = env(lex_simp(r, s.toList))
 
 // Language specific code
-val KEYWORD : Rexp = "while" | "if" | "then" | "else" | "do" | "for" | "to" | "true" | "false" | "read" | "write" | "skip" 
+// Language specific code
+val KEYWORD : Rexp = "while" | "if" | "then" | "else" | "do" | "for" | "upto" | "true" | "false" | "read" | "write" | "skip" | "break"
 val OP : Rexp = "+" | "-" | "*" | "%" | "/" | "==" | "!=" | ">" | "<" | ">=" | "<=" | ":=" | "&&" | "||"
-val LET: Rexp = RANGE(('A' to 'Z').toSet ++ ('a' to 'z'))
-val SYM : Rexp = LET | RANGE(Set('.', '_', '>', '<', '=', ';', ',', ':'))
+val LET: Rexp = RANGE(('A' to 'Z').toSet ++ ('a' to 'z').toSet)
+val SYM : Rexp = RANGE(Set('.', '_', '>', '<', '=', ';', ',', ':', ')', '('))
 val PARENS : Rexp = "(" | "{" | ")" | "}"
 val SEMI : Rexp = ";"
 val WHITESPACE : Rexp = PLUS(" ") | "\n" | "\t" | "\r"
 val DIGIT : Rexp = RANGE(('0' to '9').toSet)
 val DIGIT1 : Rexp = RANGE(('1' to '9').toSet)
-val STRING : Rexp = "\"" ~ (SYM | " " | "\\n" | DIGIT).% ~ "\""
+val STRING : Rexp = "\"" ~ (LET | SYM | DIGIT | " " | "\\n").% ~ "\""
 val ID : Rexp = LET ~ (LET | "_" | DIGIT).%
 val NUM : Rexp = "0" | (DIGIT1 ~ DIGIT.%)
 val EOL : Rexp = "\n" | "\r\n"
-val COMMENT : Rexp = "//" ~ (SYM | PARENS | " " | DIGIT).% ~ EOL
+val COMMENT : Rexp = "//" ~ (LET | SYM | PARENS | " " | DIGIT).% ~ EOL 
 
 val WHILE_REGS = (("k" $ KEYWORD) | 
                   ("o" $ OP) | 
@@ -224,6 +225,8 @@
                   ("n" $ NUM) |
 		  ("c" $ COMMENT)).%
 
+
+
 // Token
 abstract class Token extends Serializable 
 case class T_KEYWORD(s: String) extends Token