private void WhileStatement()
{
Loop l = new Loop();
StartLoop(l);
// Compile the condition.
Consume(TokenType.LeftParen, "Expect '(' after 'while'.");
Expression();
Consume(TokenType.RightParen, "Expect ')' after while condition.");
TestExitLoop();
LoopBody();
EndLoop();
}
// Marks the beginning of a loop. Keeps track of the current instruction so we
// know what to loop back to at the end of the body.
private void StartLoop(Loop l)
{
l.Enclosing = _loop;
l.Start = _bytecode.Count - 1 - 1;
l.ScopeDepth = _scopeDepth;
_loop = l;
}
private void ForStatement()
{
// A for statement like:
//
// for (i in sequence.expression) {
// System.write(i)
// }
//
// Is compiled to bytecode almost as if the source looked like this:
//
// {
// var seq_ = sequence.expression
// var iter_
// while (iter_ = seq_.iterate(iter_)) {
// var i = seq_.iteratorValue(iter_)
// System.write(i)
// }
// }
//
// It's not exactly this, because the synthetic variables `seq_` and `iter_`
// actually get names that aren't valid identfiers, but that's the basic
// idea.
//
// The important parts are:
// - The sequence expression is only evaluated once.
// - The .iterate() method is used to advance the iterator and determine if
// it should exit the loop.
// - The .iteratorValue() method is used to get the value at the current
// iterator position.
// Create a scope for the hidden local variables used for the iterator.
PushScope();
Consume(TokenType.LeftParen, "Expect '(' after 'for'.");
Consume(TokenType.Name, "Expect for loop variable name.");
// Remember the name of the loop variable.
string name = _parser.Source.Substring(_parser.Previous.Start, _parser.Previous.Length);
int length = _parser.Previous.Length;
Consume(TokenType.In, "Expect 'in' after loop variable.");
IgnoreNewlines();
// Evaluate the sequence expression and store it in a hidden local variable.
// The space in the variable name ensures it won't collide with a user-defined
// variable.
Expression();
int seqSlot = DefineLocal("seq ", 4);
// Create another hidden local for the iterator object.
null_(this, false);
int iterSlot = DefineLocal("iter ", 5);
Consume(TokenType.RightParen, "Expect ')' after loop expression.");
Loop l = new Loop();
StartLoop(l);
// Advance the iterator by calling the ".iterate" method on the sequence.
LoadLocal(seqSlot);
LoadLocal(iterSlot);
CallMethod(1, "iterate(_)");
// Store the iterator back in its local for the next iteration.
EmitByteArg(Instruction.StoreLocal, iterSlot);
// TODO: We can probably get this working with a bit less stack juggling.
TestExitLoop();
// Get the current value in the sequence by calling ".iteratorValue".
LoadLocal(seqSlot);
LoadLocal(iterSlot);
CallMethod(1, "iteratorValue(_)");
// Bind the loop variable in its own scope. This ensures we get a fresh
// variable each iteration so that closures for it don't all see the same one.
PushScope();
DefineLocal(name, length);
LoopBody();
// Loop variable.
PopScope();
EndLoop();
// Hidden variables.
PopScope();
}
// Initializes [compiler].
public Compiler(Parser parser, Compiler parent, bool isFunction)
{
_parser = parser;
_parent = parent;
// Initialize this to null before allocating in case a GC gets triggered in
// the middle of initializing the compiler.
_constants = new List<Obj>();
_numUpValues = 0;
_numParams = 0;
_loop = null;
_enclosingClass = null;
parser.Vm.Compiler = this;
if (parent == null)
{
_numLocals = 0;
// Compiling top-level code, so the initial scope is module-level.
_scopeDepth = -1;
}
else
{
// Declare a fake local variable for the receiver so that it's slot in the
// stack is taken. For methods, we call this "this", so that we can resolve
// references to that like a normal variable. For functions, they have no
// explicit "this". So we pick a bogus name. That way references to "this"
// inside a function will try to walk up the parent chain to find a method
// enclosing the function whose "this" we can close over.
_numLocals = 1;
if (isFunction)
{
_locals[0].Name = null;
_locals[0].Length = 0;
}
else
{
_locals[0].Name = "this";
_locals[0].Length = 4;
}
_locals[0].Depth = -1;
_locals[0].IsUpvalue = false;
// The initial scope for function or method is a local scope.
_scopeDepth = 0;
}
_bytecode = new List<byte>();
}
// Ends the current innermost loop. Patches up all jumps and breaks now that
// we know where the end of the loop is.
private void EndLoop()
{
int loopOffset = _bytecode.Count - 1 - _loop.Start + 2;
// TODO: Check for overflow.
EmitShortArg(Instruction.Loop, loopOffset);
PatchJump(_loop.ExitJump);
// Find any break placeholder instructions (which will be Instruction.END in the
// bytecode) and replace them with real jumps.
int i = _loop.Body;
while (i < _bytecode.Count - 1)
{
if (_bytecode[i] == (byte)Instruction.End)
{
_bytecode[i] = (byte)Instruction.Jump;
PatchJump(i + 1);
i += 3;
}
else
{
// Skip this instruction and its arguments.
i += 1 + GetNumArguments(_bytecode.ToArray(), _constants, i);
}
}
_loop = _loop.Enclosing;
}