// cons: creates a list from an expression and an existing list
private PLList cons_fun(PLObject a, PLList b)
{
PLList retval = new PLList();
retval.Add(a);
for (int i = 0; i < b.Count; ++i)
{
retval.Add(b[i]);
}
return(retval);
}
// Evaluate a list of expressions. Return a new list that gives the
// value for each expression.
private PLList EvalPLList(PLList list, PLEnvironment localEnv, Stream traceStream)
{
PLList retval = new PLList();
for (int i = 0; i < list.Count; ++i)
{
retval.Add(Eval(list[i], localEnv, traceStream));
}
return(retval);
}
// cdr: returns the remainder of a list
private PLList cdr_fun(PLList list)
{
PLList retval = new PLList();
for (int i = 1; i < list.Count; ++i)
{
retval.Add(list[i]);
}
return(retval);
}
public PLObject GetExpression()
{
string token = GetToken();
if (token == null)
{
return(null);
}
if (token.Equals("'"))
{
// The "'" character means "quote", and preserve
// the next expression.
PLList retval = new PLList();
retval.Add(new PLStringAtom("quote"));
PLObject nextExpr = GetExpression();
if (nextExpr == null)
{
throw new Exception("'quote' was not followed by a complete expression");
}
retval.Add(nextExpr);
return(retval);
}
if (!token.Equals("("))
{
// Not the beginning of a list; the expression is
// simply this token. See if it's a number or a
// plain string.
try {
return(new PLNumberAtom(token));
}
catch (Exception) {
// Just treat it as a regular string
return(new PLStringAtom(token));
}
}
else
{
// We have the beginning of a list, which can contain
// any number of expressions. Keep building our list
// until we encounter the closing paren.
PLList retval = new PLList();
PLObject nextExpr = GetExpression();
while (!nextExpr.Equals(")"))
{
retval.Add(nextExpr);
nextExpr = GetExpression();
if (nextExpr == null)
{
throw new Exception("Incomplete expression (unbalanced parens?");
}
}
return(retval);
}
}
