The language we looked at in the previous lecture was rather

primitive and the compiler rather crude---everything was

essentially compiled into a big monolithic chunk of code

inside the main function. In this handout we like to have a

look at a slightly more comfortable language, which I call

Fun-language, and a tiny-teeny bit more realistic compiler.

The Fun-language is a functional programming language. A small

\noindent Compare the code of the fib-program with the same

program written in the While-language\ldots Fun is definitely

more comfortable. We will still focus on programs involving

integers only, that means for example that every function is

expected to return an integer. The point of the Fun language

is to compile each function to a separate method in JVM

bytecode (not just a big monolithic code chunk). The means we

need to adapt to some of the conventions of the JVM about

methods.

The grammar of the Fun-language is slightly simpler than the

While-language, because the main syntactic category are

expressions (we do not have statements). The grammar rules are

as follows:

: \meta{Exp} ::= \meta{Id} | \meta{Num} {\hspace{3cm}}

             |   \meta{Exp} + \meta{Exp} | ... | (\meta{Exp})

: \meta{Def} ::= \code{def} \textit{FunName} ($x_1$, ..., $x_n$) = \meta{Exp}\\               

\noindent where, as usual, \meta{Id} stands for variables and

\meta{Num} for numbers. We can call a function by applying the

arguments to a function name (as shown in the last clause of

\meta{Exp}). The arguments in such a function call can be

again expressions, including other function calls. In

contrast, when defining a function (see \meta{Def}-clause) the

arguments need to be variables, say $x_1$ to $x_n$. We call

the expression on the right of = in a function definition as

the \emph{body of the function}. We have the restriction that

the variables inside a function body can only be those that

are mentioned as arguments of the function. A Fun-program is

then a sequence of function definitions separated by

semicolons, and a final ``main'' call of a function that

starts the computation in the program. For example

def fact(n) = if n == 0 then 1 else n * fact(n - 1);

write(fact(5))

\noindent would be a valid Fun-program. The parser of the

Fun-language produces abstract syntax trees which in Scala

can be represented as follows:

{\small

\begin{lstlisting}[numbers=none]

abstract class Exp

abstract class BExp 

abstract class Decl

case class Var(s: String) extends Exp

case class Num(i: Int) extends Exp

case class Aop(o: String, a1: Exp, a2: Exp) extends Exp

case class If(a: BExp, e1: Exp, e2: Exp) extends Exp

case class Write(e: Exp) extends Exp

case class Sequ(e1: Exp, e2: Exp) extends Exp

case class Call(name: String, args: List[Exp]) extends Exp

case class Bop(o: String, a1: Exp, a2: Exp) extends BExp

case class Def(name: String, 

               args: List[String], 

               body: Exp) extends Decl

case class Main(e: Exp) extends Decl

\end{lstlisting}}

Let us first look at some clauses for compiling expressions.

The compilation of arithmetic and boolean expressions is just

like for the While-language and do not need any modification.

(recall that the \textit{compile}-function for boolean

expression takes a third argument for the label where the

contro-flow should jump when the boolean expression is

\emph{not} true---this is needed for compiling \pcode{if}s).

One additional feature in the Fun-language are sequences.

Their purpose is to do one calculation after another. The

reason why we need to be careful however is the convention

that every expression can only produce s single result

(including sequences). Since this result will be on the

top of the stack, we need to generate a 

\pcode{pop}-instruction in order to clean-up the stack. Given 

the expression of the form \pcode{exp1 ; exp2} we need to 

generate code where after the first code chunk

a \pcode{pop}-instruction is needed. 

\begin{lstlisting}[language=JVMIS, numbers=none,mathescape]

$\textrm{\textit{compile}}($exp1$)$

pop

$\textrm{\textit{compile}}($exp2$)$

\noindent In effect we ``forget'' about the result the first

expression calculates. I leave you to think about why this

sequence operator is still useful in the Fun-language, even 

if the first result is just ``discarded''. 

There is also one small modification we have to perform when

calling the write method. Remember in the Fun-language we have

the convention that every expression needs to return an

integer as a result (located on the top of the stack). Our

helper function implementing write, however, ``consumes'' the

top element of the stack and violates this convention.

Therefore before we call, say, \pcode{write(1+2)}, we need to

duplicate the top element of the stack like so

\begin{figure}[t]

\begin{lstlisting}[language=JVMIS, 

                   xleftmargin=2mm, 

                   numbers=none]

.method public static write(I)V 

   .limit locals 1

   .limit stack 2

   getstatic java/lang/System/out Ljava/io/PrintStream; 

   iload 0 

   invokevirtual java/io/PrintStream/println(I)V 

   return 

.end method

\caption{The helper function for printing out integers.\label{write}}

\end{figure}

\begin{lstlisting}[language=JVMIS, numbers=none,mathescape]

$\textrm{\textit{compile}}($1+2$)$

dup

\noindent We also need to first generate code for the

argument-expression of write, which in the While-language was

only allowed to be a single variable.

Most of the new code in the compiler for the Fun-language

comes from function definitions and function calls. For this

have a look again at the helper function in

Figure~\ref{write}. Assuming we have a function definition

\begin{lstlisting}[numbers=none,mathescape]

def fname (x1, ... , xn) = ...

\noindent then we have to generate

\begin{lstlisting}[numbers=none,mathescape,language=JVMIS]

.method public static fname (I...I)I

   .limit locals ??

   .limit stack ?? 

   ...

  ireturn

.method end  

\end{lstlisting}

\noindent where the number of \pcode{I}s corresponds to the

number of arguments the function has, say \pcode{x1} to

\pcode{xn}. The final \pcode{I} is needed in order to indicate

that the function returns an integer. Therefore we also have

to use \pcode{ireturn} instead of \pcode{return}. However,

more interesting are the two \pcode{.limit} lines. Locals

refers to the local variables of the method, which can be

queried and overwritten using \pcode{iload} and

\pcode{istore}, respectively.