private IEnumerable <ExecutionMessage> SLet(List <SExpression> args, Environment e)
{
Environment local = new Environment(e); //TODO BUG: (let () ...) parses the name name bindings that are the pair <EmptyList,NULL>
if (args[0] is SPair)
{
SPair nameBindings = (SPair)args[0];
if (!nameBindings.isProperList())
{
throw new Exception("Can't use impro per list in a let");
}
List <SExpression> names = nameBindings.flatten();
for (int i = 0; i < names.Count - 1; i++)
{
String name = ((SID)((SPair)names[i]).getHead()).identifier;
SExpression val = ((SPair)((SPair)names[i]).getTail()).getHead();
//Cycle through the yields until done
var elmEnum = evaluate(val, e).GetEnumerator();
SExpression elm = null;
while (elmEnum.MoveNext())
{
ExecutionMessage current = elmEnum.Current;
if (current.status == ExecStatus.DONE)
{
elm = current.returnVal; break;
}
yield return(current);
}
local.addVal(name, elm);
}
}
else if (args[0] is SEmptyList)
{
}
else
{
throw new Exception("Name bindings section of let must be a list");
}
//Lets can have an arbitrary number of statements after the name bindings, execute them for stateful effects
ExecutionMessage msg = null;
for (int i = 1; i < args.Count; i++)
{
foreach (var tmsg in evaluate(args[i], local))
{
msg = tmsg;
if (msg.status == ExecStatus.DONE)
{
continue;
}
yield return(msg);
}
}
//yield final message
yield return(msg);
}
public IEnumerable <ExecutionMessage> evaluate(SExpression expr, Environment e)
{
if (expr is SID)
{
yield return(new ExecutionMessage(ExecStatus.DONE, e.lookup((SID)expr)));
yield break;
}
else if (!(expr is SPair))
{
yield return(new ExecutionMessage(ExecStatus.DONE, expr));
yield break;
}
SPair exprL = (SPair)expr;
if (!exprL.isProperList())
{
throw new Exception("Not a proper list!");
}
//We need to get the value of the head, but that could be another step of execution that can be interrupted
//So, yield the value if it's not done. It it is, continue on
List <SExpression> elms = exprL.flatten();
var subcall = evaluate(elms[0], e).GetEnumerator();
SExpression head = null;
while (subcall.MoveNext())
{
ExecutionMessage current = subcall.Current;
if (current.status == ExecStatus.DONE)
{
head = current.returnVal;
break;
}
else
{
yield return(current);
}
}
if (!(head is SApplicable))
{
throw new Exception("SExpression not applicable!");
}
//args are going to be body. But because this is a proper list, the last element is going to be a empty list we want to drop
elms.RemoveAt(0); //drop head
elms.RemoveAt(elms.Count - 1); // remove empty list at end
SApplicable appHead = (SApplicable)head;
//Convert arguments to primatives
if (appHead.fixedArgCount)
{
if (elms.Count != appHead.argCount) // make sure expected num arguments matches num arguments
{
throw new Exception(String.Format("Expected {0} arguments, recieved {1}", appHead.argCount, elms.Count));
}
}
//Convert arguments to usable values, the goofy escape is so that specific primatives (define, lambda, let, if) are skipped
//preEval is always true for user created functions
if (appHead.preEval)
{
for (int i = 0; i < elms.Count; i++)
{
var elmEnum = evaluate(elms[i], e).GetEnumerator();
SExpression elm = null;
while (elmEnum.MoveNext())
{
ExecutionMessage current = elmEnum.Current;
if (current.status == ExecStatus.DONE)
{
elm = current.returnVal; break;
}
yield return(current);
}
elms[i] = elm;
}
}
//Actually CALL the dang thing
if (appHead is SPrimitive)
{
SPrimitive prim = (SPrimitive)appHead;
foreach (var msg in prim.func(elms, e))
{
yield return(msg);
}
yield break;
}
//Therefore is SFunc
SFunc lambda = (SFunc)appHead;
//bind names to variables to create new subenvironment
if (lambda.fixedArgCount)
{ //if there is fixed number of args, pair each evaluated argument with its name
for (int i = 0; i < elms.Count; i++)
{
lambda.env.setLocalVal(lambda.names[i].identifier, elms[i]);
}
}
else
{ //if there are not, match the generic identifier with the list of args (need to convert to SPair w/tail first)
lambda.env.setLocalVal(lambda.arglist.identifier, new SPair(elms, true));
}
callStack.Add(expr);//append current function to call stack
yield return(new ExecutionMessage(ExecStatus.PENDING_STEP, expr));
foreach (var msg in evaluate(lambda.body, lambda.env))
{
yield return(msg);
}
callStack.RemoveAt(callStack.Count - 1);
}