Add a couple of builtin functions, and the Vm::call() method

* Builtin functions print and println have been added
* If a global lookup fails, the VM will attempt to look up a builtin
* Vm::call(fun, args) allows interrupting the current execution state
  and starting a new function instead. It will return the value left on
  top of the stack.

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

src/vm/mod.rs

@@ -1,11 +1,11 @@
 pub mod consts;
-pub mod error; // TODO : not needed?
+pub mod error;
 pub mod frame;
 pub mod inst;
 pub mod signal;
 
 use crate::{
-    obj::{reserved::*, prelude::*},
+    obj::{builtin::BUILTIN_OBJS, reserved::*, prelude::*},
     vm::{consts::ConstPool, frame::*, inst::*, signal::*},
 };
 
@@ -91,6 +91,13 @@ impl<'c> Vm<'c> {
         self.condition = condition;
     }
 
+    /// Calls a function.
+    pub fn call(&mut self, fun: ObjRef, args: Vec<ObjRef>) -> ObjRef {
+        self.handle_signal(Signal::Call(fun, args));
+        self.resume_until_return();
+        self.pop().expect("return value")
+    }
+
     /// Resumes execution of the current program.
     pub fn resume(&mut self) {
         while !self.frames().is_empty() {
@@ -111,7 +118,8 @@ impl<'c> Vm<'c> {
             let signal = match frame {
                 Frame::Native(fun) => {
                     let callee = fun.callee().clone();
-                    (*fun.fun_ptr())(callee, self, vec![])
+                    let args = fun.args().clone();
+                    (*fun.fun_ptr())(callee, self, args)
                 }
                 Frame::User(_) => {
                     self.resume_user_fun()
@@ -200,6 +208,7 @@ impl<'c> Vm<'c> {
             }
             Inst::LoadGlobal(global) => {
                 let value = self.get_global(global)
+                    .or_else(|| self.get_builtin(global))
                     .expect("TODO: throw error for missing global");
                 self.push(value);
             }
@@ -332,7 +341,7 @@ impl<'c> Vm<'c> {
     }
 
     fn get_global(&self, name: Name) -> Option<ObjRef> {
-        self.frames
+        self.frames()
             .first()?
             .user_frame()?
             .bindings()
@@ -349,4 +358,15 @@ impl<'c> Vm<'c> {
         let bindings = frame.bindings_mut();
         bindings.insert(name.index(), value);
     }
+
+    fn get_builtin(&self, name: Name) -> Option<ObjRef> {
+        read_obj!(let fun = self.frames()
+            .first()?
+            .user_frame()?
+            .callee()
+        );
+        let fun: &UserFun = std::any::Any::downcast_ref(fun.as_any()).unwrap();
+        let sym = fun.locals()[name.index()];
+        BUILTIN_OBJS.get(&sym).cloned()
+    }
 }