assignment: Add variable assignment

* Syntax for x = y
* Compiler creates Inst::Store thunks
* New Vm::store function

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

examples/hello.np

@@ -1,3 +1,4 @@
 println(123);
 println(None);
-println("hell world");
+message = "hell world";
+println(message);

src/compile/compiler.rs

@@ -37,7 +37,17 @@ impl Compiler {
 
     fn emit_stmt(&mut self, stmt: &SpStmt) -> Thunk {
         match stmt.inner() {
-            Stmt::Assign(_lhs_expr, _expr) => todo!(),
+            Stmt::Assign(lhs_expr, expr) => match lhs_expr.inner() {
+                AssignLhs::Name(n) => {
+                    let name = self.scope().lookup_scoped(n).unwrap();
+                    let mut thunk = self.emit_expr(expr);
+                    thunk.push(Inst::Store(name));
+                    thunk
+                }
+                AssignLhs::Complex(_, _) => {
+                    todo!("Complex LHS foo.bar assign")
+                }
+            },
             Stmt::Expr(expr) => {
                 let mut thunk = self.emit_expr(expr);
                 thunk.push(Inst::Pop);
@@ -91,7 +101,7 @@ impl Compiler {
     fn gather_names(&mut self, stmts: &Vec<SpStmt>) {
         for stmt in stmts {
             match stmt.inner() {
-                Stmt::Assign(lhs, _) => match lhs {
+                Stmt::Assign(lhs, _) => match lhs.inner() {
                     AssignLhs::Name(name) => {
                         self.scope_mut().insert_local(name);
                     }

src/syn/ast.rs

@@ -2,7 +2,7 @@ use crate::syn::Spanned;
 
 #[derive(Debug, Clone)]
 pub enum Stmt {
-    Assign(AssignLhs, Expr),
+    Assign(SpAssignLhs, SpExpr),
     Expr(SpExpr),
     If {
         if_true: SpCondExpr,
@@ -20,6 +20,8 @@ pub enum Stmt {
     },
 }
 
+pub type SpAssignLhs = Spanned<AssignLhs>;
+
 #[derive(Debug, Clone)]
 pub enum AssignLhs {
     /// A simple assignment.

src/syn/parser.lalrpop

@@ -22,6 +22,13 @@ pub SpStmt = Spanned<Stmt>;
 
 pub Stmt: Stmt = {
     <SpExpr> => Stmt::Expr(<>),
+    <lhs:SpAssignLhs> "=" <expr:SpExpr> => Stmt::Assign(lhs, expr),
+}
+
+pub SpAssignLhs = Spanned<AssignLhs>;
+
+pub AssignLhs: AssignLhs = {
+    <Name> => { AssignLhs::Name(<>) }
 }
 
 pub SpExpr = Spanned<Expr>;

src/vm/mod.rs

@@ -91,6 +91,18 @@ impl Vm {
         None
     }
 
+    pub fn store(&mut self, name: Name, value: Option<ObjPtr>) {
+        for frame in self.frames_mut() {
+            match frame {
+                Frame::User(frame) => {
+                    frame.locals_mut().insert(name, value);
+                    return;
+                }
+                Frame::Builtin(_frame) => continue,
+            }
+        }
+    }
+
     pub fn make_frame(&self, fun: ObjPtr, args: Vec<ObjPtr>) -> Option<Frame> {
         let fun_any = fun.as_any();
         if fun_any.is::<BuiltinFun>() {
@@ -134,7 +146,11 @@ impl Vm {
                     todo!("Throw exception: Could not load name {name:?} (is it set?)");
                 }
             }
-            Inst::Store(_name) => todo!(),
+            Inst::Store(name) => {
+                let name = *name;
+                let value = self.pop_stack().expect("Stack underflow");
+                self.store(name, Some(value));
+            }
             Inst::GetAttr(_attr) => todo!(),
             Inst::Pop => {
                 self