diff -r 54a483a33763 -r 02ef5c3abc51 solutions/cw4/parser.sc
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/solutions/cw4/parser.sc	Fri Nov 04 12:07:40 2022 +0000
@@ -0,0 +1,162 @@
+// CW3
+
+import $file.lexer
+import lexer._ 
+
+
+case class ~[+A, +B](_1: A, _2: B)
+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] =
+    for ((head, tail) <- parse(ts); if tail.isEmpty) yield head
+}
+
+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 (new ~(head1, head2), tail2)
+}
+
+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 : 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)
+}
+
+// New parser that takes as input a list of tokens
+case class TokenListParser(ts: List[Token]) extends Parser[List[Token], List[Token]] {
+    def parse(tsb: List[Token]) = {
+        val (prefix, suffix) = tsb.splitAt(ts.length)
+        if (prefix == ts) Set((prefix, suffix)) else Set()
+    }
+}
+
+// Implicit definitions to go from a token 
+// or a list of tokens to a TokenListParser
+implicit def token2parser(t: Token) = TokenListParser(List(t))
+implicit def tokenList2parser(ts: List[Token]) = TokenListParser(ts)
+
+implicit def ParserOps[I : IsSeq, T](p: Parser[I, T]) = 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)
+}
+
+implicit def TokenOps(t: Token) = new {
+    def || (q : => Parser[List[Token], List[Token]]) = new AltParser[List[Token], List[Token]](List(t), q)
+    def || (qs : List[Token]) = new AltParser[List[Token], List[Token]](List(t), qs)
+    def ==>[S] (f: => List[Token] => S) = new FunParser[List[Token], List[Token], S](List(t), f)
+    def ~[S](q : => Parser[List[Token], S]) =
+        new SeqParser[List[Token], List[Token], S](List(t), q)
+    def ~ (qs : List[Token]) =
+        new SeqParser[List[Token], List[Token], List[Token]](List(t), qs)
+}
+
+implicit def TokenListOps(ts: List[Token]) = new {
+    def || (q : => Parser[List[Token], List[Token]]) = new AltParser[List[Token], List[Token]](ts, q)
+    def || (qs : List[Token]) = new AltParser[List[Token], List[Token]](ts, qs)
+    def ==>[S] (f: => List[Token] => S) = new FunParser[List[Token], List[Token], S](ts, f)
+    def ~[S](q : => Parser[List[Token], S]) =
+        new SeqParser[List[Token], List[Token], S](ts, q)
+    def ~ (qs : List[Token]) =
+        new SeqParser[List[Token], List[Token], List[Token]](ts, qs)
+}
+
+// Abstract Syntax Trees
+abstract class Stmt
+abstract class AExp
+abstract class BExp
+
+type Block = List[Stmt]
+
+case object Skip extends Stmt
+case class If(a: BExp, bl1: Block, bl2: Block) extends Stmt
+case class While(b: BExp, bl: Block) extends Stmt
+case class Assign(s: String, a: AExp) extends Stmt
+case class Read(s: String) extends Stmt
+case class WriteId(s: String) extends Stmt  // for printing values of variables
+case class WriteString(s: String) extends Stmt  // for printing words
+case class For(counter: String, lower: AExp, upper: AExp, code: Block) extends Stmt
+
+
+case class Var(s: String) extends AExp
+case class Num(i: Int) extends AExp
+case class Aop(o: String, a1: AExp, a2: AExp) extends AExp
+
+case object True extends BExp
+case object False extends BExp
+case class Bop(o: String, a1: AExp, a2: AExp) extends BExp
+case class And(b1: BExp, b2: BExp) extends BExp
