/// <summary>
/// Evaluates an expression in a given lexical environment
/// </summary>
/// <param name="form"></param>
/// <param name="environment"></param>
/// <returns></returns>
public static Object Eval(Object expression, Environment environment)
{
profiler.TraceCall(expression);
if (expression == Reader.EOFVALUE)
{
return(profiler.TraceReturn(expression));
}
if (expression == null)
{
return(profiler.TraceReturn(null));
}
// The expression is either an atom or a list
if (Primitives.IsAtom(expression))
{
// Number
if (expression is double)
{
return(profiler.TraceReturn(expression));
}
if (expression is int)
{
return(profiler.TraceReturn(expression));
}
// Character
if (expression is char)
{
return(profiler.TraceReturn(expression));
}
// String
if (expression is string)
{
return(profiler.TraceReturn(expression));
}
Symbol sym = expression as Symbol;
if (sym == Symbol.TRUE)
{
return(profiler.TraceReturn(true));
}
if (sym == Symbol.FALSE)
{
return(profiler.TraceReturn(false));
}
if (sym == Symbol.NULL)
{
return(profiler.TraceReturn(null));
}
// If the symbol is bound to a value in this lexical environment
if (environment.Contains(sym))
{
// Then it's a variable so return it's value
return(profiler.TraceReturn(environment.GetValue(sym)));
}
else
{
// Otherwise symbols evaluate to themselves
return(profiler.TraceReturn(expression));
}
}
else
{
// The expression must be a list
Cons cons = (Cons)expression;
// Lists are assumed to be of the form (function arguments)
// See if there is a binding to a function, clsoure, macro or special form
// in this lexical environment
object function = environment.GetValue((Symbol)cons.First());
// If there is no binding, then use the function name directly - it's probably
// the name of a .NET method
if (function == null)
{
function = cons.First();
}
// If it's a special form
if (function is SpecialForm)
{
return(profiler.TraceReturn(((SpecialForm)function)((Cons)cons.Cdr(), environment)));
}
// If its a macro application
if (function is Macro)
{
object expansion = ((Macro)function).Expand((Cons)cons.Cdr());
return(profiler.TraceReturn(Runtime.Eval(expansion, environment)));
}
// It must be a function, closure or method invocation,
// so call apply
Object arguments = EvalList((Cons)cons.Cdr(), environment);
return(profiler.TraceReturn(Runtime.Apply(function, arguments, environment)));
}
}
