
diff -r cf3ca219c727 -r c8ce95067c1a handouts/scala-ho.tex
--- a/handouts/scala-ho.tex	Mon Feb 22 22:09:31 2016 +0000
+++ b/handouts/scala-ho.tex	Tue Mar 22 17:09:24 2016 +0000
@@ -54,9 +54,8 @@
 
 \begin{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
 $ scala
-Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM).
-Type in expressions to have them evaluated.
-Type :help for more information.
+Welcome to Scala version 2.11.8 (Java HotSpot(TM) 64-Bit Server VM).
+Type in expressions for evaluation. Or try :help.
 
 scala>
 \end{lstlisting}
@@ -133,12 +132,12 @@
 
 \begin{center}
 \begin{tabular}{r@{\hspace{2mm}}r@{\hspace{2mm}}l@{\hspace{13mm}}l}
-  $r$ & $::=$ &   $\varnothing$         & null\\
-        & $\mid$ & $\epsilon$           & empty string\\
-        & $\mid$ & $c$                  & single character\\
-        & $\mid$ & $r_1 \cdot r_2$      & sequence\\
-        & $\mid$ & $r_1 + r_2$          & alternative / choice\\
-        & $\mid$ & $r^*$                & star (zero or more)\\
+  $r$ & $::=$ &   $\ZERO$            & null\\
+        & $\mid$ & $\ONE$            & empty string\\
+        & $\mid$ & $c$               & single character\\
+        & $\mid$ & $r_1 \cdot r_2$   & sequence\\
+        & $\mid$ & $r_1 + r_2$       & alternative / choice\\
+        & $\mid$ & $r^\star$             & star (zero or more)\\
   \end{tabular}
 \end{center}
 
@@ -155,19 +154,19 @@
 actually very simple: It first requires an \emph{abstract
 class}, say, \code{Rexp}. This will act as the type for
 regular expressions. Second, it requires a case for each
-clause in the grammar. The cases for $\varnothing$ and
-$\epsilon$ do not have any arguments, while in all the other
-cases we do have arguments. For example the character regular
-expression needs to take as an argument the character it is
-supposed to recognise. In Scala, the cases without arguments
-are called \emph{case objects}, whereas the ones with
-arguments are \emph{case classes}. The corresponding Scala
-code is as follows:
+clause in the grammar. The cases for $\ZERO$ and $\ONE$ do not
+have any arguments, while in all the other cases we do have
+arguments. For example the character regular expression needs
+to take as an argument the character it is supposed to
+recognise. In Scala, the cases without arguments are called
+\emph{case objects}, whereas the ones with arguments are
+\emph{case classes}. The corresponding Scala code is as
+follows:
 
 \begin{lstlisting}[numbers=none]
 abstract class Rexp 
-case object NULL extends Rexp
-case object EMPTY extends Rexp
+case object ZERO extends Rexp
+case object ONE extends Rexp
 case class CHAR (c: Char) extends Rexp
 case class SEQ (r1: Rexp, r2: Rexp) extends Rexp 
 case class ALT (r1: Rexp, r2: Rexp) extends Rexp 
@@ -202,20 +201,19 @@
 omit such ``tricks'' here. 
 
 Note that Scala in its response says the variable \code{r} is
-of type \lstinline[emph={ALT}]!ALT!, not \code{Rexp}. This
-might be a bit unexpected, but can be explained as follows:
-Scala always tries to find the most general type that is
-needed for a variable or expression, but does not
-``over-generalise''. In our definition the type \code{Rexp} is
-more general than \lstinline[emph={ALT}]!ALT!, since it is the
-abstract class. But in this case there is no need to give
-\code{r} the more general type of \code{Rexp}. This is
-different if you want to form a list of regular expressions,
-for example
+of type \code{ALT}, not \code{Rexp}. This might be a bit
+unexpected, but can be explained as follows: Scala always
+tries to find the most general type that is needed for a
+variable or expression, but does not ``over-generalise''. In
+our definition the type \code{Rexp} is more general than
+\code{Rexp}, since it is the abstract class. But in this case
+there is no need to give \code{r} the more general type of
+\code{Rexp}. This is different if you want
+to form a list of regular expressions, for example
 
 \begin{lstlisting}[numbers=none]
-scala> val ls = List(ALT(CHAR('a'), CHAR('b')), NULL)
-ls: List[Rexp] = List(ALT(CHAR(a),CHAR(b)), NULL)
+scala> val ls = List(ALT(CHAR('a'), CHAR('b')), ZERO)
+ls: List[Rexp] = List(ALT(CHAR(a),CHAR(b)), ZERO)
 \end{lstlisting}
 
 \noindent In this case, Scala needs to assign a common type to
@@ -301,8 +299,8 @@
 
 \begin{center}
 \begin{tabular}{r@{\hspace{2mm}}c@{\hspace{2mm}}l}
-$rev(\varnothing)$   & $\dn$ & $\varnothing$\\
-$rev(\epsilon)$      & $\dn$ & $\epsilon$\\
+$rev(\ZERO)$   & $\dn$ & $\ZERO$\\
+$rev(\ONE)$      & $\dn$ & $\ONE$\\
 $rev(c)$             & $\dn$ & $c$\\
 $rev(r_1 + r_2)$     & $\dn$ & $rev(r_1) + rev(r_2)$\\
 $rev(r_1 \cdot r_2)$ & $\dn$ & $rev(r_2) \cdot rev(r_1)$\\