+case class Or(b1: BExp, b2: BExp) extends BExp
+
+case class IdParser() extends Parser[List[Token], String] {
+    def parse(tsb: List[Token]) = tsb match {
+        case T_ID(id) :: rest => Set((id, rest))
+        case _ => Set()
+    }
+}
+
+case class NumParser() extends Parser[List[Token], Int] {
+    def parse(tsb: List[Token]) = tsb match {
+        case T_NUM(n) :: rest => Set((n, rest))
+        case _ => Set()
+    }
+}
+
+case class StringParser() extends Parser[List[Token], String] {
+    def parse(tsb: List[Token]) = tsb match {
+        case T_STRING(s) :: rest => Set((s, rest))
+        case _ => Set()
+    }
+}
+
+// WHILE Language Parsing
+lazy val AExp: Parser[List[Token], AExp] = 
+  (Te ~ T_OP("+") ~ AExp) ==> { case x ~ _ ~ z => Aop("+", x, z): AExp } ||
+  (Te ~ T_OP("-") ~ AExp) ==> { case x ~ _ ~ z => Aop("-", x, z): AExp } || Te
+lazy val Te: Parser[List[Token], AExp] = 
+  (Fa ~ T_OP("*") ~ Te) ==> { case x ~ _ ~ z => Aop("*", x, z): AExp } || 
+  (Fa ~ T_OP("/") ~ Te) ==> { case x ~ _ ~ z => Aop("/", x, z): AExp } || 
+  (Fa ~ T_OP("%") ~ Te) ==> { case x ~ _ ~ z => Aop("%", x, z): AExp } || Fa  
+lazy val Fa: Parser[List[Token], AExp] = 
+   (T_PAREN("(") ~ AExp ~ T_PAREN(")")) ==> { case _ ~ y ~ _ => y } || 
+   IdParser() ==> Var  || 
+   NumParser() ==> Num
+
+lazy val BExp: Parser[List[Token], BExp] = 
+   (AExp ~ T_OP("==") ~ AExp) ==> { case x ~ _ ~ z => Bop("==", x, z): BExp } || 
+   (AExp ~ T_OP("!=") ~ AExp) ==> { case x ~ _ ~ z => Bop("!=", x, z): BExp } || 
+   (AExp ~ T_OP("<") ~ AExp) ==> { case x ~ _ ~ z => Bop("<", x, z): BExp } || 
+   (AExp ~ T_OP(">") ~ AExp) ==> { case x ~ _ ~ z => Bop(">", x, z): BExp } ||
+   (T_PAREN("(") ~ BExp ~ List(T_PAREN(")"), T_OP("&&")) ~ BExp) ==> { case _ ~ y ~ _ ~ v => And(y, v): BExp } ||
+   (T_PAREN("(") ~ BExp ~ List(T_PAREN(")"), T_OP("||")) ~ BExp) ==> { case _ ~ y ~ _ ~ v => Or(y, v): BExp } ||
+   (T_KEYWORD("true") ==> (_ => True: BExp )) || 
+   (T_KEYWORD("false") ==> (_ => False: BExp )) ||
+   (T_PAREN("(") ~ BExp ~ T_PAREN(")")) ==> { case _ ~ x ~ _ => x }
+
+lazy val Stmt: Parser[List[Token], Stmt] =
+    T_KEYWORD("skip") ==> (_ => Skip: Stmt) ||
+    (IdParser() ~ T_OP(":=") ~ AExp) ==> { case id ~ _ ~ z => Assign(id, z): Stmt } ||
+    (T_KEYWORD("if") ~ BExp ~ T_KEYWORD("then") ~ Block ~ T_KEYWORD("else") ~ Block) ==> { case _ ~ y ~ _ ~ u ~ _ ~ w => If(y, u, w): Stmt } ||
+    (T_KEYWORD("while") ~ BExp ~ T_KEYWORD("do") ~ Block) ==> { case _ ~ y ~ _ ~ w => While(y, w) : Stmt } ||
+    (T_KEYWORD("read") ~ IdParser()) ==> { case _ ~ id => Read(id): Stmt} ||
+    (T_KEYWORD("write") ~ IdParser()) ==> { case _ ~ id => WriteId(id): Stmt} ||
+    (T_KEYWORD("write") ~ StringParser()) ==> { case _ ~ s => WriteString(s): Stmt} ||
+    (T_KEYWORD("for") ~ IdParser() ~ T_OP(":=") ~ AExp ~ T_KEYWORD("upto") ~ AExp ~ T_KEYWORD("do") ~ Block) ==> {
+      case _ ~ id ~ _ ~ lower ~ _ ~ upper ~ _ ~ blck => For(id, lower, upper, blck): Stmt
+    }
+
+lazy val Stmts: Parser[List[Token], Block] =
+    (Stmt ~ T_SEMI ~ Stmts) ==> { case x ~ _ ~ z => x :: z : Block } ||
+    (Stmt ==> (s => List(s) : Block))
+
+lazy val Block: Parser[List[Token], Block] =
+    (T_PAREN("{") ~ Stmts ~ T_PAREN("}")) ==> { case x ~ y ~ z => y} ||
+    (Stmt ==> (s => List(s)))
+