\chapter{The Scheme Interpreter}

After all the code required to generate the APL interpreter of Chapter 3,
the Scheme interpreter is simplicity in itself.  Of course, this has more
to do with the similarity of Scheme to the basic Lisp interpreter of
Chapter 2 than with any differences between APL and Scheme.

To implement Scheme it is only necessary to provide an implementation of
the lambda function.  This is accomplished by the class {\sf Lambda}, shown
in Figure~\ref{lambda}.  The actual implementation of lambda uses the same
class UserFunction we have seen in previous chapters.

\begin{figure}
\begin{cprog}
class LambdaFunction : public Function {
public:
	virtual void apply(Expr &, ListNode *, Environment *);
};

void LambdaFunction::apply(Expr & target, ListNode * args, Environment * rho)
{
	if (args->length() != 2) {
		target = error("lambda requires two arguments");
		return;
		}

	ListNode * argNames = args->head()->isList();
	if (! argNames) {
		target = error("lambda requires list of argument names");
		return;
		}

	target = new UserFunction(argNames, args->at(1), rho);
}
\end{cprog}
\caption{The class Lambda}\label{lambda}
\end{figure}

Initialization of the Scheme interpreter differs slightly from the code
used to initialize the Lisp interpreter (Figure~\ref{schemeinit}).  The 
{\sf define} command is no longer recognized, having been replaced by the
{\sf set}/{\sf lambda} pair.  The built-in arithmetic functions are now
considred to be global symbols, and not value-ops.  Indeed, there are no
comands or value-ops in this language.

\begin{figure}
\begin{cprog}
initialize()
{

	// initialize global variables
	reader = new LispReader;

	// initialize the value of true
	Symbol * truesym = new Symbol("T");
	true = truesym;
	false = emptyList();

	// initialize the command environment
	// there are no command or value-ops as such in scheme

	// initialize the global environment
	Environment * ge = globalEnvironment;
	ge->add(new Symbol("if"), new IfStatement);
	ge->add(new Symbol("while"), new WhileStatement);
	ge->add(new Symbol("set"), new SetStatement);
	ge->add(new Symbol("begin"), new BeginStatement);
	ge->add(new Symbol("+"), new IntegerBinaryFunction(PlusFunction));
	ge->add(new Symbol("-"), new IntegerBinaryFunction(MinusFunction));
	ge->add(new Symbol("*"), new IntegerBinaryFunction(TimesFunction));
	ge->add(new Symbol("/"), new IntegerBinaryFunction(DivideFunction));
	ge->add(new Symbol("="), new BinaryFunction(EqualFunction));
	ge->add(new Symbol("<"), new BooleanBinaryFunction(LessThanFunction));
	ge->add(new Symbol(">"), new BooleanBinaryFunction(GreaterThanFunction));
	ge->add(new Symbol("cons"), new BinaryFunction(ConsFunction));
	ge->add(new Symbol("car"), new UnaryFunction(CarFunction));
	ge->add(new Symbol("cdr"), new UnaryFunction(CdrFunction));
	ge->add(new Symbol("number?"), new BooleanUnary(NumberpFunction));
	ge->add(new Symbol("symbol?"), new BooleanUnary(SymbolpFunction));
	ge->add(new Symbol("list?"), new BooleanUnary(ListpFunction));
	ge->add(new Symbol("null?"), new BooleanUnary(NullpFunction));
	ge->add(new Symbol("primop?"), new BooleanUnary(PrimoppFunction));
	ge->add(new Symbol("closure?"), new BooleanUnary(ClosurepFunction));
	ge->add(new Symbol("print"), new UnaryFunction(PrintFunction));
	ge->add(new Symbol("lambda"), new LambdaFunction);
	ge->add(truesym, truesym);
	ge->add(new Symbol("nil"), emptyList());
}
\end{cprog}
\caption{Initialization of the Scheme Interpreter}\label{schemeinit}
\end{figure}