Update calling convention

Previously, the stack would look like this when calling a function:

TOP
| arg2     |
| arg1     |
| function |
BOTTOM

This order is now reversed, with the function coming first and then the
args:

TOP
| function |
| arg2     |
| arg1     |
BOTTOM

This is a little more friendly to the stack-based machine. It is
slightly less intuitive as far as order of operations goes, but having a
function's arguments depend on the state of the function itself is a
little suspect and easy to work around if you truly need that.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile/compiler.rs

@@ -142,10 +142,10 @@ impl Compiler {
     fn emit_expr(&mut self, expr: &SpExpr) -> Thunk {
         match expr.inner() {
             Expr::Call(expr, args) => {
-                let mut thunk = VecDeque::with_capacity(args.len() + 1);
+                let mut thunk_items = VecDeque::with_capacity(args.len() + 1);
-                thunk.push_back(self.emit_expr(expr));
+                thunk_items.extend(args.iter().map(|arg| self.emit_expr(arg)));
-                thunk.extend(args.iter().map(|arg| self.emit_expr(arg)));
+                thunk_items.push_back(self.emit_expr(expr));
-                let mut thunk = Thunk::List(thunk);
+                let mut thunk = Thunk::List(thunk_items);
                 thunk.push_back(Inst::Call(args.len()));
                 thunk
             }

src/vm/inst.rs

@@ -25,8 +25,8 @@ pub enum Inst {
     /// Pops the top item off of the stack, discarding it.
     Pop,
 
-    /// Pops the top N function arguments off the stack, and attempt to call the
+    /// Pops the top item off of the stack to call, and then pops off the
-    /// next stack item.
+    /// following N arguments.
     Call(usize),
 
     /// Exits the current function stack frame and returns control to the

src/vm/mod.rs

@@ -195,6 +195,7 @@ impl Vm {
             Inst::Call(argc) => {
                 let argc = *argc;
                 let stack = self.stack_mut();
+                let fun = stack.pop().expect("No function pointer in Inst::Call");
                 let split_at = stack.len() - argc;
                 let args = stack.split_off(split_at);
 
@@ -204,7 +205,6 @@ impl Vm {
                     "Not enough arguments popped in Inst::Call"
                 );
 
-                let fun = stack.pop().expect("No function pointer in Inst::Call");
                 next_frame = self.make_frame(fun, args);
             }
             Inst::Return => {